Hardness results that fall below specification are not always caused by heat treatment errors. In this Technical Tuesday installment, Ana Laura Hernández, founder of Consultoría Carnegie, examines how decarburization develops, how it affects hardness and surface properties, and the practical methods heat treaters can use to evaluate, control, and prevent it through metallurgical analysis and process discipline.
This informative piece was first released in Heat Treat Today’sJuly 2026 Annual Super Brands Issue print edition.
In heat treatment operations, hardness results outside of specification are often attributed to deviations in process parameters, operational errors, or variations in the chemical composition of the material. However, there are cases where the root cause is related to metallurgical phenomena that directly affect the surface of the material.
A representative case involved a batch of ten forged components weighing more than 5 tons each, manufactured from AISI 4340 steel. The applied heat treatment process was known and previously validated: normalizing, quenching, followed by tempering in a range of 600–610°C (1112–1130°F), with the objective of meeting the required mechanical properties. After completing the heat treatment, the inspection team reported hardness values approximately 20–30 HB below specification. This result was unexpected, considering that AISI 4340 steel, due to its alloying elements, maintains relatively high hardness values even at relatively high tempering temperatures. A hardness reevaluation was requested, increasing the surface preparation depth up to 5 mm. However, the hardness values only increased slightly (~5 HB), without reaching the required levels.
Under pressure due to delivery timelines, it was decided to reprocess the components. The complete heat treatment cycle was applied again, adjusting the tempering temperature to 580°C (1076°F), which, from a metallurgical standpoint, should increase hardness. Nevertheless, the results remained practically unchanged. The chemical composition was verified using PMI, confirming that the material was indeed AISI 4340 steel. Additionally, the evaluation piece “Qualification Test Coupon” (QTC) indicated high hardness values, consistent with the expected behavior of the material.
Given the inconsistency between experimental results, metallurgical fundamentals, and prior experience, a deeper analysis of the process history was conducted, correlating variables from each stage of processing. This is where the key to a proper failure analysis lies: thoroughly understanding the history of the component.
It was identified that, prior to heat treatment, the components had remained inside a furnace within a temperature range of 800–900°C (1472–1652°F) for approximately three days, due to delays in the rolling process caused by maintenance issues. As a result of prolonged exposure and poor communication between shift operators, the components remained inside the furnace at “lower temperatures” while waiting to complete the final rolling step. Although not at rolling temperatures, this prolonged exposure in a non-controlled atmosphere promoted decarburization of the material.
To validate this hypothesis, a longitudinal cut was performed on one of the components, and hardness measurements were carried out in the core. The results confirmed that the material exhibited significantly higher values in the interior, revealing a decarburized surface layer. This case demonstrates the importance of: Analyzing the full material and process history before concluding a non-conformance.
What Is Decarburization?
Figure 1. Representative schematic of the decarburization and oxidation process. | Image credit: Consultoría Carnegie
Steel is a solid solution based on iron and carbon, where mechanical properties depend directly on the carbon content and the alloying elements present. Decarburization is a metallurgical phenomenon consisting of the loss of carbon from the surface of steel when it is exposed to high temperatures in atmospheres with oxidizing potential or low carbon potential (Figure 1). This phenomenon commonly occurs during forging, hot rolling, normalizing, annealing, and heat treatment in conventional furnaces without atmosphere control.
From a thermodynamic perspective, decarburization occurs when the chemical potential of carbon in the steel is higher than that of the surrounding environment, which drives carbon diffusion toward the surface. In the presence of oxidizing gases, carbon reacts to form gaseous species:
It can also react with water vapor or carbon dioxide, accelerating the process. As a result, a carbon gradient is generated (surface vs. core), the microstructure is altered, and mechanical properties are degraded, especially at the surface.
Oxidation and Its Relationship with Decarburization
Decarburization is closely related to oxidation processes. Oxidation occurs when oxygen reacts with the metal, forming oxides on the surface. This process depends on the thermodynamic equilibrium between the partial pressures of the gases present, which can be analyzed using diagrams such as the Ellingham-Richardson.
An increase in temperature accelerates both oxidation and decarburization, which is why the use of controlled atmosphere furnaces is essential to prevent these phenomena.
Process Kinetics: Carbon Diffusion
Decarburization is a diffusion-controlled process. When steel is at high temperatures, atoms gain higher mobility and carbon diffuses from regions of high concentration (core) to regions of lower concentration (surface). This process intensifies when the material is fully in the austenitic phase (above Ac₃), where carbon diffusivity is significantly higher.
The depth of the decarburized layer depends on temperature, exposure time, steel composition and furnace atmosphere conditions. This phenomenon is critical because the reduction of carbon content at the surface directly affects wear resistance and fatigue performance of the material.
Methods for Determining Decarburized Layer and Oxides
Prediction of Oxides Using Computational Tools
The use of tools like Thermo-Calc software allows the prediction of stable phases as a function of temperature and oxygen partial pressure. These diagrams enable identification of which oxides will form at the surface, evaluation of whether these oxides act as a barrier or facilitate carbon loss, and optimization of heat treatment conditions.
Figure 2. Thermodynamically stable phases formed during high-temperature oxidation as a function of oxygen activity for H11 steel at 600°C (1112°F) (Balaško, et al. 2021)
For example, in steels such as H11, phase stability diagrams allow visualization of how oxide formation varies as a function of temperature and oxygen activity (Figure 2). Understanding these relationships is key to preventing decarburization, designing controlled atmospheres, and avoiding undesirable mechanical property outcomes.
Determination of Decarburized Layer Depth
The ASTM E1077 standard establishes different methods to evaluate the depth of the decarburized layer, including microscopic methods, microhardness, and chemical analysis. These methods determine whether a component meets specifications and define the proper preparation required for hardness measurement. The following sections present an example of the use of these methods. The objective of this analysis was to determine the depth of this layer to generate internal instructions for proper sample preparation through grinding, ensuring accurate hardness measurements.
A QTC sample of AISI 4340 steel in forged condition was selected and subjected to normalizing, quenching, and tempering. Samples were obtained through cross-sectional cutting to perform hardness testing, optical microscopy, and carbon chemical analysis (by LECO, Figure 3).
Results
Hardness Profile
Measurements were taken at 1 mm intervals from the surface toward the interior. The results showed 0–3 mm low hardness values (~30 HRC) and an increase up to 35–36 HRC at 5 mm, remaining constant (Figure 4). This indicates that the surface region does not represent the actual properties of the material.
Figure 4. a) Hardness profile results, b) sample used for hardness profile analysis. | Image credit: Consultoría CarnegieFigure 5. a) Hardness evaluation zones every 1 mm, b) hardness results shown as a color map (blue represents the softer surface region and red represents the harder region at 5 mm depth). | Image credit: Consultoría Carnegie and Mikra QATM
One of the challenges in this method is the precision of hardness indentations. It can be observed in Figure 4b that a fine marker was used to define the measurement zones; however, nowadays the use of advanced equipment allows automated programming of measurement locations (Figure 5).
Chemical Analysis (LECO)
Figure 6. a) Carbon analysis results using a LECO system, b) sample used for carbon analysis. | Image credit: Consultoría Carnegie
The LECO analysis showed an increase in carbon percentage starting at approximately 5 mm depth, confirming the presence of a decarburized layer (Figure 6).
Optical Microscopy
Figure 7. Results of decarburized layer analysis using optical microscopy. | Image credit: Consultoría Carnegie
The optical microscopy analysis allowed observing microstructural changes between the surface and the core associated with carbon loss (Figure 7).
Practical Application: Shop Floor Control
Table A. Work Instruction in the Hardness Testing Area
Based on the results, work instructions were established to ensure reliable hardness measurements (Table A).
Operational Implementation
It is essential to involve operational personnel in understanding these phenomena, explaining why proper surface preparation is critical, how it impacts hardness measurement, and how this helps prevent rework. Additionally, it must be considered whether the dimensions of the components allow such preparation. Otherwise, it is necessary to communicate possible deviations or limitations to the client.
This analysis should be conducted for different steel grades, as well as at different temperatures and processing times, in order to estimate the effect on decarburization depth. This will facilitate the implementation of preventive actions for the operational team, instructing them to avoid overheating the parts or exposing them in the furnace for prolonged periods.
Conclusion
It is highly recommended that heat treatment companies conduct this type of metallurgical study, since it allows a deeper understanding of the real behavior of their processes. As previously mentioned, the depth of the decarburized layer is influenced by multiple variables, which may impact each company differently depending on factors such as steel type, furnace type, and processing time, among others.
Based on the results, it is possible to establish clear guidelines for operators, specifically regarding the depth at which samples must be prepared or ground to ensure that hardness values reported to the client are accurate and representative. Likewise, these analyses can be extended to different steel grades, allowing the development of specific work instructions indicating the required grinding depth depending on the material and processing conditions.
Decarburization is a critical phenomenon in heat treatment processes that can lead to incorrect interpretation of hardness results. A comprehensive analysis combining metallurgical fundamentals, standard methodologies, computational tools, and process history allows avoiding unnecessary rejection, improving process control, and ensuring final product quality.
References
Balaško, T., Vončina, M., Burja, J., Šetina Batič, B., & Medved, J. 2021. High-Temperature Oxidation Behaviour of AISI H11 Tool Steel. Metals, 11(5), 758. https://doi.org/10.3390/met11050758.
Herring, Daniel H. 2014. Atmosphere Heat Treatment: Atmospheres, Quenching, Testing. Vol. 2. Troy, MI: BNP Media.
Juan Hou, Fen-Fen Han, Xiang-Xi Ye, Bin Leng, Min Liu, Yan-Ling Lu, Xing-Tai Zhou. 2019. Effect of Surface Decarburization on Corrosion Behavior of GH3535 Alloy in Molten Fluoride Salts[J]. Acta Metallurgica Sinica (English Letters). 32(3): 401-412. https://doi.org/10.1007/s40195-018-0814-5.
Krauss, George. 2015. Steels: Processing, Structure, and Performance. 2nd ed. Materials Park, OH: ASM International.
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.
Los resultados de dureza inferiores a las especificaciones no siempre se deben a errores en el tratamiento térmico. En esta entrega de Technical Tuesday, Ana Laura Hernández Sustaita, fundadora de Consultoría Carnegie, analiza cómo se desarrolla la descarburización, cómo afecta a la dureza y a las propiedades superficiales, y los métodos prácticos que los técnicos de tratamiento térmico pueden utilizar para evaluarla, controlarla y prevenirla mediante análisis metalúrgicos y una estricta disciplina en el proceso.
Este artículo informativo se publicó por primera vez enHeat Treat Today’sJuly 2026 Annual Super Brands Issue print edition.
