Heat treating solutions are important for more than keeping an airplane flying in the sky or a bridge suspended above the water. These two examples are high profile, but what about the heat treating solutions that do not zoom through the air or mark the skyline above rivers? In the medical industry, heat treating solutions are often unseen unless something goes wrong.
When it comes to medical implant and device heat treating, what options are available to manufacturers that will benefit patients? What should we know about the heat treating processes that make metal parts functional as knees, hips, and elbows? Find out in this expert analysis from Quintus Technologies and ECM USA, Inc.
This Technical Tuesday article was first published in Heat Treat Today's December 2022 Medical and Energy print edition.
Introduction
Dan McCurdy, former president at Bodycote, Automotive and General Industrial Heat Treatment for North America and Asia, knows full well just how much time, energy, and pain the right medical heat treating practice and alloy composition can save a patient. Dan’s wife suffered from complications due to a nickel allergy in a traditionally thermally-processed ASTM F75 knee implant. She dealt with constant inflammation, swelling, and pain. Physical therapy and a second procedure did nothing to ease the discomfort. The best medicine for Dan’s wife? A specially heat treated medical implant (more of Dan's story can be found at the end of this article).
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To understand the stories behind final medical products, Heat Treat Today asked Quintus Technologies and ECM USA, Inc. to share two different approaches on medical implant and device heat treatment. These two companies at the forefront of the medical heat treating industry shared about hot isostatic pressing (HIP) with additive manufacturing, and vacuum heat treating. Read their answers to our questions and learn how, when it comes to implantable medical devices, heat treating can be the best medicine.
How do you ensure your equipment maintains the precise specifications required in the medical industry? What specifically is necessary to maintain compliance when it comes to medical implants?
Quintus Technologies
Chad Beamer Applications Engineer Quintus Technologies
Quintus Technologies has observed a trend in bringing Nadcap to the medical industry. Historically the medical industry has focused on the standards and regulations for the quality management system of their approved supplier, but a consistent transition to technical aspects of critical processes (including HIPing) is becoming the norm. Quintus Technologies’ background is one of delivering HIP equipment in line with Nadcap and AMS2750 specifications. The medical industry requires best-in-class temperature uniformity and accuracy; systems designed with production driven flexibility (such as thermocouple quick-connectors for T/C sensor installation
to minimize downtime); HIP furnaces equipped with uniform rapid cooling (URC®) for optimized cycle productivity; active involvement in standards committees; and working directly with the industry.
Requirements are increasing in terms of productivity and the introduction of more complex surface requirements. It is crucial to work closely with the industry to reduce oxidation of orthopedic implants during the HIP and heat treatment processes.
Steering of the HIP cycle is key, along with in-HIP heat treatments to achieve the desired microstructure for the application, which is a standard offering for High Pressure Heat Treatment™ (HPHT™) equipment.
ECM USA, Inc.
Dennis Beauchesne General Manager ECM USA, Inc.
Some of the features that are most important are leak rate at deep vacuum along with a chamber and furnace design that does not contribute to any contamination. In our systems, these features, along with others, are of the utmost importance when supplying equipment for the medical implant market.
What are the top 3–5 key requirements or compliance/quality issues needed to heat treat medical implants?
Quintus Technologies
There are several industry standards that have been released to establish key requirements for the HIP process that are often leveraged for medical applications demanding performance and reliability. For example, Nadcap has released AC 7102/6 which details the audit criteria for HIP. This document was developed with significant input from the industry and the government to define operational requirements for quality assurance. It offers a checklist for the HIP processing of metal products and includes requirements for:
managing the equipment per pyrometry standard AMS2750
qualifying technical instructions and personnel training
handling product during the loading and unloading operations
complying with gas purity requirements of the pressure medium
controlling temperature, including uniformity and accuracy evaluations and management
These aspects are critical to ensure product quality meeting medical customer requirements and expectations. Recent additions beyond conventional requirements highlighted above include high speed cooling in the HIP process (>200 K/min) for some materials which is important for metallurgical results.
ECM USA, Inc.
Key requirements include thermal performance (both uniformity and ramp control); real-time vacuum and gas management; traceability and production lot follow up through human machine interface (HMI); quality procedures for all sensor calibrations; and remote access for control and troubleshooting.
Can you share an example of how your equipment could be used to heat treat a medical implant/device from start to finish?
Quintus Technologies
Many medical implants — whether fabricated using conventional processing techniques such as casting, or more novel approaches such as additive manufacturing — require HIP to eliminate process related material defects. Defects include shrinkage porosity for castings and lack-of-fusion and keyhole defects for fusion based additive manufacturing techniques. These defects can have a negative impact on product quality, impacting performance and reliability. Once HIP has been applied to a material, post processing is often not complete, with additional thermal treatments required to achieve the optimum microstructure leading to the desired material properties and performance. Such thermal treatments are material and process dependent, but could include a stress relief, solution anneal, rapid cooling or quenching, and aging and are often applied in separate heat treat equipment.
Hot Isostatic Press QIH 60 offering our most advanced Uniform Rapid Cooling (URC®) furnace technology with industry leading temperature control and accuracy
Quintus Technologies has introduced HIP systems providing capabilities beyond conventional densification. Decades’ worth of work in equipment design, system functionality, and control now offers an opportunity to perform HIP and heat treatment in a combined cycle, referred to as HPHT. Combined HIP and heat treatment for castings and AM implants can mitigate the risk of thermally induced porosity, as well as grain growth, which can offer advantages for mechanical and chemical properties in implants. This methodology provides a more sustainable processing route with improved productivity and energy efficiency. A joint HIP and heat treatment offers significant advantages with lead time, and this improvement in lead time couples well with the demands placed on the personalized medical implants. It also offers opportunities to further optimize microstructures for improvement in material properties coupled with ease of manufacturability. HPHT and modern HIP equipment may allow for a higher performing material system, which produces an implant with improved reliability and life.
Within the medical industry, fine grain AM microstructure, repeatability, and low porosity are key concerns. There are many reported benefits by applying the combined HPHT route such as reduced number of process steps, reduced cycle time and lead time, and improved process and quality control. Other advantages include spending less time at elevated temperatures helping to preserve the fine grain AM microstructure by minimizing grain growth. Tight control and steering of the cooling rates during the different steps of the HPHT cycle ensures repeatability of the properties. Manufacturability can be improved through HPHT as this approach reduces the cooling or quench severity during cooling segments which can often lead to part distortion or cracking. Improved functionality and
control go hand-in-hand with the high quality and reliability demanded in the medical industry.
ECM USA, Inc.
We have several customers making titanium alloy prothesis for various applications: shoulders, hips. Our furnaces are used for post printing processes, such as stress relieving and solution annealing.
Given concerns of metal poisoning, do you know of any changes in alloy composition of medical devices over the last decade?
Quintus Technologies
There are some metals that are becoming more common for implants, including tantalum, magnesium, CP Titanium, etc., and there have been major steps in improving ceramic materials to compete with metals for many applications.
ECM USA, Inc.
As a vacuum furnace equipment supplier, we are not deeply involved in the entire process of material selection. In the early stages of 3D printing joint replacements, from 2013 to 2014, we saw cobalt being part of some alloys. Lately it seems, indeed, that there is a trend in removing that element from the finished parts.
A Happy Ending
Dan McCurdy Former president, Bodycote, Automotive and General Industrial Heat Treatment for North America and Asia
(The rest of Dan's story from the beginning of the article....) The effects of metal poisoning and metal allergies post-surgery can be devastating. In the narrative below, Dan McCurdy shares the story of his wife’s struggle with an allergic reaction to a knee implant, and the heat treating solution that proved to be the best medicine for her.
