Part discoloration after vacuum heat treating? What can heat treaters do to prevent this? In this best of the web, Q&A-style article, witness the heat treating industry gather around to exchange ideas and find a solution to the problem. Part position, backfill gas level, contaminated quench gas, or an air leak could all be to blame in this Technical Tuesday.
Dan Herring weighs in on the issue as well. To read The Heat Treat Doctor's®diagnosis, click the link below. Learn how the color and position of the discoloration give clues as to the source of the problem.
An excerpt:
"So, what else could be happening? Let The Doctor add a few thoughts to the discussion. First, the fact that the discoloration (staining) is brown in coloration suggests that the oxide is forming on the part surface during cooling when the temperature is in the range of (approximate) 245ºC – 270ºC (475ºF – 520ºF). This is supported by the fact that the oxidation does not occur “during natural cooling” (which we assume to mean cooling under vacuum). Second, the fact that the discoloration is more evident at the bottom of the load suggests the phenomenon is (gas exposure) time dependent, that is, the longer the parts take to cool through the critical range, the greater the chance for discoloration."
Ever heard of binder jetting (BJT)? It's an evolving technology that is quickly catching up to metal injection molding (MIM). Compared to MIM, BJT has a lower cost per part rate, produces larger parts, and, because BJT is a cold process, it does not introduce residual stress inside the part.
Even though BJT is a cold process, sintering is a key step in BJT. Read this best of the web article to learn the ins-and-outs of sintering with binder jetting.
An excerpt:
"Vacuum sintering furnaces are usually the go-to choice for sintering of [binder jetting] parts, thanks to the ability to provide bright and shiny sintered parts, the tight process parameters control and the possibility to work with different debinding and sintering atmospheres."
There is no way to validate the heat treating process without completely destroying the job. Here’s where pyrometry becomes crucial. The precision, accuracy, and uniformity standards of specifications like AMS2750 and CQI-9 provide peace of mind without destructive testing. Read how the requirements of these regulations are benefiting the industry through standardization and defect prevention.
"El tratamiento térmico como la mayoría de los procesos especiales, tiene la particularidad de ser una operación crítica que para su validación requiere de pruebas destructivas. . . "
Read the English translation of this article by Víctor Zacarías, general director at Global Thermal Solutions Mexico, in the version below, or the Spanish translation when you click the image to the right.
Both Spanish and English translations of the article were originally published in Heat Treat Today's March 2022 Vacuum Furnace print edition.
Víctor Zacarías General Director Global Thermal Solutions Mexico
Introduction
Heat treatment operations are generally perceived as black boxes whose results are not very predictable. Although we understand the physical mechanisms involved in modifying the properties of a certain material, heat treatment furnaces are thermodynamically imperfect, and sometimes the final results are too.
An extra variable must be added to this picture. As the properties of the final product can only be validated through destructive testing, we must have a high level of process control in place if we want to ensure repeatability in heat treat operations. This is where pyrometry specifications play an important role, particularly in defining the correct temperature controls for consistent heat treatment.
Picture 1. Temperature uniformity survey performed in a vacuum furnace
Pyrometry standards/specifications define the temperature control requirements for thermal processing equipment used in heat treatment operations (furnaces, ovens, muffles, etc.). These specifications are very comprehensive documents that allow us to solve the following problems:
How do you know that the temperature readings are accurate?
How do you know the temperature variation of your measurement system?
How do you know that the entire load was exposed to a consistent temperature during the cycle?
How do you know what you know? (Documented evidence)
The most widely accepted and proven pyrometry specifications in the industry are:
AMS2750: issued by SAE International, it is the universally accepted standard for thermal processing certification purposes in the aerospace industry (Nadcap).
AIAG CQI-9: this assessment provides the pyrometry requirements for the evaluation of heat treatment in the automotive industry.
API 6A & 16A: annexes establish the pyrometric requirements for the components treated in the energy industry (oil and gas).
All of these specifications describe in their content at least the following four items:
Calibration of thermocouples (or any other temperature sensor), as well as the limit of use depending on its
application
Calibration of control and test instrumentation
The procedure and acceptance criteria for conducting a System Accuracy Test (SAT)
The method and acceptance criteria for a Temperature Uniformity Survey (TUS)
These specifications are subject to continuous revisions to ensure that the requirements are understood. However, it does not change the fact that they are very extensive documents, generally misinterpreted and which require experienced personnel for their implementation. As an example of these difficulties, in Nadcap accreditation audits, eight out of 10 findings are directly related to pyrometry. CQI-9 assessments in the automotive industry show similar figures.
Despite the above, the right implementation of the pyrometry requirements has proven for years that a consistent heat treatment process can be achieved, providing data that allows defect prevention in an effective way.
Thermocouple Requirements
A thermocouple is a very simple temperature sensor that consists of two conductors with different thermoelectric characteristics. The conductors are joined at one end (hot junction) which will be in contact with the element whose temperature is to be measured. When the conductors are exposed to a temperature gradient, a difference of electrical potential (mV) is generated due to the phenomenon known as Seebeck effect. At the other end (cold junction), a voltmeter is used to measure the potential generated by the temperature difference between the two ends (See Figure 1).
Figure 1. Schematic of a thermocouple
Pyrometry standards defi ne the calibration requirements for the thermocouples used in thermal processing equipment. In order to acquire thermocouples in accordance with these regulations, we must consider the final use of the sensor to define the maximum error allowed at the time of calibration (See Table 1).