Introducción: cuando los resultados no coinciden con la teoría
En operaciones de tratamiento térmico, los resultados de dureza fuera de especificación suelen atribuirse a desviaciones en los parámetros del proceso, errores operativos o variaciones en la composición química del material. Sin embargo, existen casos donde la causa raíz está relacionada con fenómenos metalúrgicos que afectan directamente la superficie del material.
Un caso representativo involucró una carga de diez piezas forjadas de más de 5 toneladas cada una, fabricadas en acero AISI 4340. El proceso térmico aplicado era conocido y previamente validado: normalizado, temple seguido de revenido en un rango de 600-610°C (1112–1130°F), con el objetivo de cumplir con las propiedades mecánicas requeridas por el cliente. Tras finalizar el tratamiento térmico, el equipo de inspección reportó valores de dureza de las piezas aproximadamente 20-30 HB por debajo de la especificación. Este resultado fue inesperado, considerando que el acero 4340, debido a su contenido de elementos aleantes, mantiene valores elevados de dureza incluso a temperaturas relativamente altas de revenido. Se solicitó una reevaluación de dureza, incrementando la profundidad de preparación de la superficie hasta 5 mm. Sin embargo, los valores de dureza únicamente aumentaron ligeramente (~5 HB), sin alcanzar los niveles requeridos.
Ante la presión por los tiempos de entrega, se decidió reprocesar las piezas. Se aplicó nuevamente el ciclo completo de tratamiento térmico, ajustando la temperatura de revenido a 580°C (1076°F), lo cual, desde el punto de vista metalúrgico, debería incrementar la dureza. No obstante, los resultados permanecieron prácticamente sin cambios. Se procedió a verificar la composición química mediante PMI, confirmando que el material correspondía efectivamente a un acero 4340. Adicionalmente, la pieza de evaluación “Qualification Test Cupon” (QTC) indicaba valores de dureza elevados, consistentes con el comportamiento esperado del material.
Ante la inconsistencia entre resultados experimentales, fundamentos metalúrgicos y la previa experiencia personal, se decidió profundizar en el análisis del historial del proceso y correlacionar las variables de cada uno de los procesos. Y aquí es donde se encuentra la clave de un buen análisis de fallas, el detallar el historial de la pieza con la que se está trabajando.
Se identificó que, previo al tratamiento térmico, las piezas habían permanecido dentro de un horno en un rango de 800-900°C (1472–1652°F) durante aproximadamente tres días, debido al retraso que el equipo de mantenimiento tenía en la máquina de rolado, debido a esto y a una falta de comunicación entre los operadores de turno, la pieza continuaba en espera en el horno de calentamiento a “baja temperaturas” para concluir su último paso. Aunque no fue a temperaturas de rolado, esta exposición prolongada en una atmósfera no controlada promovió la descarburización del material.
Para validar esta hipótesis, se realizó un corte longitudinal en una de las piezas y se llevaron a cabo mediciones de dureza en el núcleo. Los resultados confirmaron que el material presentaba valores significativamente mayores en el interior, evidenciando una capa superficial descarburada. Este caso demuestra la importancia de: Analizar integralmente la historia del material y del proceso antes de concluir con una no conformidad.
¿Qué es la descarburización?
Figura 1. Esquema representativo del proceso de descarburización y oxidación. | Referencia: Consultoría Carnegie
El acero es una solución sólida basada en hierro y carbono, donde las propiedades mecánicas dependen directamente del contenido de carbono y los elementos aleantes presentes. La descarburización es un fenómeno metalúrgico que consiste en la pérdida de carbono desde la superficie del acero cuando este es expuesto a altas temperaturas en atmósferas con potencial oxidante o bajo potencial de carbono (Figura 1). Este fenómeno ocurre comúnmente durante: forja, laminación en caliente, normalizado, recocido y tratamientos térmicos en hornos convencionales sin control de atmósfera.
Desde el punto de vista termodinámico, la descarburización ocurre cuando el potencial químico del carbono en el acero es mayor que el del ambiente circundante, lo que impulsa la difusión del carbono hacia la superficie. En presencia de gases oxidantes, el carbono reacciona formando especies gaseosas:
También puede reaccionar con vapor de agua o dióxido de carbono, lo que acelera el proceso. Como consecuencia: se genera un gradiente de carbono (superficie vs núcleo), se altera la microestructura, se degradan propiedades mecánicas, especialmente en la superficie.
Oxidación y su relación con la descarburización
La descarburización está estrechamente relacionada con los procesos de oxidación. La oxidación ocurre cuando el oxígeno reacciona con el metal formando óxidos en la superficie. Este proceso depende del equilibrio termodinámico entre las presiones parciales de los gases presentes, lo cual puede analizarse mediante diagramas como el de Ellingham-Richardson.
El incremento de la temperatura acelera tanto la oxidación como la descarburización, por lo que el uso de hornos con atmósfera controlada es fundamental para prevenir estos fenómenos.
Cinética del proceso: difusión del carbono
La descarburización es un proceso gobernado por difusión. Cuando el acero se encuentra a altas temperaturas los átomos adquieren mayor movilidad y el carbono difunde desde regiones de alta concentración (interior) hacia regiones de menor concentración (superficie). Este proceso se intensifica cuando el material se encuentra completamente en fase austenítica (por encima de Ac3), donde la difusividad del carbono es significativamente mayor.
La profundidad de la capa descarburada depende de: temperatura, tiempo de exposición, composición del acero y condiciones de la atmósfera del horno. Este fenómeno es crítico, ya que la reducción del contenido de carbono en la superficie afecta directamente la resistencia al desgaste y fatiga del material.
Métodos para la determinación de la capa descarburada y óxidos
Predicción de óxidos mediante herramientas computacionales
El uso de herramientas como Thermo-Calc software, permite predecir las fases estables en función de la temperatura y la presión parcial del oxígeno. Estos diagramas permiten identificar que óxidos se formarán en la superficie, evaluar si los óxidos actúan como barrera o facilitan la salida del carbono y optimizar las condiciones del proceso térmico.
Figura 2. Fases termodinámicamente estables formadas durante la oxidación a alta temperatura (600°C [1112°F]) en función de la actividad del oxígeno para el acero H11 (Balaško, et al. 2021)
Por ejemplo, en aceros como H11, los diagramas de estabilidad de fases permiten visualizar cómo varían los óxidos formados en función de la temperatura y la actividad de oxígeno (Figura 2). Comprender estas relaciones es clave para prevenir la descarburización, diseñar atmósferas controladas y evitar resultados no deseados en propiedades mecánicas.
Determinación de la profundidad de capa descarburada
La norma ASTM E1077 establece diferentes métodos para evaluar la profundidad de la capa descarburada, incluyendo: métodos microscópicos, microdureza y análisis químico. Estos métodos permiten determinar si una pieza cumple con las especificaciones y definir la preparación adecuada para medición de dureza. A continuación, se muestra un ejemplo del uso de los métodos. El objetivo de este análisis fue determinar la profundidad de esta capa para generar instrucciones internas para la preparación adecuada mediante del pulido de las piezas para la medición correcta de dureza.
Metodología experimental
Figura 3. Preparación de QTC para extracción de muestras para análisis. | Referencia: Consultoría Carnegie
Se seleccionó un QTC de acero AISI 4340 en condición forja, sometido a un ciclo de normalizado, temple y revenido. Las muestras se obtuvieron de un corte transversal para realizar el análisis de dureza, microscopía óptica y análisis químico de carbono (LECO, Figura 3).
Resultados
Perfil de dureza
Se realizaron mediciones a intervalos de 1 mm desde la superficie hacia el interior. Los resultados mostraron: 0–3mm valores bajos de dureza (~30 HRC), a 5 mm: incremento hasta 35–36 HRC, manteniéndose constante (Figura 4). Esto indica que la zona superficial no representa las propiedades reales del material.
Figura 4. a) Resultados de análisis de perfil de dureza, b) muestra utilizada para el análisis de perfil de dureza. | Referencia: Consultoría CarnegieFigura 5. a) Zonas para evaluación de dureza a cada 1 mm b) resultados de dureza en mapa de color (en azul la zona más suave es la superficie y en rojo la zona más dura a 5 mm de profundidad). | Referencia: Consultoría Carnegie and Mikra QATM
Uno de los retos en este método es la precisión de las huellas de dureza, se puede observar como la muestra Figura 4b, se usó un marcador fino para determinar las zonas a medir, sin embargo, hoy en día el uso de equipos sofisticados nos permite realizar una programación automática de las zonas a evaluar (Figura 5).
Análisis químico (LECO)
Figura 6. a) Resultados de análisis de %C en un equipo LECO, b) muestra utilizada para análisis de %C. | Referencia: Consultoría Carnegie
El análisis mostró un incremento en el porcentaje de carbono a partir de aproximadamente 5 mm de profundidad, confirmando la existencia de una capa descarburada (Figura 6).
Microscopía óptica
Figura 7. Resultados de análisis de capa descarburada mediante microscopía óptica. | Referencia: Consultoría Carnegie
Este análisis permitió observar cambios microestructurales entre la superficie y el núcleo, asociados a la pérdida de carbono (Figura 7).
Aplicación práctica: control en planta
Tabla A. Instrucción de trabajo en área de pruebas de Dureza
Con base en los resultados, se establecieron instrucciones de trabajo para asegurar mediciones confiables de dureza (Tabla A).
Implementación operativa
Es fundamental involucrar al personal operativo en la comprensión de estos fenómenos, explicando por qué es crítico el pulido adecuado, cómo impacta en la medición de dureza y cómo esto evitará retrabajos. Así mismo, se debe considerar si las dimensiones de las piezas permiten dicha preparación. En caso contrario es necesario comunicar al cliente posibles desviaciones o limitaciones en la medición.
Se recomienda llevar a cabo este análisis para los diferentes grados de aceros, temperaturas y tiempos de procesamiento, que ayude a estimar el efecto sobre la profundidad de capa descarburizada y con esto implementar acciones preventivas al equipo operativo sobre evitar un sobrecalentamiento o someter las piezas a periodos prolongados de tiempo en un horno.
Conclusión
Es altamente recomendable que las empresas dedicadas al tratamiento térmico lleven a cabo este tipo de estudios metalúrgicos, ya que estos permiten comprender con mayor profundidad el comportamiento real de sus procesos. Como se mencionó previamente, la profundidad de la capa descarburada está influenciada por múltiples variables, las cuales pueden impactar de manera distinta en cada empresa, dependiendo de factores como el tipo de acero, el tipo de horno utilizado, los tiempos de tratamiento entre otros.