My wife, an avid runner up and down the hills of Cincinnati, was diagnosed with osteoarthritis in both knees at the age of 53. Her orthopedist suggested a knee replacement for the most degraded one. The replacement was a well-known brand, made from investment-cast ASTM F75 (nominally a Co-Cr-Mo alloy) with full FDA-approval. After a successful surgery and diligent physical therapy, her recovery plateaued, and she experienced chronic inflammation, swelling, and pain.
A blood test, designed to detect allergies to materials used in orthopedic implants, showed a reaction to nickel that was nearly off the charts. We were surprised, as she had previously tested negative for nickel allergies through skin patch testing. The ASTM F75 specification allows for up to 0.5% bulk nickel as a tramp element in implantable devices; however, depending on foundry practices, the concentration of tramp alloys at any point on the surface of a casting can vary significantly. Titanium implants may be the solution to this, but FDA-approved titanium alloys can still contain up to 0.1% Ni.
The solution for my wife, as it turned out, was a different material, originally developed for the nuclear industry, along with an innovative heat treatment process. Created with an alloy of zirconium and niobium (with a maximum nickel content of 0.0035%), her new knee was heat treated at a high temperature in an oxidizing environment, which converts the soft zirconium surface into hard ceramic zirconia, increasing hardness and wear resistance. With this specially heat treated implant in place, my wife is back to nearly 10K steps a day.
References
[1] Magnus Ahlfors and Chad Beamer. “Hot Isostatic Pressing for Orthopedic Implants.” quintustechnologies.com/knowledge-center/hot-isostatic-pressing-for-orthopedic-implants. Quintus Technologies. 2020.
[2] Chad Beamer and Derek Denlinger. “Hot Isostatic Pressing: A Seasoned Player with New Technologies in Heat Treatment — Expert Analysis.” www.heattreattoday.com/processes/hot-isostatic-pressing/hot-isostatic-pressing-technical-content/hot-isostatic-pressing-a-seasoned-player-with-new-technologies-in-heat-treatment-expert-analysis/. Heat Treat Today. 2020.
ThermTech, heat treat service provider in Waukesha, WI, has increased their capabilities to provide services for the medical, aerospace, mining and oil, nuclear, and agricultural industries.
Jason Kupkovits, vice president of Sales & Strategic Direction at the company, commented on that ThermTech will be continuing their 40 years of quality assurance, turnaround time, on-site engineering, and customer service standards.
Ben Gasbarre Executive Vice President of Sales Gasbarre Thermal Processing Systems
Partnering with Gasbarre Thermal Processing Systems, ThermTech significantly increased their normalizing, annealing, stress relieving, tempering, and neutral hardening capacity through the acquisition of three new furnaces. These three furnaces --- now fully operational --- include: a dual zone, direct-fired box austenitizing furnace; a large batch tempering furnace; and an additional tempering furnace. These furnaces are compliant with AMS2750 at different class certifications.
ThermTech has also added two additional vacuum furnaces from Ipsen, USA. The furnaces have dimensions of 36” wide x 36” tall x 48” long with capabilities of quenching up to 6 bars of pressure utilizing nitrogen or argon gas as the quench medium. These large vacuum furnaces are AMS class 3 (+/-15°F) certified capable of AMS2750.
ThermTech added a solution annealing furnace from Williams Industrial Service to give their operational aluminum line additional heat treat capabilities. This line is capable of a sub-15 second transfer to air blast quench, a water quench range of 55°F up to boiling, a sub-7 second transfer to water quench which exceeds AMS 2770/AMS2771 specifications, as well as load thermocouple monitoring during the solution treatment, quenching, and aging.
Daniel Hill, PE Sales Engineer AFC-Holcroft Source: AFC-Holcroft
Another recent acquisition includes a new austempering/marquenching furnace from Michigan based AFC-Holcroft. This furnace can handle a single part racked in the vertical orientation up to 56" long. The working dimension of the furnace is 36" W x 72" L x 56" H and is capable of operating with salt temperatures ranging from 350°F -- 750°F. "The UBQA system is an environmentally friendly ‘green technology,’" commented Dan Hill, sales engineer at AFC-Holcroft, "which can be used to impart resistance to distorting, cracking or warping of heat-treated components.” Applicable processes include marquenching, austempering, and carburizing with additional washing and tempering capacity accompanying the new marquenching/austempering furnace. Installation is expected in early 2023.
The heat treat service provider's long-term strategy is to increase growth in the Midwest and on a national scale. This includes adding more workers and integrating the use of a robotics handling systems, which is expected to be installed in late 2022.
Find heat treating products and services when you search on Heat Treat Buyers Guide.com
A firearms manufacturer based in the U.S. has ordered a vacuum heat treating furnace from a Pennsylvania manufacturer.
Solar Manufacturing Inc. announced the receipt of the heat treat furnace, a model HFL-5748-2IQ, which has a hot zone of 36” x 36” x 48” deep with a weight capacity of 5,000 lbs. Its maximum operating temperature is 2400°F and it heats to 2500°F for hot zone bake out. The furnace design also has a temperature uniformity of ±10°F and is AMS2750 compliant with vacuum levels in the low micron range.
For rapid turnaround for work cooling, a 100 HP gas blower is provided for operating at 15 PSIG (2-bar) in nitrogen gas. The furnace is complete with a SolarVac® Polaris fully automated and programmable industrial controls package, and a Eurotherm digital chart recorder.
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.
AMS2750F? What are the new changes? How do you implement them? This informative article from Heat TreatToday's Aerospace 2021 issue will help you navigate through the uncertainty of these changes to ensure successful compliance.
This Technical Tuesday is an original content contribution from Jason Schulze, the director of technical services at Conrad Kacsik Instrument Systems, Inc. Check out other technical articles here.
Jason Schulze Director of Technical Services Conrad Kacsik Instrument Systems, Inc.
Introduction
AMS2750F has been released for approximately 7 months now. This specification applies to manufacturers and suppliers who heat treat aerospace material. AMS2750F is typically communicated via industry standards such as SAE/AMS specifications as well as customer purchase orders and part prints. This specification gets even more complex when you apply Nadcap heat treat accreditation to the equation as Nadcap has a checklist dedicated to AMS2750, which, as of January 2021, has yet to be released.
In this article we will examine some of the changes within AMS2750F as well as discuss the implementation process for suppliers.
AMS2750F Changes
General Changes
AMS2750F now has 25 tables, where there were previously 11. These tables are no longer at the end of the specification (like most SAE/AMS specifications); they are now placed throughout the specification adjacent to paragraphs to which the rewrite team thought they applied. The challenge with this is that all aspects of AMS2750 are interconnected. For example, one change in the qualified operating range of a furnace will directly affect other areas, such as instrument calibration and the temperature at which an SAT is performed.
Previously, temperature values were expressed in whole numbers. They are now expressed to the tenth of a degree (X.X°F). With this change, I would recommend suppliers follow suit in their own pyrometry procedures and associated documents: think of it as comparing apples to apples.
Scope and Definitions
The definitions section is important, especially to those who are new to AMS2750F who may be working to interpret some of the verbiage within the specification. The specification has increased the number of definitions from 79 to 87. A good example of these definition changes is the comparison of expendable thermocouples versus nonexpendable thermocouples.
EXPENDABLE SENSOR: Sensors where any portion of the thermal elements are exposed to the thermal process equipment environment.
NONEXPENDABLE SENSOR: Sensors having no portion of the thermal elements exposed to the thermal process equipment environment.
This example is especially important because it is such a major change from the previous revision of AMS2750. The definitions section within AMS2750F should be utilized often by suppliers to ensure comprehension and conformance.
