Ask The Heat Treat Doctor® has returned to bring sage advice to Heat Treat Today readers and to answer your questions about heat treating, brazing, sintering, and other types of thermal treatments as well as questions on metallurgy, equipment, and process-related issues. In this installment, Dan Herring continues his discussion on gear heat treatment, exploring vacuum and induction hardening methods for gears — from low-pressure carburizing for advanced materials to single shot and tooth-by-tooth induction techniques — and how each can be matched to the specific demands of any gear application.
This informative piece was first released in Heat Treat Today’sMarch 2026 Annual Aerospace Heat Treating print edition.
In Part One of this discussion (Air & Atmospheres Heat Treating, February 2026), we discussed various gear types, materials, and how they can be atmosphere heat treated. This month, we are focusing on vacuum and induction heat treating methods. Let’s learn more.
Vacuum Heat Treatment Processing Methods
Table A. Advanced Materials Processed by LPC
Vacuum processing can be used for most of the atmosphere treatments mentioned in Part One including carburizing (Figure 1). Low pressure carburizing (LPC) is a proven technology and the choice for many advanced applications in aerospace, automotive, off-highway, and motorsports markets, as well as the development of carburizing cycles for high-performance materials (Table A).
Figure 1. Typical commercial heat treat load of gears for vacuum carburizing (Otto and Herring 2007) | Image Credit: Photo courtesy of Midwest Thermal-VacFigure 2. Pyrowear 675 – LPC – anneal – double normalize – harden – anneal – deep freeze – double temper | Image Credit: The HERRING GROUP, Inc.
The range of effective case depths for most of these grades can range up to 2.0–3.0 mm (0.080–0.120 inches) without significant sacrifice of microstructure (Figure 2). Furnace variables, such as temperature uniformity (± 3°C or ± 5°F), control of cycle parameters (boost/diffuse times, gas flow rate, pressure, hydrocarbon type) and surface carbon optimize the microstructure, producing case uniformities of ± 0.05 mm (± 0.002 inches). Where permitted, the range of carburizing temperatures now includes the use of high temperature (> 980°C, or 1800°F) techniques.
All these advanced materials required extensive development testing to produce custom designed recipes to optimize cycle parameters. Also, quenching methods (Otto and Herring 2002) have improved, allowing us to achieve desired core properties with quenching parameter selection (high-pressure gas or oil) for distortion-sensitive and distortion-prone part geometries (Otto and Herring 2005, 2008).
Induction Hardening Methods
Various methods of hardening via applied energy are used in manufacturing gears, including flame hardening, laser surface hardening, and induction hardening.
Of the various types of applied energy processing, induction hardening is the most common. Induction heating is a process that uses alternating electrical current that induces a magnetic field, causing the surface of the gear teeth to heat. The area is then quenched resulting in an increase in hardness within the heated area. This process is typically accomplished in a relatively short time. The final desired gear performance characteristics are determined not only by the hardness profile and stresses but also by the steel composition and prior microstructure. External spur and helical gears, bevel and worm gears, racks, and sprockets are commonly induction hardened. Typical gear steels include AISI/SAE grades 1050, 1060, 1144, 4140, 4150, 4350, 5150, and 8650.
Figure 3. Patterns produced by induction hardening (Rudnev 2000)
The hardness pattern produced by induction heating (Figure 3) is a function of the type and shape of inductor used, as well as the heating method. Quenching or rapidly cooling the workpiece can be accomplished by spray or submerged quench. The media typically used for the quench is a water-based polymer. The severity of this quenchant can be controlled by the polymer’s concentration. Cooling rates are usually somewhere in between what would be obtained from pure water and oil. In some unusual situations compressed air or nitrogen is used to quench the part.
The most common methods for hardening gears and sprockets are by single shot (Figure 4) or the tooth-by-tooth method (Figure 5). Single shot often requires large kW power supplies but results in short heat/quench times and higher production rates. This technique uses a circumferential copper inductor, which will harden the teeth from the tips downward.
Figure 4. Typical single shot induction hardening operation | Image Credit: Photo courtesy of Ajax-Tocco-MagnethermicFigure 5. Tooth-by-tooth induction hardening of a helical gear | Image Credit: Photo courtesy of Ajax-Tocco-Magnethermic
The larger and heavier loaded gears (where pitting, spalling, tooth fatigue, and endurance are issues) need a hardness pattern that is more profiled like those produced by carburizing, which can be obtained by tooth-by-tooth hardening. This method is limited to gear tooth sizes with modulus 4.23–5.08 (6 or 5 DP) using frequencies from 2 to 10 kHz and about 2.54 (10 DP) using a range of 25 to 50 kHz.
The lower the frequency, the deeper the case depth. Tooth-by-tooth hardening is a slow process and usually reserved for gears and sprockets that are too large to single shot due to power constraints. The process involves heating the root area and side flanks simultaneously, while cooling each side of the adjacent tooth to prevent temper-back on the backside of each tooth. The induction system moves the coil at a pre-programmed rate along the length of the gear. The coil progressively heats the entire length of the gear segment while a quench follower immediately cools the previously heated area. The distance from the coil to the tooth is known as coupling or air gap. Any changes in this distance can yield variation in case depth, hardness, and tooth distortion. The gear is indexed after each tooth has been hardened, often skipping a tooth. This requires at least two full revolutions in the process to complete the hardening of all teeth. Straight, spur, and helical gears up to 5.5 m (210 inches) weighing 6,800 kg (15,000 lb) have been processed with this method. The entire process yields a repeatable soft tip of the tooth with hard root and flank. In other applications, the tip and both flanks can be hardened simultaneously and yield a soft root.
In Summary
Today’s design engineer has the good fortune of being able to choose from a number of heat treatment technologies for any given type of gear material and design. When selecting a gear hardening method, it is essential to specify not only the desired mechanical and metallurgical properties, but the critical dimensions that must be held and even the desired stress state of the gears themselves. The secret to success is understanding the advantages and limitations of each technology and taking these into consideration when determining the overall cost of gear manufacturing.
References
Herring, Daniel H. 2004a. “Gear Heat Treatment: The Influence of Materials and Geometry.” Gear Technology, March/April.
Herring, Daniel H. 2004b. “Reducing Distortion in Heat-Treated Gears.” Gear Solutions, June.
Herring, Daniel H. 2007a. “Oil Quenching Technologies for Gears.” With Steven D. Balme. Gear Solutions, July.
Herring, Daniel H. 2007b. “Heat Treating Heavy Duty Gears.” With Gerald D. Lindell. Gear Solutions, October.
Herring, Daniel H. 2012–2016. Vacuum Heat Treatment. Vols. 1–2. BNP Media Group.
Herring, Daniel H. 2014–2015. Atmosphere Heat Treatment. Vols. 1–2. BNP Media Group.
Herring, Daniel H., Gerald D. Lindell, D. J. Breuer, and B. Matlock. 2001. “Atmosphere vs. Vacuum Carburizing.” Heat Treating Progress, November.
Herring, Daniel H., Gerald D. Lindell, D. J. Breuer, and B. Matlock. 2002. “An Evaluation of Atmosphere and Vacuum Carburizing Methods for the Heat Treatment of Gears.” In Off-Highway Conference Proceedings. SAE International.
Otto, Frederick J., and Daniel H. Herring. 2002a. “Gear Heat Treatment: Today and Tomorrow, Part 1.” Heat Treating Progress, June.
Otto, Frederick J., and Daniel H. Herring. 2002b. “Gear Heat Treatment: Today and Tomorrow, Part 2.” Heat Treating Progress, July/August.
Otto, Frederick J., and Daniel H. Herring. 2005. “Vacuum Carburizing of Aerospace and Automotive Materials.” Heat Treating Progress, January/February.
Otto, Frederick J., and Daniel H. Herring. 2007. “Advancements in Precision Carburizing of Aerospace and Motorsports Materials.” Heat Treating Progress, May/June.
Otto, Frederick J., and Daniel H. Herring. 2008. “Improvements in Dimensional Control of Heat Treated Gears.” Gear Solutions, June.
Rudnev, V. 2000. “Gear Heat Treating by Induction.” Gear Technology, March/April.
About the Author
Dan Herring “The Heat Treat Doctor” The HERRING GROUP, Inc.
Dan Herring has been in the industry for over 50 years and has gained vast experience in fields that include materials science, engineering, metallurgy, new product research, and many other areas. He is the author of six books and over 700 technical articles.
When a load hangs up during quenching, seconds matter and improvised decisions can escalate risk. In this Technical Tuesday installment, Bruno Scomazzon, general manager of Precision Heat Treat Ltd., outlines a step-by-step emergency response procedure for exactly this scenario, which is one of the most dangerous in atmosphere heat treating. Drawing on real-world experience, this guide is intended to help companies develop their own effective procedures for maintaining safety, controlling furnace conditions, and coordinating with emergency responders in high-risk situations.
This informative piece was first released in Heat Treat Today’sFebruary 2026 Annual Air & Atmosphere Heat Treating print edition.
Scenario Overview
A load has been transferred to the quench and the elevator is lowering into the oil, but the load becomes hung up and fails to fully submerge. The inner door successfully closes, and the outer (front) door remains closed.
This is an extremely high-risk situation requiring strict adherence to emergency procedures. The goal is to protect: first the personnel (minimize the chance of injury or escalation of the situation), then the facility, and finally the equipment.
1. Immediate Actions
DO NOT Open Outer Door
There may be a natural urge to assess the situation but resist temptation. DO NOT stand in front of or directly beside the outer door and never open it during an active hang-up. Opening this door can introduce oxygen to a hot chamber, causing:
Explosions or flash fires.
Loss of containment due to door warping or mechanical failure.
In extreme cases, the outer door may be compromised (blown off, stuck open, or partially open) with visible flames. This warrants immediate escalation to the fire department.
If Outer Door Cannot Be Closed
In this scenario, immediately notify the fire department and advise them to prepare for a foam response. DO NOT allow the use of water. This may trigger violent reactions with oil or atmosphere and spread the fire!
Internal trained responders should:
Don PPE.
Retrieve fire suppression gear.
Be ready to protect critical systems until responders arrive.
DO NOT shut down the furnace.
Figure 1. Atmosphere furnace during normal operation | Image Credit: Precision Heat Treat Ltd.Figure 2. Vestibule door partially opened during a controlled simulation to illustrate gas release behavior — not an actual incident | Image Credit: Precision Heat Treat Ltd.
2. Maintain Electrical Power
To ensure essential systems stay active, you must maintain electrical power. Ensure these systems stay active:
Set the furnace cycle to manual mode from auto mode. This will bypass any PLC sequencing from auto cycling doors, elevators, and handlers.
Keep the pilots lit.
Keep the oil cooler running to prevent tank overheating.
Shut off oil heaters to prevent additional heat loading in the quench tank.
Keep quench agitation on low during the entire period to assist in lowering the temperature at the interface surface area between the hot load and the oil. This prevents stratification and dissipates radiant heat into the oil.
Keep the recirculating fan running.
Keep the instrumentation functioning for monitoring.
NOTE: Loss of these systems eliminates visibility, atmosphere control, and safe response options.
3. Atmosphere Management
Maintain a protective atmosphere and positive furnace pressure to prevent oxygen ingress and uncontrolled combustion:
Set the carbon control to “0”.
Shut off the enriching gas.
Shut off the ammonia.
Shut off the dilution air.
Nitrogen Purge
These steps depend on whether a nitrogen purge is available; it is highly advised that nitrogen purge be available for all IQ or straight through units. Be sure you understand how long it takes for your specific furnace to fully purge endothermic gas. While NFPA 86 recommends five volume turnovers, some experts advise planning for up to ten per hour in an emergency. Each furnace should have established purge data under normal conditions so operators can act with confidence when time is critical.
Figure 3. Bulk nitrogen supply used for emergency purging and atmosphere control | Image Credit: Precision Heat Treat Ltd.
Begin a nitrogen purge immediately (if available) and maintain it throughout the event.
Use at least the minimum flow rate specified in your documentation. If safe, higher flow may be used to help displace flammable gases from the heating and quench chambers.
Maintain furnace temperature at 1500°F during the purge.
Residual pockets of Endo gas may remain trapped in less ventilated areas. If the chamber temperature drops below the ignition point before all flammable gas has been displaced, the introduction of oxygen could trigger an explosion. In some cases, trapped Endo and pressure imbalances can lead to sudden releases (“furnace burp”), where oil or gas is expelled due to internal pressure buildup.
After the Purge
The goal of the nitrogen purge is to displace Endothermic gas with an inert atmosphere while maintaining elevated temperature to assist in burning off residual flammable gases and preventing dangerous mixtures. This process must ensure positive pressure throughout the furnace.
A purge followed by plunge cooling in nitrogen is a valid approach if the purge is verifiably complete.
Depending on furnace size and cooling rate:
Larger furnaces may cool slowly enough for a complete purge.
Smaller or faster-cooling units may require a brief temperature hold before controlled cooling or plunge cooling.
NOTE: Once the hung-up load cools to a safe temperature (~150°F), perform a standard shutdown.
Without Nitrogen (in Endo)
If there is no nitrogen purge, or it is insufficient, the only option is to let the hung-up load cool in the vestibule while continuing to burn Endo and maintain the furnace temperature at 1500°F. Once the vestibule/oil tank cools below 150°F and the danger has passed, initiate a standard furnace shutdown.
4. Safety Management
Alert the local fire department immediately. If the situation becomes unmanageable, or if there is any doubt about the ability to maintain control, evacuate the facility and wait for trained professionals. The safety of plant personnel is paramount.
Notify plant safety and site management.
Evacuate all non-essential personnel from the heat treat area.
Inform all departments that a high-risk incident is in progress.
Fire departments are most effective when they are familiar with your facility before an emergency occurs. Make sure they know the layout of your operation, including:
Oil tank locations and sizes
Electrical panels
Gas shutoffs
Hot zones
5. Controlled Cooling Period
Maintain atmosphere protection throughout the event.
DO NOT open doors until the vestibule’s temperature is low and stable.
Cooling time will depend on load mass and heat retention. Expect five or more hours.
Use furnace pressure stability, effluent observations, and gas behavior as indirect temperature indicators.
6. Load Recovery Procedure
Once cooled and stabilized, perform a standard shutdown, starting with the removal of endothermic gas if applicable.
DO NOT attempt manual load removal until the system is verified safe.
Only maintenance personnel may retrieve the load, using PPE and appropriate tools.
7. Fire Department Familiarization
Every facility should build rapport with the local fire department before an emergency ever happens. Schedule annual walkthroughs and identify the following:
Number of furnaces
Quench oil tank volumes
Hot zone and live panel locations
Emergency shutoff points
Stuck doors are commonly caused by failed pneumatic valves. Shutting off and bleeding compressed air may allow the mechanism to reset. Always consult your equipment manual or the manufacturer before attempting corrective action.
The fire inspector conducting walkthroughs is not the one coming to fight your fires — train the ones who are.
8. Post-Incident Protocol
Before returning the furnace to service:
Conduct a formal investigation.
Identify and correct root cause(s).
Document all key parameters and actions taken.
Re-train operators as needed.
Furnace Signage
An operator is likely to read your safety plan but may forget a vital protocol during an emergency. Having bold, brightly colored warnings printed and posted at the panel that the operator can remove and use in an emergency can be invaluable.