A partir de los resultados obtenidos, es posible establecer lineamientos claros para los operadores, específicamente en relación con la profundidad a la que deben preparar o pulir las muestras, con el objetivo de asegurar que los valores de dureza reportados al cliente sean representativos y correctos. Así mismo estos análisis pueden desarrollarse para distintos grados de acero, permitiendo la generación de fichas o instrucciones de trabajo específicas para cada caso donde se indique de manera precisa la profundidad de pulido requerida en función del material y las condiciones de proceso.
La descarburización es un fenómeno crítico en procesos de tratamiento térmico que puede llevar a interpretaciones incorrectas de los resultados de dureza. El análisis integral que combine: Fundamentos metalúrgicos, normativas estándar, herramientas computacionales, historial del proceso, permite: evitar rechazos innecesarios, mejorar el control del proceso y asegurar la calidad del producto final.
Referencias
Balaško, T., Vončina, M., Burja, J., Šetina Batič, B., & Medved, J. 2021. High-Temperature Oxidation Behaviour of AISI H11 Tool Steel. Metals, 11(5), 758. https://doi.org/10.3390/met11050758.
Herring, Daniel H. 2014. Atmosphere Heat Treatment: Atmospheres, Quenching, Testing. Vol. 2. Troy, MI: BNP Media.
Juan Hou, Fen-Fen Han, Xiang-Xi Ye, Bin Leng, Min Liu, Yan-Ling Lu, Xing-Tai Zhou. 2019. Effect of Surface Decarburization on Corrosion Behavior of GH3535 Alloy in Molten Fluoride Salts[J]. Acta Metallurgica Sinica (English Letters). 32(3): 401-412. https://doi.org/10.1007/s40195-018-0814-5.
Krauss, George. 2015. Steels: Processing, Structure, and Performance. 2nd ed. Materials Park, OH: ASM International.
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.
This Technical Tuesday installment summarizes the highlights from a site visit by Heat Treat Today’sDoug Glenn, publisher and founder, and Karen Gantzer, manager of events. Learn how OMG Building Products, LLC took control of efficiency, product quality, and R&D in the middle of New England.
This informative piece was first released inHeat Treat Today’sJuly 2026 Annual Super Brands Issue print edition.
In 2003, OMG‘s decision to vertically integrate manufacturing operations meant that heat treating needed to be brought in-house, making R&D a prized cornerstone of operations that guarantee high quality products. The Northeast operation in Agawam, Massachusetts, now moves 20 million pounds of steel per year (two to three tractor trailer trucks of steel wire per day) and features electrically powered IQ batch furnaces and continuous mesh belt furnaces.
Founding and Evolution
Five guys in Agawam, Massachusetts, gathered around a backyard BBQ in 1981 talking about the challenge of finding decent roofing screws. Pooling their talents across fabrication, finance, and more, they formed the company that has expanded to be OMG Building Products LLC. Their singular focus: making better roofing fasteners. To this day, roofing fasteners still make up 60% of OMG’s business.
In 2003, following years of sustained growth and product expansion, they took steps to bring increased oversight to their product. The main objective was to further ensure quality, gain greater material velocity, and increase customer service. The first step was to invest in an ECoat painting line for the fasteners, the process closest to OMG’s clients. Kevin Walters, now OMG’s senior process engineering manager, was hired as quality manager to oversee these transitions.
Making room for roofing heat treating operations soon followed, as well as several building expansions in the industrial park, including a residential construction fastener business and increased warehouse space. These capacity developments brought OMG’s low-carbon steel and yet highly efficient roofing product to a global market.
The Next Step: In-House Heat Treat
Bringing heat treating in-house would be a major move to further secure the manufacturing line. Product was being shipped to as many as ten different locations, including facilities in the Midwest and Canada. In fact, the biggest percentage was at risk since that Waterbury, Connecticut, heat treat supplier was looking at closing its doors with just a 90-days’ notice.
It was 2012 when OMG decided to investigate installing systems at the industrial park in Agawam. After three to four years’ worth of technical and business research, the parent company approved the funds to embark on the installation that is there today.
Research and Design
Jeff Hotham, Heat Treat Manager, and Kevin Walters, R&D Lead, collaborate to make OMG’s heat treat operations more efficient and effective. | Image Credit: Heat Treat Today
When it comes to the success of their heat treat operations, Jeffrey (Jeff) Hotham, OMG heat treat manager, put it bluntly: “Kevin is our secret weapon.” Since joining the team in 2003 at the apex of change, the senior process engineering manager brought heat treating operations in-house, facing every obstacle that comes with it.
The first challenge was logistics. OMG is situated in a light industrial park that cannot provide enough energy to power gas or electric furnace operations. After much analysis, it was determined that the most cost-efficient solution was to build an electric line connecting to an Eversource utility grid. This has had welcome maintenance implications as the electrically powered furnaces do not require an on-site burner tech and the accompanying nickel element ceramic tubes are also low maintenance.
Next was determining the thermal processing equipment. For an entire year, Kevin researched heat treating methods to optimize the company’s fasteners. His boots-on-the-ground research included visiting commercial heat treat facilities, attending trade shows, and speaking directly with industry professionals.
Because OMG produces a variety of lengthy fasteners, distortion control drove the decision of an integral quench furnace design, while productivity needs were met through a continuous mesh belt furnace design. Additionally, size of the furnace systems was all important in balancing flexibility and time-to-market realities. This resulted in a competitive heat treat system design that would leverage a significant amount of automation and a loading system that minimized manual intervention. This original furnace line included two endo generators, smart oil filtration, cooling equipment, and material handling.
The Right Fit: Finding a Furnace OEM
The competitive system design, however, would hinge on the ability to find a vendor who could deliver dependable, customizable equipment. Kevin and a cross-functional team of OMG business, operations, finance, purchasing, and safety representatives were tasked with selecting the equipment supplier.
When reviewing their options, OMG’s choice of furnace supplier came down to three key factors: design flexibility, locality, and customer service. It was essential that the system integrate design decisions to accommodate Kevin’s researched solutions.
Some furnace OEMs had reservations about adapting their systems to accommodate these special requests. Other suppliers had systems completed at various locations, with features sometimes being subcontracted off-site. Ultimately, the team found their perfect fit with Williams Industrial Service, located in Bowling Green, Ohio, who were eager to accommodate design changes, maintained full control of furnace build under one roof, and demonstrated a service-minded team.
Once ordered, the next step was to monitor job progress. OMG was proactive, providing timeline-focused goals. The furnace build progressed on schedule with Kevin making quarterly visits to complete milestone reports.
Within three years, OMG’s full heat treating line was installed.
Optimizing Productivity in Current Operations
Operations have increasingly shifted to focus on cost savings and control over inefficiencies. From a 30,000-foot view, wire arrives, is skim drawn, goes through cold forming, then heat treating, and finally painting and packing.
Facility design is critical in operational performance. According to Kevin, the working environment surpasses that of many heat treat operations he has visited, a credit to how well Jeff manages the plant floor. A key example of this is safety. OMG’s safety culture can be seen throughout their operations, extending to the racking department where a climate-controlled room away from loud manufacturing supports racking staff. Kevin also remarked that the air ventilation system is a point of pride: “We literally have guys wear hoodies in the heat treat department in the wintertime… because we have so much fresh air intake and exhaust in the room; the climate here is pretty good.”
Mesh belt line | Image Credit: Heat Treat Today
When roofing fasteners are heat treated, they either go through one of the three medium- to large-scale continuous mesh belt furnace lines or through the automated integral quench (IQ) furnace line. The mesh belt line receives shorter, 1.5- to 6-inch parts that are poured from a tub onto the belt with ease as well as longer 7- to 8-inch product. On the other hand, the IQ furnace line includes two furnaces which receive longer parts up to 24 inches long from a fully automated delivery system; fasteners are hung from their heads on the rack to prevent distortion. In the future, the team looks forward to automating this part of the process.
Kevin Walters, Doug Glenn, Karen Gantzer, and Jeff Hotham stand in OMG’s testing lab. | Image Credit: Heat Treat TodayLong roofing screws are hand-racked to best control distortion. | Image Credit: Heat Treat Today
The IQ line is a point of pride. Kevin commented that he has reason to believe that his IQ line may be the most automated in the country. “My operators do not have to get on a charge cart at all,” he explains, “[because] it’s fully automated; it knows when loads are done in the furnaces, takes them, puts them in, takes them out, puts them in the washer, and takes them out. Everything is automatically programmed.” These loads are monitored based on weights for baskets and piece count for racks.
Jeff and Kevin have collaborated to increase equipment efficiency and throughput while maintaining quality.
Operations feature an in-house laboratory to test product, particularly for distortion, the leading quality concern for roofing product. As mentioned earlier, furnace line automation is a goal, yet longer parts — like thin-shanked 8-inch parts — may still need to be hand-racked on the mesh belt to attain the quality results.
The Power of Planned Maintenance
Regular planned maintenance schedules work to keep this operation up and running. Jeff coordinates any maintenance requiring extended furnace downtime to occur at the same time.
On an annual basis, temperature uniformity is prioritized through extensive furnace care, including replacing the thermocouples and carbon probes. As Jeff asserts, “We try to stay ahead of the game.” This planned maintenance takes two weeks of the furnace being out of commission to ensure the furnace is thoroughly cleaned and any equipment replacements completed.
Biannually (twice every year), furnace ductwork is cleaned — a critical measure to stop risk of fire since, according to Jeff, most industry fires start in the ducts. OMG outsources experts to complete this maintenance over the course of a full week.
Finally, on a quarterly basis, all equipment undergoes vibration and infrared analysis. Following this proactive maintenance schedule has brought the operation from producing 9 million pounds in his first year to 19.2 million pounds just six years later in 2025 — same equipment, simply leveraged by Jeff’s proactive planning and coordinating Kevin’s R&D efficiency endeavors.
Future
Kevin Walters, the R&D manager at OMG, explains how OMG forms spools into screw and nail roofing product to Doug Glenn, publisher of Heat Treat Today. | Image Credit: Heat Treat Today
As the business grows, OMG is exploring other ways to accomplish tasks better and more efficiently: induction, vacuum furnace, continuous or pass-through IQ, and rack furnaces are all being considered.
Kevin addressed their laser-focused approach in heat treating operations, saying, “As time goes on though, we are perfecting productivity with Kaizen events and controlling our own destiny and lowering overall costs. Last year, process optimization on the mesh belt lines boosted our productivity by more than 30% and our automated IQ line is up 20% this year.”
OMG’s operation demonstrates what curious innovation and on-the-ground industry research can unlock for in-house heat treating. If the past is any indication of the future, we’ll see OMG leading the way in thoughtful, quality-focused integration of advanced technology in the industry.
This article was written by Heat Treat Today’s editorial team. For more information, contact editor@heattreattoday.com.
