VACUUM FURNACES TECHNICAL CONTENT

Pyrometry: The Standard for Reliable Thermal Processing in Automotive and Aerospace Industries

/ FEATURED NEWS, MANUFACTURING HEAT TREAT, VACUUM FURNACES TECHNICAL CONTENT

OCThere 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:

  1. AMS2750: issued by SAE International, it is the universally accepted standard for thermal processing certification purposes in the aerospace industry (Nadcap).
  2. AIAG CQI-9: this assessment provides the pyrometry requirements for the evaluation of heat treatment in the automotive industry.
  3. 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).

Contact Víctor: victor@globalthermalsolutions.com

Find heat treating products and services when you search on Heat Treat Buyers Guide.com

 

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Pirometría: la normativa para un Procesamiento Térmico confiable en la Industria Automotriz y Aeroespacial

/ FEATURED NEWS, MANUFACTURING HEAT TREAT, VACUUM FURNACES TECHNICAL CONTENT

OCThere 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:

  1. AMS2750, emitida por SAE International, es la norma universalmente aceptada para fines de certificación de procesamiento térmico en la industria aeroespacial
  2. 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
  3. 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:

  1. Calibración de los termopares (o cualquier otro sensor de temperatura), así como los requisitos y tiempo límite de uso en función de su aplicación.
  2. Calibración de la instrumentación de control y prueba
  3. El procedimiento y los criterios de aceptación para la realización de la prueba System accuracy Test (SAT).
  4. 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:

  1. Documentar que el equipo ha fallado la prueba,
  2. Determinar la causa raíz de la falla y
  3. 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.

Contacto Víctor: victor@globalthermalsolutions.com

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Vacuum Gas Cooling: Pressure vs. Velocity, Part 1 of 2

/ AEROSPACE HEAT TREAT, FEATURED NEWS, QUENCHING TECHNICAL CONTENT, VACUUM FURNACES TECHNICAL CONTENT

OCThere is an age-old adage that exists in the heat treating world. That supposition states that “the smaller the vacuum furnace, the faster it will quench.” Is this adage true? Explore Solar Atmospheres’ journey as they designed an experiment to discover if pressure or velocity most affects cooling performance.

This Technical Tuesday was written by Robert Hill, FASM, president, and Gregory Scheuring, plant metallurgist, both from Solar Atmospheres. The article originally appeared in Heat Treat Today’s March 2022 Aerospace Heat Treating print edition.

Introduction

Our study compared the cooling rates of two distinctly sized High Pressure Gas Quenching (HPGQ) vacuum furnaces — a large 10-bar vacuum furnace equipped with a 600 HP blower motor versus a smaller 10-bar vacuum furnace equipped with a 300 HP motor. Both furnaces, one with a 110 cubic feet hot zone, the other with a 40 cubic feet hot zone, were exclusively engineered and manufactured by Solar Manufacturing located in Sellersville, PA.

History

High Pressure Gas Quenching in the heat treatment of metals has made tremendous strides over recent years. Varying gas pressures within the chamber have been shown to be more governable than their oil and water quenching counterparts. The number one benefit of gas cooling versus liquid cooling remains the dimensional stability of the component being heat treated. In addition, using gas as a quench media dramatically mitigates the risk of crack initiation in a component. This is primarily due to the temperature differentials during cooling. Gas quenching cools strictly by convection. However, the three distinct phases of liquid quenching (vapor, vapor transport, and convection) impart undue stress into the part causing more distortion (Figure 1).

Figure 1. Three phases of liquid quenchants
Source: Solar Atmospheres

There are multiple variables involved with optimizing gas cooling. These include the furnace design, blower designs, heat exchanger efficiency, gas pressure, gas velocities, cooling water temperatures, the gas species used, and the surface area of the workpieces. Whenever these variables remain constant, the relative gas cooling performance of a vacuum furnace typically increases as the volume of the furnace size decreases.

The Furnace

Solar Manufacturing has built multiple high pressure gas quenching furnaces of varying sizes over the years ranging from 2 to 20-bar pressure. We have learned that vacuum furnaces, rated at 20-bar and above, became restrictive in both cost constraints and diminishing cooling improvements. Therefore, Solar Manufacturing engineers began to study gas velocities to improve cooling rates. They determined increasing the blower fan from 300 HP to 600 HP, along with other gas flow improvements, would substantially increase metallurgical cooling rates. The technology was reviewed and determined to be sound. A 48” wide x 48” high x 96” deep HPGQ 10-bar furnace, equipped with this newest technology, was purchased by Solar Atmospheres of Western PA located in Hermitage, PA.

The Test

Image 3. Test load with thermocouple placement
Source: Solar Atmospheres

Once this new vacuum furnace was installed, a cooling test was immediately conducted. A heavy load would be quenched at 10-bar nitrogen in an existing HFL 50 sized furnace (36” x 36” x 48”). The same cycle was repeated in the newly designed vacuum furnace almost three times its size! (Images 1 and 2).

The load chosen for the experiment was 75 steel bars 3” OD x 17” OAL weighing 34 lbs each. The basket and grid system supporting the load weighed 510 lbs. The total weight of the entire load was 3060 lbs. Both test runs were identically thermocoupled at the four corners and in the center of the load. All five thermocouples were deeply inserted (6" deep) into ¼" holes at the end of the bars (Image 3). Each load also contained two 1" OD x 6" OAL metallographic test specimens of H13 hot working tool steel. These specimens were placed near the center thermocouple to ensure the “worst case” in terms of quench rate severity. All tests were heated to 1850°F for one hour and 10-bar nitrogen quenched.

Results

The comparative cooling curves between both HPGQ vacuum furnaces are shown in Chart 1. Table 1 reveals that in the critical span of 1850°F to 1250°F for H13 tool steel, the cooling rate in the larger furnace with more horsepower nearly matched the cooling rate of the furnace three times smaller in size.