Final Reflections
We cannot predict every consequence. No procedure can account for every possible variable in a live emergency. Once an event is in motion, all we can do is respond with the best judgment, training, and intentions — always with the safety of people as the highest priority.
This document is intended as a working reference: a structured reference developed with care, real-world experience, and best practices. It is not a one-size-fits-all solution, but a tool to help teams create or enhance their own effective procedures and respond adaptively in high-risk situations.
Fire preparedness is essential in every heat treating facility. Fires happen, and they are not always small. It is critical to know when to act, when to evacuate, and when to call for help. Equipment manuals provide a foundation, but preparedness through training and planning is the best defense.
Acknowledgments: The author would like to thank Daniel H. Herring, “The Heat Treat Doctor,” The HERRING GROUP, Inc., and Avery Bell with Service Heat Treat in Milwaukee for their valuable input.
About The Author:
Bruno Scomazzon General Manager Precision Heat Treat Ltd.
Bruno Scomazzon is the general manager of Precision Heat Treat Ltd. in Surrey, British Columbia, Canada, with over 40 years of experience in metallurgical processes and heat treating operations.
Un austenizado insuficiente afecta mucho más que la dureza final. Interrumpe la transformación de fase, debilita el rendimiento mecánico y aumenta el riesgo de deformación o fallo en condiciones de servicio exigentes. En esta entrega de Technical Tuesday, Ana Laura Hernández Sustaita, fundadora de Consultoría Carnegie, explica los orígenes metalúrgicos de la formación incompleta de la austenita; como la uniformidad del horno, la velocidad calentamiento, la composición química del acero y la geometría de la pieza, contribuyen a ese problema; y las estrategias modernas de control de procesos y simulación que garantizan una transformación completa y resultados repetibles de alta calidad.
Este artículo informativo se publicó por primera vez enHeat Treat Today’sJanuary 2026 Annual Technologies To Watch print edition.
En inglés, el término underhardening se utiliza para describir aceros que no alcanzan una austenización completa, lo que se traduce en una pérdida de dureza después del temple. Sin embargo, en este artículo ampliaremos el análisis más allá de la dureza, centrándonos en el fenómeno de la austenización insuficiente, analizando sus causas, su influencia directa en la microestructura y en las propiedades mecánicas, así como las acciones que podemos implementar en el proceso para prevenirla.
El rol del proceso de austenización
El objetivo principal del tratamiento térmico es obtener una microestructura homogénea o mixta que garantice las propiedades mecánicas requeridas para las condiciones de servicio establecidas: resistencia a la tracción, resistencia al impacto, límite elástico, entre otras.
El proceso de austenización es el primer paso crítico para muchos procesos. Consiste en calentar el acero por encima de la temperatura A3 (normalmente entre 30 y 50°C/85 y 120°F adicionales) para obtener una microestructura con red cúbica centrada en las caras (FCC) durante un tiempo determinado. Este paso es fundamental después de procesos como solidificación, forja o laminado, ya que “reinicia” la historia microestructural del acero.
¿Qué es la austenización insuficiente?
Figura 1. Diagrama tiempo-temperatura de austenización para acero Ck 45 (SAE/AISI 1045). | Image Credit: Figure 7, ASM International 2013
La formación de austenita implica cambios estructurales y composicionales influenciados tanto por la microestructura inicial como por la composición química del acero. Cuando los parámetros de austenización no se establecen adecuadamente: temperatura insuficiente, tiempo de permanencia corto o problemas de desempeño del equipo, como la falta de uniformidad térmica del horno, la transformación no se completa. El resultado es una microestructura que conserva fases no deseadas, lo que afecta la dureza, la estabilidad dimensional y la resistencia mecánica. Por lo tanto, cualquier microestructura que no logre transformarse completamente a austenita debido a los factores mencionados puede considerarse un caso de austenización insuficiente.
Causas de la Austenización Insuficiente:
Temperatura de austenización inadecuada: si la temperatura es demasiado baja, no se logra la disolución completa de ferrita o carburos.
Tiempo de empape insuficiente: un tiempo de empape (permanencia) demasiado corto impide la difusión homogénea del carbono en la austenita.
Distribución térmica no uniforme en el horno: produce zonas con distintos grados de transformación.
Composición química del acero: los elementos de aleación modifican la cinética de difusión y desplazan las temperaturas críticas de transformación.
Geometría y dimensiones de la pieza: las secciones más grandes demandan mayor tiempo de empape, para alcanzar el calentamiento completo.
Velocidades de calentamiento rápidas: pueden impedir la homogeneización de la microestructura y generar una transformación incompleta, especialmente en procesos por inducción.
Efectos de una austenización insuficiente
Microestructura heterogénea
Tal como se ilustra en el ASM Handbook, Volume 4A: Steel Heat Treating Fundamentals and Processes, la cinética de formación de la austenita depende fuertemente de la velocidad de calentamiento. A bajas velocidades, la homogeneización por difusión ocurre a temperaturas relativamente menores; en contraste, los calentamientos rápidos generan heterogeneidad microestructural, un efecto especialmente crítico en procesos como el endurecimiento por inducción o el calentamiento directo por flama. En otras palabras, la austenización insuficiente se presenta con mayor frecuencia cuando se emplean altas velocidades de calentamiento.
En consecuencia, una microestructura con composición heterogénea provoca variaciones en las temperaturas de transformación martensítica (Ms y Mf) a lo largo de la pieza. Durante el temple, las regiones con menor contenido de carbono transforman primero, originando una martensita más suave, mientras que las zonas más ricas en carbono transforman a menores temperaturas, generando tensiones internas y una microestructura inconsistente.
Mayor riesgo de deformaciones y fallas prematuras en servicio
Anteriormente se mencionó que el proceso de austenización implica un cambio en la estructura cristalina del material. Si este cambio no es homogéneo a lo largo de la pieza, se presentarán diferentes fases, resultando en un arreglo cristalográfico variado y, por ende, un cambio volumétrico. Por otra parte, calentar una pieza muy rápidamente provoca que el calor no se distribuya ni penetre de manera uniforme, causando transformaciones heterogéneas y, por lo tanto, tensiones debido a los cambios volumétricos en la estructura cristalina.
Reducción en la dureza y resistencia mecánica
Una austenización incompleta deja restos de ferrita o carburos no disueltos en la microestructura, que impide la transformación completa a martensita durante el temple, reduciendo la dureza final. Además, una menor cantidad de carbono en solución afecta negativamente la resistencia mecánica.
Aumento de la fragilidad y menor tenacidad
Una microestructura heterogénea (ferrita y perlita con martensita parcial y austenita retenida) disminuye la resistencia mecánica. Esto se traduce en menor capacidad para soportar cargas sin fracturarse.
Como prevenir la austenización ineficiente
Control preciso de temperatura y tiempo del horno
Figura 2. Ejemplo de un análisis de carga | Image Credit: Consultoría Carnegie
Para garantizar un control adecuado durante el mantenimiento, es fundamental utilizar termopares calibrados y ubicarlos estratégicamente dentro del horno para asegurar mediciones precisas. La calibración periódica previene errores en la lectura de temperatura, lo que contribuye directamente a la calidad del proceso. Además, es indispensable contar con la asesoría de un experto para determinar la vida útil recomendada de los termopares. Mantener una trazabilidad adecuada y realizar los reemplazos en tiempo y forma asegurará un funcionamiento óptimo del sistema.
Por otra parte, el uso de ventiladores internos en hornos de convecciones nos ayudara a mantener una uniformidad térmica dentro del horno, evitando zonas frías o calientes.
Otra forma de poder controlar la temperatura del proceso es el uso de registradores de temperatura o graficadores de temperatura. Estos dispositivos, conectados a termopares de contacto instalados directamente en las piezas, son especialmente recomendables para componentes con geometrías complejas con grandes espesores. Su función es registrar la temperatura en tiempo real y verificar que no existan fluctuaciones durante el tiempo de mantenimiento.
Distribución adecuada de la carga
En cargas donde es necesario realizar el tratamiento térmico de una cantidad considerable de piezas, es recomendable llevar a cabo un estudio para determinar la altura máxima de apilamiento que permita un flujo de calor adecuado y un calentamiento homogéneo. Un análisis preliminar puede realizarse colocando termopares estratégicamente en diferentes ubicaciones y en distintas piezas: por ejemplo, en la primera pieza de la carga, otra en la parte media y una más en la parte inferior de la torre de apilamiento.
Una vez que las piezas ingresan al proceso, es posible monitorear el comportamiento térmico de cada una de ellas, verificando que el tiempo de empape sea suficiente para que todas alcancen la transformación requerida al llegar a la temperatura objetivo, o bien, determinar si es necesario realizar ajustes en la configuración de la carga.
Uso simulación termodinámica para optimizar los parámetros del proceso
Cada grado de acero tiene una temperatura óptima de austenización determinada por su composición química. En los aceros al carbono (serie 10xx), estas temperaturas pueden estimarse mediante el diagrama Fe–C; sin embargo, cuando se incorporan elementos de aleación, dicho diagrama deja de ser suficiente. En esos casos, es necesario recurrir al cálculo de temperaturas críticas o al uso de herramientas más precisas, como simulaciones termodinámicas mediante software especializado, por ejemplo, Thermo-Calc®.
Aunque lo ideal sería tratar cada material a su temperatura específica, en la producción industrial esto no es eficiente, ya que implicaría procesar cada pieza de manera individual, lo cual ralentizaría la línea de fabricación y aumentaría el consumo de recursos, como tiempo y gas.
El uso de herramientas termodinámicas como ThermoCalc software ® permite evaluar cómo las variaciones en la composición química (debidas a tolerancias de colada o ajustes en elementos de aleación) afectan las temperaturas de transformación. Esto facilita la selección de una temperatura óptima de proceso que garantice que, para cada composición posible dentro de las especificaciones, las temperaturas de austenización sean las adecuadas. Con ello se optimiza el rendimiento del tratamiento térmico y se mejora la reproducibilidad del proceso.
Por ejemplo, en la figura 3, si un acero 4140 se calienta únicamente a 750°C (1380°F) en lugar de 850°C (1560°F), la ferrita no se disolverá por completo. Como resultado, después del temple se obtendrá una microestructura compuesta por martensita blanda y ferrita residual, en lugar de una martensita homogénea y dura. Esto reduce significativamente la dureza y la resistencia mecánica del material.
Figura 3. Diagrama de un eje para un acero 4140, (Fe, 0.4C, 0.8Mn, 0.2Si, 0.8Cr, 0.2Mo, 0.02Ni) | Image Credit: Consultoría Carnegie Figura 4. Histograma de la temperatura de transformación Ac3 para un acero AISI 4140 dentro del rango de especificación. | Image Credit: Consultoría Carnegie
En el histograma (figura 4) podemos observar que, incluso tratándose del mismo grado de acero, la temperatura A₃ puede variar aproximadamente 760−776°C (1400−1429°F) únicamente debido a las tolerancias químicas establecidas en la especificación. Si además consideramos la presencia de elementos residuales o microaleantes, es evidente que no podemos esperar el mismo comportamiento durante el tratamiento térmico ni las mismas propiedades mecánicas en todas las coladas.
En estos casos, herramientas termodinámicas como ThermoCalc software® permiten evaluar un conjunto amplio de posibles composiciones químicas y determinar una temperatura de austenización óptima que sea adecuada para todas las variaciones permitidas dentro de la especificación.
Diseño de curvas/rampas de calentamiento
Para asegurar que las temperaturas de transformación se alcancen de manera homogénea (tanto en procesos con cargas de alto volumen, como en piezas con geometrías variables) es recomendable implementar un calentamiento controlado. Aunque esto puede aumentar el tiempo de procesamiento, los beneficios incluyen una menor probabilidad de distorsión y la garantía de lograr una transformación austenítica completa.
La clave radica en diseñar un perfil adecuado de tiempo–temperatura, el cual dependerá de factores como las dimensiones de la pieza y las propiedades del material, entre ellas: difusividad térmica, capacidad calorífica, densidad y conductividad térmica.
Conclusión
La austenización insuficiente, conocida como underhardening, representa mucho más que una simple pérdida de dureza. Es una deficiencia metalúrgica que afecta la homogeneidad microestructural, la estabilidad dimensional y el desempeño mecánico.
Mediante un control riguroso de la temperatura, el tiempo y la uniformidad del horno, combinado con herramientas modernas de simulación, los ingenieros pueden asegurar transformaciones confiables, minimizar la distorsión y lograr resultados constantes y de alta calidad en el tratamiento térmico de los aceros.
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Herring, Dan. Metallurgical Fundamentals of Heat Treatment. Industrial Heating.
Krauss, G. 1980. Principles of Heat Treatment of Steel. ASM International.
Nuñez González, G. 1990. Fallas en los Tratamientos Térmicos para Aceros Herramienta.
Thomas, L. 2018. “Austenitizing Part 2: Effects on Properties.” Knife Steel Nerds. https://knifesteelnerds.com/2018/03/01/austenitizing-part-2-effects-on-properties/.
Totten, G. E. 2007. Steel Heat Treatment: Metallurgy and Technologies. Boca Raton, FL: CRC Press.
Acerca de la autora:
Ana Laura Hernández Sustaita Fundadora Consultoría Carnegie
Ana Laura Hernández Sustaita cuenta con Maestría en Ciencia e Ingeniería de los Materiales, Es fundadora de Consultoría Carnegie, una firma de consultoría y capacitación técnica especializada en el tratamiento térmico de aceros en México. Asimismo, se desempeña como Ingeniera de Soporte Técnico en Thermo-Calc Software, brindando asistencia a clientes en México, Canada y Estados Unidos de América. Ana promueve activamente la educación metalúrgica en Latinoamérica y fomenta la integración de herramientas computacionales en la práctica industrial del tratamiento térmico.
Insufficient austenitizing affects far more than final hardness. It disrupts phase transformation, weakens mechanical performance, and increases the risk of distortion or failure in demanding service conditions. In this Technical Tuesday installment, Ana Laura Hernández Sustaita, founder of Consultoría Carnegie, explains the metallurgical origins of incomplete austenite formation, how furnace uniformity, heating rate, steel chemistry, and part geometry contribute to the problem, and modern process-control and simulation strategies that ensure full transformation and repeatable, high-quality results.
This informative piece was first released inHeat Treat Today’sJanuary 2026 Annual Technologies To Watch print edition.
When a steel part is insufficiently austenitized, it is commonly referred to as underhardening, the resulting loss of hardness after quenching. However, in this article, we will extend the discussion beyond hardness alone, exploring the phenomenon of insufficient austenitizing, analyzing its causes and direct influence on microstructure and mechanical properties, and discussing modern strategies to prevent it.
The Role of Austenitizing in Heat Treatment
The main purpose of heat treatment is to produce a homogeneous or a desired mixed microstructure that ensures the required mechanical properties for the intended service conditions: tensile strength, impact resistance, yield strength, etc. Austenitizing is the first critical step for many processes. It involves heating the steel above the A3 temperature (typically 30–50°C or 85–120°F higher) to transform its microstructure into a face-centered cubic (FCC) lattice for a certain period of time. This step resets the steel’s structural history, particularly after casting, forging, or rolling, and defines the baseline for subsequent quenching and tempering operations.
What Is Insufficient Austenitizing?
