HARDENING TECHNICAL CONTENT Archives

Heat Treat Radio #115: Lunch & Learn: Decarbonization Part 2

November 21, 2024 / HARDENING TECHNICAL CONTENT, HEAT TREAT RADIO

In this episode of Heat Treat Radio, Doug Glenn and guest Michael Mouilleseaux, general manager at Erie Steel LTD, continue their discussion of case hardness, delving into the hardening ability of materials, focusing on case hardening and effective case depth. Michael explains the differences between total and effective case depth, the impact of core hardness, and the role of material chemistry. They also discuss practical applications for heat treaters, emphasizing the importance of understanding material properties.

Below, you can watch the video, listen to the podcast by clicking on the audio play button, or read an edited transcript.

HTT · Heat Treat Radio #115: Lunch & Learn: Material Hardenability and Effective Case Depth

The following transcript has been edited for your reading enjoyment.

The Influence of Core Hardness on Effective Case Depth Measurement (01:03)

Doug Glenn: Today we are going to talk about a pretty interesting topic, and some interesting terminology, that has to do with hardness and hardenability of metals. For people who are not metallurgists, this may seem like a strange topic because isn’t all metal hard?

But we are going to talk more in depth about hardness of metal, hardenability of metal, and effective case depth. What we want to do is get a run down on the influence of core hardness on effective case depth measurements.

Michael Mouilleseaux: We are going to get a little bit into the weeds today on some things specific to metallurgy.

Those who are involved in high volume production carburizing know that consistency of results is extremely important. It is not just important in that we have the process centered in that the results are that way, but ultimately it has something to do with the dimensional control.

Specifically with gears, if the output from the process is not consistent, then one of the things that is going to suffer is going to be the dimensions. So, we’re going to be talking about effective case depths today.

Effective Case Depth vs. Total Case Depth (02:23)

Effective case depth is a little bit different from total case depth. Total case depth is the total depth that carbon is diffused into a part. That is very much a function of time and temperature. And there are some nuances with grain size and alloy content, but it is essentially a time and temperature phenomena.

Effective case depth vs total case depth (02:59)

Effective case depth is a little bit different. If we look at this graph, the x axis is the distance to the surface, and the y axis is hardness in Rockwell C.

If you look at the green line, this is a micro hardness traverse of a carburized part. It tells us many things. If you look at the left-hand side of the line at .005 in depth, the hardness there is 60 Rockwell C. Then it diminishes as we go further into the part: 0.010, 0.020, 0.040, 0.050.

We get to the end of that line, and we see that is the core strength. The core is a function of the material hardenability.

So, what is the effective case depth? If we look at the second vertical blue line on the right, it says “Total Visual Case.” So that’s exactly what that is. If we were to look at this part and etch it — I am presupposing that everybody understands that we would section the part — we would mount it, we would polish it, and then we would look at it in the microscope at 100x. Then, we would see a darkened area, which would be the total depth of carbon diffusion into the part. That is not a function of the material grade; there are some nuances there.

But the effective case depth is a measurement. And in North America’s SAE Standard J423, we say that we measure the case effective depth to Rockwell C 50. The surface hardness is 60, we measure the hardness in increments, and when we reach this hardness the depth that hardness achieves is 50 Rockwell. That is the effective case depth. If we look at the core hardness on that part, we can see that on this particular sample it is somewhere between 45 and 50.

Finding Material Hardenability (05:17)

What causes this core hardness? It has to do with the hardenability of the material.

Here we are looking at an SAE chart J1268. It is for H band material for 4320, a common gear material. This tells us a lot. It has the chemistry on it, below that it has some information for approximate diameters, and then it has, on the far right side of the diameters, we see specs for cooling in water or cooling in oil.

And between them there is the surface, the three-quarter radius, and the center. If we look at the surface of an oil quench at two inches, it has a distance from the surface of something like 4 or 5. So, if you go over the chart on the left-hand side, go to 4/16” or 5/16”, which has an HRC of 29 to 41. Even though this is a hardenability guaranteed material, for a two-inch round you would expect to have something between 29 and 41 for the surface hardness.

Now let’s look at what you would get at three-quarter radius in an oil quench. If you look at two inches, the Jominy position is [eight]. You can see that at three-quarter radius on a two-inch bar, that is an inch and three quarters, I believe, the hardness is going to be something between 23 and 34. In the center of that bar for a two-inch round, it is going to be J12, which has a hardness of 20 to 29.

That is the definition of hardenability. It is the depth that a material can be hardened. And it’s totally a function of chemistry. Davenport and Bain did the algorithms for this in the 1920s leading up to World War II.

Effect of Core Hardness (07:20)

If we are going to evaluate the effect of core hardness, we are going to look at parts that are heat treated in the same furnace to the same cycle in the same basket under all of the same conditions — the only thing different is going to be the hardenability of the material.

Go all the way down to number three on this “Methodology” slide. The anticipated total case is going to be about 0.040 for all of these samples.

This data graph has four samples on there. The red line is the measurement of Rockwell C 50. If we look at the highest hardenability sample, the blue sample has the highest core hardness and also the deepest effective case depth. And as the core hardness is reduced, you can see that where the line crosses the plane of Rockwell C 50, that is reduced as well.

Doug Glenn: Am I correct in thinking the yellow line here at the bottom has the lowest core hardness or hardenability?

Michael Mouilleseaux: Both. You’re correct.

Doug Glenn: That’s why it is crossing the red line much earlier than the others.