Table 1. Critical cooling rates for H13 (1850°F –1250°F)
Source: Solar Atmospheres

Chart 1. Average quench rate for five thermocouples
Source: Solar Atmospheres

Micrographs of the H13 test specimens processed in each load were prepared (Images 4 and 5). The microstructure of each test specimen is characterized by a predominantly tempered martensitic microstructure with fine, undissolved carbides. The consistency of the microstructure across both trial loads further demonstrates that while the larger furnace utilized the higher horsepower, both resulted in a critical cooling rate sufficient to develop a fully martensitic microstructure.

 

Conclusions

These tests prove that the greatest impact on the cooling performance in a vacuum furnace is to increase the gas velocity within that chamber. This was achieved primarily by increasing the horsepower of the blower fan. By doing this, the ultimate cost to the customer is significantly less than manufacturing a higher pressure coded vessel. This newly designed vacuum furnace has proven to be a game changer.

Part II of this article will discuss real life case studies and how both Solar and Solar’s customers have mutually benefited from this newest technology.

About the Author:

Source: Solar
Robert (Bob) Hill, FASM
President
Solar Atmospheres of Western PA
Source: Solar Atmospheres

Robert Hill, FASM, president of Solar Atmospheres of Western PA, began his career with Solar Atmospheres in 1995 at the headquarters plant located in Souderton, Pennsylvania. In 2000, Mr. Hill was assigned the responsibility of starting Solar Atmospheres’ second plant, Solar Atmospheres of Western PA, in Hermitage, Pennsylvania, where he has specialized in the development of large vacuum furnace technology and titanium processing capabilities. Additionally, he was awarded the prestigious Titanium Achievement Award in 2009 by the International Titanium Association.

For more information contact Robert at bob@solaratm.com

Find heat treating products and services when you search on Heat Treat Buyers Guide.com

 

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Time To Evacuate Atmospheric Pressure

/ FEATURED NEWS, MANUFACTURING HEAT TREAT, VACUUM FURNACES TECHNICAL CONTENT

Source: Ipsen Harold

Time to evacuate! When it comes to evacuating atmospheric pressure from vacuum furnace chambers, the addition of a diffusion pump can help attain a lower system pressure than the typical roughing pump and vacuum booster pump allow.

This best of the web article identifies the basics of vacuum furnace pumps and then explains how diffusion pumps in particular work and identifies a few considerations to think about to determine if you need this addition or not.

An excerpt:

"For the diffusion pump to function properly, the main and foreline valves must be open, allowing the furnace to operate in high vacuum. Fluid at the bottom of the pump is heated to boiling and forced up through the center of the jet assembly."

Read more at "Ask the Expert: How Does a Diffusion Pump Really Work?"

Find heat treating products and services when you search on Heat Treat Buyers Guide.com

 

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Keeping Up With Vacuum Furnaces: Balancing What You WANT With What You NEED

/ FEATURED NEWS, MANUFACTURING HEAT TREAT, VACUUM FURNACES TECHNICAL CONTENT

OCYou can’t always get what you want. With frequently changing specifications and a volatile economy, what heat treaters want is always evolving. But what they need changes, too. Steven Christopher of Super Systems, Inc. discusses how to balance what vacuum furnace operators NEED and what they WANT. Is the difference between those two things too great?

This article originally appeared in Heat Treat Today’s March 2022 Aerospace print edition.

Steven Christopher
West Coast Operations Manager
Super Systems, Inc.
Photo Credit: Super Systems, Inc.

I love metaphors and think of vacuum furnaces as automobiles. As an owner, the goal is to keep our cars on the road for 100,000+ miles — and why not the same for furnaces? Accomplishing this feat requires the same in both cases: (1) routine maintenance — literally changing oil and (2) addressing warnings before they become problems — such as check engine lights or vacuum leaks.

The similarities stop there, however, with a key difference in how each is upgraded. In the near future, if you want a self-driving vehicle you will have no choice but to turn in the keys of your 10-year-old sedan and buy a shiny new Tesla, opting for the autonomous driving upgrade.

But what about your vacuum furnace? As the industry releases all these new standards and specifications, do we also need a newer furnace? Or can we retrofit what we have? That answer is complicated because so much is influenced by what we WANT versus what we NEED.

Day-to-day production shapes what we want. We learn from both experiences and failures, shaping features we want to improve operations, customer experience, and reduce rejected work. Specifications and customers drive what we need. Most recently AMS2750F (and 2769C) have been revised and place a burden on operating aging equipment while maintaining compliance. Before these, NFPA86 was modified in 2019, improving furnace design and safety “best practices."

These requirements levy real costs in terms of both hardware investment and increased labor (additional quality employees). We are expected to perform additional labor with the same workforce; however, the reality is that a worsening domestic labor shortage often means we are doing more work with even fewer people. This article navigates this delicate balance, maximizing each investment dollar’s impact while reducing our reliance on labor.

What We Need

It becomes impossible to completely address such large specifications in a short article, so let me highlight a few important considerations from AMS2769C:

  • Section 3.2.3.2 requires decimal precision for thermocouples (AMS2750F)
  • Section 3.2.4.1.2 outlines leak-up rate requirements
  • Section 3.3.1 reviews partial pressure and dew point requirements
  • Section 3.5.2.1 addresses permissible outgassing
  • Section 3.5.3 covers load thermocouples

Perhaps the most talked about change is the requirement of thermocouples to record to a tenth of a degree. It is important to distinguish the difference between a temperature controller and recorder. Section 3.2.3.2 does not require a furnace to control with decimal precision, only record to it. However, best practice lends itself to controllers supporting this ability as well.