Figure 1. Time-temperature-austenitization diagram for Ck 45 (SAE/AISI 1045) steel. | Image Credit: Figure 7, ASM International 2013
Austenite formation involves structural and compositional changes influenced by the initial microstructure and the steel’s chemical composition. When austenitizing parameters are not properly established, such as insufficient temperature, inadequate soaking time, or poor furnace performance (e.g., lack of thermal uniformity), the transformation remains incomplete. The result is a microstructure containing undesired residual phases that compromise hardness, dimensional stability, and mechanical strength. Therefore, any microstructure that fails to fully transform to austenite due to these factors can be directly associated with insufficient austenitizing.
Common causes of insufficient austentizing include:
Inadequate austenitizing temperature: Ferrite and carbides do not fully dissolve if the temperature is too low.
Insufficient holding time: A short soak time prevents uniform carbon diffusion throughout the austenite.
Thermal non-uniformity in the furnace (cold zones): This leads to regions with different degrees of transformations.
Chemical composition of the steel: Alloying elements modify diffusion kinetics and impact the critical transformation temperatures.
Geometry and dimensions of the part: Larger cross-sections require longer soak times for full heat diffusivity.
Rapid heating rates: Excessive heating, especially during induction hardening, can result in structural inhomogeneity and incomplete transformation.
Effects of Insufficient Austentizing
Heterogeneous Microstructure
As illustrated in the ASM Handbook, Volume 4A: Steel Heat Treating Fundamentals and Processes (2013), the kinetics of austenite formation depend strongly on the heating rate. At lower heating rates, diffusion-driven homogenization occurs at relatively lower temperatures, whereas rapid heating produces microstructural heterogeneity, an effect that is especially critical in induction or direct-flame heating. In other words, insufficient austenitizing is more likely to occur when high heating rates are used.
Consequently, a microstructure with heterogeneous composition leads to variations in the martensite transformation temperatures (Ms and Mf) throughout the part. During quenching, regions with lower carbon content transform earlier, producing softer martensite, while areas with higher carbon content transform at lower temperatures, resulting in internal stresses and an overall inconsistent microstructure.
Risk of Distortion and Premature Failure
The transformation from BCC or BCT to FCC (Defined: BCC: body-centered cubic; BCT: body-centered tetragonal; FCC: face-centered cubic) lattice during austenitizing involves a specific volume change. If this transformation occurs unevenly, differential expansion generates internal stresses, distortion, and in severe cases, microcracks. Rapid heating or poor furnace convection exacerbates these effects by producing steep temperature gradients across the part.
Reduced Hardness and Mechanical Strength
Incomplete transformation leaves undissolved carbides and residual ferrite, reducing hardenability and the amount of carbon in solid solution. This limits the formation of martensite during quenching and lowers final hardness and strength.
Increased Brittleness and Lower Toughness
A mixed structure of ferrite, pearlite, partial martensite, and retained austenite results in mechanical anisotropy and reduced energy absorption under impact loading. This condition increases the risk of brittle fracture, particularly in high-stress or cyclic applications.
How to Prevent Insufficient Austenitizing
Accurate Furnace Control
Figure 2. Example of loading analysis | Image Credit: Consultoría Carnegie
To ensure proper process control during the soaking stage, it is essential to use calibrated thermocouples strategically positioned inside the furnace to obtain accurate temperature measurements. Regular calibration prevents temperature reading errors and directly contributes to heat treatment quality. It is also important to get advice from an expert to determine the recommended service life of the thermocouples. Maintaining proper traceability and replacing them at the appropriate intervals ensures optimal system performance.
Additionally, the use of internal circulation fans in convection furnaces helps maintain thermal uniformity, preventing the formation of hot or cold zones.
Another method to monitor and control process temperature is using temperature data loggers. These devices, which are connected to contact thermocouples and placed directly on the parts, are especially recommended for components with complex geometries or large cross-sections. They record real-time temperature data throughout the process, allowing verification that no transient fluctuations occur during the soaking period.
Accurate Loading Distribution
For loads where heat treatment must be applied to a significant number of parts, it is recommended that a study be conducted to determine the maximum stacking height that will ensure proper heat flow and uniform heating. A preliminary assessment can be performed by strategically placing thermocouples in different locations and on different parts, for example, on the first part in the load, one in the middle section, and another at the bottom of the stacking tower.
Once the parts enter the process, their heating behavior can be monitored to verify that the soaking time is sufficient for all pieces in the stack to complete their transformation upon reaching the target temperature or to determine whether adjustments to the loading configuration are necessary.
Use of Thermodynamic Simulation to Optimize Process Parameters
Each steel grade has an optimum austenitizing temperature in function of its chemical composition. For carbon steels (10XX series), these temperatures can be estimated using the Fe–C diagram; however, once alloying elements are added, this diagram is no longer sufficient. In such cases, it becomes necessary to rely on critical temperature calculations or on more advanced tools such as thermodynamic simulations using specialized software, like Thermo-Calc®.
Although the ideal scenario would be to heat treat each material at its specific optimum temperature, this approach is impractical in industrial production; the required processing of each part individually would slow the manufacturing line and increasing resource consumption, including time and fuel.
Thermodynamic tools such as Thermo-Calc allow engineers to evaluate how variations in chemical composition (arising from casting tolerances or adjustments in alloying elements) affect transformation temperatures. This enables the selection of an optimum processing temperature that ensures complete austenitization for all possible compositional variations within the specification. As a result, the heat treatment operation becomes more robust, more reproducible, and more energy efficient.
For example, in Figure 3, if a 4140 steel is heated only to 750°C (1380°F) instead of 850°C (1560°F), the ferrite will not fully dissolve. As a result, the microstructure will consist of soft martensite and residual ferrite after quenching, rather than a fully homogeneous and hard martensitic structure. This significantly reduces the material’s hardness and mechanical strength.
Figure 3. Equilibrium diagram, AISI 4140 0.38C, 0.78Mn, 0.85Cr, 0.22Mo (%wt.) | Image Credit: Consultoría CarnegieFigure 4. Histogram of Ac3 transformation temperature for AISI 4140 steel within the specification range. | Image Credit: Consultoría Carnegie
We can observe in the histogram (Figure 4) that even within the same steel grade, the A3 temperature can vary from approximately 760−776°C (1400−1429°F) solely due to the composition tolerances specified for the alloy. If we also consider the presence of residual or microalloying elements, it becomes clear that we cannot expect identical behavior during heat treatment or identical mechanical properties across all heats.
In such cases, thermodynamic tools allow us to evaluate a batch of possible chemistries and determine an optimal austenitizing temperature that is suitable for all compositional variations.
Heating Curve Design
To ensure that transformation temperatures are reached uniformly (whether in processes involving large loads or parts with variable geometries), it is advisable to implement controlled heating rates. Although this approach may increase processing time, the benefits include reduced distortion risk and assurance of complete austenitic transformation.
The key is to design an appropriate time–temperature profile, which depends on factors such as part dimensions and material properties, including thermal diffusivity, heat capacity, density, and thermal conductivity.
Conclusion
Insufficient austenitizing, also known as underhardening, represents far more than a loss of hardness. It is a metallurgical deficiency that affects microstructural homogeneity, dimensional stability, and mechanical performance. Through rigorous control of temperature, time, and furnace uniformity combined with modern simulation tools, engineers can ensure reliable transformations, minimize distortion, and achieve consistent high-quality results in steel heat treatment.
Callister, W. D. 2019. Materials Science and Engineering: An Introduction. Hoboken, NJ: Wiley.
Herring, Dan. Metallurgical Fundamentals of Heat Treatment. Industrial Heating.
Krauss, G. 1980. Principles of Heat Treatment of Steel. ASM International.
Nuñez González, G. 1990. Fallas en los Tratamientos Térmicos para Aceros Herramienta.
Thomas, L. 2018. “Austenitizing Part 2: Effects on Properties.” Knife Steel Nerds. https://knifesteelnerds.com/2018/03/01/austenitizing-part-2-effects-on-properties/.
Totten, G. E. 2007. Steel Heat Treatment: Metallurgy and Technologies. Boca Raton, FL: CRC Press.
About The Author:
Ana Laura Hernández Sustaita Founder Consultoría Carnegie
Ana Laura Hernández Sustaita holds a Master’s degree in Materials Science and engineering. She is the founder of Consultoría Carnegie, a technical consulting and training firm specializing in steel heat treatment in Mexico. Additionally, she works as a technical support engineer at Thermo-Calc Software, providing assistance to clients across México, Canada, and United States of America. Ana actively promotes metallurgical education throughout Latin America and advocates for the integration of computational tools into industrial heat treatment practice.
In this episode of Heat TreatRadio, host Doug Glenn invites Dennis Beauchesne of ECM USA to explore the technology, benefits, scalability, and sustainability of modular heat treating systems. Together, they discuss how shared utilities, automated transfers, and adaptable heating cells can replace multiple standalone furnaces without compromising quality or precision. Learn how these systems streamline and simplify operations for future expansion — one cell at a time.
Below, you can watch the video, listen to the podcast by clicking on the audio play button, or read an edited transcript.
The following transcript has been edited for your reading enjoyment.
Introduction
Doug Glenn: I am very privileged to have with me today, Dennis Beauchesne from ECM USA. We’re going to be talking about modular heat treating systems, which is a growing category of equipment.
ECM Synergy Center (00:50)
Doug Glenn: Tell me about ECM’s Synergy Center, which is where you are at right now, on the shop floor.
The ECM Flex 600TG vacuum furnace located in the ECM Synergy Center Source: ECM USA
Dennis Beauchesne: I’m standing here in the middle of our Synergy Center. It’s about a 5,000-square-foot facility that is dedicated to proving out client parts for testing various processes, mostly LPC, but we also do a number of other processes here. We have a full metallurgical lab, 3D microscope, a number of tools, including a CMM that we can do before and after heat treat distortion testing for clients that want to know how much their parts move.
It’s a dedicated center just for clients to use. We also use the center for pre-completion of installations, final testing, and training, such as training on maintenance, understanding the software, and how everything works together.
Doug Glenn: It’s proof of process plus much more — helping clients’ proof of process.
Dennis Beauchesne: Absolutely. That’s a big part of convincing people that this process is for them and that it works on their part. We can send them ten different reports of an exact same material and part, but they want to know what their part will do.
What is Modular Heat Treating? (02:50)
Doug Glenn: On a very basic, rudimentary level, what is modular heat treating and how does it differ from what might be considered standard or normal heat treating?
Dennis Beauchesne: A modular heat treat system is one that works together to have more than one furnace working in the same platform. You may have a shop that has five or six vacuum furnaces separated — they each have their own door, vacuum system, electrical supply, quench motors…those types of components. Or you may have a series of batch IQ furnaces for carburizing; those systems are one off, which means they are individual, independent systems.
In a modular system, you try to utilize those facilities for the use of multiple heating chambers. Instead of having one vacuum furnace with one set of pumps and one gas quench motor, what we would do is have three to eight heating cells that would be utilizing one quench, depending on the process timing; that’s all done with an internal transfer car and we try to utilize one vacuum system. It’s much smaller than what you would have for three, four, or even eight cells.
If you had oil or high pressure gas quenching, which is what’s dominating right now in the modular heat treat business, you could supply basically six batch IQ hot zones to one oil quench.
The savings then are huge simply by removing five or six other quench tanks in front of this system, as well as leveraging the floor space (and the number of pits you have to dig). Other advantages including utility savings and utilizing equipment across a number of heating chambers.
Doug Glenn: This modular approach is basically separate chambers that are dedicated to doing whatever that chamber is doing, and they are all in some way interconnected. For standard units, you would heat up, pre-process, do the actual process itself, cool down, all in the same chamber. In a modular unit, you move from chamber to chamber to do each of those separate steps.
Dennis Beauchesne: Yes, I refer to it as a continuous batch.
Doug Glenn: Continuous batch. We were talking before we actually hit the record button with your colleague there, Allison DeAngelo, who just got done visiting the Heat Treat Boot Camp. We were talking about different types of furnaces, and we started talking about continuous vacuum, which of course, is almost a misnomer — a vacuum can’t be continuous because you have to open it up and break the vacuum to get stuff out. Anyhow, we talked about it basically being a batch, right? A batch furnace that’s continuous, a continuous batch furnace.
Benefits of Modular Heat Treating (06:35)
Now that we have a basic understanding of what these modular systems are, why would companies want to move from the standard type of heat treating system to a modular system?
Dennis Beauchesne: Manpower. If you are running five or six vacuum furnaces, you are going to need a number of people to open the doors, put new loads in, those kinds of tasks. With a modular system, you only have one entry or one exit area. Therefore, you are only going to load once every 15-20 minutes, and the system is going to take over and control that load going through the system.
In addition, especially in a carburizing atmosphere situation, you can have every load be a different case depth — a different process in each cell — and then the next load that goes in that same cell can be totally different from the one before. For instance, if you had a batch IQ, you typically use the same carbon potential, and you are typically going to run the next load almost identical to the one before. In contrast, with the modular system, each cell can run a different process every load.
It’s also easier to integrate automation if you are doing capacity increases.
Throughput Comparison (08:00)
Doug Glenn: What is the comparison of throughput between a standard unit and a modular unit?
Dennis Beauchesne: The throughput comparison is interesting because you typically can use a little higher temperature for a carburizing and a little higher carbon potential, and of course that’s what we specialize in here with the modular systems. You can achieve about a 30-40% gain in your cycle time. That furnace is operating very close to 100% occupancy, because when that load is done, you are moving it out right into the gas quench. Then, the next load comes and goes right into it.
Doug Glenn: You are able to increase your throughput because you have basically 100% utilization of the equipment or very close to that. Comparatively, you don’t necessarily have that in the standard equipment.
Product Quality Comparison (09:15)
Doug Glenn: Do modular systems produce higher quality products?
Dennis Beauchesne: The quality of the parts coming out of the system is improved. A vacuum environment is a very clean environment, especially if we are considering atmosphere and low pressure carburizing — it’s in a vacuum. We typically do everything in high pressure gas quenching. However, even in oil quenching under vacuum, you are going to have a much cleaner part.
Also in low pressure carburizing, the carburizing is much more uniform throughout the part because we heat it to temperature under nitrogen before the part gets to austenitizing temperature to start attracting carbon. We make sure that the full part, that’s the tooth, the root, every piece of the part, is at temperature before we start adding carbon to the load, which makes a more uniform case depth, and therefore makes a stronger part.
Doug Glenn: Since each module, each chamber, is dedicated to doing what it is supposed to do, it seems like the consistency and the reliability of the parts being processed in a modular system have a much better chance of being higher quality.
Dennis Beauchesne: You do not have six different variable chambers or six different variable systems. You just have to look at monitoring the connection between those and understanding that the vacuum levels are all the same across the levels and across the cells. Each cell can meet a different temperature and run a different process, but those are consistent across the board.
Typical Dedicated Cells/Chambers (11:10)
Doug Glenn: What would be the typical dedicated cells/chambers of a modular system?
Dennis Beauchesne: It is dependent on the processes. They are most widely used for vacuum carburizing. For pre-oxidation and preheating, we usually use an air oven outside of the system, and we connect that with an external loader. Before the load goes into the modular system, the load will go through a regular air oven, be heated to around 700°F (400°C), and then the load will be moved in.