Michael Mouilleseaux: Yellow also has the lowest effective case depth.

If we look at this in a tabular form, this is the data, and what you looked at were the microhardness traverses, per the standard using an MT-90: the hardness was (the effective case depth was) measured to Rockwell C, the total case depth was determined visually on these things, and, you’re going to say, that Michael, you’ve got four different materials there. That is correct. We also have four different hardenabilities.

In answer to the question, if these were all the same heat, would we have these same results? We would with the exception of the bottom one at 1018. There is no way that we could take an alloy steel and reduce the hardenability of that amount.

Here is what we are talking about: We know that they were all run at the same process when we look at the total enrichment on this; it’s within the margin of error 0.038 to 0.042.

We look at the effective case depth, interestingly, we have quite a variation there. The first one has the highest core hardness at 46, and the effective case depth is 0.039. Second, we have a sample where the core hardness is 44, and the effective case depth is 0.036. Third, we have a sample where the core hardness is Rockwell C 39, and we have 0.029 effective case depth. And finally, there’s the 1018 sample that was put in there just as a reference. The core hardness on that was 24 with 0.015 effective case depth.

There is a direct proportion between the core hardness and the effective case depth that you are going to be able to achieve.

Referencing back to that hardenability chart that we looked at (the very bottom, half inch section quench in moderately agitated oil), it has a Jominy position of 3. If we look at J3 on the chart, we can see that at the lower end of the chemistry composition, we could have a core hardness as low as 35, and at the upper end of the chemistry composition we could have a core hardness as high as 45.

Let’s go back to the tabular data. That column for J3 is the data that was provided to us by our client from the steel mill.

When they melt a sheet of steel, it is a high value part. So, they use what is called SBQ (special bar quality). Special bar quality is subject to a lot of scrutiny and a lot of controls. One of the things provided, in addition to things like the chemistry and the internal cleanliness in the steel certification, is the hardenability of that specific heat.

You can see that the 8620, the first line, had a J3 of 44. We actually had a 46. The way to understand that is that when you’re going to melt 200 tons of steel, 100 tons of steel, or whatever amount it’s going to be, it’s not all done at one time in a single pour. It’s multiple pours out of a tundish.

The chemistry and the hardenability numbers that you got in a steel certification is going to be very close to an arithmetic average of what you would get when they test the first pours and the middle pours and the end pours. They’re going to average out.

Applying the Data (12:44)

When we’re using this data internally, we say we want to be plus or minus two points Rockwell C within the steel certified hardenability data. I can say that experientially over the years, Gerdau, SDI, Nucor (the domestic sources of SBQ bar )are very consistent in the way they make this stuff, and this is something that we can depend upon.

You could use this as a check for what you’re doing. If the steel that you have has a hardenability of 44 and if you’re not plus or minus two, you have to ask yourself why. There are only a couple of reasons that it would be outside those limits. If it’s above you, it probably is not the heat that it’s purported to be. If it is lower, it could either not be the heat that it’s purported to be, or there could be an issue with the heat treating.

As I said at the outset, we’re going to assume in this discussion that the reason that we have these numbers — the differential and core hardness — is not attributable to heat treating; it’s solely attributable to the chemical composition of the material or the hardenability.

We can use this information if we are an in-house or captive operation and are purchasing the material. We have an opportunity to define in our purchasing practice what the hardenability of the material is going to be.

As I mentioned before, the domestic sources are very consistent in the material that they produce. To produce a heat of 4320°F that has a J3 of 40 or 42 or 44, there is no cost penalty to that (in my experience involved in a major automotive supplier). It is a definition of what you want.

They are not making heats by randomly selecting chemistries for these heats and selling them. They make a recipe for a specific client. And my experience has been that they hold very true to that recipe.

If you are introducing a lot of variation into your process, not only is the output from that variable, but the cost of handling that is variable as well. A material such as this, to specify a J3 of 42 to 44, is something that is eminently doable. My experience is that the steel companies have been able to do that over time with a great amount of consistency.

Now, for those who are not involved in high volume production and do not have control of the source of the material, this chart remains usable. If someone is running a job shop or shorter term things who does not have furnace load sizes of parts, the key is to be able to mix and match things into specific processes. At least in carburizing, if we understand what the hardenability of the material is, then we have a much better opportunity of taking multiple parts and putting them into a load and determining ahead of time whether or not we are going to have consistent results.

Just one more thing that we would like to look at here is this next graph — the Caterpillar hardenability calculator. This is available from Caterpillar, and they readily share it with most all of their suppliers. I have been involved in numerous businesses and have never been refused this. You have to ask them for a copy of it.

SAE Chart J1268, which measures hardenability band for 4320 (05:47)

Michael Mouilleseaux: Using this calculator you import the chemistry of a heat, and then it automatically calculates the hardenability of that heat.

If you recall the J3 on the 4320 material that we looked at, the hardenability guaranteed it had a ten-point range. If you look at this particular heat, and this is what we call the open chemistry, this would not be a hardenability guaranteed material. The upper limit is higher than what you would see on a hardenability guaranteed material, and the lower limit is lower than what you would see. So the variation in a “Standard SAE J 48620” is going to be much wider – it will be much different.

If we look at that same J3 position, we are looking at 25 to 45, a 20-point swing in core hardness. If we go back and revisit the results we had, 39 to 46 with a seven-point swing, we had a 0.010 difference in effective case stuff. If we had a 20-point swing, you could imagine it is going to be significantly greater than that.