Once we have a calibrated thermocouple, the date of the installation must be documented to track the sensor life. Thermocouples have a finite lifetime because of the natural degradation of the materials of which they are made, leading to a decrease in their accuracy. Therefore, the replacement of temperature sensors must be calendarized depending on the thermocouple type and the temperature to which they are exposed.
Instrumentation Requirements
Instruments receive electrical communication from thermocouples and convert potential (mV ) to a usable format.
Pyrometry specifications like AMS2750 and CQI-9 define the resolution and accuracy requirements for the instrumentation used in heat treating equipment, as well as the frequency at which these instruments must be calibrated. The level of accuracy of the instrumentation is based on the applicable specification and the purpose of the instrument, as shown in Table 1.
Table 1. Accuracy required for temperature sensors according to AMS2750 and CQI-9
It is important to consider the manufacturer’s instructions when installing and calibrating control and recording instruments. From a metrological standpoint, documentation must evidence that the calibrations are traceable to a national reference standard (NIST, CENAM, etc.) and, in most industries, carried out in accordance with ISO/IEC 17025.
The System Accuracy Test
A System Accuracy Test (SAT) or probe check is a very simple test to ensure that the entire measurement system (thermocouple and instrument together) provides an accurate representation of the temperature. It is an on-site comparison of the furnace’s measurement system against an independent calibrated measurement system (See Figure 2). The purpose of this test is to determine if the natural deviation of the temperature measurement system is still acceptable.
Figura 2. Diagrama de un Ensayo de Exactitud del Sistema (SAT)
The criteria to determine whether the results of an SAT test are acceptable or not will depend on the applicable regulations, AMS2750 or CQI-9. If the difference in the SAT exceeds the limits allowed by the standard, internal procedures must take into account the following considerations before reprocessing parts:
Document that the equipment has failed a test
Determine the root cause of the failure
Implement corrective actions
When an SAT test result fails, corrective actions can generally be reduced to two options: replace the thermocouple and/or recalibrate and adjust the instrument.
A SAT is performed to assure the accuracy of all the systems in the furnace which are used to make decisions about the product, both control and recording. It is important to note that SAT test results change over time, therefore historic SAT data is very useful to identify trends and proactively take action before a deviation shows.
Temperature Uniformity Surveys
Figure 3. Schematic of a temperature uniformity survey (TUS)
A Temperature Uniformity Survey (TUS) is a test where a calibrated instrument (data logger) and several calibrated thermocouples measure the temperature variation inside the furnace. The result of a TUS test indicates where the hottest and/or coldest spots are in a furnace and provides elements to determine how to correct them.
For most commercially available furnace volumes, TUSs are conducted introducing nine thermocouples for batch type furnaces, and three tracking thermocouples for continuous furnaces.
A TUS is considered acceptable if the test thermocouple readings are within the limits set by the specification for the required time. TUS is highly recommended to be performed after the initial installation of the equipment or after a modification that could alter the heating characteristics of the furnace. Subsequently, they must be carried out periodically in accordance with the applicable regulation.
Importance of Pyrometry
The labor of harmonizing special processes is not easy. However, there is strong evidence that proves the effectiveness of this eff ort. For example, Supplier Technical Assistance teams at Ford Motor Co. have followed the results achieved by the implementation of CQI-9 by their suppliers and have estimated cost savings of up to 20 million dollars in reduction of heat treatment defects. Similarly, the Performance Review Institute, which is the organization in charge of managing Nadcap, reports increasingly positive results each year by the implementation of the program, impacting directly on continuous improvement of aerospace organizations that accredit it (Figure 4).
Figure 4. Perception in quality improvement from Nadcap audits
Pyrometry testing provides valuable information that encourages preventive maintenance of furnaces and related equipment. At the same time, it provides understanding of the measurement systems that allow achieving repeatable metallurgical results. In both cases, the information generated in pyrometry allows heat treaters to reduce scrap and quality claims and most importantly, ensures business continuity by showing compliance with customers’ requirements.
About the author: Víctor Zacarías is a metallurgical engineer from the University of Querétaro with studies in Strategic Management from Tec de Monterrey. With over 15 years of experience in heat treatment management, he is currently the managing director of Global Thermal Solutions México. Victor has conducted numerous courses, workshops, and assessments in México, United States, Brazil, Argentina, and Costa Rica and has been a member of the AIAG Heat Treat Work Group (CQI-9 committee).
There is no way to validate the heat treating process without completely destroying the job. Here’s where pyrometry becomes crucial. The precision, accuracy, and uniformity standards of specifications like AMS2750 and CQI-9 provide peace of mind without destructive testing.
Read the Spanish translation of this article by Víctor Zacarías, director general de Global Thermal Solutions México, in the version below, or read both the Spanish and the English translation of the article where it was originally published: Heat Treat Today's March 2022 Vacuum Furnace print edition.
El tratamiento térmico como la mayoría de los procesos especiales, tiene la particularidad de ser una operación crítica que para su validación requiere de pruebas destructivas. Al no poder medir el 100% del producto, las normas de pirometría juegan un papel fundamental en el control y documentación de los procesos de tratamiento térmico. La norma AMS2750 y la evaluación CQI-9 son los estándares mas aceptados en la industria aeroespacial y automotriz respectivamente, y describen los requisitos de precisión, exactitud y uniformidad para los sistemas de medición de temperatura y los equipos empleados en el procesamiento térmico. Este artículo sintetiza los requerimientos de estas normativas e ilustra los beneficios en la industria de contar con un enfoque homologado para la reducción de la variación y la prevención de defectos.