Figure 1. Maximum permissible leak rates
Photo Credit: AMS2769 Table 4

Exposure to oxygen at elevated temperatures is detrimental to part metallurgy, be it aesthetics or integrity. Leak-up rates are so important because they prove such exposure is eliminated (or significantly reduced). AMS2769C attempts to mitigate this exposure by standardizing the best practices for performing such tests. Leak-up rate tests are required weekly for (minimum) 15 minutes. Figure 1 identifies a maximum allowable leak-up rate based on the material being processed.

Historically this requires an operator to initiate a cycle, stop the evacuation (pumping), then document the beginning and ending vacuum levels by hand. While simple, this requires both time and attention, preventing any operator from performing other tasks.

AMS2769C proceeds by addressing outgassing, requiring ramp/soak controllers to either be placed on hold or to disable the heating elements if the vacuum level exceeds (1) the partial pressure target or (2) the diffusion pump operating range. Aging controllers require well-trained operators, constantly monitoring vacuum instrumentation and manually adjusting the controller. This introduces potential for operator error, again limiting their ability to perform other tasks.

Section 3.5.3 details placement and requirements for load thermocouples. Assuming load thermocouples are required, runs may be rejected should thermocouples fail below the minimum processing temperature. Disconnected control systems monitor load thermocouples using a recorder separate from the ramp/soak controller. This complicates the control system’s ability to alert operators to such failed conditions — the recorder not knowing which thermocouples are required.

AMS2769C progresses to cover partial pressure. Partial pressure has been automated for years with minimal changes to control mechanisms, though some have replaced solenoid valves with mass flow controllers (MFCs). System upgrades should strongly consider automatic gas type compensation and digital communications of vacuum levels.

Thermocouple (or pirani) vacuum sensors estimate the heat emitted from a heating filament within the sensor. This measurement represents an exact vacuum level, though the gaseous media separating the fi lament from the measuring tip influences the reading (thermodynamics heat transfer). This phenomenon (represented in Figure 2) explains why nitrogen and argon result in very different vacuum estimates.

Figure 2. Gas compensation graph
Photo Credit: Televac MM200 User Manual

NOTE: Thermocouple gauges operate in vacuum ranges where enough gas molecules remain (e.g., in excess of 1 micron) to influence this reading; unlike cold cathode sensors which operate under complete vacuum, excluding them from such compensation.

As an example, consider a vacuum furnace operating under nitrogen partial pressure. The vacuum instrument correctly displays 200 microns (refer to the AIR curve). Now consider the same cycle, only the operator introduces argon. The display now incorrectly displays (and controls to) 200 microns; however, the furnace is truly operating closer to 100 microns (refer to the ARGON curve).

Figure 3. Dew point requirements
Photo Credit: AMS2769 Section 3.3.1.1

Historically vacuum signals have transmitted a 0-10vdc analog signal representing the vacuum level. As with all analog signals, error is introduced by both the accuracy of the instrument generating the signal as well the recorder interpreting it. This error is mitigated by routine calibrations — often aligned with temperature uniformity survey (TUS) schedules. Modern control systems replace such signals with vacuum instrumentation supporting digital communications, eliminating error in the process. As a bonus, the reduction in calibration points reduces time when performing calibrations. Such systems may even automatically compensate thermocouple sensors resolving the sensitivity of thermocouple sensors to multiple gas types.

AMS2769C references other specifications, namely AMS2750 and the Compressed Gas Association (CGA). CGA establishes minimum requirements ensuring inert gas quality. In addition to supplier certification, gas quality is proven by dew point. All gasses have a dew point, with outside air relatively high (e.g., +50°F) and inert gas very low (e.g., -100°F). Purchasing supplier certified gas results in a facilities bulk storage tank having a very low dew point, with any leaks in gas delivery system (pipe threads, fittings, etc.) resulting in a less negative dew point. The concept that dew point can only raise once exiting the storage tank illustrates the importance of sampling “as the gas enters the furnace” — measurements taken upstream fail to detect leaks downstream. The intensity of this increase directly correlates to the amount of air (oxygen) entering the gas supply, compromising the gas purity, which as previously discussed negatively impacts the parts being processed. Proving a dew point below -60°F proves the inert gas mostly free of oxygen. Measurements have long been a manual process; an operator samples gas using a portable sensor and records the findings in an entry log. Modern systems seamlessly integrate dedicated sensors continuously sampling gas quality which alert upon compromised gas.

What We Want

This article’s first draft opened this section listing a handful of features — that was November. Fast forward three calendar months (what feels like an entire year), it is now January, and priorities have changed. Three months ago we wanted features, now we just want parts. The growing supply chain disruption is feeling less temporary and more permanent. This final draft opens with availability. Any upgrades should factor both (1) component lead-time and (2) their flexibility. Lead-time should focus not just on immediate project delivery, but the long-term availability of the product. Is it in its infancy? Or near the end of its life? What is the current lead-time and strategies to maintain inventory? Flexibility should focus on limitations of the product. Is it limited to specific applications? Or can it be used in other equipment? Flexibility paired with planning results in standardization. Keeping with the automobile theme, standardization is what made Henry Ford’s Model T so special. Standardization reduces on-site spare parts, as the same component can be installed in many locations. Standardization should be a primary focus when purchasing programmable logic controllers (PLCs), vacuum instruments, and temperature controllers.

As if the supply chain worries are not enough, the U.S. faces a labor shortage projected to worsen over the next decade. This highlights another late addition to this article, stressing the importance that any upgrade considers the availability of the most important resource: people. New furnaces and upgrades alike (like it or not) develop a co-dependence between multiple parties. This relationship may be internal, between operations and engineering; or external, between an end user and a supplier. No matter the specific situation, all parties should discuss availability and access to information. Failure to discuss this early on is often exacerbated, especially when upgrades are performed by a supplier who is considered (1) unresponsive or slow to respond and (2) unwilling to share information. Purchase orders should document expectations in terms of deliverables (PLC logic, schematics, etc.) and support.