For sintering and those kinds of applications, there is a debind step or a preheat step that would be done in one cell. Some of the processes that can be done in a modular system include:
Low pressure carburizing
Low pressure carbon nitriding (LPC)
FNC (ferritic nitrocarburizing)
Nitriding
Debinding
Sintering
Neutral hardening
The most prominent process right now is LPC, and that is being used all over the world in these systems.
Advantages of a Modular Unit for Captive Heat Treaters (12:53)
Doug Glenn: Why would a modular unit be beneficial for a captive heat treater, someone who does their own in-house heat treating, which probably means they’ve got potentially high volume, low variability as far as their workloads?
Dennis Beauchesne: The modular unit has many different advantages. First of all, floor space. You are going to save a lot of floor space by not having multiple furnaces set up separately. You will also save utilities because you would not have as many vacuum pumps or electrical systems running these furnaces on their own. You will have some shared service and utilities in that fashion.
Doug Glenn: That would also likely lead to maintenance cost savings as well, correct?
Dennis Beauchesne: Yes, it all goes down the line. Anything that you have multiples of, you are going to have much less costs than on a joint system. The modular system might be a little larger than one singular unit, but there will be fewer of them.
For vacuum carburizing applications in a captive shop, the quality and cleanliness of the part is very, very important. Gas quenching lends itself to no oil in your plant, no washers necessary for a post-quench. Typically, there’s a washer before the process starts, but you do not have to have any wash to get the oil off of the parts with a modular unit — you do not have to reclaim the oil or the water from the washer. You would not have waste oil in your plant either or any oil on your plant floor. These are some of the reasons some of the larger captive shops have gone to the modular systems.
Also, safety: There are no open flames with a modular unit, no risks of fire on the systems. They are also easier to maintain. For a fully operational, let’s say, eight-cell system for high production, captive operation, it would only take about five hours to cool that whole system down if you had to go in and work on the whole system. In comparison, it’s going to take you three to four days sometimes to cool down a typical atmosphere, high-temperature furnace.
It also takes time to heat the system up again. In a modular system, it takes about an hour and a half to heat the system up again and then you are ready to start running. That means now you can schedule your downtime on weekends or holidays. You do not have to have staff present to run anything.
You also do not have to have a secondary equipment, like Endo generators running to feed the carburizing gas. The carburizing gas is using acetylene out of cylinders, it’s not a regenerative system. You do not need a separate piece of equipment to feed to the furnace.
Another benefit is CapEx expansion. Typically, captive heat treaters do not want to buy everything upfront because their volumes are going to increase over time. In the beginning, they typically only need one or two cells ready to do a small amount of production so they can prove out the production and prove out the system. Then they can start building the system with more cells and more capacity later on. Generally, it’s two to three days of downtime to add a cell to a system. It’s very convenient to do that with a modular system. All of the utilities are typically alongside the modular system so that you can easily add those or add a cell to it over a short period of time, and those cells can be ordered a year or two down the road whenever you might need that.
You also can order peripheral equipment, like extra temper ovens or additional automation. You can add a robotics system to the layout as well. That’s why captive shops are very interested.
Finally, workforce: It’s a little bit easier to get someone to work on a modular system. These systems are completely clean and white. The one located in our Synergy Center has been there for eight years. We use it every single day, and it’s a very clean aesthetic environment for someone to work in. These systems are also water cooled, which means not a lot of extra heat in the building around you to work in.
Advantages of a Modular Unit for Commercial Heat Treaters (17:59)
Doug Glenn: What are some advantages of modular units for commercial heat treating?
Dennis Beauchesne: On the commercial heat treat side, modular units are typically useful because you can get multiple processes out of similar cells and you can have a system that has oil and a gas quench.
You can have a lot of flexibility in that one system that you have in the plant. I’ve visited hundreds of captive and commercial heat treaters. They generally have a number of furnaces in one area of the plant, and a number of furnaces in another area of the plant. A modular system gives you all the capability in one machine and one tool: oil quenching, gas quenching, FNC, nitriding low pressure, carburizing, carbonitriding, and neutral hardening all in one piece of equipment.
Automation and Robotics with Modular Heat Treating (18:57)
Doug Glenn: What automation and robotics advantages are there with modular systems?
Dennis Beauchesne: This is the new trend. People that have modular systems are now considering, “How do I automate the system to get more production out of it?” And what we’ve been doing the last five years especially is implementing systems that use CFC fixtures.
CFC fixtures are very robust in the furnace but sensitive to being controlled outside. Therefore, what we try to do is have the CFC fixtures be utilized in an automation that no humans have to interact with it. We usually use robots for external loaders and internal loaders to move the fixtures through the process.
This causes you to have a lighter load, which means less heating time, less energy being consumed. Also, the fixtures last three to four times longer if they’re not damaged. But of course, all of these systems can be using regular alloy steel as well, and we can fixture different parts. You can use baskets, we are now doing bulk loading where we have parts that are filled into baskets and then processed. We are doing that with vacuum carbonizing as well, not just neutral hardening.
So it’s really interesting to see how the limits are being pushed, as well as the different materials that we are gas quenching now. I know 20-25 years ago, we were quenching some simple materials that were very high hardenability, and today we’re quenching a lot of less hardenability steels.
Doug Glenn: Is that primarily due to increase of pressure in the quench?
Dennis Beauchesne: It’s pressure, it’s flow, it’s the intensity of the gas going through the parts. It’s also heat removal as well — heat exchangers, removing the heat out of the load faster. We also have reversing gas quench motors to reverse the flow inside from top to bottom, bottom to top, in the middle of the cycle.
Sustainability of Modular Heat Treating (22:24)
Doug Glenn: Do these systems promote sustainability and greenness?
Dennis Beauchesne: Absolutely, especially when it comes to carburizing. These systems have been compared against typical atmosphere carburizing cycles, and only about 4% of the carburizing time has gas injection, when we are actually injecting acetylene and having hydrocarbons being used in the process.
If you took the same cycle times, seven or eight hours of a carburizing cycle, you are flowing Endo gas or nitrogen methanol in the system for that full time. In contrast in a vacuum carburizing system, it’s 4-5% of the time of the cycle that you’re injecting into the furnace. Ultimately, you only have about 10% of the CO2 output that you would have in a typical atmosphere furnace.
As mentioned previously, there’s also no oil in your plant. You’re not reclaiming oil out of the water and the wash or off the floor or in your car when you leave your heat treat shop.
How Does the Modular Heat Treating System Work? (23:40)
Doug Glenn: Let’s talk through the process a little bit. You provided us with figures to aid in describing the process. We have included these. Describe how the system works.
Dennis Beauchesne: This animation is a plan view of one of our Flex systems. In the center, going left to right, is a tunnel section. This tunnel section is about an 8-foot diameter. It has an automated loader that moves down left to right or horizontally, and it transfers loads from each cell to another, in and out.
On the bottom left is a loading/unloading chamber. In that loading/unloading chamber, we remove the air once the load is put in there, and then we balance the vacuum on that cell to the tunnel’s vacuum. Then we’re capable of moving that load to an available heating cell, and that would be on the right of the system — on the top right or the bottom right of the tunnel, those are heating cells. Then recipe for that particular load will be loaded into that cell. While that load is processing, another load will be moving into the tunnel and into the other heating cell as well.
On the top left is the gas quench cell, which could be in this orientation or instead have an exit on the back as well. In this system, you could do neutral hardening, carbon nitriding, LPC, a number of the processes. This is a very valuable tool, especially in a commercial heat treat heat treat shop.
Doug Glenn: Is this whole unit, including all four chambers under vacuum? I noted there are separation doors on the purge and the entry chamber. Can this area be vacuum sealed?
Dennis Beauchesne: Yes. There are vacuum seals on the loading/unloading chamber on the bottom left and then the top left. The gas quench also has a seal from a pressure standpoint. The two heating chambers have a graphite door — we call it the flap door, and it just flaps and it doesn’t really seal actually against another face of graphite. It’s graphite-to-graphite. We pull vacuum out of there through the tunnel to create the central vacuum pressure in the system. We also pull vacuum from the cell itself, and we could also have a separate door on the front of the unit if the process necessitates that or if we feel that a door is needed there by a client.
In a normal state or a standard unit, there are no hot seals on the door, only vacuum seals on the loading/unloading chamber and the gas quench.
Doug Glenn: In the animation, your vacuum pumps are down in the bottom right, correct?
Dennis Beauchesne: Exactly, that’s a process pump.
Doug Glenn: What is located in the top left?
Dennis Beauchesne: On the top left, we have a gas quench tank. We want to ensure we have enough gas pressure and volume there to quench the load quickly. It’s very important to get the gas through the gas quench quickly.
ECM Flex 600TG vacuum furnace with two added heating cells / Source: ECM USA
Now, we have added two more additional heating cells and a central tunnel section. In essence, you just doubled the space, doubled the capacity of the unit, where you only added 50% of the space of what you had for capacity before.
We are still utilizing the same gas quench and the same loading/unloading cell. We only added utilities for the two heating cells, not for a whole gas quench or oil quench capability there; this can be added in a very short time.
Doug Glenn: Now I’m gonna go let this video roll here for a minute. There we go.
ECM Flex 600TG vacuum furnace with four added heating cells for six heating cells total
Dennis Beauchesne: So now we added another 50% capacity with two more heating cells (six heating cells total) and a tunnel section. Typically, what you want to do is to have the tunnel sized for about five years out for your capacity and then buy the cells as you need them and have it grow so then the tunnel is ready to implement.
We have just tripled the capacity of this installation, and we are only still using the same gas quench and the same loading/unloading cell. Generally, this system could go to eight cells and have just one gas quench, that’s our typical orientation.
Doug Glenn: It looks like we also added a discharge side here. Whereas before we were going in and out.
Dennis Beauchesne: Yes, this adds to the efficiency of the system because the load is already in the gas quench when it’s finishing, so it just exits out the back, out the door.
Doug Glenn: Now what do we have here?
ECM Flex 600TG vacuum furnace processing different treatments in each cell. See animation above to watch the animation in motion.
Dennis Beauchesne: We have the loads entering, and the loads will go to the first cell that is available (empty). Then that recipe would be downloaded for that cell, and then the next load will go to the next available heating cell and download that recipe into that cell. These could be two different loads.
One load could be for neutral hardening; one could be for carburizing. One could be for carburizing in a low case depth. The other one could be carburizing at a deeper case. In this case, we just see the gas quench on here, but this tunnel could also be outfitted with an oil quench as well, and you could have one load go into gas, quench one load, go into oil quench or both going to either.
Doug Glenn: This gives people a sense of what the process looks like.
Processes and Materials for the Modular System (30:29)
Doug Glenn: Are there any processes or materials that do not make sense to process them through one of these systems?
Dennis Beauchesne: If you are doing a lot of annealing and normalizing, those are longer cycles. There is some regulated cooling that occurs. This is not really the type of equipment investment that you would want to make for those processes. If you were going to use it for a few loads in your plant where you received parts that weren’t annealed or you wanted to try to anneal a part for a particular process before you went to full production, you could certainly use a modular system for that, but it’s not a cost effective methodology. Neither would we recommend preheating in the cell. However, it is very flexible for a number of other processes that we have mentioned.
The size of the part is also important to note. These systems are typically 24 inches wide and about 39 inches long and about 28 inches high. However, we will soon have a new system, the Flex Max, a 12-9-9 system. It’s a 36×48 unit that comes with an oil quench and is modular, like this. We can either do an oil quench or a slow cool cell on that system. So, we will have that capability of 36×48 in that modular system.
Other than that, restrictions on material? Very few there. Like I said, you would not want to do annealing and normalizing on a lot of parts, but you could do it in these units.
Doug Glenn: It sounds like the sweet spot is surface modification type applications, and some sintering is possible with dedicated chambers.
Dennis Beauchesne: Yes, sintering and brazing is also possible.
Doug Glenn: Does that include aluminum brazing?
Dennis Beauchesne: Not aluminum brazing, but some brazing applications.
Expenses with Modular Heat Treating Systems (33:03)
Doug Glenn: What would be considered capital expenses for this modular system?
Dennis Beauchesne: As far as capital expenses, it’s not a furnace-to-furnace comparison. Clients always ask how much our furnace is. But companies need to first take two steps back and take a look at their incoming material, how they would like to be able to modify that incoming material in their heat treat process to make sure that their outgoing quality is higher than it is today. That’s the kind of benefit that this type of modular system gives you — a better quality part, safety in your plant, and a better quality work environment with being able to turn the system off and not need additional personnel around.
These are all factors that have to be considered when thinking about the CapEx expenditure and investment. When we consider these factors, a modular system investment is a much better situation than looking at a furnace-to-furnace replacement, and that’s really the thought process that clients need to go through to understand the actual investment and value of the system.
Doug Glenn: What about the operational expenses?
Dennis Beauchesne: For instances, if you had a batch IQ sitting there, you would typically keep it running whether it has a load in it or not. With a modular system, you just shut off that cell that you’re not using. It does not take any more energy. If you are not working five days a week, you do not use it on the weekends — you shut it off. You do not use it during Christmas shutdown or any holiday shutdown, vacation shutdown. You’re able to shut it off and that means saving a lot of energy and labor by having it off.
Also, in the opposite way, you could run it lights out if you wanted, as well. You could stock up a number of loads on the automation before you leave, have the system operate it, run it, and have the load come back out before the morning. You could have it time start as well, if you wanted to start it on Monday at 5 AM, but you will not be there till 8 AM. You would come in and the furnace would be hot and ready to run a process.
There are a number of operational advances over the typical operational heat treat that’s out there today.
Doug Glenn: How does maintenance work with these systems? Say your heating element goes bad in cell number three, do I have to shut the whole system down to fix or can I fix number three and leave the rest of the system up and running?
Dennis Beauchesne: In this situation if you had a tunnel like we showed, you would typically shut off that cell; that is, if you knew that heating element was out or it wasn’t heating properly, you could shut off that cell, de-validate is what we call it, and then keep running the rest of the system until you had a window in your production that you could shut the whole system to get into that heating element.
If you had a system with doors on the front, it could be possible to go in the back while the system is operating. Then, it would be all based on your safety requirements for your plant and those kinds of things.
To do that, we have another system called the Jumbo, and it is much more flexible in the maintenance world. It has a vacuum car that moves down on rails and docks and mates with every heating cell on the system. In that line, the heating cell can actually be isolated from the rest of the line. You would just slide it back (It’s on wheels, it slides back about three feet away from the line), you put in a new piece of safety fence, and you continue to run your line. You can completely lock out/tag out that cell and work on it completely.
Doug Glenn: How would you approach a vacuum leak since the whole system is connected, right? I believe you mentioned these are graphite-on-graphite doors.
Dennis Beauchesne: You would want to fix the leak before you move on. Especially if it’s a bad leak. If it’s something that’s causing you to not maintain your process pressure, you certainly don’t want to do that, and that’s true with every vacuum piece of equipment.
ECM Modular Systems (38:55)
Doug Glenn: How many of these modular type systems does ECM have out in the marketplace?
Dennis Beauchesne: The Flex is the most popular modular system, which we discussed with the animation. We also have a number of Jumbos systems, and the unit in our Synergy Center is called a Nano, which has become more and more popular these days. The Nano has three different size chambers, but they’re typically smaller, 20x24x10 inch high size chamber. I explained a little bit about the Flex and the Jumbo is the same.