Víctor Zacarías Director General Global Thermal Solutions México
Introducción
Las operaciones de tratamiento térmico son percibidas generalmente como cajas negras cuyos resultados son poco predecibles. Si bien, entendemos los mecanismos físicos involucrados para modificar las propiedades de un material, los hornos de tratamiento térmico son sistemas termodinámicamente imperfectos, y por ende los resultados finales en ocasiones también lo son.
A esta situación hay que agregar una variable adicional. Al tratarse de operaciones en las cuales las características del producto final solamente pueden ser validadas a través ensayos destructivos, debemos de contar con un nivel particular de control de proceso si queremos asegurar la repetibilidad en las operaciones de tratamiento térmico.
Fotografía 1. Ensayo de uniformidad de temperatura conducido en horno de vacío
Las normas y especificaciones de Pirometría definen los requerimientos de control de temperatura para los equipos de procesamiento térmico (hornos, muflas, estufas, etc) empleados en las operaciones de tratamientos térmicos. Se trata de estándares muy completos que nos permite resolver las incógnitas que los auditores de proceso ponemos sobre la mesa
¿Cómo sabes que las lecturas de temperatura de tu horno son precisas?,
¿Cómo sabes cuál es la variación de temperatura de tu sistema de medición?
¿Cómo sabes que la totalidad de la carga fue expuesta a una temperatura consistente durante el ciclo completo de tratamiento térmico?,
¿Cómo sabes que lo sabes?
Las especificaciones de pirometría mayormente aceptadas y probadas en la industria son:
AMS2750, emitida por SAE International, es la norma universalmente aceptada para fines de certificación de procesamiento térmico en la industria aeroespacial
CQI-9 de la Automotive Industry Action Group (AIAG). Las secciones 3.1, 3.2, 3.3 y 3.4 definen los requerimientos de pirometría para la evaluación de tratamientos térmicos en la industria automotriz y
API 6A y 16A, cuyos anexos establecen los requisitos pirométricos para los componentes tratados en la industria de energía (oil & gas)
Todas estas especificaciones contemplan en su contenido al menos los siguientes 4 aspectos:
Calibración de los termopares (o cualquier otro sensor de temperatura), así como los requisitos y tiempo límite de uso en función de su aplicación.
Calibración de la instrumentación de control y prueba
El procedimiento y los criterios de aceptación para la realización de la prueba System accuracy Test (SAT).
El método y los criterios de aceptación para la prueba de uniformidad de temperatura o Temperature Uniformity Survey (TUS).
Las normas de pirometría son sometidas procesos de revisión profunda de manera frecuente por las organizaciones que las emiten para asegurar que los requerimientos sean entendidos. Sin embargo, no cambia el hecho de que se trata de documentos complejos, generalmente malinterpretados y que requieren de personal experimentado para su implementación. Cómo ejemplo de estas dificultades, en auditorías de certificación Nadcap (industria aeroespacial) 8 de cada 10 hallazgos levantados están relacionados directamente con pirometría. Las evaluaciones de CQI-9 en la industria automotriz presentan cifras similares.
A pesar de lo anterior, la implementación correcta de los requerimientos de pirometría ha probado por años que se puede alcanzar un proceso de tratamiento térmico consistente y arrojar datos que permiten prevenir defectos de manera efectiva.
Termopares
Un termopar es un sensor de temperatura que consiste de dos conductores con características termoeléctricas distintas. Los conductores están unidos en un extremo (unión de medición o hot junction), el cual estará en contacto con el elemento cuya temperatura se quiere medir. Cuando los conductores se exponen a un gradiente de temperatura se genera una diferencial de potencial (mv) debido al fenómeno conocido como Efecto Seebeck. En el otro extremo (cold junction), se empleará un voltímetro para medir el potencial generado por la diferencia de temperatura entre los dos extremos (ver figura a continuación).
Figura 1. Diagrama de un termopar
La normas de pirometría definen los requisitos de calibración para los termopares usados en el equipo de procesamiento térmico. Para adquirir termopares acordes con la normatividad, debemos considerar la aplicación final del sensor para definir el error máximo permitido al momento de la calibración (ver tabla a continuación).
Una vez que contamos con termopares calibrados, se debe documentar la fecha en la que se realiza la instalación para monitorear el tiempo de vida del sensor. Los termopares tienen un tiempo de vida finito debido a que la exposición a la temperatura provoca la degradación de los conductores y por ende la disminución de su precisión. El reemplazo por lo tanto de un sensor de temperatura estará determinado por el tipo de temopar (K, N, E, T, J, B, R, o S) y la temperatura a la que se expone.
Instrumentación
Los instrumentos reciben comunicación eléctrica de los termopares y convierten fuerza electromotriz (fem) a un formato usable.
La especificaciones de pirometría como AMS2750 y CQI-9 definen los requisitos de resolución y precisión para la instrumentación empleada en Tratamientos Térmicos, así como la frecuencia a la que se deben calibrar dichos instrumentos. El nivel de precisión de la instrumentación está en función la norma aplicable y el propósito del instrumento como se muestra en la siguiente tabla.