Heat TreatRadio host Heather Falcone and guest Vincent Lelong, Senior Synergy Center Manager and Metallurgist at ECM USA, explore the realities of process qualification and recipe development in modern heat treating. Vincent shares decades of experience developing vacuum carburizing processes for automotive, aerospace, and high-volume manufacturing applications. Together, they discuss how heat treaters can balance metallurgy, fixturing, quench strategy, and production demands to achieve repeatable results. From practical troubleshooting insights to the evolution of vacuum carburizing technology, this conversation offers a grounded look at what it takes to optimize heat treating.
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 (00:05)
Heather Falcone: Hi, I’m Heather Falcone, and welcome to Heat TreatRadio. Today, we are asking a metallurgist, and we are talking about the ins and outs of process qualification and recipe development. We are on-site with the sponsor of our episode, ECM USA, in beautiful Pleasant Prairie, Wisconsin. It’s a real treat to be able to record on site. We hardly ever get that chance. Joining me today is Vincent Lelong, the Senior Synergy Center Manager and Metallurgist. Thanks for joining me today, Vincent.
Vincent Lelong: Thank you, Heather.
Heather Falcone: So you have the luxury, since we are in your beautiful facility, to tell me not just about yourself, but also about ECM. Start off with your background because it is extensive.
Vincent Lelong: I am a metallurgist, I went to college in France, and I started with ECM in 1999. Since then, I’ve been across the world in the U.S. for 20 years. I do presentation and testing for the furnace behind us and our larger furnace. I go on-site. This is our Synergy Center; it’s a nice environment, clean — quiet today because we’re filming.
Heather Falcone: Tell me a little bit about what ECM does.
Vincent Lelong: At ECM, we manufacture and install heat treatment vacuum furnaces. Our main focus is on low-pressure carburizing modular furnaces, large and small. We integrate not just heat treatment itself, but the pre-treatment, like washing, storage, preparation of the load with robots, heat treatment, temper, cryo… Everything is set up together, and that is our main goal: integration of a fully automatic installation.
Heather Falcone: A one-stop shop. You go to one place, and you’re going to make it all work.
Vincent Lelong: There’s a part and everything will be ready in few hours later, sometimes days, depending on the treatment.
Heather Falcone: Hopefully it all works.
ECM Nano vacuum furnace used for client cycle development and qualification.
Vincent Lelong: Well, it always works. It’s a really repeatable modular furnace. In the U.S., we are focusing on vacuum carburizing, but we also manufacture other types of furnaces for crystal growth, silicon heat treatment, melting silicon for solar panel, and others. Within ECM Group, there’s a range of heat treatment processes that we manufacture for. And not just steel; it can be other materials. I’m specialized in steel and very restricted heat treatment with vacuum carburizing, but maybe one day we can do other materials.
Heather Falcone: Steel makes the world run.
Vincent Lelong: With our modular furnace, we can do hardening, gas quench, oil quench, carburizing, gas quench, oil quench, and now we do vacuum carbonitriding and we can do nitriding in the furnace. So you can have one installation for multitask heat treatment. It’s the purpose of the modular furnace, the beauty of it.
Heather Falcone: More flexibility, more capabilities.
Biggest Challenges of Process Qualification and Recipe Development (4:08)
Heather Falcone: That brings us into the first core question that I want to ask, because when you’re evaluating bringing in another piece of equipment or if you’re trying to bring on a new process, it can be a little challenging. What do you see are the biggest pitfalls or what do you see people struggling with most when it comes to process qualification and recipe development?
Vincent Lelong: Most customers say, “I would like a good metallurgy. I would like mechanical properties.”
Heather Falcone: Right, make good parts. That’s number one.
Vincent Lelong: But you need to look at what kind of furnace you would like to use, as well as the size, the type of part, and the process. If you have a small part, do you want a bulk load? Do you want special fixtures because the main target is distortion-free? Everybody would like everything…
Heather Falcone: …distortion-free!
Vincent Lelong: Always. No problem. So, as a metallurgist, you need to think not just about heat treatment because you’ve been asked to heat treat a part. I could heat and quench in oil and say, “You have the metallurgy. My job is done.”
Heather Falcone: Right. It hit hardness.
Vincent Lelong: Exactly. Hardness, good. Vacuum carburizing, no oxidation. Good. How about the distortion? Okay, let’s speak about what we can do to reduce the power of the heat treatment itself. Then, let’s do a gas quench, but let’s work on fixtures also. It’s working with the supplier for fixtures and working with the customer with the machinists.
Sometimes, because we propose gas quench, people say, “Oh, gas quench means no distortion.” Well, the first step is to get the metallurgy right. If you need 20 bar gas quench, you might have some distortion. From experience, we know that the fixtures are also important. So we can work with the customer and supplier to propose the right fixtures and test the fixtures to target the best cooling rate and other properties.
But we also need to work with the machinists. If you have a challenging part, machinists may say, “It’s the fault of the metallurgist.” And the heat treater will say, “No, let’s work together at the table. We’ll sit, and we do the testing.” At the Synergy Center we have also the CMM, so we can measure before heat treatment and after heat treatment. It’s the best feeling when you bring not just the metallurgists together, but also the people who make the part. We work together to have the best of the best. It’s a lot of work, but we have years of experience, so we can reach the target faster than 20 years ago.
Technology has changed. As such, you also need to work with the steel manufacturer, the way the steel is made and the composition. Research has shown that if you improve the steel, you can reduce the quench pressure. If you need oil quench, then you already know that distortion will be potentially higher. If you need 20 bar, it’s one thing. Well, I did a presentation not long ago, and I found that the distortion with 10 bar or 20 bar in a Nano furnace was much better than in a larger load. For one part, that is. This might not be true if it’s other parts. There’s always that phrase in heat treatment, “that depends.” I don’t like this phrase.
Heather Falcone: It always depends.
Vincent Lelong: But it’s true, unfortunately. So, when you have worked all that together, you can bring the best analysis. I’m working before that to sell a furnace.
I need to choose whether it’s better for the customer to have a smaller furnace or a larger installation. That really depends on how many parts, the diversity of parts. If the customer has mostly small parts, the Nano may be the better choice. You have a faster answer because it’s in and out. You don’t need to buy so many fixtures. But if you have a larger part, a larger furnace is better. It’s not whether one furnace is better than the other. You need to choose which one will achieve your production target.
A customer in facility number A may need a larger furnace, but in facility B, there are other types of part, so that facility may need a smaller furnace. That’s how we work with the customer to target what is important.
Heather Falcone: Pick the right atmosphere, pick the right hot zone size, pick the right fixturing, raw material specs. All of those are going to influence how the runs going to go. I bet you have some stories about TCE from fixturing and eutectic and inadvertent bonding.
Vincent Lelong: I certainly have had a few mistakes in testing. I used a higher temperature because with vacuum carburizing we say we can go to a higher temperature. But then I used CFC fixtures at the wrong temperature!
Today however, the mixtures of fixtures can work and reduce the weight. As a metallurgist and a heat treater, I prefer to heat treat the part rather than the fixtures. When I see a customer running a load with almost more fixtures than parts, I’m asking, “What do you heat treat?” Are you losing money on heat treating fixtures more than parts?”
Heather Falcone: There is such a big weight differential between fixturing and the parts. How much lag time on your heat up and cool down are you wasting on having too much fixturing?
Vincent Lelong: It’s true.
Balancing Technical Requirements with Production (11:30)
Heather Falcone: Once you’re in recipe development, how do you balance technical requirements, the repeatability, and the realities of production?
Vincent Lelong: Repeatability is firstly about how you design the load. Then you need to know the quantity of parts. You also may know the surface of your load, but it may always be necessary, I will say. Most of the time we don’t know that and it still runs correctly.
Heather Falcone: Still a black box. It’s an art what we do.
Vincent Lelong: Technically, it’s a good thing because when we put something inside the furnace, it’s coming back, and we don’t see much difference. But the mechanical properties are completely different, so it is kind of like magic.
So you define your load, you define your heating time to get temperature, and when you’re sure you’re at temperature, you start to carburize. I will speak about vacuum carburizing. First, we have software for the carburizing boost and diffusion; you input your parameters and then you have your recipe. You can run it generic, or you can go into detail and improve it. But as built, the software will give you some set parameters and will work.
Heather Falcone: Technology is so cool.
Vincent Lelong: It’s getting better and better. You will also need to select the type of furnace. When you have one part, it’s not the same as when you have 3,000 parts. The density of the load will influence your gas flow and the capacity of your installation.
Most of our larger furnaces have a maximum of acetylene of 4,000 liters. But you can play with the gas and the duration of the boost and diffusion so it goes inside the part; when you have a blind hole or you need 3 millimeters, you carburize. (Acetylene is beautiful, but molecules can go everywhere, sometimes where we don’t want acetylene, which is why we have the stop off.) But you define your load, you define your recipe, your gas flow, and then you run a test and analyze the metallurgy. If it’s good, then you’re done and you don’t move from that.
Heather Falcone: That’s what production wants to hear.
Vincent Lelong: When you are a heat treater, you may receive 10 parts to heat treat. Tomorrow you may receive 20 parts, and maybe you need to run the same recipe. In general, you can run the same recipe for 10 parts or 20 parts. You would use the parameters of the larger load for the smaller, if possible. The result will be mostly the same, but the cost will not be the same.
This is where you can optimize your recipe for different types of load, like half of the load versus a full load. You can change the flow of gas, and then reduce your heating time because why set for two hours when in one hour, it is at temperature. One hour is money.
Heather Falcone: Got to turn and burn.
Vincent Lelong: This is also where you define your database and your repeatability. I once had a customer that had me create a recipe for a specific quantity of parts and a design of load. A few years later, that customer called and said, “It doesn’t work.” With modern heat treating controls, we record everything, so you have a database, the curves, and you can go back and see what was wrong.
Heather Falcone: Right. What changed?
Vincent Lelong: In this case, I discovered the customer was running the double quantity of parts in the same load without changing the recipe at all.
Heather Falcone: Makes sense. Start there.
Vincent Lelong: Heating time was not long enough. Gas flow was not long enough. And they were not working at the right pressure in the furnace, so failure occurred. I said, “Change that.” No news is good news usually. If the customer doesn’t call you, it’s because…
Heather Falcone: …Everything works.
Vincent Lelong: That’s the beauty of this furnace.
With repeatability, you always need to look at the curves. If you have an issue of temperature out of the range, there will be an alarm. So, you can check if the part is good without checking the metallurgy. If you heat treat a big part, usually you won’t cut the part. If you have six parts in a load or 10, you have samples. You have to validate your sample and your real part at the beginning.
Heather Falcone: To make sure it’s representative.
Vincent Lelong: Exactly, or if there are differences because you cannot find exactly the same material, you know the difference, and the difference would be always the same.