Out of those three systems, we have more than 350 modular systems, not just the heating cells, but more than 350 systems that are out in the marketplace today operating, running parts every day, running millions and millions of parts every week. Those systems are comprised of about 2,000 heating cells. As much as people hear about this being a new technology, it has actually been around about 30 years, and many companies have been using these systems and have replaced a number of pusher furnaces and those style furnaces for high-capacity installations especially.
Doug Glenn: Okay, that sounds good. I really appreciate your time.
About the Guest
Dennis Beauchesne General Manager ECM USA
Dennis Beauchesne joined ECM over 25 years ago and has since amassed extensive vacuum furnace technology experience with over 200 vacuum carburizing cells installed on high pressure gas quenching and oil quenching installations. Within the last 10 years, his expertise has expanded to include robotics and advanced automation with the heat treat industry high-demand for complete furnace system solutions. As General Manager of ECM USA, Dennis oversees customer supply, operations and metallurgical support for Canada, U.S., and Mexico for ECM Technologies. He has worked in the thermal transfer equipment supply industry for over 30 years.
The Heat Treat Doctor® has returned to offer sage advice to Heat Treat Today readers and to answer your questions about heat treating, brazing, sintering, and other types of thermal treatments as well as questions on metallurgy, equipment, and process-related issues.
This informative piece was first released in Heat Treat Today’sAugust 2025 Automotive Heat Treating print edition.
Quench cracking during heat treatment can turn expensive components into scrap metal in seconds. In today’s Technical Tuesday article, Dan Herring (The Heat Treat Doctor®) explores more about the underlying mechanisms and proper preventative measures to save you time, money, and ensure reliable part performance.
As a young heat treater, I learned first-hand about quench cracking while running various dies for our tool and die shop — and succeeded in cracking all of them! I have never forgotten the foreman’s (rather animated) critique of my heat treating abilities. Quench cracking can be a significant problem for heat treaters, its potential consequences ranging from costly rework to premature failure in the field. Let’s learn more.
We must not only understand the mechanisms involved but also take proactive steps to avoid it. This includes careful consideration of such items as:
Material (e.g., chemistry, hardenability, form, mill processing)
Component part design (e.g., sharp radii, thin and thick sections next to one another)
Manufacturing processing steps (e.g., the effect of stress relief after rough machining)
Part loading (e.g., part orientation in relation to the quench, fixturing, total load weight)
Equipment choice (i.e., limitations and capabilities)
Quench medium (e.g., type, agitation, flow characteristics, temperature, temperature rise)
Process parameters (e.g., ramp rates, atmospheres, vacuum levels)
The Heat Treatment Challenge
Quench cracking primarily occurs during the hardening process, typically when materials are rapidly cooled via quenching. Since the cooling process introduces internal stresses within the material, it can result in crack formation. These stresses are a result of the rapid transformation of the material’s microstructure, most notably when transforming to martensite, a very hard, brittle structure.
Figure 1. Quench crack in a 4140 axle shaft
Mechanisms Involved
Failure mechanisms related to quench cracking include the following seven factors.
Material Imperfections
As material is heated, thermally induced stresses can cause existing surface or subsurface defects, such as inclusions, laps, and seams. These defects act as stress risers to open and propagate into cracks. Once a defect reaches “critical flaw size” — the smallest flaw that can lead to failure under expected operational stress levels — crack propagation will begin and lead to part failure.
Rapid or uneven heating only exacerbates this issue, especially when a material undergoes phase transformations that introduce volumetric changes.
Stress Risers
Sharp corners, steep edges around holes, and even grooves in parts create stress concentration points where quench cracking is most likely to occur. These features also result in localized heating and cooling, causing differential stresses that can initiate cracks.
The sharp edges of a part, for instance, cool much faster than the rest of the material, leading to a high risk of cracking.
Proper design modifications, such as adding radii to sharp corners, can reduce the likelihood of stress concentrations.
Rapid Cooling and Phase Transformation
The transformation from austenite to martensite during quenching is a key contributor to internal stresses. The rate at which the material cools can greatly influence these stresses. If cooling is too rapid or if tempering is delayed, the material can become overly brittle, leading to quench cracking.
Improper Heating and Overheating
Overheating during the austenitizing process can lead to coarse-grained structures that are more prone to quench cracking. Coarse grains increase the depth of hardening but reduce the material’s resistance to cracking. It is critical to avoid temperature overshoot, high ramp rates, and excessively long dwell times when heating.
Inadequate Quenching Methods
The choice of quench medium (brine, water, oil, polymer, high pressure gas, etc.) can also contribute to quench cracking. Overly aggressive quenchants may create excessive thermal stresses.
Improper Fixturing
The way parts are positioned during quenching can create problems. If parts are bunched together in a basket, uneven cooling rates will occur, with parts on the edges cooling faster than those in the center. This can lead to differential stresses and increase the risk of cracking.
Delays Between Quenching and Tempering
Quenching produces high residual stresses in the material, and if parts are not tempered soon after quenching, these stresses can lead to cracking. For materials with high hardenability, such as 4340 steel, immediate tempering (usually within 15 minutes of quenching) is critical to prevent in-service failure.
Understanding Fracture Mechanics
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Understanding these mechanisms is critically important. A material’s fracture toughness, which is the ability to resist crack growth, is defined by the stress intensity factor (KIC). This value varies based on the material’s properties and the size and geometry of the crack. The important point to remember is that when the applied stress reaches a critical threshold, cracks begin to propagate (literally at the speed of sound), leading to catastrophic failure.
Digging a bit deeper, there are three primary modes of fracture:
Tensile (Mode I): Fracture caused by tensile stress at the crack tip.
Sliding (Mode II): Fracture caused by shear stress that causes the two sides of the crack to slide.
Tearing (Mode III): Fracture caused by shear forces in a direction perpendicular to the crack plane.
Preventive Measures
Several strategies can be employed to minimize the risk of quench cracking during heat treatment. They broadly fall into the following categories.
Material Selection
Choosing the right material for the job is essential. Many designers select materials with high hardenability, forgetting that they can be prone to cracking. Additionally, one should take special care with materials that have high carbon content or are heavily alloyed.
Design Considerations
Ensure that part designs minimize stress risers. Avoid sharp corners and incorporate radii where necessary. Proper design can reduce the likelihood of cracks forming at critical locations.
Improved Manufacturing Practices
Proper stock removal during machining and addressing surface imperfections before heat treatment can prevent the initiation of cracks. Machining should aim to eliminate any seams or inclusions that might act as nucleation sites for cracks. Stress relief after rough machining is almost always a good idea.
Control of Heat Treatment Parameters
Maintain tight control over the heating and quenching processes to ensure uniformity. Avoid overheating and try to ensure that the part enters the quench medium in the best possible orientation to reduce the likelihood of creating differential cooling rates.
Figure 2. Quench crack due to a combination of rapid heating, overheating and improper
polymer quench medium concentration in a motor shaft (50x, as polished)
Quenching Media
Select the appropriate quenching medium based on the material, part geometry, and load. Less aggressive quenchants or minimizing time in the quench should be considered for materials with moderate to high hardenability.
Post-Quench Tempering
Temper parts as soon as practical after quenching to avoid concerns with internal stresses. High-hardness materials should be tempered immediately to prevent quench cracking.
Quench Cracking in Other Materials
Quench cracking is not exclusive to steel. Other materials, such as nickel and cobalt superalloys, can also experience cracking due to similar mechanisms. In these materials, the phenomena are often referred to as “fire cracking,” “strain-age cracking,” or “stress cracking.” As with steel, cracks in these materials are often linked to high residual tensile stresses on the surface and the presence of stress raisers. Strategies, such as shot peening, redesigning part geometries, and improving surface finishes, can help mitigate cracking in superalloys.
Summing Up
Quench cracking represents a significant challenge in heat treatment, but by understanding its underlying mechanisms, heat treaters and engineers can take steps to mitigate the risk. Material selection, part design, proper heat treatment procedures, and timely tempering are all critical factors in preventing quench cracking and ensuring the integrity of components. A proactive approach to addressing flaws and stress concentrators combined with careful attention to detail in every stage of the manufacturing and heat treatment process can greatly reduce the likelihood of failure and contribute to the long-term success of heat treated products.
References
Herring, Daniel H. 2012. “Quench Cracking.” Industrial Heating, April.
Herring, Daniel H. 2015. Atmosphere Heat Treatment, Volume 2. BNP Media.
Johnson, D. D. 2005. “Thermal and Mechanical Behavior of Materials.” University of Illinois.
Klarstrom, Dwaine L. 1996. “Heat Treat Cracking of Superalloys.” Advanced Materials and Processes, April.
Krauss, George. 2005. Steels: Processing, Structure and Performance. ASM International.
About the Author
Dan Herring “The Heat Treat Doctor” The HERRING GROUP, Inc.
Dan Herring has been in the industry for over 50 years and has gained vast experience in fields that include materials science, engineering, metallurgy, new product research, and many other areas. He is the author of six books and over 700 technical articles.
What do Mars rovers, sniper pods, and rotor grips have in common? Uphill quenching — a thermal-mechanical technique that uses liquid nitrogen and high-velocity steam to dramatically reduce stress and distortion.
In today’s episode of Heat TreatRadio,Greg Newton, Newton Heat Treating CEO, joins host Doug Glenn to take a dive deep into this little-known but highly effective process for controlling residual stress in aluminum alloys. Guest John Avalos, Newton’s quality engineer and IT/Digital Transformation Manager, joins the conversation.
Get the full picture of how this thermal-mechanical method improves machinability, enhances precision, and extends component life, especially in aerospace and optical applications.
Below, you can watch the video, listen to the podcast by clicking on the audio play button, or read an edited transcript.
The following transcript has been edited for your reading enjoyment.
Introduction (2:20)
Before we even start talking about the process, let’s talk about your qualifications and experience. How did you get in heat treating and aluminum heat treating?
Greg Newton: In 1968, my father opened up a heat treating facility in the city of industry. From age 13 on, I had a summer job and weekend job. It’s part of my blood. In the early ‘70s, we were the first heat treater to purchase an X-ray diffraction machine, which is a non-destructive way of checking for residual stresses beneath the surface of aluminum alloy and different alloys; we concentrated on aluminum. It’s an old analog Rigaku machine; it’s still running. It gives me great data, so why change it out for a half-million-dollar new machine? That’s how we got started.
There was a pilot project with Northrop Grumman for controlling residual stresses, taking glycol out of the laboratory and putting it in production. Now, one thing I didn’t like about that project was that we gave all the data to Northrop and then they wrote a spec and gave it to the world. I didn’t feel we got our fair payback for it.
When theM1 tank optics came along and they knew we had X-ray diffraction on premises, they wanted to take something basically out of the laboratories. The patent names it “thermal mechanical uphill quenching,” which describes the process perfectly. We use it because of the residual stresses created during the quench cycle. When you go from roughly 1000°F down to room temperature rapidly, that’s what sets up your mechanical properties in aluminum.
What Is Uphill Quenching (5:02)
Doug Glenn: Let’s take a 30,000-foot view for someone who has no concept of what an aluminum alloy is. What is uphill quenching?
Greg Newton: It’s the inverse process of the quenching cycle in the solution heat treat cycle. You’re going roughly from 1000°F to room temperature, hot to cold. A part can’t cool instantly. What happens? The outside cools first. It shrinks, and you get a compressive shell. By the laws of thermodynamics, I have an equal and opposite action happening in the core of that part. So, it develops tensile stresses to hold up that compressive shell. They’re in equilibrium when I’m done with the part and I send it back to the machine shop.
Then, they’re going to remove material from one side; they’re going to gun drill it. That’s when challenges arise, because at the point of after-quench, we have the compressive shell and the tensile stresses in the core. They are in equilibrium. When I remove material away, that compressive shell moves, and that’s where aluminum becomes very difficult to machine.
Newton Heat Treating’s thermal processing equipment
Source: Newton Heat Treating
Doug Glenn: Does uphill quenching solve this problem?
Greg Newton: It solves the problem, for all parts, all shapes, all sizes. Parts that don’t lend themselves to flip flopping, which never solves the problem. You might machine away some compressive shell, rejig the part, flip it over, remove a little of the compression on the other side, but you still have those tensiles. The tensiles are the bad guys. That’s what’s causing a failure in use and propagating cracks.
Doug Glenn: Tensile stresses are the ones pushing out, whereas the compressive strengths are the ones that are pulling in.
Greg Newton: And a compressor shell can actually be advantageous for certain types of fatigue, like creep.
Doug Glenn: Is uphill quenching predominantly done on aluminum or exclusively done on aluminum?
Greg Newton: It is predominantly done on aluminum. We’ve done a little bit on titanium. It had fair results with it. Alcoa developed uphill quenching in the late ‘50s. That’s how old this technology; it’s nothing new. Back then, though, engineers used to design things 2.5 times as robust as they needed to be, just because we didn’t know how much residual stresses were inherent in the manufacturing of these parts. But now, with trying to get aircraft, car, and all other types of components to be as light and as thin as possible, this process comes into play. It has finally come of age.
Neutralizing Stresses with Uphill Quenching (7:50)
Doug Glenn: So you have the compressive and tensile stresses, and uphill quenching basically is helping to neutralize or to balance those so that when you go to machining and you’re doing some machining, you’re not going to get what you would anticipate with a distortion or something of that sort.
Greg Newton: Well, again, we go back to the original patent name that describes the process perfectly. Thermal, mechanical, uphill grade. We’re not stretching it with a hydraulic press to 1.5–3% to dislocate the lattices. We’re using a thermal gradient. That’s our energy. That’s our machine.
It’sa little hard to wrap your head around. We’re going to compress and get the dislocation that way. Well, what put those stresses in was that thermal gradient of the quench roughly going from 1000°F to room temperature. How can we reverse that? Aluminum, unlike steels, is almost annealed soft in an as-quenched (AQ) condition.
So that is the optimum time, as the original patent tells you. There are so many misconceptions out there. When you do it in a hardened condition, you’ve lined up everything against yourself. You’ve increased yield strength. You want to do it when the material is as soft as possible. For aluminum, you want to either do it immediately after quench, within an hour, or retard the natural aging by putting it in a sub-zero freezer.
Doug Glenn: The uphill quenching is neutralizing those stresses, so there could be further processing without as much “fear.”
Greg Newton: That’s correct. We’re going to go from -320°F and heat it up with a high-velocity steam blast, back up past room temperature.
Doug Glenn: We’ll get to the actual process, I just wanted to make sure we’re understanding why we’re doing it.
Greg Newton: The machinability of aluminum are close-tolerance parts: They diamond hone our laser optics to a millionth of an inch in aluminum.
Doug Glenn: Wow.
John Avalos: That’s a tight tolerance.
Doug Glenn: Yeah, that’s a tight tolerance. So basically, uphill quenching is just the inverse of the quench.
Greg Newton: That’s all it is.
Doug Glenn: Coming downhill on the quench, then we’re going back uphill. Is this similar to a temper process for a ferrous material?
Greg Newton: We’re not changing any of the mechanical properties. All we’re doing is a realignment of the lattice parameter of the inner crystalline structure.
Doug Glenn: That sounds so different.