Thermocouples
As simple as thermocouple technology is, there are many requirements within AMS2750F governing thermocouple usage, error, replacement, etc. Previously, AMS2750 did not address resistance temperature devices (RTDs). It now requires RTDs be nonexpendable, noble metal, and ASTM E1137 or IEC60751 (Grade A).
I do not see this next change as anything major, because what I’m witnessing in my consulting all around the US and Mexico are that suppliers already conform. Thermocouple hot junctions (the tips of the thermocouple measuring temperature) are made by either twisting, welding, or a combination of both.
In my experience, it is rare to see a thermocouple supplier/manufacturer issue a thermocouple certification that is nonconforming. Whenever there are issues with thermocouples, it is typically because the supplier did not communicate the correct information. With that, thermocouple error should be considered and communicated correctly.
Thermocouple permitted error has changed to the following:
Type R & S: ±1.0°F or ±0.1%
Type B: ±1.0°F or ±0.25%
Base metal: ±2.0°F or ±0.4%
AMS2750E permitted ±4.0°F or 0.75% for TUS, load and furnace thermocouples.
Exceptions:
Note 11: For temperatures <32°F or <0°C for Types E and T only, calibration accuracy shall meet the following:
Type E: -328 to 32°F, ±3.0°F or ±1.0 % for either, whichever is greater
Type T: -328 to 32°F, ±1.8°F or ±1.5 % for either, whichever is greater
Note 13: When correction factors are used, type B load sensors shall meet calibration accuracy of ±2.7°F or ±0.5% and types R and S load sensors shall meet calibration accuracy of ±2.7°F or ±0.25%
AMS2750 has always required that the results of an SAT and TUS must reflect corrected temperatures. This would mean when expressing the final ± readings of a TUS, those readings must be identified as corrected values. The challenge may come when you need a correction factor from a thermocouple certification where there is not a temperature value for the test. AMS2750F now addresses this situation:
PARA 3.1.4.8 - Interpolation of correction factors between two known calibration points is permitted using the linear method.
PARA 3.1.4.9 - Alternatively, the correction factor of the nearest calibration point shall be used.
PARA 3.1.4.10 - Whichever method is used shall be defined and applied consistently.
Each supplier must decide what method they will utilize and document this. Know your customer requirements; some customers may not permit certain methods.
Sensor usage has changed dramatically, especially for expendable test sensors. These thermocouples are now limited to a single use above 1200°F regardless of the type. Between 500°F and 1200°F, Type K may be used five times or three months, whichever occurs first and for Type N, 10 times or three months, whichever comes first. Below 500°F, Type K may be used for three months with no limit to the number of uses, and Type N may be used for six months, with no limit to the number of uses. I can understand how this may seem like a lot to understand and filter through, but I can assure you, we will get used to it as we did with AMS2750E.
Thermocouple certification requirements have also changed. I do not foresee any issue with this as what is listed is, for the most part, already on existing thermocouple certifications. I would advise suppliers to check the requirement in bullet point “E.” (Figure 1)
Figure 1
Instrumentation
There were several major changes within the instrumentation section. The first one is readability of furnace recording and field test instruments. Previously, readability for all furnace and field test instruments was 1.0°F; it is now 0.1°F, or to the tenth of a degree. Suppliers may find this challenging to meet as not all field test instruments on the market are capable of this. An easy way to test yours is to either source or read the value on your field test instrument at 999°F. Then, increase the temperature to 1000°F.
On some units, when a temperature is reading/sourced below 1000°F, it will show to the tenth of a degree, but when increased above a tenth of a degree, the value in the tenths place will be removed and only whole numbers will be shown. If this is the case, you will need to purchase a new field test instrument which displays values to the tenth of a degree regardless of whether values go above 1000°F.
The second major change is timers or digital clocks on recording devices. This change makes sense, as most thermal cycles used to achieve metallurgical transformation are time-dependent and have specific tolerances that apply. AMS2750F now requires that these timing devices must be accurate to within ±1 minute per hour. There is a caveat which states that as an alternative, suppliers may have a synchronized system linked to NIST via internet system which is verified monthly and will support the ±1 minute per hour requirement. With that, a new paragraph, regarding stopwatch calibration and accuracy requirements, has been inserted adjacent to the recording device timing calibration requirements.
The third change, simple and straight forward, is that the instrument number or furnace number must be stated on the calibration sticker.
Additionally, changes have been made to what is required on an instrument calibration report. (Figure 2)
Figure 2
System Accuracy Testing (SAT)
There are several changes within the SAT section that should get attention. One which may continue to be overlooked is whenever an SAT cannot be performed (not that one fails), but if no product was run and the furnace was locked out, the SAT could be performed with the first production run (AMS2750E, para 3.4.2.4). This is no longer an option. AMS2750F now states that, in this situation, the SAT must be performed prior to putting the furnace back into service (or prior to production).
Furnaces that have multiple qualified operating ranges (i.e., CL2 from 1000°F to 1600°F and CL5 from 1601°F to 2000°F) must have the SAT performed in each range, at least annually. This means that if you typically run production at 1550°F and SATs are run at the same time, at least annually, an SAT must be processed above 1600°F to catch the CL5 range.
The alternate SAT process was the source of much confusion when revision E was released. Previously, single use thermocouples (i.e., load thermocouples) did not require an SAT per AMS2750D, para 3.4.1.2. When AMS2750E introduced the alternate SAT, the wording was so poor it caused suppliers to misunderstand the requirement, and subsequent audits yielded quite a few related findings. I have written previous articles explaining the alternate SAT process in detail, so I will not be going into this topic too deeply. For information, please visit www.heattreattoday.com and search Jason Schulze.
The changes within the alternate SAT section primarily amount to clarification and incorporation of what was previously in Nadcap’s pyrometry reference guide. That being said, there really isn’t much to speak of in this section for existing Nadcap suppliers, but one item to point out is how the wording has changed. Previously, it applied to single use sensors or sensors which were replaced more frequently than the SAT frequency requirement. This has been changed to state that the alternate SAT applies to load sensors used only once. Nadcap heat treat auditor advisory HT-20-010 has clarified this further. If load sensors are used more than once, the alternate SAT does not apply, and the comparison SAT must be used.
There were some minor changes to what is required on the comparison SAT report. (Figure 3)
Figure 3
Documentation related to the alternate SAT as well as the SAT waiver have been introduced. These should be examined closely by those suppliers to whom it may apply.
Temperature Uniformity Surveys (TUS)
Among many of the changes in this section, there is one that is not stated outright but is based on verbiage changes within Tables 18 and 19 of revision F regarding frequency. In AMS2750E, Tables 8 and 9, the statement reads “Initial TUS Interval” and “Extended Periodic TUS Interval.” Due to the wording, it was assumed that if four passing consecutive TUSs were needed before going to a reduced frequency, the initial TUS would count as part of the four needed. The modified wording in Tables 18 and 19 of AMS2750F now reads “Normal Periodic Test Interval” and “Extended Periodic Test Interval.” With this change in verbiage, the initial TUS does not count toward the needed consecutive tests to reduce TUS frequencies.
If a supplier uses vacuum furnaces for thermal processes and both partial pressure and low vacuum is used, a TUS must be performed annually in the partial pressure range using the gas applied during production. This is a rather simple change, although it is important to recognize that partial pressure gases, depending on certain variables, can affect the uniformity in the area in which the gas enters the furnace.