Tabla 1. Precisión requerida sensores de temperatura de acuerdo a AMS2750 y CQI-9
Es importante considerar las instrucciones del fabricante al momento de instalar y calibrar los instrumentos de control del horno. Desde el punto de vista metrológico, la documentación debe demostrar que la calibración de los equipos es trazable a un patrón nacional (NIST, CENAM, etc) y, en la mayoría de los casos, realizada de conformidad a la norma ISO/IEC 17025:2017 correspondiente a los laboratorios de ensayo y calibración.
Prueba de Exactitud del Sistema (System Accuracy Test o Probe Check)
La prueba System Accuracy Test (SAT) o Probe Check es una comparación en sitio del sistema de medición del horno contra un sistema de medición calibrado. El objetivo de esta prueba es determinar si la desviación natural del sistema de medición de temperatura se encuentra dentro de límites aceptables.
Figura 2. Diagrama de un Ensayo de Exactitud del Sistema (SAT)
El criterio de aceptación para determinar si los resultados de una prueba SAT son aceptables o no, dependerá de la normativa aplicable. Si la diferencia del SAT excediera los límites permitidos por la norma, los procedimientos internos deben tomar en cuenta la siguientes consideraciones antes de volver a procesar piezas:
Documentar que el equipo ha fallado la prueba,
Determinar la causa raíz de la falla y
Implementar acciones correctivas
Cuando el resultado de la prueba SAT excede los límites permitidos, las acciones correctivas generalmente se pueden reducir a dos alternativas: (1) Reemplazo del termopar o (2) Recalibración y ajuste del instrumento.
Una vez aplicadas las acciones correctivas y, antes de procesar cualquier material adicional, la prueba SAT debe repetirse conforme al procedimiento de la norma para confirmar la efectividad de las acciones correspondientes.
Un SAT es una prueba muy simple para asegurar que el todo el sistema de medición (termopar mas instrumento en conjunto) provee una representación exacta de la temperatura. Es importante tomar en cuenta que los resultados de la prueba SAT cambian con el tiempo, por lo tanto se trata de un chequeo muy útil para identificar tendencias y tomar acciones de manera proactiva antes de una desviación.
Prueba de Uniformidad de Temperatura (Temperature Uniformity Survey)
Figura 3. Diagrama de un Ensayo de Uniformidad de Temperatura (TUS)
Un Temperature Uniformity Survey (TUS) es una prueba en donde un instrumento y varios termopares calibrados miden la variación de temperatura dentro del volumen de trabajo del horno. La prueba TUS indica dónde se encuentran los puntos mas fríos y/o calientes de un horno y proporciona elementos para determinar el porqué de esos puntos y cómo corregirlos.
El primer aspecto a considerar es la cantidad de termopares a emplear durante la prueba, que está en función del volumen de trabajo del horno y la normativa aplicable. Para la mayoría de los volúmenes de los hornos disponibles comercialmente, la cantidad de termopares requeridos es de 9 para hornos tipo batch (lote) y 3 para hornos continuos.
Un TUS se considera aceptable si las lecturas de los termopares se encuentran dentro de los límites establecidos por la especificación durante el tiempo requerido en todo momento. La prueba TUS se recomienda realizar después de la instalación inicial del equipo o después de una modificación que pudiera alterar las características de uniformidad del horno. Posteriormente se deben realizar de manera periódica de acuerdo a la normativa.
Importancia de la pirometría
La labor para armonizar los procesos especiales no es sencilla, sin embargo existen datos contundentes que prueban la efectividad de este esfuerzo. El equipo de STAs de Ford Motor Co. ha realizado estimaciones de los beneficios obtenidos al implementar CQI-9 en su cadena de proveduría y han cuantificado ahorros de hasta 20 millones de dolares por conceptos de reducción de defectos en Tratamientos Térmicos. De igual manera, el Performance Review Institute, quien es la organización encargada de administrar el programa Nadcap, reporta cada año el impacto en la mejora continua en las organizaciones aeroespaciales que acreditan este programa.
Figura 4. Percepción de la mejora en la calidad en relación con su acreditación Nadcap
Las pruebas de pirometría proporcionan información valiosa que fomenta el mantenimiento preventivo de los hornos y equipos relacionados. Al mismo tiempo, el entendimiento y control de los sistemas de medición ayudan de manera proactiva a obtener resultados metalúrgicos repetibles. En ambos casos la información generada en estas pruebas nos permite reducir la probabilidad de scrap o reclamos de calidad y asegurar la continuidad del negocio al mostrar conformidad con los mandatos del cliente.
Sobre el autor: Víctor Zacarías es ingeniero metalúrgico egresado de la Universidad Autónoma de Querétaro con estudios en Gerencia Estratégica por parte del Tec de Monterrey. Con más de 15 años de experiencia en la gestión de tratamientos térmicos, actualmente es director general de Global Thermal Solutions México. Víctor ha realizado numerosos cursos, talleres y evaluaciones en México, Estados Unidos, Brasil, Argentina y Costa Rica y ha participado en el Grupo de Trabajo de Tratamiento Térmico de AIAG (CQI-9) y en el Comité de Ingeniería de Materiales Aeroespaciales de SAE.
There is an age-old adage that exists in the heat treating world. That supposition states that “the smaller the vacuum furnace, the faster it will quench.” Is this adage true? Explore Solar Atmospheres’ journey as they designed an experiment to discover if pressure or velocity most affects cooling performance.
This Technical Tuesday was written by Robert Hill, FASM, president, and Gregory Scheuring, plant metallurgist, both from Solar Atmospheres. The article originally appeared inHeat TreatToday’sMarch 2022 Aerospace Heat Treating print edition.