Heather Falcone: Right. Make it predictable.
Vincent Lelong: We have some customers that would check one load per day or per shift and not every load. If you have 3,000 parts in a load, you can check a part. When you have six parts, you will likely not cut a part. If the furnace tells you it’s good, why check? When you check repeatability, you still need a lab. Not checking the metallurgy is difficult. You should always check again. Repeatability shouldn’t be left to chance or statistics. Take a part and check. Is it good? Then continue.
Heather Falcone: That’s kind of the target of qualification, right? To get those parameters that you predefined.
Vincent Lelong: In time you need to be sure nothing changes.
Heather Falcone: What does re-qualifying look like?
Vincent Lelong: You want to be sure that nothing changes, material-wise. Sometimes, you run the same material, and you achieve 35 HRC. But at one point in time, you achieved only 25 HRC. This is the same material on the paper, but something has happened. So you need to go back in time and figure out what was originally going on. Is it the heat treatment? Is the installation of something around it?
Heather Falcone: Use all the data that’s available to you.
Vincent Lelong: This is where you check the productivity.
Quench Media (19:20)
Heather Falcone: How about the quench media?
Vincent Lelong: When you develop a recipe, if you do oil quenching you will always do austenitizing. You don’t want a crack. You will carburize, austenitize, and go to oil quench. It’s pretty easy to switch from atmosphere oil quench to vacuum oil quench because technically the recipe is pretty much the same, and we’re going to cover that ground extensively because I know it’s kind of scary to even consider that possibility.
When you go with gas quench, if you don’t know the target or are unsure, you select 20 bar.
Heather Falcone: Sure, 20 bar. That’s the easiest in the world.
Vincent Lelong: Exactly. You do 20 bar and you get what you get. There is an advantage of the gas quench. Many customers will ask, “Do I need to do a direct quench? Do I need to do austenitizing?” Most of the time we say direct quench, and we found that you don’t crack with gas quench. Whatever the pressure, we never really have much cracking. Or a customer may ask, “Will the part break due to the gas quench?” That will never happen. I once asked the competition if they ever saw a crack with gas quenching and they said no. It’s the way the gas quench quenches and cools the part down, then it’s less powerful than the liquid, so you don’t have this potential issue.
But if you don’t know, you stay at 20 bar. If you would like to optimize, you can reduce, but you need to achieve the target metallurgy.
Heather Falcone: Right. You’ve got to get your core hardness.
Vincent Lelong: Your limit is when your metallurgy is not right. When you reduce, you reduce the cost. If you reduce the pressure and the speed, you will reduce the distortion potential.
Heather Falcone: Which is always a good thing.
Vincent Lelong: If you have a shaft, you should not place it horizontal, because whenever you quench that, it will not be straight.
Heather Falcone: We’ve potato chipped a few parts over the years, yes.
Vincent Lelong: Me too. I have tried horizontal in some cases. It’s interesting, but you need more support. I think it’s possible, but nobody wants to try it.
Heather Falcone: Why bother?
Vincent Lelong: Vertical is easier.
Heather Falcone: If you’re qualifying, just make the fixture that’s going to support it.
Vincent Lelong: Yes. Why should I change? It’s always a big question; we just quench it vertically this way. We do it this way. Now for gears, in heat treat sites I see a lot of vertical positions for gears strung on a rod. As an operator, I don’t like that.
Heather Falcone: Tell me why.
Vincent Lelong: Because it’s heavy. We already have difficulty finding operators in heat treatment. If it’s heavy, nobody wants to do it.
From a robotic point of view, it’s more difficult, too. Today, with most vacuum carburized and gas quenched gears are heat treated horizontally on fixtures and most of the time with offset position. That will give you the best metallurgy, but also the least distortion overall. Vertically for machinists, it’s very difficult to re-machine something round to oval. When you place it horizontally, you can do potato chips but machinists can grind and reshape the part easily, if it’s possible.
Heather Falcone: If you’re already near net, it’s going to be a different story.
Vincent Lelong: Exactly. That’s where when you check your distortion and repeatability of process — it’s the fixtures.
Heather Falcone: I would think working with the customer as much as you can to see if material can be left on the part too, if we do need to have grinding, if we do need to have repair or recovery for any possible distortion.
Vincent Lelong: So, let’s say we are working on a six-speed. For the six-speed, everybody wants to vacuum carburizing gas quench. The objective: zero distortion, heat treat, and assemble.
Heather Falcone: In theory.
Vincent Lelong: No, in truth.
Heather Falcone: Really? Okay.
Vincent Lelong: That was the target.
Heather Falcone: Ambitious, I like it.
Vincent Lelong: Yes, but when you have a new product, you can use new technology, you can work on the fixtures, you can work on everything. We worked a lot with car manufacturers to do the best heat treatment, the best fixtures, the maximum of parts, of course, and repeatability. This furnace is running millions of parts.
That is why we know vacuum carburizing works and it’s repeatable. For this high volume, we had to work on zero distortion. But the specification then didn’t change, metallurgically speaking. Most of the time it was 0.3 to 0.7 millimeters. It’s a large gap. No problem. Then we went to the 10-speed for most of the automotive, and then the distortion was the target because of the experience we gained with the six-speed, which was the noise. People don’t want the noise. Today, with the 10-speed, we start to grind at 0.1 millimeter.
You have to compensate for your carburizing process so that it is longer and deeper, but also most customers will reduce the metallurgical requirements. From 0.3 to 0.7, they want 0.5 plus or minus 0.05, which is much thinner. With electrical applications today, you want zero noise, because you can hear everything. There’s a lot of grinding, and when I say a lot, I mean exact, 0.2 millimeter.
Heather Falcone: That is a lot of post-process work.
Vincent Lelong: You have the perfect teeth. You need to anticipate longer carburizing, and it’s great! Also, what I see with metallurgy, it’s not that you don’t have a general metallurgical specification, but each area of a part will have its own metallurgy. That means you have the pitch of the gear, the roots, a minimum, but sometimes you have a specification of the tip.
A customer may specify, “I don’t want more than 0.8.”
Heather Falcone: Just for that area.
Vincent Lelong: Yes. As the parts get more and more complex, they have more than just one application or function. You have the spline. You have double teeth. Each one will have its own requirement. With an atmospheric furnace, to get that, it doesn’t work very well. But with vacuum carburizing, you can achieve very precise requirements.
Switching from Atmospheric to Vacuum Carburizing (28:11)
Heather Falcone: To that point, is that one of the things that stops people from considering the change from atmospheric to vacuum carburizing, or is it part complexity?
Considering the switch? See how different carburizing technologies and furnace features stack up when you click on the image above.
Vincent Lelong: The larger companies do not seem to be afraid, because they know what they want and they already have experience. For the heat treaters, the smaller companies, it’s very difficult to switch with the requirements today. When you see the requirement on the drawing, it’s funny because before there was just heat treatment.
Heather Falcone: Yeah, it’s on this process sheet.
Vincent Lelong: One line. Surface condition, effective case depth, core hardness.
Heather Falcone: Right.
Vincent Lelong: Today there are different requirements, and there are several requirements: before heat treatment, after, and final. You know how much you need to take off, and not every area you take off. You said keep more material. The advantage of that is more material, less distortion. But you will have to carburize more.
Heather Falcone: Ultimately it may be more expensive for everybody.
Vincent Lelong: Exactly. The machining behind the grinding is also costly. When we develop a recipe, we have the customer machine to the final dimension, do the heat treatment, and then we will see where we are in terms of metallurgy and distortion. If we are not where we need to be, don’t take off too much. Then you adjust.
One other story is about a thread.
Heather Falcone: Oh, God, threads. The bane of every heat treater’s existence, threads.
Vincent Lelong: I get a lot of questions about threads. Do I need to make them before heat treatment? Do I need to put a mask on, paint, or make the fixture?
Vincent Lelong: Or to not do them and do them after?
Heather Falcone: That is also expensive.
Vincent Lelong: Yes, also expensive; but I think it could be a robotic application.
Heather Falcone: Oh true. Very good point.
Vincent Lelong: It’s the way I would go.
Heather Falcone: How interesting.
Vincent Lelong: To not do the thread before.
Heather Falcone: Lower risk.
Vincent Lelong: First, when you carburize, you can create a brittleness of the thread. But also operator movement of the thread from crate to the fixtures can cause damage to the thread. What do you do? Can you save the thread after heat treatment? Not always. Then that is garbage. You had to manufacture the part, heat treat the part, just to put in the garbage, which is a cost.
Heather Falcone: Probably 80 or 90% of your whole cost, gone in an instant.
Vincent Lelong: In my opinion, you could have a robot preparing the load. You would have a robot take off, and every time it’s the same movement. Then, thread or not, it’s easier.
Heather Falcone: More predictable.
Vincent Lelong: Then the robot, if you don’t do the thread, can put the shaft, usually it’s a shaft with a right connection on the thread on the end, put to a little induction machine, reheat, and then put in a crate and go back to machining. Then it’s all done.
In that instance, I think it’s my preference not to do the thread first. I have customers who ask me for paint or for a mask. Paint is not, I would say, 100% safe. You need a specialist to put on the paint. There are some tricks, as I know heat treaters know. They have been doing this for years and they know their stuff.
Heather Falcone: That’s their secret.
Vincent Lelong: I think heat treaters have more secrets about painting and protection of the part than the big companies do. They know it better.
Heather Falcone: Their lives are on the line. That’s all they do, so they have to make it perfect.
Vincent Lelong: You can learn a lot from heat treaters. They know their work.
Heather Falcone: That’s what I always recommend to the captives. Get out to as many heat treat shops as you can, because it’s going to make your in-house heat treat better.
Vincent Lelong: They have years of experience to learn from. Heat treatment is tricks after tricks. Some customers are afraid to go to in-house heat treatment. These heat treaters hold a variety of information that is helpful for these customers.
Heather Falcone: It’s an interesting point that you brought up about being afraid, because vacuum carburizing has gotten that reputation over the years that it’s difficult. It’s tough to figure out.
Vincent Lelong: Because we did a good job. We did a good job to say you can optimize. In reality, if you understand what you want to heat treat, the carburizing process is all made by software. Every company has their own software. So it is pretty simple, and though it can still be scary for the heat treater.
Heather Falcone: Process change is scary.
Vincent Lelong: There are some companies that develop vacuum carburizing software where you need to know everything about the steel, the chemistry, all the parameters. We work with carbon. For our software, you enter the carbon content originally, roughly the temperature you would like to heat treat, and what you would like on the final. The software will give you something, and it will be 95% of what you’re looking for.
Now you need to quench. Like I said, with oil quenching, it’s no problem. Gas quenching, 20 bar, no problem, very easy and straightforward. Optimization can be where it’s trickier. But it’s just like an atmospheric furnace. I work with atmospheric furnaces, and they all have their cheat sheets.