Figure 4. Projected US labor deficit
Photo Credit: US Department of Labor

This third paragraph was that ill-fated November draft’s first. Today’s buzzword, the Internet of Things (IoT ). As we are well on our way to the quarter mark of the 21st century, we have all become accustomed to a lot of quickly accessible information. Why should vacuum furnace recorders not meet the same lofty expectations? Control system upgrades should be capable of recording information and displaying it in an easily retrievable format. Recorded data should expand beyond the required process data into the status of the furnace itself (valve position, state of limit/thermal/vacuum switches, motor status, etc.). Such data can be evaluated postmortem to troubleshoot a failed production run’s root cause of the failure. Advanced systems should be able to notify personnel of issues via email or text messaging.

Often the information gathered above is passed into a Supervisory Control and Data Acquisition (SCADA) System. This system must meet industry compliance for data integrity and security. As every new software seems to have its own system, daily operation requires most to juggle many of these systems, often sharing common data. A SCADA System should be designed to operate in this unknown environment and be capable of sharing data between itself and Enterprise Resource Planning (ERP) and other supervisory systems. The first step here is to build upon common platforms; and today the most widely accepted platform is Microsoft SQL Server. SCADA Systems should be able to “offer up” data using any number of industry standard protocols (Modbus, API, OPC, etc.).

The biggest invisible threat to our industry is internet security. For those fortunate enough to have avoided a cybersecurity attack, IT’s work seems a burden. For those unfortunate to have experienced such an event, IT’s work is beloved. This rapidly changing frontier is our reality and programs like Cybersecurity Maturity Model Certification (CMMC) become a necessary (even required) precaution. Hardware for upgrades should be vetted for compliance with these evolving precautions.

Thus far this article has focused on people, hardware, and features. I now turn the focus to the vacuum furnace itself. Furnaces routinely struggle with passing TUS at both lower (<1000°F) and elevated (>2000°F) temperatures. The issue itself varies between graphite and molybdenum hot zones but the root cause remains the same: inflexibility with rheostats to adjust across a wide temperature range or the furnace’s incapability of reaching elevated temperatures. Users manually adjust the applied power to each zone in attempt to minimize the difference between the coldest and hottest TUS thermocouples. Rheostats force the user to settle for a configuration “just good enough” for all temperatures but “not perfect for any.” Modern systems replace rheostats with individual silicon controlled rectifiers (SCRs) driving each variable reactance transformer (VRT), a feature commonly called digital trim. All furnaces are candidates for digital trim, though older VRT packages using slide wire (or “corn cob”) resistors may require the addition of direct current (DC) rectifiers in addition to SCRs. The benefit of digital trim is these settings can automatically adjust with temperature allowing for the ideal configuration at every temperature.

How Do We Get There?

Resurrecting the automobile analogy which opened this article, have you ever wondered why so many people love Jeep Wranglers (and I realize Jeep could easily have been Harley Davidson or a new home purchase)? The reason is not what they are, rather what they can become. Owners see upgrades and features in their mind long before anything is modified. The key concept here is customization. This same vision applies to vacuum furnaces, any upgrades should consider robust and powerful control systems, flexible enough to evolve with the industry.

PLCs and process instrumentation should always be sourced with room to grow. Modular designed platforms easily expand to integrate new hardware. Ask suppliers how their hardware handles additional inputs, outputs, and sensors. Instrumentation should be integration-friendly and be capable of monitoring the entire vacuum ecosystem — considering the temperature, load thermocouples, and vacuum and gas control systems. Ideally, instrumentation will communicate with each other, passing relevant information between each while simultaneously eliminating calibration points.

Control systems should be sourced with an Evolution Plan in place; compliant solutions today in no way assure compliance tomorrow. Suppliers should be asked their plan for AMS2750G, H, and I. Doing so positions you to make large investments once, then grow hand-in-hand with the industry rather than fight it every time it changes.

Summary: Have a Plan

Modern controllers consolidate a furnace’s self-contained subsystems (vacuum, load thermocouples, valve control, etc.) into a singular control system. This provides the transparency necessary for the controller to alert operators or place itself on hold when necessary. The outcome is that operators require less time monitoring the subtleties of production, meaning they focus their time on more urgent tasks. A happy byproduct becomes the natural progression of data (the recorded values from all these subsystems) into information (meaningful, document values presentable to customers, reviewable by auditors, or referenced for troubleshooting).

I was once told to either open or conclude an article with a poignant quote, so let me offer this advice: When considering upgrades for any furnace “have a plan or become part of someone else’s.” Early conversations between engineers/suppliers and quality/production ensures the delivered product shares everyone’s goals.

About the Author

Steven Christopher is a Purdue University engineering graduate and a 15-year veteran of the heat treating industry. He began his career in pharmaceutical maintenance before joining a commercial heat treat facility focusing on the automotive and aerospace industries. He now manages Super Systems' West Coast operations supporting all types of industries west of the Rocky Mountains.

For more information, contact Steven at schristopher@supersystems.com

Find heat treating products and services when you search on Heat Treat Buyers Guide.com

 

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Don’t Vacuum-Braze Metals Containing Zinc

/ BRAZING TECHNICAL CONTENT, FEATURED NEWS, MANUFACTURING HEAT TREAT, VACUUM FURNACES TECHNICAL CONTENT

Source: Kay & Associates Brazing Consultants 

Are you sure you should vacuum braze that? As the title of this best of the web article suggests, vacuum brazing materials containing zinc is not a good idea. Volatized zinc can contaminate, and maybe even ruin, your vacuum furnace. But what about cadmium, lead, chromium, and magnesium? Is vacuum brazing safe for those materials?