Thermocouple location for work zone volumes less than three cubic feet has changed. AMS2750E/Nadcap previously required that the five TUS thermocouples be placed on a single plane. AMS2750F has revised this to require each test thermocouple be placed diagonally opposite of each other. Using Figure 4, this could mean suppliers may choose locations 1, 4, 5, 7, and 8 or 2, 3, 5, 6, and 9.
Suppliers familiar with GE’s P10TF3 specification will recognize this next change as it was a GE requirement long before SAE/AMS introduced it into AMS2750F. Previously, data collection during TUSs needed to start prior to the first furnace or test thermocouple reaches the lower end of the tolerance (AMS2750E, para 3.5.13.3.1). This has changed and now requires data collection to begin when the furnace and TUS thermocouples are no fewer than 100°F below the survey temperature.
The documentation or TUS certification requirements have also changed. Considering that there are so many changes within this section, I will merely point out the letter annotations that apply to changes within Para 3.5.16.1: B, D, F, G, H, J, L, O, R, S, and Y. Some of these items contain simple verbiage changes, although most of them are solid changes and should be incorporated into suppliers’ procedures and forms.
Figure 4
Rounding Requirements
Previously, AMS2750E permitted rounding in accordance with ASTM E29. To the delight of many users, I am sure, this has changed. AMS2750F now permits rounding in accordance with the following options:
All rounding must be applied in accordance with a documented procedure and used in a consistent manner.
Rounding to the number of significant digits imposed by the requirement is permitted in accordance with ASTM E29 using the absolute method or other equivalent international standards. (Previously, the only method permitted was the rounding method.)
The rounding method built into commercial spreadsheet programs is also acceptable.
All specified limits in this specification are absolute and out of tolerance test data cannot be rounded into tolerance.
Rounding must only be applied to the final calibration or test result.
Quality Provisions
The only change in this section is in regard to pyrometry service providers. The requirement now states, “Beginning 2 years after the release of this specification, third-party pyrometry service provider companies shall have a quality system accredited to ISO/IEC 17025 from an ILAC (International Laboratory Accreditation Cooperation) recognized regional cooperation body. The scope of accreditation shall include the laboratory standards and/or field service as applicable.” It is important to keep in mind that, when verifying conformance to this, the supplier’s scope of accreditation should include reference to AMS2750 with regards to instrument calibration, SAT, or TUS or all three if that is what is performed at your facility by an outside service provider.
Implementation of AMS2750F
he implementation of AMS2750F with suppliers’ systems should be two-fold: not only what is implemented but when it is implemented. Right now, AC7102/8 Rev A, as it applies to AMS2750F, is in the review stage. Its projected release date is April 2021. Regardless, once the new revision of AC7102/8 is released, suppliers will have 90 days to implement AMS2750F.
Implementing AMS2750F must be done in its entirety, not partially. This means internal procedures, forms, purchase orders, etc. should be revised in the background in conjunction with training. Once your team is familiar with the new changes, then all the revised documents should be released at one time. This ensures the whole of AMS2750F is implemented at once and not in stages.
Nadcap heat treat auditor advisory HT-20-007 requires that all thermocouples issued on or after Jan. 1, 2021 must be certified in accordance with AMS2750F. By this time, suppliers should have already revised purchase orders to require this and may have thermocouple certifications reflecting AMS2750F.
About the Author: Jason Schulze is the director of technical services at Conrad Kacsik Instrument Systems, Inc. As a metallurgical engineer with over 20 years in aerospace, he assists potential and existing Nadcap suppliers in conformance as well as metallurgical consulting. He is contracted by eQualearn to teach multiple PRI courses, including pyrometry, RCCA, and Checklists Review for heat treat.
This brief original content column by Heat Treat Today’s publisher, Doug Glenn, is from the most recent print magazine,Air and Atmosphere 2021. Are standardization and innovation in competition with one another, or do they assist each other? Which one is better to have? Read this article weighing the economics, business, and cultural realities of both.
Doug Glenn Publisher and Founder Heat Treat Today
In the heat treat industry, I wonder what effect standardization has had on innovation. This is a somewhat loaded question given the number of companies in the North American heat treat industry that are invested in industry standards such as AMS2750, CQI-9, and a large alphabet soup bowl of other standards. I’d like to hear your specific stories about how standardization has been helpful or harmful. Maybe Heat Treat Today can do a future article on the topic if we get enough responses. But in lieu of those real-life anecdotes, let’s think for a moment about the relationship between innovation and standardization.
First, I think that nearly everyone would agree that innovation is a good thing and should be encouraged. Many of today’s conveniences are the result of yesterday’s innovations. Certainly, not EVERY innovation is good, but encouraging a company, economy, or culture of innovation is far and away preferred to the absence of innovation.
Second, we should also acknowledge the benefits of standardization. Repeatability is the hallmark of high production societies. Knowing that you’re always going to get the same burger at any McDonald’s across the country is a huge selling point for that fast food giant. And when it comes to mission-critical or life-critical goods or services, who would not want the assurance that “past performance is a good indicator of future results.” I prefer my heart surgeon to do the same thing every time!
Third, let’s be clear that standardization and innovation are, by nature, mortal enemies in the sense that each tends to destroy the other. An atmosphere of standardization, where everything is always done the same – over and over again – is antithetical to shaking things up and trying new and sometimes odd things. Likewise, an atmosphere of innovation, cuts directly across the same sameness of standardization. If you do it differently one time, standardization is destroyed.
There is wonderfully simple and brilliant book written by the towering mind of Ludwig von Mises called Bureaucracy which contrasts bureaucratic organizations with profit-driven organizations. I recommend it highly (search Bureaucracy, von Mises) and it has something to say about the differences between bureaucratic organizations, which are highly standardized by nature, as well as being profit-driven organizations that tend to be less standardized and more innovative. One of his points is that there is a place for both in the world. The military, for example, is not a good place for question-asking and innovation, especially in the midst of a battle. In a military setting, do what you’re told without question and don’t deviate/innovate. In a profit- driven business, however, this same mindset is not so healthy – take for example the postal system or another bureaucratic organization where responsiveness to customer needs is not highly valued.
Some may say that there is a standardized process for being innovative. Could be.
Where’s the balance and how do we know if/when we’ve gone too far in either direction?
I’d be interested to hear your heat treat stories of when and why standardization or innovation is good, and especially how these two live comfortably together.
Considerable investment is made when purchasing a batch integral quench (BIQ) furnace. These popular furnaces need specific care and maintenance to keep them in prime operating condition. In this informative article by Ben Gasbarre, president of Industrial Furnace Systems at Gasbarre Thermal Processing Systems, learn how you can protect your BIQ from avoidable downtime.
This original content article appears in Heat TreatToday’s Air and Atmosphere’s February 2021 magazine. When the print edition is distributed, the full magazine will be accessible here.
Ben Gasbarre President, Industrial Furnace Systems Gasbarre Thermal Processing Systems
The batch integral quench furnace, or sealed quench furnace, is one of the most popular pieces of equipment in the heat treating industry. The core benefit is its versatility as it can easily adjust to changes in load weight, configurations, and heat treating processes. This makes
it a highly efficient and profitable piece of equipment for both captive and commercial heat treaters.
With all the good that is done in these furnaces, the downside comes in the maintenance of the equipment. By nature, these furnaces are hot, dirty, and have many moving parts, including multiple doors, load handlers, elevators, fans, quench agitators, and pumping systems; this furnace has it all! Although there are many areas of an integral quench furnace, understanding the subassemblies and having a good maintenance program can ensure the equipment operates safely and maintains its highest level of performance year after year.