You need carbon potential, temperature, time, and in vacuum carburizing, it’s the same thing. The temperature, your carbon potential, or what you expect for carbon, the time, the case depth you would like to achieve, and here is your process.
Heather Falcone: Then you don’t need the cheat sheet. It’s gone. Then it’s documented and repeatable.
Vincent Lelong: Right, then you have to heat up and quench. So, straight heat treatment: heat it up to be sure you’re at temperature, then carburize, and you quench.
Heather Falcone: One test?
Vincent Lelong: Yes, I often do just one test.
Customers may ask me to do a test for a Flex system, a larger system. I do it here at the Synergy Center first because it’s cheaper for me because it just value add and it’s here. I can mix a different part, different design, and run. “Oh, it’s a 8620? No problem.” Usually, it’s good.
For a Nano load, it’s like a one-fifth or it’s a one fixture and the bigger load, it’s like two column of fixtures, then you stack. So, it’s not much different. You start on a smaller scale and you go bigger, and you just add a little heat up time.
And if you ask me, “What do you need?” Put everything to the maximum!! (We are here to sell you a spare part, so, you know.) But really, if you put everything to the maximum you will be good. Maybe too good, and you might have a more maintenance, but we’d be able to provide a quote to reduce the maintenance. (*joking laughter*)
Closing Thoughts (37:36)
Heather Falcone: As we finish up, if there’s one thing that you’d want our listeners to take away, rethinking about their approach to process qualification and recipe development, what do you want them to know?
Vincent Lelong: It’s not difficult. It’s like every other heat treatment; you have to test it and you will quickly see that it’s easy. At ECM USA, we provide training and testing to show you what can be achieved and answer your questions. If you’re worried, we will show you how easy it is, how clean it is. There’s no flame. If you have oil quench, there’s no flame because everything is protected. So it’s not difficult, it’s just one step. Do not be afraid.
Heather Falcone: Take that first step, and explore the process.
Vincent Lelong: I think there’s enough research and evidence in the last 25 years in the U.S., especially with large automotive and aerospace companies to know that it’s not a big deal. Most people — in heat treating or not — don’t like change.
Heather Falcone: But they can partner with you, Vincent, who has decades of experience. Reach out to ECM. Get in touch. Start exploring.
Vincent Lelong: The funny thing is, with decades of experience, what we are capable of heat treating 20 years ago we can do way better now. The technology is better. Gas quenching is made to quench, not to cool. It’s quenching. It’s hard. It’s almost as hard of a quench as an oil quench. You can do bulk load carburizing. I did carbonitriding in bulk load not that long ago. If you had asked me to do that 20 years ago, I would say, “No way that works.” But today that works. Just contact us.
Heather Falcone: Start the conversation, right?
Vincent Lelong: And I would be happy to show you. I like my job.
Heather Falcone: You love your job. I’m going to say it. You’re very passionate.
Vincent Lelong: I like testing because it’s a challenge every day. It’s pushing the limit. It’s like a movie. Is it possible? If you follow the book or internet, they will say no. I would say, “Let’s try.” I’m testing on materials other than steel as well that I would not have expected to work. Modular furnaces can be a very versatile.
Heather Falcone: Well it sounds like production is getting ready to get things done. Thanks so much, Vincent. It was great spending time with you.
About the Guest
Vincent Lelong Synergy Center Manager / Sr. Metallurgist ECM USA, INC.
Vincent Lelong, ECM USA Synergy Center Manager, transferred to ECM USA in 2005 to manage the North American Testing Program in Wisconsin after 6 years’ experience with production/testing furnaces at ECM Technologies headquarters in Grenoble, France. Vincent has degrees in Chemistry & Physics from the University of Reims, and Treatment of Materials (specializing in Heat Treatment) from BTS Roosevelt, also located in Reims, France. He began his career as a production and laboratory technician for a commercial heat treater, and joined the ECM Group in 1999 as an ECM Technician running LPC testing/metallurgical analysis within ECM vacuum furnace systems.
Vacuum furnaces performing hardening have been in use for over 50 years, yet many heat treaters may not be taking full advantage of newer, more advanced analysis tools and methods. Controlling the cooling pressure can dramatically improve toughness and tool life, but only if applied with precision. In this Technical Tuesday installment, Paulo Duarte, technical director at Treatnorte, explores the science behind gas quenching, the role of step cooling, and why measuring and adjusting cooling curves is critical for consistent, high-performance results.
This informative piece was first released in Heat Treat Today’sMarch 2026 Annual Aerospace Heat Treating print edition.
Introduction
It has been a long time since the invention of the vacuum hardening process, yet innovation in this field continues. In recent years, industrial furnaces capable of operating with higher cooling gas pressures — up to 15 bar now commonly offered on the market — have become standard. But do we truly know how to make the best use of such high pressures?
Pressures up to 10 bar were first applied to cool small parts made from cold-work tool steels, such as sheet metal stamping tools. However, such high pressures can lead to cracking in larger hot-work steel dies when cooled directly. Step cooling was introduced as a solution: start with a fast initial cooling at higher temperatures to avoid carbide formation, then gradually lower pressure stages during the final cooling phase to reduce distortion and minimize the risk of crack appearance.
Despite this empirical knowledge, the question remains: do we really understand what we are doing? Are we routinely measuring cooling rates to determine where they stand on the CCT diagram, predicting microstructure and properties, and adjusting quenching parameters accordingly? And are we certain about which pressures to use for producing high-performance, demanding tools?
Cooling in Vacuum Furnaces
Quenching is one of the most critical steps in the hardening cycle. It transforms austenite into the optimal final microstructure, avoiding the formation of coarse carbides and pearlitic constituents during cooling. This ensures the finest possible microstructure.
Figure 1. Gas quenching in a vacuum hardening furnace | Image Credit: SECO/WARWICKFigure 2. Surface cooling rates region on systematic analysis of parts quenching in a 600 mm x 600 mm x 900 mm furnace. Parts comprising weights from 500 up to 1,000 kg. Cooling pressures varies from 4 to 5 bar. Hot work tool steel. | Image Credit: Metaltec Solutions
In vacuum furnaces, this is typically achieved by injecting cooling gas through nozzles directed at the surface of the parts located in the furnace hot zone. During cooling, the gas circulates through the chamber, being drawn through furnace ports into contact with the heat exchanger tubes. A turbine then blows the cooled gas back into the hot zone where the load is located (Figure 1).
The higher the programmed cooling pressure, the greater the volume of gas passing through the nozzles over the same period of time. This increases the heat transfer from the parts to the cooling gas, resulting in a faster cooling rate.
By measuring successive cooling curves for different loads, specifically for single hot-work steel tools weighing over 500 kg, surface cooling rates pass through the bainitic–martensitic domain (the green area of the CCT diagram shown in Figure 2). Thinner parts tend to cool closer to the martensitic end at the Ms-Bs intersection, while larger tools tend to approach the pearlitic nose.
These observations highlight the importance of adjusting cooling pressure to produce the desired microstructure and account for the different cooling behaviors of large, medium, and small parts.
Investigative Approach: Testing Furnace Data Against CCT Diagrams
Measuring part temperatures during cooling began over 20 years ago, using thermocouples and data loggers, and comparing the results to steel continuous cooling transformation (CCT) diagrams. Most vacuum furnaces do not include this capability as standard, and when available as optional software, many companies choose not to invest in it. In 2005, it was discovered what few in the industry knew at the time: hardening hot-work tool steels in industrial vacuum furnaces often results in a bainitic–martensitic microstructure. This phenomenon is now more widely recognized, with published cooling curves overlaid on CCT diagrams for larger tools becoming more available.
Even so, open discussion remains rare, partly because many heat treaters are reluctant to present this evidence to academia, fearing criticism that their results do not match the fully martensitic microstructure taught at universities. This is not a debate about right or wrong, but rather an opportunity for research and improvement in heat treatment practices worldwide.
After initial testing with a 600 mm × 600 mm × 900 mm French-made single-chamber furnace, trials continued with a larger 900 mm × 900 mm × 1,800 mm German-made vacuum furnace. These tests began by measuring both surface and core temperatures for repeated cycles with small and large charges ranging from small cold-work tools to hot-work tool steel parts weighing 500–1,500 kg. Leading vacuum furnace manufacturers in North America and Europe have developed technologies capable of successfully heat treating small, medium, and large tools, resulting in microstructures that often contain both bainite and martensite. This is, in fact, an inherent characteristic of the technology. Such tools have performed well in service for decades. That said, heat treaters using higher cooling pressures have seen improved tool life significantly, while also increasing the risk of treatment failures if the pressure is too high.
In the last 10 years, properties and microstructure analyses have shown that variations in cooling rate can significantly change the microstructure and toughness of the part even within the same bainitic–martensitic domain of the CCT diagram.
With the emergence of Industry 4.0 and 5.0, along with digitalization and AI, systematic research into heat treatment processes combined with quenching deformation simulation can lead to better selection of cooling pressures. This is a critical parameter in controlling the hardening process, and it has a direct impact on part toughness and service performance. Metaltec Solutions introduced one of the first software tools aimed at improving vacuum heat treatment through Industry 4.0 concepts in 2017. This technology represents a step toward greater awareness and precision in tool steel hardening, helping heat treaters program their cycles for optimal performance in demanding applications.
Regulating Pressure in Vacuum Hardening Furnaces
To obtain the best possible microstructures, gas quenching must be programmed in the furnace so that the cooling rate is kept as close as possible to the martensitic end, i.e., at the Ms-Bs intersection, of the CCT diagram, avoiding the formation of coarse and undesirable microconstituents in the steel. This is achieved by selecting the highest permissible cooling pressure that still prevents cracking or excessive deformation. While small parts can withstand direct high-pressure cooling, larger tools require a reduction in cooling pressure.
Preliminary Pressure Comparison
For optimal quenching of large parts, the cooling pressure should not remain constant throughout the entire cooling cycle. Instead, high pressure should be applied during the initial cooling stage to prevent coarse carbides and pearlite formation and then reduced when the surface temperature reaches approximately 550°C (1022°F). This creates a martempering stage at lower pressures, reducing the risk of distortion and cracking.
Figure 3a. Cooling pressure effect on Vidar Superior (an H11 steel grade variation) part surface toughness | Image Credit: Metaltec SolutionsFigure 3b. Cooling pressure effect on 400 mm x 400 mm x 400 mm block surface toughness | Image Credit: Metaltec Solutions
If we measure the toughness of steel pieces quenched at different cooling pressures, then tempered together to achieve a typical 46–48 HRC hardness (in hot work tool steel), we find that higher cooling pressures result in greater toughness. Using older furnace pressures (around 3 bar) yields lower toughness, whereas increasing cooling pressure can improve toughness by approximately 60% (Figure 3a). This translates into longer tool life, since high-pressure-quenched tools better absorb stress, delaying the initiation and propagation of cracks. These benefits result from higher cooling rates (Figure 3b) and the corresponding finer microstructures achieved.