Two things, if you have the lower hardenability grade of material, it allows you to modify your process ahead of time to compensate for the fact that the core hardness is going to be lower in this part. Vice versa, if you have an extremely hot heat or it is high hardenability, similarly, you may be able to reduce some time and not put as much total case on the part in order to achieve what specified as effective case.

The hardenability charts are great guides in helping to establish a process and then to evaluate the consistency of that process.

One other comment about the chart is this is not a full-blown Lamont chart, which has various quench severities for different sizes. And that can be utilized to help pinpoint this. As you can see on the SAE chart, you essentially have two different quench rates. You have mild oil and water.

There are a lot of different types of quenchants that are available. The moderate quench rate that is on this chart very closely mimics what we at Erie have been able to achieve modified marquenching. Therefore, I’m able to use this chart without any offset.

Now, if you had a fast oil — petroleum-based oil is very fast — and a heat that had a J3 of 40 in which you are consistently seeing 44 out of it, then in your specific instance, your quenchant is more aggressive than what this chart was built to simulate. However, you can continue to use the chart. It’s just that you must use your experience in doing it.

So again: The strategy to control it is getting the hardenability data so that you can utilize that ahead of time — understanding what your specific heat treating operation is and, more specifically, what your quenching operation allows you to achieve.

Then, knowing that a typical section size of X in this furnace is going to give a Jominy position of Y, you can take that information and say over time, “If I have a variation here, it’s going to be an effective case depth. Is that variation attributable to the core hardness?” If it is, there is a strategy which will possibly change and tighten up the purchasing practice. If it is attributable to something else, then that gives good information to say, “There’s something in my heat treating process that I should be looking at that is attributable for this variation in case depth.”

Conclusion (22:14)

So, we waded into the weeds, and hopefully we have found our way out.

Doug Glenn: I think that explanation is going to be especially good for those who already know a little bit of metallurgy and know those charts.

Bethany Leone: Michael, for in-house heat treaters, how often do they need to be aware of the materials coming into their operations, testing it, or asking about changes that could be happening?

Michael Mouilleseaux: Hopefully this would give heat treaters worth their salt a reason to pause if they previously assumed the material does not come into play.

The next thing would be in high valued components — gears, shaft, power transmission, those kinds of things — heat lot control is typically mandated by the end user. If you have heat lot control and the unique data that goes with that, utilizing the strategy we just talked about is going to give you the ability of evaluating variation. If the primary source of variation is the material, that needs to be addressed. If the material is very consistent and yet you continue to have variation, there is obviously something in the heat treating process that needs to be addressed to reduce that variation.

Doug Glenn: Thanks for listening and thanks to Michael for presenting today. Appreciate your work, Michael.

About The Guest

Michael Mouilleseaux
General Manager at Erie Steel, Ltd.
Sourced from the author

Michael Mouilleseaux is general manager at Erie Steel LTD. Michael has been at Erie Steel in Toledo, OH, since 2006 with previous metallurgical experience at New Process Gear in Syracuse, NY, and as the Director of Technology in Marketing at FPM Heat Treating LLC in Elk Grove, IL. Having graduated from the University of Michigan with a degree in Metallurgical Engineering, Michael has proved his expertise in the field of heat treat, co-presenting at the 2019 Heat Treat show and currently serving on the Board of Trustees at the Metal Treating Institute.

Contact Michael at mmouilleseaux@erie.com.

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Heat Treat Radio #115: Lunch & Learn: Decarbonization Part 2 Read More »

Ask the Heat Treat Doctor®: How Does One Determine Which Quench Medium To Use?

The Heat Treat Doctor® has returned to offer sage advice to Heat Treat Today readers and to answer your questions about heat treating, brazing, sintering, and other types of thermal treatments as well as questions on metallurgy, equipment, and process-related issues.

The Heat Treat Doctor® ha vuelto para ofrecer sabios consejos a los lectores de Heat Treat Today y para responder a suspreguntas sobre el tratamiento térmico, brazing, sinterizado y otros tipos de procesamiento térmico, así como preguntassobre metalurgia, equipos y problemasrelacionados con los procesos.

This article was originally published in Heat Treat Today‘s September 2024 People of Heat Treat print edition.

To read the article in Spanish, click here.

Quenching is a critical step in the heat treating process. And while there are often several choices available to the heat treater, a delicate balance exists between what is available to us and how we can optimize its performance characteristics to meet our client’s requirements/specifications. Material, part design (geometry), pre-and post-manufacturing requirements, loading, allowable dimensional change (i.e., distortion), and the process itself must be taken into careful consideration. Let’s learn more.

Quenchants — A Brief Overview

Today’s quenchants offer a broad and, in some instances, overlapping range of capabilities. But at a fundamental level, the role of a quenchant is to extract heat from the part surface to meet a specified critical cooling rate and achieve the desired microstructure in the component part necessary to achieve the required mechanical and physical properties. In hardening of steels, for example, one must miss the “nose” of the time-temperature transformation (TTT) curve if the desired end-result is a martensitic (or bainitic) microstructure. By contrast, the cooling rate for a normalizing process requires cooling in “still air” — a term that is often misunderstood and which we will cover in a future discussion.

Figure 1. Common types of quenchants and their effect on distortion (See Reference 1)

However, a quenchant (Figure 1) is more than just its cooling rate. Quenchants should be stable over their service life, especially with respect to degradation (e.g., oxidation), be safe, be easy to service and maintain, have a high vaporization point, ideally not interact with the part surface, be used within their optimum performance range, have long life, be easily removed by cleaning after quenching, and be cost effective.