Introduction
Our study compared the cooling rates of two distinctly sized High Pressure Gas Quenching (HPGQ) vacuum furnaces — a large 10-bar vacuum furnace equipped with a 600 HP blower motor versus a smaller 10-bar vacuum furnace equipped with a 300 HP motor. Both furnaces, one with a 110 cubic feet hot zone, the other with a 40 cubic feet hot zone, were exclusively engineered and manufactured by Solar Manufacturing located in Sellersville, PA.
History
High Pressure Gas Quenching in the heat treatment of metals has made tremendous strides over recent years. Varying gas pressures within the chamber have been shown to be more governable than their oil and water quenching counterparts. The number one benefit of gas cooling versus liquid cooling remains the dimensional stability of the component being heat treated. In addition, using gas as a quench media dramatically mitigates the risk of crack initiation in a component. This is primarily due to the temperature differentials during cooling. Gas quenching cools strictly by convection. However, the three distinct phases of liquid quenching (vapor, vapor transport, and convection) impart undue stress into the part causing more distortion (Figure 1).
Figure 1. Three phases of liquid quenchants Source: Solar Atmospheres
There are multiple variables involved with optimizing gas cooling. These include the furnace design, blower designs, heat exchanger efficiency, gas pressure, gas velocities, cooling water temperatures, the gas species used, and the surface area of the workpieces. Whenever these variables remain constant, the relative gas cooling performance of a vacuum furnace typically increases as the volume of the furnace size decreases.
The Furnace
Solar Manufacturing has built multiple high pressure gas quenching furnaces of varying sizes over the years ranging from 2 to 20-bar pressure. We have learned that vacuum furnaces, rated at 20-bar and above, became restrictive in both cost constraints and diminishing cooling improvements. Therefore, Solar Manufacturing engineers began to study gas velocities to improve cooling rates. They determined increasing the blower fan from 300 HP to 600 HP, along with other gas flow improvements, would substantially increase metallurgical cooling rates. The technology was reviewed and determined to be sound. A 48” wide x 48” high x 96” deep HPGQ 10-bar furnace, equipped with this newest technology, was purchased by Solar Atmospheres of Western PA located in Hermitage, PA.
Image 1. HFL50 furnace (36” x 36” x 48”)
Source: Solar Atmospheres
Image 2. HFL74 furnace (48” x 48” x 96”) Source: Solar Atmospheres
The Test
Image 3. Test load with thermocouple placement Source: Solar Atmospheres
Once this new vacuum furnace was installed, a cooling test was immediately conducted. A heavy load would be quenched at 10-bar nitrogen in an existing HFL 50 sized furnace (36” x 36” x 48”). The same cycle was repeated in the newly designed vacuum furnace almost three times its size! (Images 1 and 2).
The load chosen for the experiment was 75 steel bars 3” OD x 17” OAL weighing 34 lbs each. The basket and grid system supporting the load weighed 510 lbs. The total weight of the entire load was 3060 lbs. Both test runs were identically thermocoupled at the four corners and in the center of the load. All five thermocouples were deeply inserted (6" deep) into ¼" holes at the end of the bars (Image 3). Each load also contained two 1" OD x 6" OAL metallographic test specimens of H13 hot working tool steel. These specimens were placed near the center thermocouple to ensure the “worst case” in terms of quench rate severity. All tests were heated to 1850°F for one hour and 10-bar nitrogen quenched.
Results
The comparative cooling curves between both HPGQ vacuum furnaces are shown in Chart 1. Table 1 reveals that in the critical span of 1850°F to 1250°F for H13 tool steel, the cooling rate in the larger furnace with more horsepower nearly matched the cooling rate of the furnace three times smaller in size.
Table 1. Critical cooling rates for H13 (1850°F –1250°F) Source: Solar Atmospheres
Chart 1. Average quench rate for five thermocouples Source: Solar Atmospheres
Micrographs of the H13 test specimens processed in each load were prepared (Images 4 and 5). The microstructure of each test specimen is characterized by a predominantly tempered martensitic microstructure with fine, undissolved carbides. The consistency of the microstructure across both trial loads further demonstrates that while the larger furnace utilized the higher horsepower, both resulted in a critical cooling rate sufficient to develop a fully martensitic microstructure.
These tests prove that the greatest impact on the cooling performance in a vacuum furnace is to increase the gas velocity within that chamber. This was achieved primarily by increasing the horsepower of the blower fan. By doing this, the ultimate cost to the customer is significantly less than manufacturing a higher pressure coded vessel. This newly designed vacuum furnace has proven to be a game changer.
Part II of this article will discuss real life case studies and how both Solar and Solar’s customers have mutually benefited from this newest technology.
About the Author:
Robert (Bob) Hill, FASM President Solar Atmospheres of Western PA Source: Solar Atmospheres
Robert Hill, FASM, president of Solar Atmospheres of Western PA, began his career with Solar Atmospheres in 1995 at the headquarters plant located in Souderton, Pennsylvania. In 2000, Mr. Hill was assigned the responsibility of starting Solar Atmospheres’ second plant, Solar Atmospheres of Western PA, in Hermitage, Pennsylvania, where he has specialized in the development of large vacuum furnace technology and titanium processing capabilities. Additionally, he was awarded the prestigious Titanium Achievement Award in 2009 by the International Titanium Association.
Time to evacuate! When it comes to evacuating atmospheric pressure from vacuum furnace chambers, the addition of a diffusion pump can help attain a lower system pressure than the typical roughing pump and vacuum booster pump allow.