Maintenance Safety
The discussion on maintenance of any piece of equipment begins and ends with safety. Prior to any work being done on the equipment, safety measures need to be considered based on the work being performed. Certain maintenance activities must be completed while the equipment is in operation; in these cases, proper personal protective equipment must be considered for work being done around hot surfaces, high voltages, elevated work, and potentially hazardous gases. If work is necessary while the equipment is offline, additional safety procedures must be followed, including lockout/tagout of all major power sources, special atmospheres, and natural gas supplies to the furnace.
Integral quench furnaces are considered confined spaces. Prior to entry into the quench vestibule, furnace chamber, and even quench pit, confined space procedures must be followed; hard stops must be in place for doors and elevators. Technicians need to ensure proper oxygen levels and air circulation prior to entry. The buddy system is always recommended when someone is entering the furnace. Prior to returning the furnace to operation, it is important to ensure all necessary safety and maintenance equipment has been removed, all supply lines are receiving designed gas pressures, and proper startup procedures are followed.
For furnace safety during shut down periods, it is wise to review furnace interlock systems and safeties to ensure proper operation. This includes items such as high-limit controllers, solenoid valves, burn off pilots, and other components critical to emergency situations. Additionally, per NFPA 86 requirements, valves and piping should be leak-checked periodically.
Reporting and Metrics for Optimum Performance
Image Source: Gasbarre Thermal Processing Systems
While Industry 4.0 is a popular concept in today’s manufacturing environment, the basic concepts behind the technology are what is important to any good maintenance plan. First, having an asset management system that enables engineers, operations, and maintenance personnel to access maintenance records is critical to ensure they can troubleshoot issues and perform maintenance activities more efficiently. Asset management tools are readily available and can range from well-established cloud-based software systems to simple Excel spreadsheet records. Ensuring important information, such as alloy replacements, burner tuning, or control calibration information, can help operations and maintenance personnel as they plan and assess future equipment needs.
The second concept is preventive or predictive maintenance plans. While these are not interchangeable concepts, the goal of implementing either is to reduce the likelihood of significant unplanned downtime, which can be costly to an organization. Preventive maintenance is a schedule of planned maintenance activities on a piece of equipment using best practices that give the best chance to catch a problem before it arises.
Predictive maintenance uses data and analytics from equipment operations that can be used to predict when problems are likely to occur. There are considerations for either approach, and the evaluation criteria for preventive versus predictive maintenance plans could be an article in and of itself.
Integral Quench Furnace Maintenance
As stated previously, breaking the furnace down into a series of subassemblies is the easiest way to develop an overall maintenance plan for equipment that has many sections and components. Discussed items will include mechanical assemblies, the heating system, the filtration system, atmosphere controls, temperature controls, and furnace seals. Each has its own importance to ensuring reliable equipment performance.
Mechanical Assemblies
Typical load transfer system alignment.
The mechanical system includes the load transfer system, recirculation fans, quench agitators, door assemblies, and elevator system. There are many exterior items that can cause abnormal equipment operation, including position sensors, rotary cam switches or encoders, and proximity switches, that if not operating properly can interrupt or cause failure within the furnace. Position settings should be logged for future reference, and sensors should be inspected regularly. Belts that may be used on recirculation fans and quench agitators should be inspected regularly for damage and excessive wear. Vibration of these items should be monitored as excess vibration can be an indication of damage or wear to the fan or agitator bearings, shaft, or blades.
The largest item of concern in this system is the alignment of the load transfer system. Unsuccessful load transfer due to misalignment or obstruction can cause significant furnace damage and create unsafe conditions within the furnace. Internal alloy components should be evaluated for integrity and alignment every six to twelve months. Elevator alignment should be reviewed to ensure smooth operation during the same period. Frequent visual inspection through sight glasses, quench time monitoring, and motor load data can give valuable information of future potential transfer issues within the furnace.
Heating Systems
Whether your furnace is gas or electrically heated, well-maintained systems can have significant impact on the operating efficiency of a furnace. For gas-heated systems, proper burner tuning and combustion blower filter cleaning can ensure optimum gas usage and can also improve radiant tube life. Burners, pilots, and flame curtains should be cleaned at least once or twice a year to ensure proper performance.
Electrically heated systems typically require less general maintenance and have fewer components that are susceptible to failure. Regular checks of heating element connections and electrical current resistance can help to identify upcoming element failure.
The largest and most critical components of reliable process performance are the radiant tubes. A crack or leak in a radiant tube can cause part quality issues. Changes in your furnace atmosphere gas consumption or troubles from controlling carbon potential can be signs of tube leaks. If the radiant tube failure is unexpected, it can also cause significant downtime if replacement tubes are not available. Cycle logs and run hour timers are the best metrics for preventive or predictive maintenance on radiant tubes.
Filtration Systems
Filtration systems are recommended for most integral quench applications. They help to eliminate build up and contamination in the oil recirculation system that flows through the heat exchanger and top/atmosphere cooler on the furnace quench vestibule. Filtration systems typically are comprised of a pump, dual filters, and an alarm system to alert users when it is time to change filters. Maintenance on your quench oil can vary by composition. Quarterly analysis of the quench oil performance is common. However, it is recommended to consult with your quench oil supplier to ensure safe and effective performance.
Atmosphere Controls
Integral quench furnace atmosphere systems can vary both by manufacturer and in overall gas composition. The most common being endothermic gas, nitrogen/methanol, along with options for ammonia or other process gases. Although these items may vary, maintenance remains consistent. Users need to ensure the integrity of the piping system including regulators, solenoid valves, and safety switches.
Endothermic gas lines should be cleaned out at least once or twice a year. Many furnace atmosphere problems can be traced back to endothermic gas generator issues, so it is important to have a well-maintained atmosphere generator to ensure peak performance in your integral quench furnace.
Typical integral quench furnace atmosphere system.
Recent technology allows for automatic burn-off of carbon probes and automated atmosphere sampling. However, probes should be burned off once per week if they are manual. Probes will require calibration and periodic replacement, and they can be rebuilt to like-new specifications. Controllers or gas analyzers that support carbon potential control should be calibrated quarterly, biannually, or annually depending on heat treat specification requirements.
Updates in the automotive CQI-9 specification will require calibration of all atmosphere flowmeters on a periodic basis. Users will need to be aware of this requirement and understand how their gas flowmeters should be calibrated. In some cases, control upgrades may be required.
Temperature Controls
Temperature control maintenance typically follows AMS2750 or CQI-9 specifications. This would relate to thermocouple replacement, system accuracy test procedures, and controller calibrations. Depending on the age of the equipment and specification requirement, these items may need to be done as frequently as once per quarter or annually.
Temperature uniformity surveys (TUS) follow similar specifications for frequency. However, a TUS can diagnose areas of the furnace that may need maintenance attention. Having a baseline TUS to reference will help identify changes in furnace performance. Changes to a TUS can indicate burner or element tuning requirements, an inner door leak, refractory damage, fan wear, or radiant tube failure.
Furnace Seals
Integral quench furnace seals can be a source of heartache for any maintenance technicians working to troubleshoot a furnace. Typical seal areas include the inner door cylinder rod, elevator cylinder rods, inner door seal against furnace refractory, outer door seal against quench vestibule, fan shaft(s), and an elevator seal if there is a top atmosphere cooler.
Typical sealing of cylinder shafts are glands comprised of refractory rope and grease. Greasing of these areas should be completed weekly. Outer door and elevator seals are typically fiber rope and may have adjustment built in as they wear, but ultimately will need to be replaced. Frequent inspection of these areas will help identify early issues. Using a flame wand or gas sniffer can help find leaks in unwanted locations. Small furnace leaks can cause part quality issues, and larger leaks can also create safety concerns within the furnace.