Greg Newton: If you picture that compression pushing in and the tensiles pulling out, we’re relaxing them back to a neutral state.
Want to read more about the Newton Heat Treating’s story? Click the image for a full article.
John Avalos: But the main point is that it doesn’t change the temper at all.
Greg Newton: It does not change any of the mechanical properties.
Doug Glenn: Is uphill quenching predominantly or exclusively used in aerospace or are there other markets where you use it as well?
Greg Newton: There are other markets — any close-tolerance parts in aluminum and the alloys. It’s extremely effective on all alloys; 6061 is used in the laser industries or laser optics. We do a lot with the optical industry.
Doug Glenn: So it’s not just aerospace, but a good chunk of it is.
Greg Newton: Nothing on Mars hasn’t come through our hands. I mean, all the gating and sending antennas, all the optical housings, the wheels even were cold stabilized, because they’re trying to make them so light. They’d gun drill them and they would collapse.
Doug Glenn: Did you say “nothing on Mars”?
Greg Newton: All the parts for the Mars rovers have come through our facility.
Actually, our first parts were on Voyager. We’d been looking at this process, and JPL (NASA Jet Propulsion Laboratory) came to us requesting us to try uphill quenching the parts. Dr. Martin Lo from JPL hand-carried these parts over that are still sending data on Voyager that is outside the influence of our sun. Isn’t that incredible?
Doug Glenn: That is incredible. I think it’s just so fascinating what this industry does that people don’t know about.
Getting Technical: The Uphill Quenching Process (12:37)
Doug Glenn: Let’s jump into it and talk technical. What is involved in the uphill quenching process?
Greg Newton: You take these heat treated parts and either perform the uphill quench within an hour or retard the natural agent, that’s key. There are companies that try to uphill quench in a hardened state, and you will get some reduction in stresses, probably more than you will get from any straight thermal stress relief where you’re just lowering the yield strength and popping some of the lattices, but this is nowhere near what you’ll get in an AQ condition.
Doug Glenn: Timeliness is important here. That’s probably the first point.
Greg Newton: Very, very important. So some of the equipment you’ll need includes a large door, depending on how big the part is. And you know, we have a 3,000-gallon tank here on premises and we are ready to put a 6,000-gallon one in. Then, all you’re utilizing the LN2 for is its coldness. It’s not like other steel heat treaters and stuff where it’s in the atmosphere. We’re just using it for…
Doug Glenn: Let me interrupt you, Greg. You said an acronym. What is LN2?
Greg Newton: Liquid nitrogen.
Doug Glenn: I assumed, but just want to make sure.
Greg Newton: The boiling point at sea level is -320°F.
Doug Glenn: So you’re taking it down.
Greg Newton: Right. You also need some sort of steam boiler or steam generator; we have both on premises. You may need an accumulator depending on the size of the parts you’re doing, because you’re using the steam, trying to reverse the delta T of the quench as fast as possible.
John Avalos: It’s a rapid process.
Doug Glenn: That’s why steam is very effective at rapidly heating.
Greg Newton: As the original report tells you, the difference is that you’ll get over 80% reduction in stresses utilizing LN2 and steam versus boiling water. The maximum’s around 19%. We’ve done our own testing and have gotten about 20% — so, significantly higher. Doing it in an AQ condition is key. The original report tells you that you get nothing out of doing the process in a hardened condition, which is done by many of my competitors.
We’ll do it any way the client wants it. While we have boiling water capabilities, but I try to talk the client into doing it the preferred way, which is in an AQ condition with LN2 to steam. That’s how you get to your biggest temperature differential, your delta T. You’re trying to match the delta T of the quench of the heat treat quench in reverse. That’s all you’re trying to do.
Doug Glenn: It sounds simple. So far, we have covered needing aluminum as-quenched, as soon as possible. You’re dipping it into LN2 to take it down to -320°F, roughly. Right?
Greg Newton: Depending on the thickness of part, it’s not a soaking cycle like solution heat treating would be, but you do want to make sure that part is completely at that temperature.
Doug Glenn: So you’re taking it down to -320°F, then immediately taking it out, and you’re hitting it with steam for how long, and what’s the criteria?
Greg Newton: It depends on the size, the shape, and the configuration. We have many, many steam fixtures out here that can be slightly modified. If you have a good production run, it’s best to design a fixture specific to that part. Bell Helicopter does this for the rotor grips for the Hueys when they were re-engineered.
Doug Glenn: Are you taking it up then to a specific temperature?
Greg Newton: Yes, we want be above 160°F for casting; 180°F, we prefer, for raw product.
Doug Glenn: Okay, and once it’s back up to that temperature, is the process done?
Greg Newton: You are done. Now there are many specs that repeat the process. I think this is mostly to make up for lack of fixturing, a part-specific fixture, so you can make up with subsequent processing. It does come out of the history of the past of when they really didn’t understand, before the original patent. There used to be tricyclic stress relieving where they would take it from dry ice into boiling water.
One of the advantages of steam, and the reason why you get much better results with steam versus boiling water, is the fact that it’s a higher temperature. It blasts away any ice that’s forming on that part, on the surface of it and it’s a turbulent flow over that part. So it readily transmits that energy quicker.
John Avalos: Can you also talk about the X-ray diffraction and how you use that to measure how effective the process is?
Greg Newton: When we took over this project and we wanted to prove it out, we learned a lot of things. When an engineer patents something, he usually controls everything. And it’s not that they’re wrong, it’s that they are .000001% right. In the real world, it makes no difference. So, you tend to throw those things away because they have no real relevance here on earth or in space.
So,we stumbled upon some other things that were advantageous to buy X-ray diffraction.
The standard operation involved first, getting the part, heat treating it, and then directly after quench, and take a reading because we know after a solution heat treated, we have that perfect setup between the compressive shell and the tensiles and the core. They’re going to be equal. Or close to it.
The thicker the section of the part, the more stresses, because it takes longer to cool. When you get into parts with two-inch cross sections and quarter-inch webbing, that’s when you get a lot of oil canning and all hell breaks loose. We can solve that.
I remember there was a bot part we had for the 767 or 757. It was the pilot’s window, and they were failing in service. The bot had a whole shift Boeing was paying to re-machine all out-of-tolerance parts on the shelf, until they finally they were over-machined and had to be thrown away. We had a hard time. I did parts for nothing to prove it to them, and they adopted it. But then the union fought them, and now that division is closed.
You have to evolve or else you will go the way of the dinosaurs.
Doug Glenn: You can’t fight with science. Ultimately you can’t fight with the truth of metallurgy.
I think we have the basic process down; it doesn’t sound that complicated. It’s a reverse of the quench process, essentially.
“Aluminum alloy 6061 is a forgiving alloy…It lends itself to uphill quenching because of its lower yield than the 7,000 series. We also do work in the 7,000 series.” Source: Theworldmaterial.com
Greg Newton: Attention must be paid to the details, though.
Doug Glenn: Yes, exactly. I have talked with a couple of other people about this process, and I’ve been told that the aluminum alloy is somewhat important in the process depending on what alloy you have. Is that the case?
Greg Newton: Aluminum alloy 6061 is a forgiving alloy, and most of the optics we do are some form of that. It’s a forgiving alloy in many, many ways. It lends itself to uphill quenching because of its lower yield than the 7000 series. We also do work in the 7000 series. It takes a little better steam fixture, perhaps a little more attention to detail. Rough machining comes into play, regarding how much rough machining is done prior to the final solution heat treat and the uphill quench.
John Avalos: There are lots of factors.
Greg Newton: We like to be involved in the beginning, not as an after fact. The best successes we’ve had is when the company knows it’s going to be a problem part, so they get us involved in the beginning. Then, we set it up right and everything goes smoothly, instead of after.
Doug Glenn: You had mentioned the X-ray diffraction and the testing of it. Is there anything more we want to say on that?
Greg Newton: After the solution heat treat, I’m going to get that perfect ratio of my compressive shell and the tensiles. After the uphill quench, we’ll measure again, and then once after aging, because aging can have a slight effect on your stress levels.
That will give us an internal baseline, and we do it for all clients on all first articles. I encourage clients to pay for it, but to a lot of machinists, it’s just an extra cost. But should they ever have a problem in the future? The proof has always been in the pudding. I send it back to them because I can’t tell you how many skeptics we’ve had that call me back and say, “dang, it really worked.” And then they think it’s that magic. Some of the failures that have come from the successes and thinking, “Now I can make up the lost time. I’m going to make twice the cuts, twice as deep, twice as fast.” Then you induce stresses by machining parts.
Newton Heat Treating’s equipment for cold stabilization Source: Newton Heat Treating
Doug Glenn: You mentioned that when the engineer initially does the patent, they control everything; they put a lot of standards in there. It sounds to me that in your practical application of this process, you found out which one of those instructions are important, and which ones are maybe not as important.
Greg Newton: We have completely refined the process.
Doug Glenn: Now you know you don’t need to waste time on item X because it really doesn’t matter so much. The correlation for success may be more tied with another item.
Greg Newton: The boiling water aspect becomes so appealing to my competition because you don’t need to use your brain to design steam fixtures and other processes. We have designed many steam fixtures over the years, and they’re semi generic. We can change the inserts for cylindrical parts. We have found it’s very advantageous to steam inside and out, simultaneously. When it says high-pressure steam, I have engineers up with their cameras and I say, “No, no, back away about 30 feet.”
Doug Glenn: Step back from the part. That sounds interesting. The design of the fixtures for the impingement of the steam sounds very similar to me to something we’ve talked to Joe Powell of Akron Steel about. He talks about that high-intensity quench, not uphill quenching, but downhill quenching in this case, where it’s really super critical that you quickly and uniformly cool the entire outer shell at the same time.
It sounds like these fixtures you’re talking about are somewhat along that same line that they need to be hitting the part at the right place, right time, right volume.
John Avalos: They represent the configuration of the part as close as we can anyways, so that we get a nice even steam blast.
Greg Newton: We’ll tend to concentrate steam in thicker areas, back off on thinner areas.
Challenges in Uphill Quenching (25:00)
Doug Glenn: What are the biggest challenges that you face when performing uphill quenching?
Greg Newton: Overcoming the misconceptions of when and how to do it can be challenging as there are so much different variables. We have capacity for boiling water and steam, but we prefer to do the best method possible, and give my clients the best, because the price is the same. I’d rather have a happy client. Then, I think, boiling water sometimes gives it a bad name when it doesn’t work. They often throw out the entire system, the baby with the bath water.
Cyclic thermal shock process Source: Newton Heat Treating
Doug Glenn: In the actual process itself, fixturing can be an issue, placement and configuration of the steam is an issue. I’m guessing part configuration can be challenging, the thick to thin cross-section. What are some of the difficult aspects of uphill quenching or difficult parts.
Greg Newton: One day, Lockheed calls me, and they had a sniper pod for the F16. They tread machined this 1,600-pound hand forging three times and were trying to go to a one piece, monolithic part. They had one more shot until they were going to lose the contract.
So, Don of Lockheed came to me asking if we could do it. They wanted to send me 1,600-pound hand forging and I said, “No, no, you need to rough machine this thing.” I asked how much the part weighed when they were done — “168 pounds.” That’s crazy!
I told them they needed to rough machine the part and then send to me. So, they rough machined it, and I get a part that is 1,200 pounds, but it was 6061. I told them we’ll give it our best shot. We did do multiple stabilizations on that part — I think we stabilized it three times, but it worked.
He was worried about getting this big hand forging back on the machine, because it did move a lot during uphill quenching. We did, in between post-heat treat, straighten it, uphill quench it, then straighten it; each run time it moved less, and, you know, you’re inducing stresses by straightening through the process as well. The third time, we uphill quenched it, checked if we needed to straighten it, and we didn’t. We shipped it, and they got through this. We saw another two or three more.
The challenge is what they think the process will do and what it’s capable of. I don’t think that would’ve worked for the 7000 series. You really want to get it within 150 thousandth to 100 thousands of control, because of the dispersoids they put in the super alloys, making it tougher to uphill clench.
Doug Glenn: What is your most interesting part that you have uphill quenched?
Greg Newton: The rotor grips for the old Hueys. When they re-engineered them and doubled the horsepower, they went from the two blades that you see on the mash that they could hear from 30 to 40 miles away. They increased the horsepower of the engines and went to four composite blades, but the rotor grip itself that they wrapped the carbon fibers around was a 2014 die forging.
But they had machining problems. They would make one pass over it and it would curl up about three quarters of an inch. So, Gene Williams came down from Bell Helicopter and spent a week with me. Bell doesn’t like anybody else’s data; they want to create their own data. So, he was out there with his camera, measuring and doing everything for a week. We got through the machining and they’re dead flat. Now, when I get rid of the stresses, I get rid of all the stresses: the compressive shell and the tensiles. So, they went back to these rotor grips and peened them, glass beaded them. This gave it a nice, even compressed shell without the negative effect of the tensiles in the core.
Now they are getting 8 to 10 times the life expectancy out of these parts, which makes sense on a fatigue curve, because you don’t know where you’re starting on that fatigue curve. Most of the curves go “whoop” [Editor’s Note: Greg demonstrates the exponential swoop of the graphic arc.], and you know you’re in that quarter and then you’re done. They store parts at 50% of their intended life for when they can’t get new parts and pray they get the new ones.
Weget the problem parts, and that usually gets my foot in the door.
Doug Glenn: You mentioned earlier that if a company is developing a part or if they’re having an issue, it’s better for the client and for you guys that the sooner they talk to you the better. Most people don’t think the commercial heat treater or the processor can be that helpful, but with guys like you who have an expertise in the area, it’s probably well worth having an early phone call.
Greg Newton: No heat treater really loves to see final finished parts. It’s a violent process. We would rather have a little beat on that.
Ideal Parts and Benefits (30:45)
Doug Glenn: What type of parts should uphill quenching be performed? Can you give us a quick overview of the types of parts that you’ve uphill quenched?
Greg Newton: Any close-tolerance parts or any parts that are moving and machining out of tolerance are good for uphill quenching.
Doug Glenn: What benefit does uphill quenching have over similar or competitive processes?
Greg Newton: With straight thermal stress relieving, in which you’re just raising the temperature of the part, you have to be careful of losing your temper when doing it. To get a real stress relieve, you need to go up 600-700 degrees, and in doing that, you’re going to blow your temperatures right out in aluminum. So, you tend to use 25 degrees below zero for longer periods of time, and you might lower it. That tends to break the most highly strained lattices because you’ve lowered that yield strength a little bit and they’ll pop. That might be enough to get you through that part, the machining.
Is it going to move later in service? Probably. When heating up and cooling it down, especially in space; when you have an unstable part in space and it turns towards the sun gets 200-300 degrees (turns away from space in the vacuum), now you’re thermo cycling. It is a different type of stress relieving, and it can move those mirrors. Any slight movement in those mirrors, and you’ve lost your integrity.
They can figure out mathematically the coefficient of thermal expansion out in space, but warpage is difficult.
Radius of Industry (32:43)
Doug Glenn: You have an expertise in aluminum. What is the radius out of the city of industry that you’re getting clients from?
Greg Newton: We have received Israeli tank mirrors and German tank mirrors. We get parts shipped from the East Coast daily. Hamilton’s products, they attribute their position with the success of their uphill quenching on almost of all their cylindrical parts. They have a better product than anybody else, and they told me that they attribute much of that success the stability of their, their aluminum.