As a very broad-based characterization, quenchants can be divided into the following general categories:

Liquid quenchants (e.g., water-based, oils, polymers, molten salts, and molten metals)
Gaseous quenchants (e.g., air, nitrogen, argon, hydrogen, steam, carbon dioxide, sulphur dioxide, reducing gases, protective atmospheres — synthetic or generated, high-pressure gases)
Solid quenchants (e.g., water-cooled dies, plates, powders)
Mixed media quenchants (e.g., mist or fog quenching, fluidized beds)

Figure 2. Ishikawa (aka fishbone) diagram of quenching variables (See Reference 1)

Selection of the Optimal Quench Medium

Various factors must be taken into consideration when selecting the best quench medium. The following are some of the important considerations when selecting the proper quench medium (Figure 2):

Material — chemistry, hardenability, form (e.g., bar, plate, forging, casting), type (e.g., wrought, powder metal), and cleanliness to name a few
Part geometry/design — shape, size, weight, complexity
Mill or preheat treatment condition — annealed, normalized, pre-hardened, stress-relieved
Stress state — the cumulative effect of both mill operations and customer manufacturing operations prior to heat treatment
Loading — baskets (alloy, C/C composites, graphite plates, etc.)
Process parameters — temperature, time, preheating
Equipment selection — is it optimal or simply adequate for the job?
Quench medium(s) available — their limitations as well as their advantages

It is important to talk briefly here about two aspects of the quench medium selection process. First, note the difference between hardness and hardenability (which we will discuss in more detail in the future). Heat treaters tend to focus on hardness (since we can easily measure it in our shops), but hardenability is a critical consideration in quench medium selection. Hardenability is a material property independent of cooling rate and dependent on chemical composition and grain size. When evaluated by hardness testing, hardenability is defined as the capacity of the material under a given set of heat treatment conditions to harden “in-depth.” In other words, hardenability is concerned with the “depth of hardening,” or the hardness profile obtained, not the ability to achieve a particular hardness value. When evaluated by microstructural techniques, hardenability is defined (for steels) as the capacity of the steel to transform partially or completely from austenite to a defined percentage of martensite.

Table 1. Average and instantaneous values of the heat transfer coefficient (See Reference 3)

Second, one must be aware of both the average and instantaneous value of the heat transfer coefficient alpha of the quench medium. Although the maximum quenching “power” may be described by the instantaneous heat transfer coefficient, the average heat transfer coefficient (Table 1) provides a better relative comparison of the various quenching media since it represents the value of the heat transfer coefficient over the entire range of cooling (from the start to the end of quenching). It is important to remember that the ability to manage (not control) distortion is a delicate balancing act between uniform heat extraction and proper transformation.

A Common Example — Quench Oil Selection

Important factors to consider when selecting a quench oil, which hold true in a slightly modified form for most liquid quenchants, are: the type of quenchant (i.e., quench characteristics, cooling curve data — new and over time); quench speed (see Table 2); usage temperature; effective quench tank volume (i.e., the one gallon per pound of steel [8.4 L/kg] rule); and the client’s requirements.

Table 2. Classification of quench oils (See Reference 1)

Quench tank design factors also play an important role and involve the following:

Volume of oil in the quench tank
Number of agitators or pumps
Location of agitators
Type of agitators (fixed or variable speed)
Internal tank baffle arrangement (draft tubes, directional flow vanes, etc.)
Quench elevator design (i.e., flow restrictions)
Quenchant flow direction (up or down through the load)
Propeller size (diameter, clearance in draft tube)
Maximum (design) temperature rise of the oil after quenching
Height of the oil over the workload
Heat exchanger — type, size, heat removal rate (instantaneous BTU/minute)
Oil recovery time to setpoint

Finally, consideration must be given to factors such as: part mass; part geometry (e.g., thin and thick sections, sharp corners and holes, gear tooth profile/modulus, thread profile, etc.); part spacing in the load; effective flow velocity through the quench area (empty and with a load); stress state from prior (manufacturing) operations; post heat treat operations to be performed (if any); loading including the grids, baskets, and fixture (material and design); and the material (chemistry and hardenability).

Final Thoughts

Quenching, considered by many to be a complex and multi-faceted subject, is one heat treaters must constantly monitor and control. In future installments we will be discussing many of the individual aspects of quenching. What is important here is to recognize that done correctly, quenching
(in whatever form) will optimize a given heat treatment and help produce the highest quality parts demanded by the industries we serve.

References

Daniel Herring, Atmosphere Heat Treatment, Volume II: Atmospheres | Quenching | Testing (BNP Media Group, 2015).

Božidar Liščić et al., Quenching Theory and Technology, Second Edition (CRC Press, Taylor Francis Group, 2010).

Daniel Herring, “A Review of Gas Quenching from the Perspective of the Heat Transfer Coefficient,” Industrial Heating, February 2006.

About the Author

Dan Herring
“The Heat Treat Doctor”
The HERRING GROUP, Inc.

Dan Herring has been in the industry for over 50 years and has gained vast experience in fields that include materials
science, engineering, metallurgy, new product research, and many other areas. He is the author of six books and over
700 technical articles.

For more information: Contact Dan at dherring@heat-treat-doctor.com.

For more information about Dan’s books: see his page at the Heat Treat Store.

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Ask the Heat Treat Doctor®: How Does One Determine Which Quench Medium To Use? Read More »

Consulta a The Heat Treat Doctor®:¿Cómo determinar cuál medio detemple utilizar?