Although quenching at 3, 6, and 9 bar passes through the same transformation domain on the CCT curve, differences in the resulting internal steel structure, whether coarser or finer, are clearly observable.
True Toughness and Speed
Looking in more detail at the above findings, we can observe that when parts are cooled in a 900 mm × 900 mm × 1,800 mm vacuum furnace, the gas temperature drops below the Ms temperature (for typical hot work tool steels) in less than one minute. The gas temperature then remains near room temperature during the subsequent cooling of the parts (Figure 4a).
Figure 4a. Cooling NADCA block in a large vacuum hardening furnace; gas cooling rate according to gas pressure used | Image Credit: Metaltec SolutionsFigure 4b. Cooling NADCA block in a large vacuum hardening furnace; surface cooling curves and its respective toughness after tempering, with the alteration of the cooling curve behavior provided by the martempering (final hardness level 46–48HRC hot work tool steel | Image Credit: Metaltec Solutions
The parts, however, take considerably longer to cool down to the furnace unloading temperature, depending on the cooling pressure applied. When analyzing the cooling of large dies using the NADCA block as the standard size for comparison, the surface cooling curves vary according to the applied pressure, falling into the bainitic–martensitic domain for 3, 6, and 9 bar cooling pressures.
From this data, it can be seen that hardness is not significantly affected by using 3, 6, or 9 bar cooling pressures, even though the higher pressures produce cooling rates up to twice as fast as the slower ones. Toughness, however, is largely influenced by the way the cooling curves pass through the bainitic–martensitic domain, whether crossing the Bs and Ms intersection closer to the martensitic end (9 bar), near the center (6 bar), or closer to the pearlitic nose (3 bar).
Tuning Pressure and Time
These results show that, within the typical cooling rates of vacuum hardening (Figure 2), toughness varies significantly with cooling pressure, corresponding to finely tuned cooling speeds ranging from approximately 9 to 16°C/min (48 to 61°F/min) between 800°C and 500°C (932°F and 1472°F). This highlights the need to use the highest possible cooling pressures to achieve excellent properties while avoiding direct high-pressure cooling of large parts by applying step cooling with an initial fast cooling phase, followed by reduced pressure.
How Microstructure Drives Toughness
The reason for achieving better properties at higher cooling pressures lies in the resulting microstructure, as shown in Figure 5. Fine bainite and martensitic needles, formed through faster cooling rates, are responsible for the higher toughness observed. When lower cooling pressures are used, the cooling rate decreases, leading to coarser needle sizes (Figres 5a–c) and, consequently, lower toughness values.
Figure 5a-c. Microstructures obtained after quenching Orvar Supreme (premium H13 steel): a) 100°C/min; b) 12°C/min; c) 3°C/min (or, a) 180°F/min; b) 22°F/min; c) 5°F/min) | Image Credit: Metaltec SolutionsFigure 6. Toughness model | Image Credit: Metaltec Solutions
This can be explained by Figure 6. In a coarser microstructure, cracks can propagate more easily because there are fewer obstacles to their advance. In finer microstructures, the higher density of needles forces cracks to deviate repeatedly from their path due to the branching effect, altering the directions of crack propagation. This “shock absorber” effect — caused by the frequent detours a crack experiences when traveling through a greater number of fine needles — is the reason for the toughness improvement observed when higher cooling pressures are used to achieve faster cooling rates.
Figure 7. Convection coefficients for a 900 mm × 900 mm × 1,800 mm vacuum hardening furnace according to the pressure being used | Image Credit: Metaltec Solutions
Each furnace behaves differently, from one furnace builder to another and also depending on the level of maintenance of a furnace. So a similar furnace to the one used for obtaining cooling curves and corresponding toughness values (Figure 4b) was used to obtain the convection coefficients (Figure 7). We can see a strong correlation between convection coefficient, pressure, and final toughness obtained, indicating that these features must be carefully adjusted to reach optimal part properties and longer service life.
Conclusion
Properly applying cooling pressures, through direct high-pressure cooling for small loads or step cooling for larger tools, can significantly increase part toughness and extend tool life. The key lies in understanding how cooling curves interact with the bainitic–martensitic microstructure and adjusting pressure according to part size, geometry, and furnace characteristics.
By measuring temperatures, analyzing microstructures, and fine-tuning cooling cycles, heat treat operators can achieve consistent, high-performance results, as demonstrated with the above studies on tool steels. Faster, well-controlled cooling typically produces finer bainitic–martensitic microstructures which results in a part with “shock absorber” qualities.
Ultimately, maximizing cooling pressure, not just for minimal distortion, creates more durable tools, reduces downtime, and strengthens competitiveness through part performance.
About The Author:
Paulo Duarte Technical Director Treatnorte
Paulo Duarte is an independent researcher and consultant on heat treat technologies, also working as technical director at Treatnorte. His education and expertise in metallurgy have culminated in several articles and patents. Previously, he was the project manager at Metalsolvus and also had been the technical manager and heat treatment manager within bohler-uddeholm group for the Portuguese market. Currently, Paulo focuses on helping heat treaters by providing innovative, more efficient, and profitable heat treatment services to companies.
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.
Ask The Heat Treat Doctor® has returned to bring sage advice to Heat Treat Today readers, answer questions about heat treating, brazing, sintering, and other types of thermal treatments, as well as metallurgy, equipment, and process-related issues. In this installment, Dan Herring examines the essential role of heat treatment in gear performance: exploring the key material and design considerations for power transmission gears, the difference between through hardening and case hardening, and the atmosphere heat treatment processes — from carburizing and carbonitriding to nitriding and nitrocarburizing — that determine how well a gear handles load, wear, and fatigue in heavy-duty applications.
This informative piece was first released in Heat Treat Today’sFebruary 2026 Annual Air & Atmosphere Heat Treating print edition.
Have questions or feedback? We’d love to hear from you — reach out to our editorial team at editor@heattreattoday.com.
Gears play an essential role in the performance of many products that we rely on in our everyday lives. When we think about gears, we generally separate them into two categories: motion-carrying and power transmission. Motion-carrying gears are generally nonferrous alloys or plastics, while load bearing power transmission gears (Figure 1) are usually manufactured from ferrous alloys and are intended for heavy-duty service applications.
Figure 1. Typical off-highway truck power transmission gears | Image Credit: The Heat Treat Doctor®
Gear Materials & Engineering
Power transmission gears involve a wide variety of steels and cast irons. In all gears, the choice of material must be made only after careful consideration of the performance demanded by the application end-use and total manufactured cost, taking into consideration such issues as pre- and post-machining economics.
Key design considerations require an analysis of the type of applied load, whether gradual or instantaneous, and the desired mechanical properties, such as bending fatigue strength or wear resistance — all of which will define core strength and heat treating requirements.
Figure 2. Stress profile in a heavy-duty transmission gear | Image Credit: The Heat Treat Doctor®
It is important for the designer to understand that each area in the gear tooth profile sees different service demands (Figure 2). Consideration must be given to the forces that will act on the gear teeth with tooth bending and contact stress, resistance to scoring and wear, and fatigue issues being paramount. For example, in the root area, good surface hardness and high residual compressive stress are desired to improve endurance or bending fatigue life. At the pitch diameter, a combination of high hardness and adequate subsurface strength are necessary to handle contract stress and wear and to prevent spalling.
Some of the factors that influence fatigue strength are:
Hardness distribution, a function of:
Case hardness
Case depth
Core hardness
Microstructure, a function of:
Retained austenite percentage
Grain size
Carbide size, type, and distribution
Non-martensitic phases
Defect control, a function of:
Residual compressive stress
Surface finish and geometry
Intergranular toughness
In the total manufacturing scheme, a synergistic relationship must exist between the material selection process, engineering design, and manufacturing (including heat treatment). A balance of the priorities in each discipline must be reached to achieve the optimization necessary for the ultimate performance of the gear design. This is often not an easy task.
Various atmosphere heat treatment methods are used for most types of gears including pre-hardening steps (e.g., annealing, normalizing, stress relief) and hardening processes (e.g., neutral hardening and case hardening).
Hardening
Neutral (aka through hardening) refers to heat treatment methods that do not produce a case. Examples of commonly through-hardened gear steels are AISI/SAE grades 1045, 4130, 4140, 4145, 4340, and 8640. It is important to note that hardness uniformity should not be assumed throughout the gear tooth. Since the outside of a gear is cooled faster than the inside, there will be a hardness gradient developed. The final hardness is dependent on the amount of carbon in the steel. The depth of hardness depends on the hardenability of the steel.
Through hardening can be performed either before or after the gear teeth are cut. When gear teeth will be cut after the part has been hardened, machinability becomes an important factor based on final hardness. The hardness is achieved by heating the material into the austenitic range, typically 815°C–875°C (1500°F–1600°F), followed by quenching and tempering.
Case Hardening
By contrast, case hardening is used to produce a hard, wear resistant case (surface layer) on top of a ductile, shock resistant interior (core). The idea behind case hardening is to keep the core of the gear tooth at a level under 40 HRC to avoid tooth breakage while hardening the outer surface to increase pitting resistance.
Carburizing
Figure 3. Atmosphere carburizing of large gears | Image Credit: Photograph courtesy of Aichelin Group
Atmosphere carburizing is the most common of the case hardening methods in use today and can handle a diverse range of part sizes and load configurations (Figure 3). In general, a properly carburized gear will be able to handle somewhere between 30–50% more load than a through-hardened gear. Examples of commonly carburized gear steels include AISI/SAE grades 1018, 4320, 5120, 8620, and 9310, as well as international grades, such as 20MnCr5, 17CrNiMo6, 18CrNiMo7-6, and 20MoCr4.
Atmosphere carburizing is typically performed in the temperature range of 870°C–955°C (1600°F–1750°F) although temperatures up to 1010°C (1800°F) are used for deep case work. Carburizing case depths can vary over a broad range, typically 0.13–8.25 mm (0.005–0.325 inches).
Carbonitriding
Carbonitriding is a modification of the carburizing process, not a form of nitriding. This modification consists of introducing ammonia into the carburizing atmosphere to add nitrogen to the carburized case as it is being produced. Examples of gear steels that are commonly carbonitrided include AISI/SAE 1018, 1117, and 12L14.