This best of the web article identifies the basics of vacuum furnace pumps and then explains how diffusion pumps in particular work and identifies a few considerations to think about to determine if you need this addition or not.
An excerpt:
"For the diffusion pump to function properly, the main and foreline valves must be open, allowing the furnace to operate in high vacuum. Fluid at the bottom of the pump is heated to boiling and forced up through the center of the jet assembly."
You can’t always get what you want. With frequently changing specifications and a volatile economy, what heat treaters want is always evolving. But what they need changes, too. Steven Christopher of Super Systems, Inc. discusses how to balance what vacuum furnace operators NEED and what they WANT. Is the difference between those two things too great?
This article originally appeared inHeat TreatToday’sMarch 2022 Aerospace print edition.
Steven Christopher West Coast Operations Manager Super Systems, Inc. Photo Credit: Super Systems, Inc.
I love metaphors and think of vacuum furnaces as automobiles. As an owner, the goal is to keep our cars on the road for 100,000+ miles — and why not the same for furnaces? Accomplishing this feat requires the same in both cases: (1) routine maintenance — literally changing oil and (2) addressing warnings before they become problems — such as check engine lights or vacuum leaks.
The similarities stop there, however, with a key difference in how each is upgraded. In the near future, if you want a self-driving vehicle you will have no choice but to turn in the keys of your 10-year-old sedan and buy a shiny new Tesla, opting for the autonomous driving upgrade.
But what about your vacuum furnace? As the industry releases all these new standards and specifications, do we also need a newer furnace? Or can we retrofit what we have? That answer is complicated because so much is influenced by what we WANT versus what we NEED.
Day-to-day production shapes what we want. We learn from both experiences and failures, shaping features we want to improve operations, customer experience, and reduce rejected work. Specifications and customers drive what we need. Most recently AMS2750F (and 2769C) have been revised and place a burden on operating aging equipment while maintaining compliance. Before these, NFPA86 was modified in 2019, improving furnace design and safety “best practices."
These requirements levy real costs in terms of both hardware investment and increased labor (additional quality employees). We are expected to perform additional labor with the same workforce; however, the reality is that a worsening domestic labor shortage often means we are doing more work with even fewer people. This article navigates this delicate balance, maximizing each investment dollar’s impact while reducing our reliance on labor.
What We Need
It becomes impossible to completely address such large specifications in a short article, so let me highlight a few important considerations from AMS2769C:
Section 3.2.3.2 requires decimal precision for thermocouples (AMS2750F)
Section 3.3.1 reviews partial pressure and dew point requirements
Section 3.5.2.1 addresses permissible outgassing
Section 3.5.3 covers load thermocouples
Perhaps the most talked about change is the requirement of thermocouples to record to a tenth of a degree. It is important to distinguish the difference between a temperature controller and recorder. Section 3.2.3.2 does not require a furnace to control with decimal precision, only record to it. However, best practice lends itself to controllers supporting this ability as well.
Exposure to oxygen at elevated temperatures is detrimental to part metallurgy, be it aesthetics or integrity. Leak-up rates are so important because they prove such exposure is eliminated (or significantly reduced). AMS2769C attempts to mitigate this exposure by standardizing the best practices for performing such tests. Leak-up rate tests are required weekly for (minimum) 15 minutes. Figure 1 identifies a maximum allowable leak-up rate based on the material being processed.
Historically this requires an operator to initiate a cycle, stop the evacuation (pumping), then document the beginning and ending vacuum levels by hand. While simple, this requires both time and attention, preventing any operator from performing other tasks.
AMS2769C proceeds by addressing outgassing, requiring ramp/soak controllers to either be placed on hold or to disable the heating elements if the vacuum level exceeds (1) the partial pressure target or (2) the diffusion pump operating range. Aging controllers require well-trained operators, constantly monitoring vacuum instrumentation and manually adjusting the controller. This introduces potential for operator error, again limiting their ability to perform other tasks.
Section 3.5.3 details placement and requirements for load thermocouples. Assuming load thermocouples are required, runs may be rejected should thermocouples fail below the minimum processing temperature. Disconnected control systems monitor load thermocouples using a recorder separate from the ramp/soak controller. This complicates the control system’s ability to alert operators to such failed conditions — the recorder not knowing which thermocouples are required.
AMS2769C progresses to cover partial pressure. Partial pressure has been automated for years with minimal changes to control mechanisms, though some have replaced solenoid valves with mass flow controllers (MFCs). System upgrades should strongly consider automatic gas type compensation and digital communications of vacuum levels.
Thermocouple (or pirani) vacuum sensors estimate the heat emitted from a heating filament within the sensor. This measurement represents an exact vacuum level, though the gaseous media separating the fi lament from the measuring tip influences the reading (thermodynamics heat transfer). This phenomenon (represented in Figure 2) explains why nitrogen and argon result in very different vacuum estimates.
Figure 2. Gas compensation graph Photo Credit: Televac MM200 User Manual
NOTE: Thermocouple gauges operate in vacuum ranges where enough gas molecules remain (e.g., in excess of 1 micron) to influence this reading; unlike cold cathode sensors which operate under complete vacuum, excluding them from such compensation.
As an example, consider a vacuum furnace operating under nitrogen partial pressure. The vacuum instrument correctly displays 200 microns (refer to the AIR curve). Now consider the same cycle, only the operator introduces argon. The display now incorrectly displays (and controls to) 200 microns; however, the furnace is truly operating closer to 100 microns (refer to the ARGON curve).