October 8, 2024 / HARDENING TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH, QUENCHING TECHNICAL CONTENT

This article was originally published in Heat Treat Today‘s September 2024 People of Heat Treat print edition.

To read the article in English, click here.

El temple es un paso fundamental en el proceso de tratamiento térmico. Y si bien el especialista en tratamiento térmico suele tener varias opciones disponibles, existe un delicado equilibrio entre lo que está disponible para nosotros y cómo podemos optimizar sus características de rendimiento para cumplir con los requisitos/especificaciones de nuestros clientes. Se deben tener en cuenta cuidadosamente el material, el diseño de la pieza (geometría), los requisitos previos y posteriores de manufactura, la carga, el cambio dimensional permitido (es decir, la distorsión) y el proceso como tal. Conozcamos más.

Medios de temple: una breve Descripción

Los medios de temple actuales ofrecen una amplia gama de capacidades que, en algunos casos, se traslapan. Sin embargo, en un nivel fundamental, la función de un medio de temple es extraer calor de la superficie de la pieza para cumplir con una velocidad crítica de enfriamiento especificada y con ello lograr la microestructura necesaria para lograr las propiedades mecánicas y físicas requeridas. En el temple de aceros, por ejemplo, se debe evitar pasar por la “nariz” de la curva de transformación-tiempo-temperatura (TTT) si el resultado final deseado es una microestructura martensítica (o bainítica). Por el contrario, la velocidad de enfriamiento para un proceso de normalización requiere enfriamiento “al aire”, un término que a menudo se malinterpreta y que abordaremos en una discusión futura.

Figura 1. Medios de Temple comunes y su efecto en la distorsión (1)

Sin embargo, un medio de temple (Figura 1) es más que solo su velocidad de enfriamiento. Los medios de temple deben ser estables durante su vida útil, especialmente con respecto a la degradación (por ejemplo, oxidación), ser seguros, ser fáciles de arreglar y mantener, tener un alto punto de vaporización, idealmente no interactuar con la superficie de la pieza, usarse dentro de su rango de rendimiento óptimo, tener una larga vida útil, eliminarse fácilmente mediante limpieza después del temple y ser rentables.

A manera de una caracterización muy amplia, los medios de temple se pueden dividir en las siguientes categorías generales:

Medios de temple líquidos (p. ej., a base de agua, aceites, polímeros, sales fundidas y metales fundidos)
Medios de temple gaseosos (p. ej., aire, nitrógeno, argón, hidrógeno, vapor, dióxido de carbono, dióxido de azufre, gases reductores, atmósferas protectoras sintéticas o generadas, gases a alta presión)
Medios de temple sólidos (p. ej., dados de prensa enfriados, placas y polvos)
Medios de medios mixtos (p. ej., temple por aspersión, lechos fluidizados)

Figura 2. Diagrama de Ishikawa (también conocido como de pescado) de las variables de temples (1)

Selección del medio de temple óptimo

Se deben tener en cuenta varios factores al seleccionar el mejor medio de temple. A continuación, se enumeran algunos de los aspectos importantes a tener en cuenta al seleccionar el medio adecuado (Figura 2):

Material: composición química, templabilidad, forma (p. ej., barra, placa, forja, fundición), tipo (p. ej., forjado, sinterizado) y limpieza, por nombrar algunos
Geometría/diseño de la pieza: forma, tamaño, peso, complejidad
Estado de laminación o tratamiento térmico previo: recocido, normalizado, preendurecido, relevado de esfuerzos
Estado de tensión: el efecto acumulativo de las operaciones de laminación y las operaciones de fabricación del cliente antes del tratamiento térmico
Carga: canastillas (aleación, compuesto C/C, placas de grafito, etc.)
Parámetros del proceso: temperatura, tiempo, precalentamiento
Selección del equipo: ¿es óptimo o simplemente adecuado para el trabajo?
Medio(s) de temple disponibles: sus limitaciones y ventajas

Es importante hablar brevemente aquí sobre dos aspectos del proceso de selección del medio de temple. Primero, observar la diferencia entre dureza y templabilidad (que analizaremos con más detalle en el futuro). Los tratadores térmicos tienden a centrarse en la dureza (ya que podemos medirla fácilmente en nuestro taller), pero la templabilidad es una consideración crítica en la selección del medio de temple. La templabilidad es una propiedad del material independiente de la velocidad de enfriamiento y dependiente de la composición química y el tamaño del grano. Cuando se evalúa mediante pruebas de dureza, la templabilidad se define como la capacidad del material bajo un conjunto dado de condiciones de tratamiento térmico para endurecerse “en profundidad”. En otras palabras, la templabilidad se relaciona con la “profundidad de endurecimiento”, o el perfil de dureza obtenido, no con la capacidad de alcanzar un valor de dureza particular. Cuando se evalúa mediante técnicas microestructurales, la templabilidad se define (para aceros) como la capacidad del acero para transformarse parcial o completamente de austenita a un porcentaje definido de martensita.

Tabla 1. Valores medios e instantáneos del coeficiente de transferencia de calor (3)

En segundo lugar, se debe tener en cuenta tanto el valor medio como el instantáneo del coeficiente de transferencia de calor alfa (α) del medio de temple. Aunque la “potencia” máxima de temple se puede describir mediante el coeficiente de transferencia de calor instantáneo, el coeficiente de transferencia de calor promedio (Tabla 1) proporciona una mejor comparación relativa de los diversos medios de temple, ya que representa el valor del coeficiente de transferencia de calor en todo el rango de enfriamiento (desde el inicio hasta el final del temple). Es importante recordar que la capacidad de gestionar (no controlar) la distorsión es un delicado acto de equilibrio entre la extracción uniforme del calor y la transformación adecuada.