Additional Maintenance Items
Other key maintenance items include a bi-monthly or monthly burn out of the furnace heating chamber. This requires the furnace to have air safely injected into the chamber at or slightly above process temperature to allow the carbon to burn out of the furnace. Doing this process on a regular basis will help improve refractory and alloy component life as well as helping to maintain good process control.
Example thermal camera image
Another helpful snapshot of furnace health is using a thermal camera to take images of the equipment. It is recommended to do this on a monthly or quarterly basis. Thermal camera images can identify hot spots on the furnace outer steel shell that may indicate refractory deterioration or a furnace atmosphere leak. Thermal images can also identify potential issues with motors or bearings on fans and agitator assemblies.
Conclusion
In the end, all furnaces have different nuances that require different maintenance approaches. This could be based on the manufacturer, types of processes being run, or utilization of the equipment. By consulting with your original equipment manufacturer or other furnace service providers, a strong maintenance plan can be developed and implemented. This can include support and training from experienced professionals on that style of furnace. Broader cost benefit analysis should be done as it relates to spare part inventories, resource allocations, frequency of preventive maintenance activities, or investments into predictive maintenance and asset management technologies and how those activities can maximize utilization of each piece of equipment.
About the Author: Ben Gasbarre is president of Gasbarre’s Industrial Furnace Systems division. Ben has been involved in the sales, engineering, and manufacturing of thermal processing equipment for 13 years. Gasbarre provides thermal processing equipment solutions for both atmosphere and vacuum furnace applications, as well as associated auxiliary equipment, and aftermarket parts and service.
All images provided by Gasbarre Thermal Processing Systems.
An eastern Pennsylvania vacuum furnace manufacturer recently shipped an external quench vacuum furnace to a West Coast aerospace manufacturer. The furnace will primarily be used for vacuum heat treating investment castings for the aerospace industry.
The Model HFL-7472-2EQ features an all-metal hot zone, a load weight capacity up to 10,000 lbs., a maximum operating temperature of 2400° F, and a 2-bar quench system optimized for argon with a 150 HP quench motor and a variable frequency drive. The furnace working zone measures 48”W x 48”H x 72”D, includes the SolarVac® Polaris control system, and is AMS2750F compliant.
In this first of a three-episode series on AMS2750F,Heat Treat Radiohost, Doug Glenn, discusses Andrew Bassett of Aerospace Testing & Pyrometry discusses the significant changes in the specification in the areas of thermocouples and calibrations.
Below, you can either listen to the podcast by clicking on the audio play button, or you can read an edited version of the transcript.
The following transcript has been edited for your reading enjoyment.
Doug Glenn (DG): This past June AMS2750 released revision F, but what does that mean to you? We caught up with AMS2750F committee participant, Andrew Bassett, to find out. Our conversation about this revision will stretch over 3 episodes with the first dealing with thermocouples and sensors, the second dealing with system accuracy tests and the third, temperature uniformity surveys. This first episode will be all about thermocouples, sensors and calibration.
Andrew, welcome to Heat Treat Radio. We're excited to have you to discuss this AMS2750F revision. If you don't mind, why don't you take a minute and introduce yourself to our listeners?
Andrew Bassett (AB): I'm president and owner of Aerospace Testing & Pyrometry, headquartered out of beautiful Bethlehem, Pennsylvania. I've been in the aerospace pyrometry field for going on 30 years, after graduating from college at Davis and Elkins college in Elkins, West Virginia with a degree in communications. I discovered by myself that I would end up starving in radio broadcasting, which my field was, and got involved with a company called Pyrometer Equipment Co., a family owned pyrometry business. They needed some help as they were expanding operations, and it was the father of my girlfriend (at the time)—now my wife--who had started that business in 1956. That's how I got my break into pyrometry.
Davis and Elkins College (photo source: dewv.edu)
This was also the time when NADCAP was starting to put its foothold on the aerospace industry. I kind of self-taught myself in the ways of aerospace pyrometry. I spent many years getting to know the specification and understanding what the requirements were, dealing with the auditors themselves, and having them teach me about what they look for during audits. I've taken that knowledge with me for the last 26 years.
After I left the family business, I worked for another start-up company in the field of pyrometry, left that company, and worked for a large commercial heat treat company based in the Southeast as their pyrometry director. At that time I started to feel like I wanted to start my own pyrometry business. So, in 2007, I started Aerospace Testing and Pyrometry (ATP). I was doing it part-time for a while, but then in 2009, I decided to go full force. To this day, it is not just me anymore: there are 16 of us in the company which is spread from coast to coast to take care of pyrometry services as well as other things we have branched off in with ATP. I call it our four headed monster. We have our pyrometry services, which includes calibration and testing of thermal processing equipment. We do get involved with other testing as well, like vacuum measuring systems for vacuum furnaces. We've also done humidity pressure gauges and gotten involved with different types of calibrations as well. Additionally, we have our laboratory, which is based in Ohio, where we do calibrations of secondary standards and field test equipment. Finally, we have our consultant and training arm, with which we have a full-time ex-NADCAP auditor on staff who is able to assist our customers with pre-assessments of NADCAP audits.
AMS2750 is the main aerospace material specification in pyrometry. If you actually try to do a Webster's Dictionary search on pyrometry, you'll find it is a made-up word. We've interpreted it as the calibration and testing of thermal processing equipment; that is, heat treating equipment and any type of thermal processing will fall under this specification when it comes to testing.
AMS2750 has also now been adopted by others; it is not just a heat treating specification anymore. Two years ago, the FDA adopted AMS2750. Those facilities that are heat treating medical implants or dental drill bits will now have to follow the requirements of AMS2750. The one industry that walked away from this specification is the automotive industry. They have their own requirements called CQI-9. I always make a joke that the one good thing about AMS2750 in dealing with aircraft is that we don't see planes falling out of the sky, but we do see a few more recalls on automobiles and automotive parts.
DG: Just as a little preview for our listeners, Heat Treat Radio will be doing probably a two to four-part series, similar to what we're doing here with Andrew, on CQI-9, so stay tuned for that.
Andrew, how exactly did your company get involved with AMS2750?
AB: So, they had started to revise—and this goes back several revisions ago—revision C to create revision D. Revision C, I always said, was the Bible: You can give it to 100 different people and you would get 100 different interpretations. It was a much-needed change that was needed in revision D. At this time in my career, I only had about 8 years experience in pyrometry, but I had to live and breathe this document day-in and day-out. So, I approached several members from the AMS2750B team to get involved with the spec. I didn't have the great experience like some of the other members of the team who were from Boeing, Bodycote, and Carpenter Technology and other folks, and they said, “Well, we kind of have our team set into place. We'll ask you questions if we need anything.” I didn't hear much from them, but one of the team members did keep me posted of some of the changes.
Then when it came to the rev. E, I heard rumblings that they were going to revise the spec again, and it was at this time that I decided to attend an AMEC meeting. AMEC is basically the think tank of all of the AMS specifications that are dealt with. AMEC stands for the Aerospace Metals Engineering Committee. The various segment specifications fall under various commodity groups, I believe it's A thru H. AMS2750 is actually owned by committee B for NSAE. So AMS guys write the specifications, the commodity committees own the specifications and that's how this process works.
I did attend my first AMEC meeting and the chairman at the time was a gentleman from Lockheed Martin. Anybody can join the AMEC meetings and be a part of them, but at that meeting he asked who I was and my background. I told him and said that I wanted to get involved with this specification and he said, “By all means you need to get involved with this specification. Since you do this for a living, I think we'd like to have that perspective.” So that's how I got on the AMS2750 team for rev. E. I'm still young enough, and dumb enough, to keep going on to this revision of rev. F and will probably be around for the next revision after that.