Doug Glenn: Is there anything that you thought of as we’re talking that you want to add into the conversation?
John Avalos: I’ll add that we’re the leaders in this process. There are a lot of similar processes Greg mentioned with boiling water. What that does is it forms the ice barrier around the part. By using steam blasting and uphill quenching, it removes that barrier — a barrier simply doesn’t form.
Greg Newton: Ice is a great insulator.
Doug Glenn: It reminds me of the vapor barrier when you’re trying to quench. It’s an insulator.
Greg Newton: Regarding the X-ray diffraction, having process control is important. You’re spending 10 times a normal heat treat, you’re throwing money in a problem, and there is nobody else that has any process control. To me, that’s playing Russian roulette with five in the chamber, not one. Your chances of success are slimmer. We want to know when something goes wrong. Why did it go wrong? Without any sort of can imagine, if we threw out EC and Rockwell out of our heat treatment and say, “Look, the charts look good! It must be good,” we’d have airplanes falling out of the sky daily.
Heat Treat Radio episode #124 with host Doug Glenn and guests Greg Newton and John Avalos
You have a very expensive problem. I would like to see a little more process control that everybody’s using. Nadcap is trying to tie that up as we speak.
Doug Glenn: Very good. Well, gentlemen, thank you very much I hope the listeners have enjoyed this as well. I think it’s a very interesting, somewhat unique process, and it’s good to talk with you two guys about it.
Greg Newton: I challenge any machine shop out there to send me their biggest nightmare in aluminum
Doug Glenn: He just threw down the gauntlet: Send him your worst stuff, and he’ll see if he can fix it. Anyhow, thanks, Greg and John, thank you so much. I appreciate you guys.
About the Guest
Greg Newton Owner, President, CEO Newton Heat Treating
Greg Newton is the owner, president, and CEO of Newton Heat Treating. Founded by his father in 1968, Greg became president of Newton Heat Treating in 1995 and has decades of experience leading numerous projects in the heat treating industry. Greg has focused specifically on aluminum alloys — specializing in heat treating, uphill quenching, and other advanced thermal processes.
La mayoría de quienes aplican el tratamiento térmico reconocen la importancia de medir la austenita retenida (RA, por sus siglas en inglés); no obstante, muchos optan por no realizar estas mediciones por razones de tiempo y/o de los costos asociados. Este artículo explica los motivos por los cuales se deben practicar las mediciones RA, los factores a favor y en contra de las tecnologías de medición tradicionales y los beneficios de realizar la medición en la planta misma, utilizando tecnologías más avanzadas.
This informative piece was first released inHeat Treat Today’sMarch 2025 Aerospace Heat Treating print edition. To read the article in English, click here.
La importancia del porcentaje de austenita retenida
Antes de entrar a examinar algunas metodologías de medición, es necesario entender lo básico en relación a la austenita retenida, al igual que la importancia que reviste el porcentaje de la misma (%RA).
Austenita retenida (RA) es el nombre que se le da a la austenita que durante el proceso de templado no se transforma en martensita. En términos sencillos, la austenita retenida (figura 1) ocurre cuando el acero se ha templado sin llegar de manera contundente a la temperatura de acabado de la martensita (Mf); es decir, la temperatura ha estado por encima de lo requerido para permitir la formación de martensita al 100%. Debido a que la Mf está por debajo de la temperatura ambiente en la mayoría de las aleaciones que contienen más del 0.30% de carbón, se pueden presentar cantidades significativas de austenita retenida en la martensita a temperatura ambiente. (Herring, Atmosphere Heat Treatment).
Al tratarse del %RA, con frecuencia existe un equilibrio muy sensible entre sus efectos benéficos (el aumento en la durabilidad de ciertos componentes manufacturados) y sus atributos negativos (la creación de piezas susceptibles de fracturas y averías). Por tal motivo es de crítica importancia que los tratadores térmicos logren el %RA óptimo para la aplicación deseada.
Por ejemplo, en las industrias de la aeronáutica y la astronáutica, con frecuencia se especifica que los niveles de RA sean inferiores al 8%, y para piezas como los cojinetes y los actuadores lineales, se requiere un RA por debajo del 3%, lo más cercano posible a cero. No obstante, en otras aplicaciones, como por ejemplo los engranajes grandes para generadores de energía, energía eólica y plataformas de rendimiento, se ha identificado que un RA en el rango del 15-30% reviste mayores beneficios. (Errichello et al., “Investigations of Bearing Failures”). De igual manera, un alto % RA es una ventaja en el caso de cojinetes que vayan a entrar en contacto con lubricantes contaminados.
Figura 1. Microestructura en la superficie de la trayectoria de un cojinete de rodamiento 12CrNi3 (o SAE/AISI 9310) compuesto por martensita templada en la que se evidencia austenita retenida (áreas blancas)
Marco DeGasperi, gerente técnico de Verichek, se pronunció al respecto señalando que el %RA es de crítica importancia para los inyectores de combustible, para piezas pequeñas en aplicaciones médicas y para aplicaciones de alto nivel y alto volumen tales como las placas de desgaste en la industria minera. Lo resumió afirmando: –Cuando tu ejercicio se trate de someter a presión y movimiento cualquier dispositivo de calibración fina…si utilizas la palabra “precisión” para darte a conocer, vas a querer hacerte a una [herramienta de medición del %RA].
Las mismas características que le dan a la austenita retenida muchas de sus propiedades particulares, son a la vez las respons ables de significativos problemas de funcionamiento. Sabemos que la austenita es la fase normal del acero a altas temperaturas, mas no a temperatura ambiente. Debido a que la austenita retenida existe por fuera del rango normal de su temperatura, es metaestable, lo que quiere decir que, cuando entre en funcionamiento, los factores como la temperatura, el estrés, y aún el tiempo, harán que se transforme en martensita no revenida. Es más, junto con dicha transformación se dará un cambio en el volumen (aumentará) generando un alto grado de estrés interno en el componente y provocando muchas veces la formación de grietas lo que podrá llevar a que las piezas fallen en el campo.
El % RA también es importante, no solo por el impacto sobre la estabilidad dimensional, sino además por las propiedades mecánicas tales como el límite elástico, la resistencia a la fatiga, la tenacidad, y la manejabilidad. (Herring, Atmosphere Heat Treatment). A manera de ejemplo, DeGasperi identifica en la industria automotriz las consecuencias de un %RA demasiado alto o demasiado bajo: –Hablemos de las piezas en una transmisión o en una caja de transferencia; aquí es donde se dan los casos en los que se empiezan a romper los cojinetes, o terminas viéndote en la obligación del retiro masivo del producto del mercado. Y por lo general toda la cadena de suministro identifica al anterior como el culpable cuando ninguno en toda la cadena se ha tomado la molestia de probar las piezas por sí mismo.
Por el contrario, en algunos casos, la RA diseminada en pequeñas cantidades aporta para que el material resista la propagación de fracturas por fatiga y disminuye el estrés por fatiga en el contacto de rodamiento, así que lograr el correcto equilibrio en la cantidad de RA es importante en muchas aplicaciones. Además, el % justo de RA es esencial para el control de calidad, al igual que para evitar problemas de seguridad y retiros masivos del mercado. El debido control y la medición precisa del % RA en las aleaciones de acero es un punto crítico para garantizar la calidad y la seguridad de los componentes terminados, salvaguardando así la reputación y el margen de ganancia tanto de los tratadores térmicos como de los fabricantes.
Métodos de medición de RA
El medir con precisión la RA es de vital importancia para establecer si existe el balance correcto entre la austenita retenida y la martensita en determinado componente. Los tratadores térmicos tienen a su disposición varias metodologías para esta medición, cada una con sus respectivas ventajas y desventajas. Para el tratador térmico entender la importancia de medir el % RA representa tan solo una parte de la batalla ganada, mientras que la otra parte se gana cuando se logra identificar un método de medición que sea rápido, preciso y rentable.
La difracción de rayos-X: el mejor y más preciso de los métodos
Figura 2a. Una unidad de sobremesa ArexD de GNR
La difracción de rayos-X, utilizada para identificar y cuantificar las fases en un material, se considera el método más preciso de medición de RA en acero ya que logra establecer los niveles de RA hasta el rango aproximado de 0.5-1% (GNR, “AreX Diffractometer,” 3). En la difracción de rayos-X, las diferentes fases cristalinas demuestran diferentes patrones de difracción, lo que permite que sean identificadas y medidas. Además del análisis de fases, la difracción de rayos-X se puede utilizar para analizar car acterísticas microestructurales tales como la textura, el esfuerzo residual y el tamaño del grano.
Hoy en día, la difracción de rayos-X es una solución segura y no-destructiva que permite valorar una región mucho más amplia que la de varios de los otros métodos disponibles, sin necesidad de gran preparación ni análisis de la muestra, haciendo de ésta una solución más eficiente y efectiva. Es la tecnología más opcionada para una empresa que requiera valorar la RA con un resultado esperado inferior al 10%,
La actual generación de difractómetros de rayos-X ostenta un diseño de sobremesa con un peso aproximado de 25 libras. Existen modelos con costos inferiores a los USD $100.000, lo que los hace rentables frente al costo de un difractómetro tradicional (USD $200.000) que tenía además la desventaja de presentar dificultades cuando la muestra tuviera fases y reflexiones adicionales, ya fuera por el tamaño del grano, por los carburos o por las texturas que pudieran provocar disturbios y variaciones en la medición. La nueva generación de equipos de rayos-X logra superar estos obstáculos utilizando múltiples picos de difracción para minimizar los efectos de la orientación preferida y detectar la interferencia de los carburos.
Figura 2b. Una unidad de sobremesa ArexD de GNR
Las máquinas modernas de difracción de rayos-X tienen la capacidad de recoger hasta siete picos de difracción (tres para la fase ferrítica/martensítica y cuatro para la fase austenítica) para luego establecer la concentración de porcentaje por volumen de RA en la muestra al comparar las intensidades de los picos y analizar las relaciones entre éstos de acuerdo con el ASTM E975-22 (práctica estándar para la determinación por rayos-X de austenita retenida en acero con orientación cristalográfica cercana a la aleatoria).
No es complicado usar los equipos modernos de difracción de rayos-X. En menos de tres minutos se logra la medición con tan solo ubicar la muestra en la máquina y oprimir el botón de inicio. Estos difractómetros realizan mediciones en muestras de diferentes tamaños y se valen de software intuitivo, dando lugar a que cualquier técnico, tenga o no experiencia previa en metalurgia o difracción, efectúe la medición de manera rápida, precisa y eficiente.
La microscopía óptica: un método a prueba del tiempo
La RA se puede medir de manera metalográfica con un microscopio óptico. En la mayoría de los casos, un metalúrgico con experiencia puede establecer el %RA en el rango hasta del 10-15%, lo cual es más que suficiente para muchas aplicaciones, con el beneficio adicional de que también caracteriza la microestructura.
Este método, que implica establecer la fracción de austenita mediante el contraste derivado del comportamiento de grabado o morfología, es de bajo costo; sin embargo, puede ser demorado. En libros de referencia existen tablas y diagramas que ayudan a determinar el porcentaje de austenita retenida utilizando métodos comparativos. La microscopía óptica es subjetiva ya que depende del individuo y la interpretación que haga de la muestra bajo el microscopio.
Figura 3. Ejemplo de la técnica para medir los picos de %RA
Métodos alternos
Los tratadores térmicos también disponen de otros varios métodos de medición de la RA. Entre los más comunes se encuentran:
La inducción magnética: Aquí se magnetiza una muestra al punto de saturación y se mide la polarización de saturación. Con esto, se calcula la diferencia entre la saturación medida y la saturación teórica de la RA utilizando la ecuación.
La inducción magnética no es destructiva y ofrece un rango más alto y amplio que el de la microscopía óptica (1-30%). Sin embargo, al ser una medición de volumen, es necesario que el instrumento sea calibrado a los materiales específicos, junto con sus tratamientos térmicos y geometrías, lo cual exige mucho tiempo y depende en un alto grado de la habilidad del técnico.
Difracción de electrones por retrodispersión (EBSD, por sus siglas en inglés): Utilizar este método de medición de RA implica ubicar la muestra en un microscopio electrónico de barrido (SEM, por sus siglas en inglés) para caracterizar la estructura cristalográfica al igual que la microestructura. Las mediciones de RA con base en esta técnica no suelen ser muy precisas y dependen de la correcta preparación de la muestra. Adicionalmente, es un método destructivo y arroja una medida sobre un volumen muy pequeño.
En conclusión
El medir acertadamente el nivel de austenita retenida permite que tanto el ingeniero de diseño como el metalúrgico maximicen los efectos benéficos que ofrece, al mismo tiempo evitando sus consecuencias negativas. El tratador térmico, por su parte, deberá tener en cuenta la química del material y las variables del proceso de tratamiento térmico tales como la temperatura de austenización, la rapidez de enfriamiento, los tratamientos criogénicos o de congelación profunda y las temperaturas de templado.
Referencias
Errichello, Robert, Robert Budny, and Rainer Eckert. “Investigations of Bearing Failures Associated with White Etching Areas (WEAs) in Wind Turbine Gearboxes.” Tribology Transactions 56, no. 6 (2013): 1069–1076.
GNR, Analytical Instruments Group. “AreX Diffractometer: GNR Proposal for measuring Retained Austenite in the industrial domain and in laboratory.”
Herring, Daniel H., Atmosphere Heat Treatment. Volume I. Chicago: BNP Media, 2014.
Agradecimientos
Queremos agradecer a los siguientes contribuyentes por su aporte en el desarrollo de este artículo: Thomas Wingens, presidente y especialista en Heat Treat, WINGENS CONSULTANTS; Dennis Beauchesne, gerente general, ECM USA; Tim Moury, presidente & CEO, Marco DeGasperi, gerente técnico, Jeff Froetschel, vicepresidente y director financiero, Verichek Technical Services, Inc.; y Dan Herring, The Heat Treat Doctor®, The HERRING GROUP, Inc.
Most heat treaters recognize the importance of measuring retained austenite (RA), yet many opt not to perform these measurements due to time and/or cost constraints. This Technical Tuesday installment explains why performing RA measurements is necessary, the pros and cons of traditional measurement techniques, and the benefits of using more current and in plant technologies.
This informative piece was first released inHeat Treat Today’sMarch 2025 Aerospace Heat Treating print edition. To read the article in Spanish, click here.
Why Retained Austenite Percentage Matters
Before examining measurement methodologies, it is important to understand the fundamentals of retained austenite and why the percentage of retained austenite (RA%) matters.
Austenite that does not transform to martensite upon quenching is called retained austenite (RA). In simple terms, retained austenite (Figure 1) occurs when steel is not fully quenched to the martensite finish (Mf) temperature; that is, low enough to form 100% martensite. Because the Mf is below room temperature in most alloys containing more than 0.30% carbon, significant amounts of retained austenite may be present within the martensite at room temperature (Herring, Atmosphere Heat Treatment).
When it comes to RA%, there is often a delicate balance between its beneficial effects (an increase in the life of certain manufactured components) and its negative attributes (the creation of parts that are prone to cracking and failure). For this reason, it is crucial that heat treaters achieve the optimal RA% for the intended application.