In this article by Dan Kay, examine the vapor pressure curves of common metallic elements to be sure you know exactly when you need to worry about vaporization. And remember, operating your furnace at partial pressure does not offset the effects of vaporization.

An excerpt: 

Many people braze stainless steels (which contains chromium) at vacuum levels approaching 10-5 Torr [. . . ] You can readily see that at 10-5 Torr the temperature at which Cr volatilizes has dropped down to only about 1800F (950°C). Since nickel-brazing of stainless typically takes place at about 2000-2100°F (1095-1150°C), please understand that you will indeed be volatilizing chromium during this brazing operation, which will condense on the furnace walls, giving them a greenish/bluish coloration.

Read more: Don't Vacuum-Braze Metals Containing Zinc

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“The Die is Cast”: What’s NADCA 207?

/ FEATURED NEWS, MANUFACTURING HEAT TREAT, VACUUM FURNACES TECHNICAL CONTENT

Source: TAV VACUUM FURNACES

When "the die is cast," heat treaters should make sure that they're using NADCA 207 standards. Prepared by the North American Die Casting Association (NADCA) for its members, they provide recommendations on how to produce dies for die casting to optimize thermal tool life in terms of thermal fatigue.

In today's best of the web article, check out what some of the essential requirements are and how this standard could help in "maximizing the resistance of tools to the occurrence of cracks from thermal fatigue."

An excerpt:

However, the content of this specification is so well processed that it is valid not only for the production of die casting dies and for thermal fatigue, but also for many other applications, and is the best information material for commercial vacuum heat treatment plants, tool shops and die casting foundries, enabling the elimination of fundamental errors in the tool making process.

Read more at "DO YOU KNOW WHAT NADCA 207 IS?"

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A Layman’s Guide to Understanding the Theory of Gases

/ FEATURED NEWS, MANUFACTURING HEAT TREAT, VACUUM FURNACES TECHNICAL CONTENT

Source: VAC AERO International, Inc.

Need a refresher course on the "gas laws" and how they relate to heat treating? What exactly is going on at a molecular level in your vacuum furnace? This best of the web article gives a helpful review of the theory of gases and practical tips to make your heat treating experience easier.

An excerpt:

"The movement of gases is an important and interesting subject but one often dismissed as a topic best left to scientists. However, the Heat Treater needs to know something about the basic nature (theory) of gases and in particular how they behave in vacuum. The main difficulty is that too much theory tends to become a distraction. Our focus here will be to better understand what goes on inside a vacuum furnace."

 

Read more: A Layman’s Guide to Understanding the Theory of Gases

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Avoiding Diffusion Bonding of Parts and Fixtures: A Case Study

/ FEATURED NEWS, FIXTURE RACKING SYSTEMS TECHNICAL CONTENT, MANUFACTURING HEAT TREAT, VACUUM FURNACES TECHNICAL CONTENT

OCAs most heat treaters know, parts and fixtures often do not mix well. Diffusion bonding can cause the two to fuse together. In this case study, learn how combining thin-film coatings with specific part and fixture design can avoid diffusion bonding. 

Read all about it in today's Technical Tuesday feature, written by Jeff Tomson, sale manager at Ionbond. This article was originally published in Heat Treat Today’s December 2021 Medical & Energy print edition.

Jeff Tomson
Sales Manager
Ionbond

A client approached Ionbond looking for a solution to a problem: They had parts diffusion bonding to their fixtures during heat treatment. The client was using 316SS fixture spacers for heat treating 17-4 SS components at 1904°F (1040°C) in a high-vacuum heat treatment furnace and 316L SS components at 1652°F (900°C) in a high-vacuum heat treatment furnace. Due to the chemical affinity of the alloying elements of the two materials, the length of the heat treatment, and the operating temperature, atoms from both materials could intersperse. The resulting diffusion bonding caused difficulty getting the subject parts to separate from the fixtures.

The coating solution needed to be chemically inactive at the processing temperature while providing a defect-free contact surface. Ceramic materials satisfy these requirements; thus, Ionbond's CVD 29 (Al₂O₃) coating was recommended. The CVD process is a method for producing low stress coatings by means of thermally-induced chemical reactions. Typically, the substrate is exposed to one or more precursors such as TiCl₄, CH4, or AlCl₃ which react on the substrate material to produce the desired film. CVD coatings typically do not maintain their characteristics at the elevated temperatures of our client's application for long periods. However, the high-vacuum environment would allow the coating to function above its 1832°F (1000°C) service temperature. The coating has an excellent record in high temperature applications (cutting, forming, etc.) since it is chemically inert and has the ability to maintain a high hardness.

CVD equipment by Bernex

The CVD 29 coating has different variations and many applications. In the cutting tool world, its ability to resist thermal stresses makes it well suited for high-volume machining of mild and stainless steels. In resistance welding it is used heavily for locating pins and splatter guards, as its electrically insulating properties prevent arcing and its high toughness allows for a long life. For high temperature forming, chemical inertness prevents aluminum buildup on die profiles. High wear resistance makes this coating an ideal solution on ferrous and non-ferrous alloys used in hot extrusion and die casting applications. The overall coating thickness varies from 6 to 16 microns, depending on the version being applied as well as the substrate material. The coating produced is multilayered with adhesion-promoting underlayers that are needed to ensure bonding of a ceramic material to steel.

Due to the high coating temperatures, austenitic stainless steel is typically not an ideal substrate for the CVD process due to its low carbon content causing issues with adhesion. It is a better option than martensitic grades as post-coat hardening is unnecessary. Popular substrates for this coating family include carbides, D2, and H13 tool steels. Some exotic materials such as platinum and nickel content alloys are also used for specialized applications in the semiconductor and aerospace industries.