I did have my inputs in both the specs. We had a great team for rev. F which included myself, Doug Matson from Boeing, who has since just retired, Marcel Cuperman, who is a staff engineer for heat treating for PRI NADCAP, Cyril Vernault from Safran Aerospace, (he is also the heat treat task group chairman in NADCAP), Brian Reynolds from Arconic, Douglas Shuler from Pyro Consulting and a NADCAP auditor, and James LaFollette from GeoCorp. Our team has consisted of people across various parts of the industry. From Arconic’s standpoint, we were looking from the raw material producers. Obviously, with GeoCorp, it was from the thermocouple side of things. And from Cyril Vernault based in France, we wanted the European influence of what's going on over there. So, a good, broad range of people from various sectors of the industry are involved with the specification.
[blocktext align="left"]“I'm an end-user, so I'm able give my input and say, ‘Hey, this doesn't make sense. What you want to add into the spec is not real world.’”[/blocktext]One of the things I always had in my mind when I first got involved with the specification was that the specifications were written by the aerospace "primes," but that's not the case; it involves people, such as myself, who are end-users of this specification. I'm an end-user, so I'm able give my input and say, “Hey, this doesn't make sense. What you want to add into the spec is not real world.” It’s nice that people such as us get involved with these specifications.
DG: Let's talk about the main sections of this specification. If you break them down, what are the main sections?
AB: There are really only five sections of the specification. You can break it down into thermocouples, calibrations and thermal processing classification, SAT (system accuracy testing), TUS (temperature uniformity surveys), and the very last five or six paragraphs are on the quality provisions (what happens if you have a failed test). Those are the 5 main sections of AMS2750.
DG: So focusing on the topic of this episode, thermocouples and sensors, let's highlight some of the profound changes that have been made in rev. F. First, what are the biggest changes regarding thermocouples and sensors?
AB: The bigger changes relate to how we address some different thermocouple types that were not addressed in previous revisions of the spec. In rev. F, we added and gave a thermocouple designation, type M, to Nickel/Nickel-Moly thermocouple. These thermocouples have been around for a long period of time. We do know that they're being used in aerospace application, especially at very high, elevated temperatures. It's more cost-effective than going into the platinum or the noble-based thermocouples. Type M was one of the newer thermocouples we added.
We also addressed the use of RTDs, which is, again, something that we had seen in the aerospace industry for quite a while. As I mentioned before, this is also a crossing over from the heat treat world into the chem-processing world. A lot of these chem-processing tanks use RTDs to measure chem temperatures, so we thought we better address these type of thermocouples.
RTDs in AMS2750F explained (photo source: Andrew Bassett, ATP)
Then we also added refractory thermocouples, which people weren't all that familiar with, unless you're dealing with the hot isostatic pressing (HIP) process. We're seeing more and more of the HIP furnaces out there now, with all of the additive manufacturing that is going on. We see people adding HIP furnaces everywhere, and a lot of those HIP furnaces are coming with type C thermocouples, because they are rated for these elevated temperatures that the HIP processes do. I think the type C thermocouples are rated close to 4,000 degrees Fahrenheit. We had to add some of these extra sensors that have been around for a while, but we wanted to bring them out a little bit further.
One of the other changes that was pretty significant—though I don't think it will affect the industry all that much—is that now we require thermocouples to be accurate to what's called “special limits of error.” The previous revision allowed for two different types: You were allowed special limits of error, which the accuracy is + or –2 degrees Fahrenheit, or .4% of reading. That was only required for a system accuracy test sensor or for a sensor that was being put in a Class 1 or 2 furnace. All other sensors, such as TUS of load sensors, and class 3-6, we allowed for standard limits of air, which was + or –4 or .75% of reading, whichever is greater.
We did some polling of major thermocouple suppliers out there. With my personal experience and that of some of the other people on the committee, we kind of said, “Hey, you know what? No one really orders the junky stuff, the standard limits; everyone orders special limits of error.” James LaFollette said, “Come to think of it, I don't think I've ever seen a purchase order that says give me the crappy stuff. We all order special limits.” So that's what we discovered – that no one was ordering the bare minimum because there wasn't a price difference between the two. Everyone had already been ordering the good stuff, so we just made that a little bit of a tighter requirement. Again, I don't think it's going to affect any suppliers out there.
I think the biggest change, when it came to thermocouples and sensors, was a big restriction that we put on what's called “expendable test sensors.” This was dealing with the base metal thermocouples. Base metal thermocouples are type K, type J, type T, type N, type M, and a couple other type base metals.
Click to read the Heat Treat Today article on thermocouples.
Primarily in the heat treating and thermal processing world, you pretty much see the K, J, N, and T. We had done some studies as a sub-team within 2750 to look at the drifting of thermocouples, that is, where thermocouples start to lose their accuracy. In the previous revision, we had some provisions in place that allowed people to use these expendable thermocouples that were attached to a temperature uniformity survey rack and were preserved. They could use them up to three years or 90 uses when below 1200 degrees. We thought that seemed kind of excessive on a 20-gauge wire that is covered with fiberglass coating. They're probably not going to hold up, but maybe we should see if there is any drifting of these thermocouples. So, we had one of the major thermocouple suppliers, Cleveland Electric Lab, run some drift studies on type K thermocouples, and we found out that these wires were actually starting to drift after three or four runs. The drift study included a cycling test where they ran it up to temperature and back down 30 different times. We asked, “Why don't we try to simulate how these thermocouples are going to interact coming in and out of thermal processing equipment? Why not pull them out every single time and do it that way?” Again, we found that thermocouples were drifting even further and even quicker.
At this point we decided we better put a restriction on this, and that gave the biggest uproar regarding the reuse of these thermocouples. Previous drafts before the final release of the spec was, if it's used above 500, your expendable wire is one and done above 500 degrees. A lot of the suppliers out there came screaming and said this is going to cost us millions and millions of dollars more in thermocouples. But we stood firm and said, “Hey look, if you're using these test thermocouples to validate your furnaces, either through a system accuracy test or uniformity survey, you really do not know what your error of that wire is after the first use.”
Most of the major thermocouple suppliers will even state on certifications that they will only guarantee accuracy at the time of calibration. Once it goes in a furnace, atmosphere and different conditions of the furnace will affect the wire. We stood our ground, but we ended up backing off a little bit. If you were using them strictly below 500, you're allowed to use them for 3 months (90 days) and you're going to have to keep a log. If you're using them between 500 and 1200, we're going to allow you to use them for 90 days, but now you're only restricted to five usages. And then again, above 1200, you use it once and throw it away. That was probably the biggest hassle, trying to get that. We did finally compromise on that three month or five usages. I do see the burden on the suppliers because they were used to three years or 90 usages, so now it's down to three months or five usages.
DG: I see on the chart that I've got here in front of me that base metal types of M, T, K, and E are all the three month or five use, but you've also got base metal type J and N which is three months or 10 uses. But all of them, above 1200, one and done.
Table for SAT and TUS Sensor Reuse (photo source: Andrew Bassett, ATP)
AB: Correct. That's one of the things I was trying to explain to some of the suppliers that were having heartache about the original change of 500 one-and-done. We only left it to the types M, T, K, and E; we always left this out of types J and N. My personal experience with type J has been (and we've switched over to type J wire a while ago for testing below 1200 degrees),that it's a little bit cheaper in price than the type K wire, and there was always this allowance for doubling the amount of usage if you just switch over to type J or type N.
DG: We have a few significant changes in the area of calibrations. What's another area of change in this section?