Carbonitriding is done at a lower temperature than carburizing, typically between 790°C–900°C (1450°F–1650°F), and for a shorter time. Combine this with the fact that nitrogen inhibits the diffusion of carbon, and what generally results is a shallower case than is typical for carburized parts. A carbonitrided case is usually between 0.075–0.75 mm (0.003–0.030 inches) deep.
Nitriding
Nitriding is another surface treatment process that has as its objective increasing surface hardness. One of the appeals of this process is that rapid quenching is not required, hence dimensional changes are kept to a minimum. It is not suitable for all gear applications; one of its limitations is that the extremely high surface hardness case produced has a more brittle nature than say that produced by the carburizing process. Despite this fact, in a number of applications, nitriding has proved to be a viable alternative. Examples of commonly nitrided gear steels include AISI/SAE 4140, 4150, 4340, and Nitralloy® 135M.
Nitriding is typically done in the range of 495°C–565°C (925°F–1050°F). Case depth and case hardness properties vary not only with the duration and type of nitriding being performed but also with steel composition, prior structure, and core hardness. Typically, case depths are between 0.20–0.65 mm (0.008–0.025 inches) and take from 10 to 80 hours to produce.
Nitrocarburizing (Ferritic or Austenitic)
Nitrocarburizing is a modification of nitriding, not a form of carburizing. In the process, nitrogen and carbon are simultaneously introduced into the steel while it is in a ferritic or at times an austenitic condition. A very thin “white” or “compound” layer is formed during the process, as well as an underlying “diffusion” zone. Like nitriding, rapid quenching is not required. Examples of gear steels that are commonly nitrocarburized include AISI/SAE grades 4140, 5160, 8620, and certain tool steels, such as H11 and H13.
Nitrocarburizing is normally performed at 550°C–600°C (1025°F–1110°F) and can be used to produce a 58 HRC minimum hardness, with this value increasing dependent on the base material. White layer depths range from 0.0013–0.056 mm (0.00005–0.0022 inches) with diffusion zones from 0.03–0.80 mm (0.0013–0.032 inches) being typical.
In Summary
There are many ways to heat treat gears. While atmosphere heat treatment (discussed above) is perhaps the most widely used technology today, other types of heat treatments, namely vacuum and induction hardening, are becoming more and more common methods. These will be discussed in Part Two.
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.
A new study from the Umweltbundesamt (the Federal Environment Agency in Germany) outlines a clear, technically grounded pathway for achieving CO2-neutral process heat across energy-intensive industries. This Technical Tuesday installment highlights the study’s key findings, offering North American heat treaters a concise look at the technical feasibility, economic pressures, and strategic choice involved in moving beyond fossil-fuel-based thermal processing.
This informative piece was first released inHeat Treat Today’sJanuary 2026 Annual Technologies To Watch print edition.
Introduction
Figure 1. Metal Industry – distribution of total annual energy consumption by energy source | Image Credit: SchwotzerFigure 2. Mineral industry – distribution of total annual energy consumption by energy source | Image Credit: SchwotzerTable A. Overview Examined Dectors, Associated Reference Technologies, and Thermal Processing Systems | Image Credit: Schwotzer
Efforts to mitigate climate change are crucial, particularly in Germany where there is a significant amount of energy-intensive industry, to achieve ambitious climate targets while preserving jobs and international competitiveness. Currently, process heat generation is heavily dependent on the use of fossil fuels, especially natural gas, with a low utilization of renewable energies. Fossil energy sources dominate the metal industry, accounting for 87.3%, while electricity represents 10.8%, and hybrid heating systems make up 2.0%. The mineral industry shows an even stronger dependence, with fossil fuels accounting for 99.7%. These figures illustrate the challenges and potential for technological innovations to provide CO2-free process heat in these sectors.
Although some sectors are already either using technologies for CO2-neutral process heat supply or are planning to do so, there is no comprehensive overview of the technical possibilities for generating process heat in energy-intensive industries in the context of future economic framework conditions.
In this study, technologies for the CO2-neutral supply of process heat are considered from a technical, economic, and ecological perspective. The study was conducted for thirteen industries and thirty-four exemplary applications in the metals and minerals industries, as well as for the cross-cutting technology steam generation industry (Table A). For each application, alternative CO2-neutral technologies are examined for their technical feasibility, economic viability, and ecological impact. The focus is on the electrification of plant technology, the use of hydrogen, but also hybrid systems, and, in some cases, the use of biomass. From this comprehensive review of the current situation and the possible alternative technologies, findings and recommendations for implementation will be developed for industry, policymakers, and researchers to support the transformation to CO2-neutral process heat generation.
Study Method
Figure 3. Study approach | Image Credit: Schwotzer
The study is based on an industry and technology assessment of the state of the technology (Figure 3). The results from the metal and mineral industries and the cross-sectional technology of steam generation were analyzed and summarized in consultation with experts. The central process chains were examined for each sector and the most important processes in terms of energy were identified. Each process chain contains several processes in which specific thermal process plants (industrial furnaces) are used, which are grouped into plant types. Based on the selected processes and plant types, applications are defined for further consideration. A reference technology and two to four CO2-neutral alternative technologies (new technologies) are assigned to each application. Key figures such as specific energy requirements, process-related emissions, or investment costs are used for comparison.
Table B. Theses Summary of Study Results | Image Credit: Schwotzer
The central findings of the study are summarized in eleven theses on the transformation of process heat generation (Table B). In this article, Theses 1, 2, 6, and 9 are presented in detail, providing a broad overview of the essential findings. For a more in-depth examination of the theses, see the link to the original study.
The Plant Fleet of Industrial Furnaces is Heterogeneous
The metal and mineral industries are characterized by numerous small process plants (throughput of less than 20 tons per hour and plant capacity of less than 20 MW). At the same time, there are large facilities with significantly higher throughput and corresponding higher plant capacities. Figure 4 shows a selection of technical examples from the study. Examples of large plants include heating and annealing furnaces in the steel industry with capacities of up to 170 tons per hour or cathode shaft furnaces in the copper industry with throughputs of up to 80 tons per hour. It is observed that the specific energy requirement of a plant correlates with the process temperature. The higher the required temperature of a process, the higher the specific energy requirement.
Figure 4. Classification of the considered applications and reference technologies in the plant fleet in Germany based on characteristic parameters | Image Credit: Schwotzer
Additionally, the cross-sectional technology of steam generation was examined. The most up to date technology includes natural gas boilers or combined heat and power (CHP) systems. Industry-specific characteristics play a minor role in the selection of technology for achieving CO2 neutrality. The technical requirements for end applications are less different compared to industrial furnaces. This includes performance, throughput, pressure, and temperature.
A transition to CO2-neutral process heat generation encompasses various technical possibilities and obstacles, as well as investment costs and space requirements, depending on the industry and application. Accordingly, the necessary adaptation measures require a differentiated approach to the transition to CO2-neutral process heat generation. An effective strategy to achieve CO2 neutrality should take into account the unique characteristics of each industry’s production processes, as well as the specific challenges and opportunities they present.
Technical Transformation to CO2-Neutral Production is Feasible
Despite the wide variety of plants and specific challenges, the transition to CO2-neutral process heat generation is technically feasible by 2045. The solutions will vary depending on the industry and application, and the effort required to transition from currently used reference technologies to CO2-neutral alternatives varies significantly.
The heterogeneity of industrial furnaces has a significant impact on the feasibility of deploying CO2-neutral technology in the future. While electrification is already highly advanced and most up to date in applications such as the foundry industry, bulk forming, or melting of aluminium with induction furnaces, it shows comparatively low technological maturity in sectors like the lime and cement industry, which are associated with fundamental technical challenges; see Figure 5. This significant heterogeneity in the existing plant stock and terms of technology readiness level (TRL) (European Commission 2014) requires consideration in transformation strategies.
Figure 5. Technology readiness level (TRL) of the alternative technologies (summarized) | Image Credit: Schwotzer
Both hydrogen and electrification can have a significant impact, although further research and development are needed in many areas. Across applications, it is evident that electrification generally requires the construction of new facilities. Transitioning from natural gas-operated reference technology to hydrogen involves less technical effort in terms of plant technology and can be accomplished by retrofitting the burner technology. Additionally, using hydrogen requires local infrastructure (pipes, valves) and its impacts on process and product quality need to be tested. Industrial-scale facilities are not yet available, resulting in a TRL of < 5, according to the study. However, with ongoing research and development in many projects, the TRL for many applications is expected to rise quickly in the coming years.
Scaling all alternative technologies to an industrial level and testing them in operational deployments are crucial. Some technologies face significant technical barriers, such as the continuous heating in steel rolling mills. These processes and their plant technology are characterized by very high process temperatures and production capacities, requiring heating technologies with a high energy density, which are not possible with current most cutting-edge electrical heating technologies. The use of hydrogen also presents a particular technological challenge, especially in areas where solid fuels like coke are currently used, such as in shaft kilns for lime burning or in cupola furnaces of iron foundries. As a result, alternative, bio-based fuels are being considered for these applications.
However, for these fuels to be a viable option, they need to be produced in sufficient quantity and quality. On the other hand, CO2-neutral techniques for steam generation using hydrogen and for electrification are already available for industrial use today.
The continuation of this article will be released in Heat TreatToday’sSustainable Heat Treating Technologies edition (May 2026) where electrification versus hydrogen and a frank reckoning with the cost of new investments will be examined.
References
European Commission. 2014. Annex G – Technology Readiness Levels (TRL). Extract from Part 19 – Commission Decision C(2014)4995, “Horizon 2020 – Work Programme 2014–2015. General Annexes.” Brussels: European Commission.
Fleiter, Tobias, et al. 2023. CO2-Neutrale Prozesswärmeerzeugung: Umbau des industriellen Anlagenparks im Rahmen der Energiewende. Dessau-Roßlau: German Environment Agency (Umweltbundesamt).
All results in this article derive from the study “CO2-neutral process heat generation” (German: „CO2-neutrale Prozesswärmeerzeugung – Umbau des industriellen Anlagenparks im Rahmen der Energiewende: Ermittlung des aktuellen SdT und des weiteren Handlungsbedarfs zum Einsatz strombasierter Prozesswärmeanlagen”). The authors of this article would like to thank everyone who contributed to the study, listed in the published study. The study and further documents are on the website of the Federal Environment Agency in Germany (Umweltbundesamt).
This editorial is published with permission from Heat TreatToday’s media partner heat processing, which published this article in March 2024.
About The Authors:
Dr. Christian Schwotzer Department for Industrial Furnaces and Heat Engineering RWTH Aachen University, Germany schwotzer@iob.rwth-aachen.de
Katharina Rothhöft, M.Sc. Department for Industrial Furnaces and Heat Engineering RWTH Aachen University, Germany rothhoeft@iob.rwth-aachen.de
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.
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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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.