Tabla 2. Clasificación de los aceites de temple (1)

Un ejemplo común: selección de aceite de temple

Los factores importantes a tener en cuenta al seleccionar un aceite de temple, que son válidos en una forma ligeramente modificada para la mayoría de los medios líquidos, son: el tipo de medio (es decir, características del temple, datos de la curva de enfriamiento, nuevo y a lo largo del tiempo); velocidad de temple (consulte a Tabla 2); temperatura de uso; volumen efectivo del tanque de enfriamiento [es decir, la regla de un galón por libra de acero (8,4 L/kg)]; y los requisitos del cliente.

Los factores de diseño del tanque de temple también juegan un papel importante e involucran lo siguiente:

Volumen de aceite en el tanque de temple
Número de recirculadores o bombas
Ubicación de los recirculadores
Tipo de recirculadores (velocidad fija ovariable)
Disposición de los deflectores internos del tanque (tubos de aspiración, álabes de flujo direccional, etc.)
Diseño del elevador de temple (es decir, restricciones de flujo)
Dirección del flujo del temple (hacia arriba o hacia abajo a través de la carga)
Tamaño de la propela (diámetro, espacio libre en el tubo de aspiración)
Máximo incremento dela temperatura (diseño) delaceite después del temple
Altura del aceite sobre la carga
Intercambiador de calor: tipo, tamaño, tasa de extracción de calor (BTU instantáneos/minuto)
Tiempo de recuperación del aceite hasta el set point

Por último, se deben tener en cuenta factores como: la masa de la pieza; la geometría de la pieza (por ejemplo, secciones delgadas y gruesas, esquinas y barrenos afilados, perfil de los dientes del engrane, perfil de la rosca, etc.); espaciamiento de la pieza en la carga; velocidad de flujo efectiva a través del área de temple (vacía y con carga); estado de tensión de operaciones anteriores (de manufactura); operaciones de tratamiento térmico posteriores a realizar (si las hay); carga, incluidas las charolas, las canastillas y el herramental (material y diseño); y el material (composición química y templabilidad).

Reflexiones finales

El temple, considerado por muchos como un tema complejo y multifacético, es un asunto que los especialistas en tratamiento térmico deben supervisar y controlar constantemente. En futuras entregas, analizaremos muchos de los aspectos individuales del temple. Lo importante aquí es reconocer que, si se realiza correctamente, el temple (en cualquier forma) optimizará un tratamiento térmico determinado y ayudará a producir las piezas de la más alta calidad que exigen las industrias a las que prestamos nuestros servicios.

Referencias

Daniel Herring, Atmosphere Heat Treatment, Volume II: Atmospheres | Quenching | Testing (BNP Media Group, 2015).

Bozidar Liscic et al., Quenching Theory and Technology, Second Edition (CRC Press, Taylor Francis Group, 2010).

Daniel Herring, “A Review of Gas Quenching from the Perspective of the Heat Transfer Coefficient,” Industrial Heating, February 2006.

Sobre el autor

Dan Herring ha trabajado en la industria durante más de 50 años y ha adquirido una vasta experiencia en campos que incluyen ciencia de materiales, ingeniería, metalurgia, investigación de nuevos productos y muchas otras áreas. Dan es
autor de seis libros y más de 700 artículos técnicos.

Para más información: Comuníquese con Dan en dherring@heat-treat-doctor.com.

For more information about Dan’s books: see his page at the Heat Treat Store.

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Thermal Processing for EV Components

August 20, 2024 / AUTOMOTIVE HEAT TREAT TECHNICAL CONTENT, HARDENING TECHNICAL CONTENT, NITROCARBURIZING TECHNICAL CONTENT

The advent and increasing adoption of electric vehicles (EVs) has brought a wave of change to the automotive supply chain, including the heat treating industry. While the internal combustion engine (ICE) and all its related components may one day become a thing of the past, there are several key areas of every vehicle that aren’t going anywhere fast. In this Technical Tuesday article, Rob Simons, metallurgical engineering manager at Paulo, discusses the difference between EV and ICE vehicles and the latest heat treating trends to be aware of.

This informative piece was first released in Heat Treat Today’s August 2024 Automotive print edition.

ICE vs. EV Technology

The most apparent difference between EVs and ICE vehicles is that, with EVs, fuel and internal combustion engines are no longer needed. The two vehicle types rely on different sets of key components, and when it comes to making the cars run, EVs use fewer parts that require heat treatment.

Table 1. Existing ICE technology vs. EV technology

Without ICE systems, EVs require fewer fasteners, shafts, gears, and rods — all parts that are typically heat treated. But that doesn’t mean heat treatment is less critical for EVs. In fact, certain parts require additional attention on EVs when compared to ICE vehicles, and many safety-critical parts remain the same across both categories. Let’s begin our discussion with the differences in braking systems between the two technologies and what that means for heat treatment.

Latest Trends in Disc Brake Rotors

How EV Brake Systems Work

There’s no question that electric power innovations have completely revolutionized the way vehicles (and the automotive industry) operate. The regenerative braking system is just one aspect of this. Instead of relying on the conventional hydraulic system every time you press the brakes (which uses friction to decelerate), manufacturers have found a way to use the vehicle’s kinetic energy to put the electric motor into reverse, slowing down the vehicle and returning energy to the battery.