For example, in the aeronautics and astronautics industries, RA levels are often specified to be under 8% and, for devices such as bearings and linear actuators, RA under 3% and as close to zero as possible is required. In other applications, however, such as large gearing for power generation, wind energy, and performance platforms, in the range of 15–30% or more RA has been found beneficial (Errichello et al., “Investigations of Bearing Failures”). Also, high RA% has been found beneficial for bearings that will be subjected to contaminated lubricants.
Figure 1. 12CrNi3 (similar to SAE/AISI 9310) bearing roller path surface microstructure consisting of
tempered martensite with evidence of retained austenite (white areas)
Marco DeGasperi, technical manager at Verichek, weighed in on this, noting that for fuel injectors, small pieces in medical applications, and high-level, high-volume applications like wear plates in the mining industry, RA% is critical. He summarized with the statement, “When you’re applying pressure and motion to anything that’s fine-tuned … If you have ‘precision’ in your name, you probably want [an RA% measurement device].”
The very characteristics that give retained austenite many of its unique properties are those responsible for significant problems in service. We know that austenite is the normal phase of steel at high temperatures, but not at room temperature. Because retained austenite exists outside of its normal temperature range, it is metastable. This means that in service, factors such as temperature, stress, and even time will see it transform into untempered martensite. In addition, a volume change (increase) accompanies this transformation and induces a great deal of internal stress in a component, often manifesting itself as cracks, which leads to parts failing in the field.
RA% is also important not only because of its influence on dimensional stability but on mechanical properties such as yield strength, fatigue strength, toughness, and machinability (Herring, Atmosphere Heat Treatment). For example, looking in the automotive industry, DeGasperi gives an example of the consequences of having too high or too low RA%: “Let’s say pieces in a transmission or a transfer case; this is when gears start breaking or you get issued wide-end recalls. And then usually the supply chain all starts blaming the guy before them when nobody throughout the supply chain has actually tested the parts themselves.”
Alternatively, in some cases, finely dispersed RA helps the material resist the propagation of fatigue cracks and improves rolling contact fatigue stress, so balancing the amount of RA is important in many applications. Also, the correct RA% is essential for quality control, and proper control and accurate measurement of RA% in steel alloys is crucial to guaranteeing the quality and safety of finished components, as well as protecting the reputation and profitability of heat treaters and manufacturers.
RA Measurement Methods
Accurate RA measurements are critical to determine whether the correct balance of retained austenite and martensite exists within a given part. Several RA measurement methodologies are available to heat treaters, each having their own unique set of advantages and disadvantages. For heat treaters, understanding why it is crucial to measure the percentage of RA is only half the battle. Finding a cost-effective, fast, and accurate measurement method is the other half.
X-Ray Diffraction: The Best and Most Accurate Method
Figure 2a. An ArexD table-top unit from GNR
X-ray diffraction, which is used to identify and quantify phases in a material, is considered the most accurate method of RA measurement in steels as it can precisely determine RA levels down to the range of approximately 0.5–1% (GNR, “AreX Diffractometer,” 3). In X-ray diffraction, different crystalline phases have different diffraction patterns, allowing them to be identified and measured. In addition to phase analysis, X-ray diffraction can be used to analyze microstructural features such as texture, residual stress, and grain size.
Today, X-ray diffraction is a non-destructive, safe solution that can sample a much larger region than many other available methods and does not involve much sample preparation and analysis, making it a more efficient and effective solution. This is the option of choice for a company that needs to test RA with expected readings under 10%.
The current generation of X-ray diffractometers are tabletop sized, weighing about 25 lbs. With models under $100,000, they are also cost-effective when compared to traditional X-ray diffractometers ($200,000), which were sometimes problematic in the presence of additional phases and reflections due to grain size, carbides, or textures that could cause disturbances and variances in measurement. The new generation of X-ray equipment compensates for these obstacles via the use of multiple diffraction peaks to minimize the effects of preferred orientation and detect interference from carbides.
2b. An ArexD table-top unit from GNR
Modern X-ray diffraction machines can collect up to seven diffraction peaks (three for ferrite/martensite phase and four for austenite phase) and then determine the volume percent concentration of RA in the sample by comparing the intensities of the peaks and analyzing the peak ratios in accordance with the ASTM E975-22 (standard practice for X-ray determination of retained austenite in steel with near random crystallographic orientation).
The use of today’s X-ray diffraction equipment is not complicated. It can be measured in under three minutes by simply placing the sample in the machine and pressing the start button. These X-ray diffractometers measure various-sized samples and use intuitive software so the measurement can be performed quickly, accurately, and efficiently by any technician — with or without prior metallurgical or diffraction experience.
Optical Microscopy — A Time-Proven Method
RA can be measured metallographically with an optical microscope. An experienced metallurgist can usually determine RA% down to approximately 10–15% RA. For many applications, this is more than adequate and has the added benefit of characterizing the microstructure as well.
This method, which involves determining the austenite fraction using contrast from etching behavior or morphology, is low cost, however, it can be somewhat time consuming. Charts and diagrams in reference books are available to help determine the percentage of retained austenite by comparative methods. Optical microscopy is subjective as it is dependent upon the individual and their interpretation of the sample under the microscope.
Figure 3. Example of how RA% peaks are measured
Alternative Methods
Several other methods for measuring RA are available to heat treaters. The most common of these methods includes:
Magnetic Induction: Here, a sample is magnetized to saturation and the saturation polarization is measured. The difference between measured and theoretical saturation of the RA can then be calculated using this equation:
Magnetic induction is non-destructive and offers a higher, broader range than optical microscopy (1–30%). However, because it is a volume measurement, the instrument needs to be calibrated to the specific materials, heat treatment, and geometries, which is time consuming and highly dependent on the skill of the technician.
Electron Backscatter Diffraction (EBSD): Using this RA measurement method involves placing a sample in a Scanning Electron Microscope (SEM) to characterize the crystallographic structure as well as the microstructure. RA measurements using this technique are not particularly accurate and are reliant upon proper sample preparation. Additionally, it provides a very small measure volume and is a destructive test method.
Conclusion
Accurate measurement of the level of retained austenite allows both the design engineer and metallurgist to maximize its beneficial effects without suffering from its negative consequences. On the part of the heat treater this means taking into account the material chemistry and the heat treat process variables such as austenitizing temperature, quench rate, deep freeze or cryogenic treatments, and tempering temperatures.
References
Errichello, Robert, Robert Budny, and Rainer Eckert. “Investigations of Bearing Failures Associated with White Etching Areas (WEAs) in Wind Turbine Gearboxes.” Tribology Transactions 56, no. 6 (2013): 1069–1076.
GNR, Analytical Instruments Group. “AreX Diffractometer: GNR Proposal for measuring Retained Austenite in the industrial domain and in laboratory.”
Herring, Daniel H., Atmosphere Heat Treatment. Volume I. Chicago: BNP Media, 2014.
Acknowledgments
We’d like to thank the following contributors for the support of this article: Thomas Wingens, President & Heat Treat Specialist, WINGENS CONSULTANTS; Dennis Beauchesne, General Manager, ECM USA; Tim Moury, President & CEO, Marco DeGasperi, Technical Manager, Jeff Froetschel, VP & CFO, Verichek Technical Services, Inc.; and Dan Herring, The Heat Treat Doctor®, The HERRING GROUP, Inc.
Given safety and performance concerns in the aerospace sector, it may be beneficial to consider quench testing that uses CQI-9 as well as AMS2759 since the automotive standard focuses on safety. Read on to understand the different approaches between these two standards in this Technical Tuesday installment, written by Michelle Bennett, quality assurance senior specialist, and Greg Steiger, senior account manager, both at Idemitsu Lubricants America.
This informative piece was first released inHeat Treat Today’sMarch 2025 Aerospace Heat Treating print edition.
In today’s world, there are many different quality systems available to heat treaters. Many of these, such as ISO, are quality management systems. These quality management systems are an important piece of running a successful business. However, to successfully run a heat treat business and compete in either the North American automotive market or the aerospace market, a heat treater must conform to either CQI-9 or AMS2759, or, in cases where a company processes both automotive and aerospace parts, both. This article will explain the requirements for both CQI-9 and AMS2759. It will also explain the differences between the two quality standards and any additional testing that could benefit a heat treater or how they operate their quench tank.
AIAG’s CQI-9
The Automotive Industry Action Group (AIAG) is a non-profit group of over 800 automotive OEMS, parts manufacturers, and service providers who oversee the requirements for CQI-9. The 4th edition is the most current edition of CQI-9. As an internal audit process, CQI-9 covers most of the heat treating process. Section 3.14 specifies the quench oil and water-soluble polymer requirements. An oil quenchant requires that the in-use oils be tested every six months and the testing must include water content, percent suspended solids, total acid number, viscosity, flash point, and cooling curve. The specification range and warning limits are based on the vendor’s requirements and recommendations. For water-based polymers, there are two tests required: concentration and quenchability. The standard does not specify a test for quenchability, however, it does make a few suggestions such as a cooling curve, viscosity, and titration.
For water-based polymers, there are two tests required: concentration and quenchability. The standard does not specify a test for quenchability, however, it does make a few suggestions such as a cooling curve, viscosity, and titration.
All the required testing of the quenchant is designed to achieve consistent metallurgy for safety reasons. Viscosity is monitored to look for oxidation or heat decomposition of the oil. Degradation can be in the form of oxidation, thermal breakdown, or the presence of various contaminants. Increased oil viscosity typically results in decreased heat transfer rates. A decrease in viscosity may indicate contamination. Some suspended solids are to be expected during the quenching process, but the majority of them should be filtered or centrifuged from the process. If the quantity of these contaminants becomes too high, then it can both affect the brightness of the parts, and the parts can get soft spots as the contaminants may not cool the parts at the same rate.
Water and flash point are both monitored for safety. If the flash point drops below the accepted range or the water content is above the acceptable range, these can cause fires during the operation. Water can also show issues with the equipment or the procedure such as leaking of anything that is water cooled, such as the outer door on a furnace. Acid value is monitored to degradation of the oil. As the oil breaks down and oxidizes, the acid value will increase. This can cause the maximum cooling rate to increase and can cause cracking or distortion on the parts. Carbon residue can be measured for two reasons. If the result is below the specification, it can show that the quench speed improver is being broken down or dragged out of the system. If the result is higher than the specification, it can show the formation of sludge, which will impact the brightness of the parts.
For water-based quenchants, the most common test items include pH, refractive index or brix, viscosity, and concentration calculation. Sometimes additional test items can be added, such as biological testing, to help determine and correct current issues.
Table 1. CQI-9 vs. AMS2759 quenchant requirements
SAE’s AMS2759
Just as AIAG is a non-profit business group responsible for CQI-9, SAE International is a non-profit organization responsible for AMS2759. The most recent revision of AMS2759 is Revision G. AMEC (the Aerospace Materials Engineering Committee) is responsible for maintaining this standard. Unlike CQI-9, AMS2759 requires a certificate of conformance for all shipments. Section 3.10.3 begins the requirements for quenchant testing and quenchant deliveries. Viscosity, flash point, and temperature at the maximum cooling rate must be reported on the certificate of compliance when dealing with mineral oil quenchants. For a polymer, the requirements are that the pH of the neat polymer and the neat viscosity of the polymer must both be reported on the certificate. Also required on the polymer certificate are the viscosity, pH, and the temperature at the maximum cooling rate for polymers at 20% dilution by weight.
Similarly to CQI-9, AMS requires that the in-use quenchants be tested biannually. This standard, however, only requires the cooling rate and temperature at max cooling rate be tested, as well as any additional tests the supplier recommends. The AMS2759 specification does not have set limitations on the cooling rate and temperature. Instead, the specification sets the allowed upper and lower deviations from the supplier’s standard for the maximum cooling rate and the temperature at the maximum cooling rate for both oils and water-soluble polymers. The supplier should have calculated the average max cooling rate and average temperature at max cooling rate using many different blend lots and multiple test runs. This average will not vary or change based on current production values or the values for the batch that the client is currently using (Table 1).
Although both standards require having the quenchant tested bi-yearly, most quenchant suppliers encourage their clients to submit their furnace samples for testing quarterly. This ensures that the medium is being monitored frequently, and if a sample is missed or late when sampling quarterly, then the client is still within compliance for the six month testing requirements.
However, because many of the test parameters in CQI-9 are run for safety reasons along with performance reasons, it is highly advised that aerospace heat treaters should run the full suite of CQI-9 testing along with the AMS2759 testing.
Taking a Quench Sample
There are many different quench methods and both standards allow for any of the following variations: ASTM D6200, ISO 9950, JIS K2242, ASTM D6482, or ASTM D6549. The type of testing that is going to be conducted will determine the size of sample that will be needed. For just this quench testing, the volume of sample needed ranges from 250 milliliters to 2 liters.
As always, when taking samples, it is important to be sure to get a good representative sample of the current quenchant being used in the process. The agitation needs to be running and collected in a clean and dry container. The sampling site should be the most convenient location to safely obtain a sample. It should also be the same location for every sample. The lid also needs to be put on before the oil cools too much because the container will draw in moisture and condensation as the oil cools if it is open to the atmosphere.
Conclusion
When examining the standards, there is one basic commonality: the need to run a complete cooling curve every six months. There is also a large difference in that AMS2759 does not require the full suite of testing that CQI-9 does. However, because many of the test parameters in CQI-9 are run for safety reasons along with performance reasons, it is highly advised that aerospace heat treaters should run the full suite of CQI-9 testing along with the AMS2759 testing. For automotive heat treaters, the maximum cooling rate and the temperature at maximum cooling rate is something that can be reported in the normal D6200 cooling curve test.
For manufacturers heat treating parts for aerospace, automotive, or both markets, we recommend quarterly quench samples at a minimum. The primary reason for more frequent testing is safety. Also, with the current labor shortage, heat treaters are busier than ever. If quench samples are routinely taken on a quarterly basis and are somehow missed and forgotten, there is still time to take another sample and remain in CQI-9 and AMS2759 compliance.
Remaining in compliance of these two important standards requires a lot of hard work from both the heat treater and the quenchant provider. Unless the quenchant supplier is working together in a true partnership, it will be very difficult to remain in compliance with the requirements for CQI-9 and AMS2759. But with routine monitoring, heat treaters can help to ensure quenchant and equipment have a longer life and achieve ever-tightening requirements from clients.
About The Authors:
Michelle Bennett Quality Assurance Senior Specialist Idemitsu Lubricants America
Michelle Bennett is the quality assurance senior specialist at Idemitsu Lubricants America, supervising the company’s I-LAS used oil analysis program. Over the past 12 years, she has worked in the quality control lab and the research and development department. Her bachelor’s degree is in Chemistry from Indiana University. Michelle is a recipient of Heat Treat Today’s40 Under 40 Class of 2023 award.
Greg Steiger Senior Account Manager Idemitsu Lubricants America
Greg Steiger is the senior account manager at Idemitsu Lubricants America. Previous to this position, Steiger served in a variety of technical service, research and development, and sales and marketing roles for Chemtool Incorporated, Witco Chemical Company, Inc., D.A. Stuart Company, and Safety-Kleen, Inc. He obtained a BS in Chemistry from the University of Illinois at Chicago and recently earned a master’s degree in Materials Engineering at Auburn University. He is also a member of ASM International.