Ionbond's Cleveland team. Ionbond is a global leader in thin-film coatings, which are used to improve durability, quality, functionality, efficiency, and aesthetics of tools and components. Its portfolio includes physical vapor deposition (PVD), plasma assisted chemical vapor deposition (PACVD), chemical vapor deposition (CVD), and chemical vapor aluminizing (CVA) technologies, including a broad range of diamond-like carbon (DLC ) coatings.

Given the nature of the CVD process, typically all surfaces receive uniform coating. In the first trial, the client's spacers were coated utilizing different fixtures to ascertain whether fixturing methods would be a factor. Subsequent client trials revealed no discernable differences.

The first test by the client using the coated parts at 1904°F (1040°C) in a high-vacuum environment was considered a success, with the client stating that the coating performed “excellently.” There was no sign of coating degradation based on the visual appearance and the subject parts were easily removed from the fixtures with no signs of diffusion bonding. The second test was performed at a lower temperature of 1652°F (900°C) and had similar positive results.

Ionbond in Cleveland, OH

Given the success of the first batch, the client ordered another trial. The second set of parts, while made from the same material, were a completely new design. There were three different parts, two that had threads and the third that was a smaller washer shape. Sharp edges can present issues for the CVD process as stresses can build up at the points of the threads and cause the coating to delaminate. The small washers presented their own concerns due to the thin dimensions sparking concerns about excessive movement. Visual inspection after coating showed good adhesion with no delamination, as the threads were not sharp enough to cause issues. The smaller washers also had negligible distortion after coating. The second set of spacers were also tested in heat treatment at 1652°F (900°C) with similar results.

Inspired by these successes, the client is currently having a third set of parts manufactured to further improve the productivity of their fixtures. The geometry of the third set is completely different as our client continues to leverage the performance of the coating with the design for a more efficient fixturing.

About the Author:

Jeff Tomson is the sales manager at Ionbond’s Cleveland, Ohio site. He has been in sales and marketing roles since graduating from the University of Michigan in 1999. He has worked in automotive, aerospace, and thin-film industries.

For more information:

Contact Jeff at Jeff.Tomson@ionbond.com

(216) 704-4395

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Microprocessor-Based SCR Power Controllers: Making Your Life Easier

/ CONTROLS TECHNICAL CONTENT, FEATURED NEWS, MANUFACTURING HEAT TREAT, VACUUM FURNACES TECHNICAL CONTENT

OC Precise temperature regulation is undoubtedly the top variable in the industrial process that influences the quality of the final product. Using intelligent power control and predictive maintenance, silicon controlled rectifiers (SCRs) play a major role in temperature regulation and in improving the industrial heat treating process. What are SCRs and how do they improve the industrial heat treat process?

In this Technical Tuesday feature, written by Tony Busch, sales application engineer at Control Concepts, Inc. and Meredith Barrett, manager of Marketing and Business Development at Weiss Industrial, discover how SCRs can help you improve temperature regulation.

(This article was originally published in Heat Treat Today’s November 2021 Vacuum Furnace print edition.)

Introduction

Meredith Barrett
Marketing and Business Development Manager,
Weiss Industrial

Tony Busch
Sales Application Engineer
Control Concepts, Inc.

In manufacturing metals and in the heat treat industry, temperature regulation is crucial. SCR power controllers regulate the flow of electricity from the grid to a major heating element in a manufacturing process. Usually, the major heating element is a furnace, kiln, or oven, and the SCR is often connected to the heating element directly or to a transformer connected to the heating element.

The ability to calculate resistance in a furnace can provide information on the overall condition of an element. The SCR collects data and communicates it back to the network. Predictive maintenance is knowing when an element has reached its useful life. This article will define what an SCR power controller is, how it functions, and the different firing modes.

Digital Thyristor/SCR Power Controller Overview

“Thyristor” is a Greek-derived word for “door.” The term is a hybrid of the word thyratron and transistor. As defined by ElectricalTechnology.org, a thyratron is a gas-filled tube that works as an SCR. SCR and thyristor are interchangeable terms in describing a device with four semiconductor layers or three PN junctions with a control mechanism. These small machines are known as latching devices. In the context of electrical engineering, a latch is a type of switch where once it’s on, it will remain on after removing the control signal.

Figure 1. Current flow

The actual power control module is an advanced electronic device with LED indicators and I/O terminals. The main internal components of an SCR power controller include:

• Semiconductor power devices (SCRs and Diodes)
• Microprocessor-based control circuits normally referred to as the firing circuit
• Heat sink (a means to dissipate the heat generated from semiconductor devices)
• Protective circuits (fuses and transient suppressors)

The diagram below is a very basic model showing one leg of an SCR controller. However, in all electrical designs of power controllers, such as the popular Control Concepts MicroFUSION series featured in this article, each controlled leg requires SCRs back-to-back within the power control module because of alternating current.

Figure 2. Basic model of one leg of SCR controller

How are Digital SCR Power Controllers Superior to Their Analog Predecessors?

“Digital” SCR power controllers are basically a concise way of referring to a power controller unit that utilizes a SCR switch (as opposed to a different switching method such as an insulated-gate bipolar transistor (IGBT)) and has all the above components. Additionally, these units contain microprocessors that make them more of a smart device. They are scalable, and easily paired with other digital units, whereas pairing analog power controllers results in potential emitter gain and bias.

Digital SCR power controllers can provide flexibility unmatched by analog units. This flexibility includes various communication options and the ability to switch through fi ring modes with ease, all without requiring the unit to be changed or rewired. The adaptable nature of digital SCR power controllers allows them to be incorporated into an industrial heat treat process much more effortlessly.

Older analog units are not highly configurable like their digital replacements. Newer SCRs not only have configurable faults and alarms, but also savable configuration files which can easily be loaded onto another unit.