Although regenerative braking is more efficient, hydraulic braking still has one key advantage: stopping power. EVs today are equipped with conventional braking mechanisms for emergency purposes.

The Rust Conundrum

To address recurring rotor corrosion, heat treaters introduced ferritic nitrocarburizing (FNC). FNC is a thermal process traditionally used for case hardening, and for brake rotors, it’s used to achieve corrosion resistance.

The Solution: Corrosion-Resistant Rotors with FNC

Figure 1 shows a perfect example of the difference that FNC makes. These are pictures of brake rotors from electric vehicles owned by two Paulo team members — one has brake rotors that were ferritic nitrocarburized and show no signs of rust, whereas the other did not go through the FNC process.

Figure 1a. EV brake rotor without FNC Source: Paulo

Ferritic Nitrocarbonizing Process

FNC is a case hardening technique that uses heat, nitrogen, and carbon to toughen up the exterior of a steel part, improving its durability, decreasing the potential for corrosion, and enhancing its appearance. FNC is unique in that it offers case hardening without the need to heat metal parts into a phase change (it’s done between 975–1125°F). Within that temperature range, nitrogen atoms can diffuse into the steel, but the risk of distortion is decreased. Due to their shape and size, carbon atoms cannot diffuse into the part in this low-temperature process. However, carbon is necessary in the FNC process to generate desirable properties in the intermetallic layer.

Heat Treated Materials for Automotive Seating Components

Safety-Critical Components

Like brake rotors, many automotive seating components (like mechanisms for seat recliners) are here to stay. Thermal processing is used to achieve stringent specifications that are put in place to keep drivers safe in the event of a collision. EV seat components and the thermal processes used to make them crash-ready are identical to those of ICE vehicle components.

Figure 2. To achieve the stringent specifications for components like seat recliners, identical thermal processing is implemented for both EVs and ICE vehicles.

Seating Components

Generally, these components are case hardened (either carburized or carbonitrided), typically using one of the following materials:

1010 and 1020 carbon steel: These are plain carbon steel with 0.10% carbon content, fairly good formability, and relatively low strength.
1018 carbon steel: 1018 is a grade that’s often chosen for parts that require greater core hardness and better heat treatment response than 1010 or 1020.
10B21 boron steel: Boron steels are becoming more popular in the automotive industry due to their excellent heat treatment response.
4130 alloy steel and 8620 alloy steel: Alloy steels are more responsive to heat treatment than plain carbon steels, so the thermal processing specifications for parts made from these materials are often adjusted to account for the material’s innate strength properties.

Seat Belt Latches

High-strength seat belt latches are usually made from the following materials:

4140 and 4130 alloy steels: 4140 alloy steel is one of the most common engineering steels used in manufacturing. For seat latches and hooks, 4140 and 4130 will be neutral hardened to increase their strength and hardness throughout due to the high performance and precision required of these parts.
1050 carbon steel: 1050 is a medium carbon steel that contains 0.47–0.55% carbon content. Carbon steels are a less expensive choice when compared to alloy steels such as 4140 or 4130.

Seat Frames and Brackets

Seat frames (also known as seat brackets) give car seats their shape using slender pieces of steel joined together to form the skeleton of the seat. These components are often made from boron steels:

10B21 or 15B24 boron steel: These are a good choice for seat brackets because they are only marginally more expensive than other steels used in seating but have impressive toughness, have a good heat treat response, and are weldable.

A Closer Look: Case Hardening for Seating Components

Case hardening diffuses carbon or carbon and nitrogen into the surface of a metal from the atmosphere within a furnace at high temperatures. Adding carbon or carbon and nitrogen to the surface of steel hardens a metal object’s surface while allowing the metal deeper underneath to remain softer, creating a part that is hard and wear-resistant on the surface while retaining a degree of flexibility with a softer, more ductile core. This softness and ductility create toughness in parts, allowing them to respond to stress without failing. Case hardening is a general term for this heat treating method. Depending on the materials and specifications for the part, we apply various case hardening techniques, including carburizing and carbonitriding.

Figure 3. When it comes to heat treating, innovations are rarely exclusive to EVs.

Carbonitriding

During carbonitriding, parts are heated in a sealed chamber well into the austenitic range — around 1600°F — before nitrogen and carbon are added. Because the part is heated into the austenitic range, a phase change occurs, and carbon and nitrogen atoms can diffuse into the part. Carbonitriding is used to harden surfaces of parts made of relatively inexpensive and easily machined or formed steels, which we often see in automotive metal stampings. This process increases wear resistance, surface hardness, and fatigue strength. It is also good for parts that require retention of hardness at elevated temperatures.

Neutral Hardening

Also called through hardening, neutral hardening is a very old method for hardening steel. It involves heating the metal to a specified temperature and then quenching it, usually in oil, to achieve high hardness/strength. In this process, the primary concern is increasing hardness throughout the part, as opposed to generating specific properties between the surface and the core of the part.

All of the metal components of a seat belt, including seat belt loops, tongues, and buckles, are neutral hardened. Specifications typically dictate that these components are hardened to up to 200 thousand pounds per square inch (ksi).

Because seat belt components are visible to the end consumer, their cosmetics are important in addition to their mechanical properties. It’s important to keep the furnace free of soot and thoroughly clean the parts both before and after heat treatment. Proper cleaning readies the part for secondary processing, ensuring the success of activities like polishing and chrome plating.