AB: One of the big things which really surprised me when we wrote it into the standard, but which was kind of overlooked by some of the suppliers, was the requirement of test instruments to have a .1 readability. So when it deals with test instruments and also now data acquisition systems. Now, if you have a chart recorder that is on your furnace (most people are going to data acquisition systems, some sort of SCADA systems), that recorder must have a .1 readability. That caused an uproar since that may create big changes.
Now, we don't put out these changes because we think it's a good idea; AMEC is data driven. The big thing with the .1 readability is that we were actually fixing a flaw that has been in the spec since the first day it was written, when it was just rev. A. We allowed for percentages of readings for your accuracy requirements. Let's say, for instance, on your instruments that are on your furnace calibrated controller an if it's in Fahrenheit, you're allowed + or –2, but if it's in Celsius, it has to be + or – 1.1. And if your instrumentation doesn't show .1 readability, how can you show compliance? That question is one of the reasons—that is, fixing a flaw in specification.
(photo source: www.atp-cal.com/laboratory/)
But we also allow for percentage of reading, which is + or –2 Fahrenheit or 1.1 Celsius or .2 % of reading, whichever is greater. Let's say you have a calibration point at 1400 degrees, you're actually allowed an error of 2.8. If you can't show that decimal point readability, how can you show compliance? That was one of the biggest issues.
Originally, the first draft said all digital instruments need to be .1 readability and then we backed that off to only say that the data acquisition system had to be .1 readability. At the end of the day, the recorders or the data acquisition system is the proof. As long as that shows the tenth of degree of readability, and it meets the requirements, then you're good to go there.
We did look at how many customers are already using digital data acquisition systems through NADCAP. There's actually a NADCAP checklist question that talks about chart speed verification, and if you answer that “N/A” then you obviously have digital data acquisition. At that time, we did look at that data and 78% of the NADCAP heat treating suppliers out there already had paperless systems. On top of that, two years after the release of 2750F, so as of June 29, 2022, you're not allowed to have paper chart recorders anymore. Everything is pushed to a digital data acquisition system 2 years after the release of this spec. I'd say, that's another one of the bigger changes when it deals with the instrumentation.
So the biggest changes are the .1 readability for your chart papers and the two years after the release requirement to go with a paperless system.
DG: Now question three: What are the changes that were made in the calibration section?
AB: There were a few changes when it came to calibration.
One of the things we added this time was the calibration of timing devices. A lot of facilities have timers or clocks that they're basing their times and temperatures, and again, there was no requirement to calibrate this. Therefore, we added a whole section on calibration of timing devices.
There was some push back on that. Certain people, who have suppliers who use certain control operated by computers and which are always synchronized in their server systems, asked if they were going to have to go out and buy calibrated stopwatches and sit at their PC to make sure it's within these new requirements. We finally said, no, you don't have to do that, but if you can procedurally address how that whole system works—that your server is always verified—you would be okay as long as you procedurally address that.
Again, we were loose on the accuracy requirements. Some of these external devices that you have only need to be calibrated every two years. Comparing it to people's standards that they use—we personally do calibration of timers as well, and our standards are required to be calibrated every two years—we ended up just tossing these devices away because it's more expensive to send them back for recalibration than it is to buy new ones. So, we gave some of the suppliers an easier way out. But we just wanted to address, again, something that has never been brought up in the specifications, which, though not technically dealing in the pyrometry world, does sit on furnaces. We need to get these things looked at every now and then as well.
“So, we gave some of the suppliers an easier way out. But we just wanted to address, again, something that has never been brought up in the specifications, which, though not technically dealing in the pyrometry world, does sit on furnaces.”
Some of the other changes come in the documentation. We did change some things that need to be required for the documentation of your calibration results. One of the things was that we need you to document the sensor that you're calibrating for that particular piece of equipment. For instance, you have a vacuum furnace and most vacuum furnace control sensors are a noble metal type S or type R thermocouple, but then the load thermocouples that measure the parts inside might be set as type K or type N. We just want you to denote that the control system is type S and the load thermocouples are type K. Not real big game changers, it's not going to cause too many issues out there from the supplier base, it's just adding basically another column in your calibration reports to say what sensor you're calibrating.
We didn't go too overly crazy on the calibration portion. The one thing, kind of in the calibration field, is we did add a new instrumentation type. When you look at thermal processing equipment, it's broken down into two different sections. You have your furnace classification which is your uniformity tolerance and then you have what's called your instrumentation type. You have class 1 - 6 and you have instrumentation A – E, now instrumentation D+. This was more for Safron Aerospace. Cyril Vernault was very adamant that we add this D+ instrumentation because Safron's specifications state that they want this extra sensor that is basically 3 inches away from the controlling sensor, so they can measure if there is a big difference between these two sensors to determine if there is drifting of your thermocouples. So we added this new D+ instrumentation. We didn't realize this was big over in Europe, but it was nice to have someone like Cyril say that a lot of European suppliers use this and that he’d like to see it in AMS2750. Again, having this broad range of people on the specification helped us find out what's going on in different parts of the world.
DG: How about we close with the fourth part of thermocouples? Could you delve into the expanded section on offsets?
AB: Absolutely. Always one of the areas, especially when it comes to NADCAP audits, is the use of offsets. We basically broke it down into two different types of offsets that are allowed. We have what's called a correction offset, which is basically either a manual or electronic means to bring an instrument back to a nominal temperature. And we have a modification offset, which is just the opposite. It takes either a manual or electronic offset or a shift in the temperature to bring it away from nominal. There are different ways that people have used these offsets. For instance, let’s say you go into a facility and you're doing your calibration of a controller, and the instrument is off linear by two degrees. People would use the offset to bring the instrument back a nominal temperature. Instead of maybe doing a full factory calibration, they would just go into the instrument, hit some magic buttons, and (say I need to offset it -2 because my instrument was two degrees high) set a two degree correction offset.
A modification offset generally is only going to be used for when you're doing a temperature uniformity survey. Let's say it is skewed to one side of your temperature median. For instance, (I always like to use this in my pyrometry training class), we know temperature uniformity and I go in and do a temperature uniformity on your furnace at 1000 degrees. I have to hold it to be + or –10. When I get my final results and I look at everything with all my calculations, I have a survey that actually comes out to be 992 – 998 degrees. It's well within the + or –10, but it’s skewed down to the lower end.
So, there's different things you can do to try to correct that. Maybe change air flow, or thermocouple location, but a lot of time, what happens is you get a furnace that was made in the 1940s and you're trying to make it comply to 2020 specifications. The only thing you can do is go in and shift the controller away from the nominal to actually make it read hotter. In this example that I'm giving you, what I would do is go in and put in an electronic offset and tell the controller to read colder now, as I will drive more heat into the furnace. So, I go in and put a -5 degree offset into the control and now, in theory, when you do the survey, you're shifting that temperature up by five degrees. Now if you look at that split, it would be 997 – 1003—it’s more centered around your set point temperature. That would be what's called a modification offset. You're taking that TUS distribution and skewing it to better center around the set point.
We really did some “spelling” on this: we put some maximums, the amount of offsets that are allowed as we don't want people to go too crazy on these things, so we did put some offsets in there. But I think we did a great job of trying to spell out what these offsets are being used for, how you're supposed to document them, and make sure that you're consistent with your practice every time. Again, procedures will have to be written to fully understand how you're going to do the offset. Am I going to put it electronically? Am I going to do a manual offset, just shift my temperature up five degrees because I know my furnace is cold by five degrees? I think with that whole new section in there, I think we did a good job of spelling that out for the suppliers.
DG: Thanks so much, Andrew for joining us on the podcast.
AB: Thanks for having me, Doug. Looking forward to chatting more with you about AMS2750F.
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