Figure 3. Dew point requirements Photo Credit: AMS2769 Section 3.3.1.1
Historically vacuum signals have transmitted a 0-10vdc analog signal representing the vacuum level. As with all analog signals, error is introduced by both the accuracy of the instrument generating the signal as well the recorder interpreting it. This error is mitigated by routine calibrations — often aligned with temperature uniformity survey (TUS) schedules. Modern control systems replace such signals with vacuum instrumentation supporting digital communications, eliminating error in the process. As a bonus, the reduction in calibration points reduces time when performing calibrations. Such systems may even automatically compensate thermocouple sensors resolving the sensitivity of thermocouple sensors to multiple gas types.
AMS2769C references other specifications, namely AMS2750 and the Compressed Gas Association (CGA). CGA establishes minimum requirements ensuring inert gas quality. In addition to supplier certification, gas quality is proven by dew point. All gasses have a dew point, with outside air relatively high (e.g., +50°F) and inert gas very low (e.g., -100°F). Purchasing supplier certified gas results in a facilities bulk storage tank having a very low dew point, with any leaks in gas delivery system (pipe threads, fittings, etc.) resulting in a less negative dew point. The concept that dew point can only raise once exiting the storage tank illustrates the importance of sampling “as the gas enters the furnace” — measurements taken upstream fail to detect leaks downstream. The intensity of this increase directly correlates to the amount of air (oxygen) entering the gas supply, compromising the gas purity, which as previously discussed negatively impacts the parts being processed. Proving a dew point below -60°F proves the inert gas mostly free of oxygen. Measurements have long been a manual process; an operator samples gas using a portable sensor and records the findings in an entry log. Modern systems seamlessly integrate dedicated sensors continuously sampling gas quality which alert upon compromised gas.
What We Want
This article’s first draft opened this section listing a handful of features — that was November. Fast forward three calendar months (what feels like an entire year), it is now January, and priorities have changed. Three months ago we wanted features, now we just want parts. The growing supply chain disruption is feeling less temporary and more permanent. This final draft opens with availability. Any upgrades should factor both (1) component lead-time and (2) their flexibility. Lead-time should focus not just on immediate project delivery, but the long-term availability of the product. Is it in its infancy? Or near the end of its life? What is the current lead-time and strategies to maintain inventory? Flexibility should focus on limitations of the product. Is it limited to specific applications? Or can it be used in other equipment? Flexibility paired with planning results in standardization. Keeping with the automobile theme, standardization is what made Henry Ford’s Model T so special. Standardization reduces on-site spare parts, as the same component can be installed in many locations. Standardization should be a primary focus when purchasing programmable logic controllers (PLCs), vacuum instruments, and temperature controllers.
As if the supply chain worries are not enough, the U.S. faces a labor shortage projected to worsen over the next decade. This highlights another late addition to this article, stressing the importance that any upgrade considers the availability of the most important resource: people. New furnaces and upgrades alike (like it or not) develop a co-dependence between multiple parties. This relationship may be internal, between operations and engineering; or external, between an end user and a supplier. No matter the specific situation, all parties should discuss availability and access to information. Failure to discuss this early on is often exacerbated, especially when upgrades are performed by a supplier who is considered (1) unresponsive or slow to respond and (2) unwilling to share information. Purchase orders should document expectations in terms of deliverables (PLC logic, schematics, etc.) and support.
Figure 4. Projected US labor deficit Photo Credit: US Department of Labor
This third paragraph was that ill-fated November draft’s first. Today’s buzzword, the Internet of Things (IoT ). As we are well on our way to the quarter mark of the 21st century, we have all become accustomed to a lot of quickly accessible information. Why should vacuum furnace recorders not meet the same lofty expectations? Control system upgrades should be capable of recording information and displaying it in an easily retrievable format. Recorded data should expand beyond the required process data into the status of the furnace itself (valve position, state of limit/thermal/vacuum switches, motor status, etc.). Such data can be evaluated postmortem to troubleshoot a failed production run’s root cause of the failure. Advanced systems should be able to notify personnel of issues via email or text messaging.
Often the information gathered above is passed into a Supervisory Control and Data Acquisition (SCADA) System. This system must meet industry compliance for data integrity and security. As every new software seems to have its own system, daily operation requires most to juggle many of these systems, often sharing common data. A SCADA System should be designed to operate in this unknown environment and be capable of sharing data between itself and Enterprise Resource Planning (ERP) and other supervisory systems. The first step here is to build upon common platforms; and today the most widely accepted platform is Microsoft SQL Server. SCADA Systems should be able to “offer up” data using any number of industry standard protocols (Modbus, API, OPC, etc.).
The biggest invisible threat to our industry is internet security. For those fortunate enough to have avoided a cybersecurity attack, IT’s work seems a burden. For those unfortunate to have experienced such an event, IT’s work is beloved. This rapidly changing frontier is our reality and programs like Cybersecurity Maturity Model Certification (CMMC) become a necessary (even required) precaution. Hardware for upgrades should be vetted for compliance with these evolving precautions.