Digital SCR power controllers can obtain accuracy and repeatability previously impossible with analog controllers. Digital units have power regulation capabilities that adjust for both variations from the mains voltage and resistance from the heating element. This form of power regulation is not only the most precise way to regulate temperature, but it also allows for process repeatability.

Synchronization of two units connected to the same power source, firing in zero-cross mode, is not ideal. This means that modules should not sync up so that they are on and off in unison. If this should happen, the process would require a large amount of current to be drawn from the source while the controllers are all on, and none when they are off.

The company’s SYNC-GUARD™ feature, not previously available on older SCR controller modules, reduces the peak current draw required from the source over time by causing each controller to attempt to find a time to turn on when fewer, or no other, controllers are firing. However, it has its limitations. The more controllers that are added to application, the probability of them syncing increases. Once ten or more controllers are utilized in an application, it becomes impossible to not have some sync up despite this feature.

Another key difference is that digital SCR power controllers are always calibrated and will never change. This allows the convenience of being able to “set it and forget it.” Newer models have an option of a digital display which was previously unavailable with analog controllers.

How the Latest SCR Power Controllers Improve Industrial Furnace Operations

SCRs can calculate electrical resistance in a furnace and provide precise power control. Intelligent power control has embedded algorithms which teach functions to calculate data and predict what is likely to happen next in the life of a heating element. This capability can determine partial load loss, resistance change, and complete load loss.

Partial load fault detection is a “watchdog” feature that monitors the system for change in resistance. This is useful for detecting an element failure for loads with multiple parallel elements. The feature monitors a user-set tolerance value that determines the drift from the target resistance in the system.

Therefore, an operator can enter the resistance manually or use the innovative “teach function” with a digital SCR controller. This is a form of artificial intelligence that will allow the SCR to learn the heating element through algorithms. The teach function auto-ramps and intelligently saves different resistance values at various setpoints in a process, eliminating guess work.

SCR power controller units attached to
industrial furnace

Heater bakeout is an aspect of industrial furnace operations where digital SCRs offer a great amount of control. Industrial furnaces, kilns, and ovens are often lined with some sort of refractory or ceramic material that allows them to withstand extremely high temperatures. Typically, this material can get stressed and crack if heated too quickly, particularly in some submersion heaters where moisture can be present.

Modern SCR power controllers have an actual heater bakeout mode that will increase the temperature to the heating element gradually, allowing the furnace to slowly equalize in temperature. If any moisture is present in the heating element, it is baked away, and either way, slowly ramping up the temperature prevents damage to the refractory. This can prevent both costly furnace repairs and downtime.

Another major advantage of digital SCR controllers is tap change indication that informs the operator when to change voltage taps. Some loads, even if they remain the same, still can influence and change the element resistance over a period of time. Because this affects the power factor, a transformer with multiple voltage taps can be used.

Additionally, digital SCR controllers can also be utilized to achieve a constant output power. The tap change indication feature signals the operator when to adjust the voltage taps to a higher or lower setting on a digital display or digitally via the alarm monitor panel.

Predictive vs. Preventative Maintenance

Predictive maintenance has become a popular buzz word related to “Industry 4.0” as we now enter what is known as the fourth industrial revolution, or digitization of a manufacturing process utilizing an interconnected network of smart devices. The goal of both predictive maintenance and preventative maintenance is to increase the reliability of assets, such as an industrial furnace, oven, or kiln used in the heat treat manufacturing process. This not only avoids costly downtime but increases the life of an asset resulting in substantial savings in maintenance costs.

The main difference between the two is preventative maintenance is simply regularly scheduled upkeep, such as a temperature uniformity survey (TUS) on an industrial furnace. Think, for example, of how you have the oil changed every 3,000 miles in your vehicle because it is common practice for extending the life of your engine: that’s preventative maintenance.

Predictive maintenance is more condition monitoring or intelligence gathering on the health of an asset. It is based on present time and continuous data monitoring from smart devices on an industrial network. Predictive maintenance is knowing when an element needs to be fixed or has reached its useful life and needs to be replaced. Knowing the life of the element allows for a structured shut down preventing expensive unscheduled downtime.

How Do SCRs Achieve Intelligent Power Control?

In the instance of intelligent power control, the SCR acts similarly to a dimmer switch on a lighting fixture. It regulates the amount of electricity going into the furnace, just like the dimmer controls the amount of brightness going into the light bulb. The purpose of regulating the electricity to the heating element is to maintain the desired temperature and prevent damage to the asset from power surges or voltage inrush.

“Resistance” is an electrical engineering term that relates to the amount of current that can flow through a heating element of a furnace, machine, or other electronic device that heats up. Technically, this can be something as simple as your household toaster. When the heating element is cold, the resistance to electricity is lower, allowing more current to pass through. When it is hot, its resistance is higher, blocking the incoming current.

Figure 3. AC supply (left) and load voltage (right)

Both variations in the electricity coming from the grid (the mains voltage) and furnace resistance can cause temperature fluctuations. SCR power controllers accommodate for both variations from the mains voltage and furnace resistance by regulating output current utilizing different firing modes.

Firing Modes of SCRs: Phase-Angle & Zero-Cross Explained

What technically is a “firing mode” when it comes to SCRs? As noted in the SCR diagram, the topology of an SCR includes a control circuit also known as a “firing circuit.” The SCR has feedback and logic to determine how it is going to fire the electric sine wave. Thyristors, as SCRs are more commonly known outside of the U.S., have two basic control modes: phase-angle and zero-cross.

Phase-Angle

When a SCR power controller adjusts the voltage using the firing angle, it is known as phase-angle mode. This is analogous to a dimmer switch on a light fixture. The SCR is acting as a dimmer switch on an industrial furnace. Using phase-angle control, each SCR in a back-to-back pair is turned on for a variable portion of the half-cycle that it conducts. This trims every single half sine wave, giving a very smooth output, hence getting the correct kilowatts to the needed load.

In a heat treat application where the SCR is firing directly into the transformer, phase-angle mode will need to be employed. This protects the transformer from saturation. (See Figure 3.)

Zero-Cross

In zero-cross firing mode, the power controller adjusts the duty cycle to regulate the voltage. Each SCR is turned on or off only when the instantaneous sinusoidal waveform is zero. In zero-cross operation, power is applied for several continuous half-cycles, and then removed for a few half cycles, to achieve the desired load power.

In other words, zero-cross is best described as a blinking on and off. You’re firing a certain amount of full wave cycles, then it is going to turn off for a period of time, and then return to the on mode. An average is taken of the cycles that fire versus do not fire, which gives you control.

The on and off nature of zero-cross is beneficial for power factor, and the overall cost is lower than running SCRs in phase-angle applications. Simply stated, running SCR power controllers in zero-cross mode versus phase-angle mode consumes less energy and saves money on the electric bill. Zero-cross also produces little to no harmonics. As illustrated below in Figure 4, you can run SCRs in two-phase versus three-phase mode using zero-cross. If the resistance is varying less than 10%, zero-cross can be applied to the heat treat process.

SCR Power Controller Configurations

Single-Phase

In a single-phase configuration, SCRs are running back-to-back to the load, which is looping back up to L1 and L2. This is the most basic SCR set up.

Figure 4. Single-phase configuration

Three-Phase/3-Leg (6SCR)

Three-phase is wired in a delta or wye and involves three SCR modules connected in a circuit. This is great for phase-angle control where the SCR is firing into transformers. The topology is beneficial for direct firing as well. Three-phase is effective in high inrush current loads that require a current limit, and it also enables the system to phase without blinking on and off.

Figure 5. Three-phase/3-leg (6SCR) configuration

Three-Phase/2-Leg (4SCR) Zero-Cross Only

This configuration involves two SCR modules controlling two of the legs, and the third leg is connected to the delta or wye but going directly back to supply voltage. This can be more cost effective for an application since it is run in zero-cross mode.

Figure 6. Three-phase/2-leg (4SCR), zero cross mode

Inside Delta

Inside delta configuration is double the wiring. However, it reduces the size of the SCRs needed. Where the SCRs are placed in the circuit in the inside delta configuration will draw less current at the point. This is a more uncommon configuration, and it is found infrequently in the field.

Figure 7. Inside delta configuration

What SCR Is Right For Your Application?

Weiss Industrial, a manufacturer’s representative company, chose to partner with one of the top OEMs to help provide their customers with uninterrupted and efficient plant operations. They teamed up with Control Concepts Inc. (CCI) on their MicroFUSION Power Controllers because they found their product to be the most reliable and their customer service superior. The company’s power controllers are manufactured in the USA in their 54,000 square foot, company-owned facility in Chanhassen, MN.

Tony Busch, sales application engineer, notes that one of the bigger factors to consider in selecting the right SCR power controller is the load type. Some loads require zero-cross fi ring modes, others phase angle only, and in certain cases it does not matter. It can be either zero-cross or phase angle.

The main rule of thumb is to never use zero-cross on fast responding loads, such as infrared lamps and low mass heaters. In this instance, zero-cross will cause too much of an inrush current and can burst lamps and/or fuses down the line. On the other hand, loads in which the resistance changes are less than 10%, such as nickel and iron chromium, zero-cross must be used. Operators also prefer zero-cross in instances where low harmonics are required, as it produces less harmonics than phase-angle firing mode.

Conclusion

In conclusion, SCRs help achieve an integral part of an industrial network that improves the modern heat treat manufacturing process by providing precise and intelligent power control. They also achieve predictive maintenance previously impossible with their analog predecessors. Their advantages are numerous in improving industrial furnace operations and the heat treat manufacturing process.

Other major advantages of SCRs are their high reliability. Since they are solid-state devices, there is no inherent wear-out mode that can be associated with other industrial mechanical machinery that has gears or moving parts. This means little to no maintenance of the SCR power controller.

They have infinite resolution, which means if there is an incoming supply voltage of 480 volts, sequentially, 480 volts will be returned out of the SCR when it is turned on fully. There is no trim back or load loss involved. You can go from zero to 100% if you want to control your voltage, power, or current.

SCRs also have an extremely fast response time, which allows the operator to turn the device on and off very quickly. In North America, voltage is mostly running on 60hz at 120 half cycles per second. SCRs allow you to target a particular half cycle and turn it on and off very quickly. This is a great feature for loads that have high inrush current, acting as a soft starter, to keep from saturating the heating element.

Want to learn more?

Weiss Industrial has partnered with Control Concepts Inc. to produce a PDF document entitled A Guide to Intelligent Power Control & Temperature Regulation Utilizing SCR Technology, which you can obtain by contacting Meredith Barrett, Marketing and Business Development manager at Weiss Industrial: meredith.barrett@weissindustrial.com.

About the Authors: 

Tony Busch, a graduate of Dunwoody College of Technology with a degree in Electrical Construction, began his career at Control Concepts, Inc.’s headquarters in Chanhassen, MN as a test technician, quickly transitioning to field service and repairs. In 2014, he began his current position as a sales application engineer and became Bussmann SCCR training certified. Contact Tony at tony.busch@ccipower.com

Meredith Barrett has a Communications degree from Penn State University and over twenty years of experience in sales, corporate communications, marketing, and business development. While her journey into the industrial and manufacturing sector began in 2014 with Siemens Industry, Meredith joined Weiss Industrial in January of 2020 as the Marketing and Business Development manager to assist in building a new marketing department and lead generation program, while also supporting business development. Contact Meredith at meredith.barrett@weissindustrial.com.

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