Thus far this article has focused on people, hardware, and features. I now turn the focus to the vacuum furnace itself. Furnaces routinely struggle with passing TUS at both lower (<1000°F) and elevated (>2000°F) temperatures. The issue itself varies between graphite and molybdenum hot zones but the root cause remains the same: inflexibility with rheostats to adjust across a wide temperature range or the furnace’s incapability of reaching elevated temperatures. Users manually adjust the applied power to each zone in attempt to minimize the difference between the coldest and hottest TUS thermocouples. Rheostats force the user to settle for a configuration “just good enough” for all temperatures but “not perfect for any.” Modern systems replace rheostats with individual silicon controlled rectifiers (SCRs) driving each variable reactance transformer (VRT), a feature commonly called digital trim. All furnaces are candidates for digital trim, though older VRT packages using slide wire (or “corn cob”) resistors may require the addition of direct current (DC) rectifiers in addition to SCRs. The benefit of digital trim is these settings can automatically adjust with temperature allowing for the ideal configuration at every temperature.
How Do We Get There?
Resurrecting the automobile analogy which opened this article, have you ever wondered why so many people love Jeep Wranglers (and I realize Jeep could easily have been Harley Davidson or a new home purchase)? The reason is not what they are, rather what they can become. Owners see upgrades and features in their mind long before anything is modified. The key concept here is customization. This same vision applies to vacuum furnaces, any upgrades should consider robust and powerful control systems, flexible enough to evolve with the industry.
PLCs and process instrumentation should always be sourced with room to grow. Modular designed platforms easily expand to integrate new hardware. Ask suppliers how their hardware handles additional inputs, outputs, and sensors. Instrumentation should be integration-friendly and be capable of monitoring the entire vacuum ecosystem — considering the temperature, load thermocouples, and vacuum and gas control systems. Ideally, instrumentation will communicate with each other, passing relevant information between each while simultaneously eliminating calibration points.
Control systems should be sourced with an Evolution Plan in place; compliant solutions today in no way assure compliance tomorrow. Suppliers should be asked their plan for AMS2750G, H, and I. Doing so positions you to make large investments once, then grow hand-in-hand with the industry rather than fight it every time it changes.
Summary: Have a Plan
Modern controllers consolidate a furnace’s self-contained subsystems (vacuum, load thermocouples, valve control, etc.) into a singular control system. This provides the transparency necessary for the controller to alert operators or place itself on hold when necessary. The outcome is that operators require less time monitoring the subtleties of production, meaning they focus their time on more urgent tasks. A happy byproduct becomes the natural progression of data (the recorded values from all these subsystems) into information (meaningful, document values presentable to customers, reviewable by auditors, or referenced for troubleshooting).
I was once told to either open or conclude an article with a poignant quote, so let me offer this advice: When considering upgrades for any furnace “have a plan or become part of someone else’s.” Early conversations between engineers/suppliers and quality/production ensures the delivered product shares everyone’s goals.
About the Author
Steven Christopher is a Purdue University engineering graduate and a 15-year veteran of the heat treating industry. He began his career in pharmaceutical maintenance before joining a commercial heat treat facility focusing on the automotive and aerospace industries. He now manages Super Systems' West Coast operations supporting all types of industries west of the Rocky Mountains.
For more information, contact Steven at schristopher@supersystems.com
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Are you sure you should vacuum braze that? As the title of this best of the web article suggests, vacuum brazing materials containing zinc is not a good idea. Volatized zinc can contaminate, and maybe even ruin, your vacuum furnace. But what about cadmium, lead, chromium, and magnesium? Is vacuum brazing safe for those materials?
In this article by Dan Kay, examine the vapor pressure curves of common metallic elements to be sure you know exactly when you need to worry about vaporization. And remember, operating your furnace at partial pressure does not offset the effects of vaporization.
An excerpt:
Many people braze stainless steels (which contains chromium) at vacuum levels approaching 10-5 Torr [. . . ] You can readily see that at 10-5 Torr the temperature at which Cr volatilizes has dropped down to only about 1800F (950°C). Since nickel-brazing of stainless typically takes place at about 2000-2100°F (1095-1150°C), please understand that you will indeed be volatilizing chromium during this brazing operation, which will condense on the furnace walls, giving them a greenish/bluish coloration.
When "the die is cast," heat treaters should make sure that they're using NADCA 207 standards. Prepared by the North American Die Casting Association (NADCA) for its members, they provide recommendations on how to produce dies for die casting to optimize thermal tool life in terms of thermal fatigue.
In today's best of the web article, check out what some of the essential requirements are and how this standard could help in "maximizing the resistance of tools to the occurrence of cracks from thermal fatigue."
An excerpt:
However, the content of this specification is so well processed that it is valid not only for the production of die casting dies and for thermal fatigue, but also for many other applications, and is the best information material for commercial vacuum heat treatment plants, tool shops and die casting foundries, enabling the elimination of fundamental errors in the tool making process.
Need a refresher course on the "gas laws" and how they relate to heat treating? What exactly is going on at a molecular level in your vacuum furnace? This best of the web article gives a helpful review of the theory of gases and practical tips to make your heat treating experience easier.
An excerpt:
"The movement of gases is an important and interesting subject but one often dismissed as a topic best left to scientists. However, the Heat Treater needs to know something about the basic nature (theory) of gases and in particular how they behave in vacuum. The main difficulty is that too much theory tends to become a distraction. Our focus here will be to better understand what goes on inside a vacuum furnace."
Source: VAC AERO International, Inc.
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There is an age-old adage that exists in the heat treating world. That supposition states that “the smaller the vacuum furnace, the faster it will quench.” Is this adage true? Explore Solar Atmospheres’ journey as they designed an experiment to discover if pressure or velocity most affects cooling performance.
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