The Convergence of EV and ICE Vehicles

To learn more about automotive heat treating, download the free Paulo Heat Treat Guide at paulo.com/AutoGuide.

The EV revolution has significantly transformed automotive manufacturing. Despite these changes, EV parts remain remarkably similar to those of their internal combustion engine (ICE) counterparts. Consequently, any advancements in materials or heat treating processes are swiftly adopted across the entire automotive sector. When it comes to heat treating, innovations are rarely exclusive to EVs.

About the Author:

Rob Simons
Metallurgical Engineering Manager
Paulo

Rob provides internal and external customer support on process design, material behavior, job development, reduction of variation, and physical analyses at Paulo. He holds a Bachelor of Science in Metallurgical Engineering from the Missouri University of Science & Technology (formerly known as the University of Mines and Metallurgy) and has worked at Paulo since 1987. Rob has analyzed several million hardness data points and/or process behaviors, leading him to develop many process innovations in the metallurgical field.

For more information: Contact Rob at rsimons@paulo.com.

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Heat Treat Radio #112: Lunch & Learn: How To Use a Hardenability Chart

August 15, 2024 / HARDENING TECHNICAL CONTENT, HEAT TREAT RADIO, QUENCHING TECHNICAL CONTENT

In this episode of Heat Treat Radio, Doug Glenn discusses the hardenability of materials with guest Michael Mouilleseaux, general manager at Erie Steel LTD. Michael walks us through how to interpret hardenability charts and provides detailed insights on reading these charts, including addressing the importance of understanding the nuances of complicated part geometry.

Below, you can watch the video, listen to the podcast by clicking on the audio play button, or read an edited transcript.

HTT · Lunch & Learn: How To Use a Hardenability Chart

The following transcript has been edited for your reading enjoyment.

Understanding a Hardenability Chart (01:59)

Doug Glenn: What I’d like to do is talk through this chart and learn how to read this a little bit better. And I’d like to ask questions about it because I’m not familiar with this, and I’m sure there are going to be some listeners and viewers who aren’t familiar with it. This will be just a quick tutorial on how to read these charts.

Go to the upper, right-hand corner. First off, SAE 4320H is the grade of the steel that we’re talking about?

The Heat Treat Lunch & Learn crew: Doug Glenn, Publisher of Heat Treat Today; Michael Mouilleseaux, General Manager at Erie Steel LTD.; Bethany Leone, Managing Editor of Heat Treat Today
Use this chart to follow along with the conversation.
Source of chart: Erie Steel, Ltd.

Michael Mouilleseaux: Correct.

Doug Glenn: Then the table right below that you’ve got percentage C (carbon). Is Mn manganese?

Michael Mouilleseaux: Manganese.

Doug Glenn: Thank you very much. Silicon, nickel, chrome, moly. My question is about those ranges. Is this basically saying the percentage carbon on the far left in 4320H goes anywhere from 0.17–0.23?

Michael Mouilleseaux: That is correct.

Doug Glenn: Okay. So that’s variability right there. All of those are basically telling you what the ranges are in those alloys in this grade of steel?

Michael Mouilleseaux: That is correct.

Doug Glenn: Then you go down to the top columns of this table below, and it says “Approximate diameter of rounds with same as quenched HRC in inches.” Right?

Approximate diameter of rounds with same as quenched HRC in inches
Source: Erie Steel, Ltd.

Michael Mouilleseaux: Yeah. Essentially, the first three rows are for water quenching. And the bottom three are for oil quenching.

Doug Glenn: If you go over to the second major column called “Location in round,” what’s the size of the round we’re working on here?

Michael Mouilleseaux: It can vary. Go down to where it says, “Mild Oil Quench,” then left to “Surface,” then left then go to “2 inches.” Then, go straight down to the bottom, and that’s approximately J5. So, the “Distance from Quenched End — Sixteenths of an Inch” is Jominy position 5.

Michael Mouilleseaux: If you go to Jominy position 5 on the left-hand chart, you can see the hardness limits for that; the maximum is Rockwell C 41, and the minimum is Rockwell C 29. So, the chemistry can vary provided the hardenability at J5 is 29–41.

Doug Glenn: That’s the acceptable range?

Michael Mouilleseaux: That’s the acceptable range. That’s one way of looking at it. The chemistry would allow you to do that.

Now, go back to the chart on the right-hand side and to “Surface,” move down one row to “¾ radius from center,” and go left to two inches. Moving down from there you see that is Jominy position 8. So, the surface of a two-inch round is Jominy position 5, and the ¾ radius is Jominy position 8.

If you go to the hardness chart on the left-hand side, that says that if you had a two-inch round of 4320H, and it was oil quenched, and you check the hardness at ¾ radius, then the expectation is that it would be 23–34.

Now, go back to the same chart that we were just at, and go to the “Center” row of “Mild oil quench.” Continue left to two inches, and that’s J12. Go back to the left-hand chart, and J12 is 20–29 in the center of the part.

So, the surface of the part could be 41, ¾ radius, center of the part would be 34, and the center of the part would be 29.And that would all meet the criteria.

Doug Glenn: The maximum for J5 would be 41.But at J12 you could get a 20 in the middle.

Michael Mouilleseaux: Right. That is one way to look at this chart. But there is another way.

Notice that it says “rounds.”There are some nuances to having flats and rectangles because, if you think about it, for the cross-sectional area of a rectangle, the hardenability is going to be determined by the direction that it is thinnest, not by the direction that it is thickest.

Take a gear tooth, for example: in the chart that we just made up the gear teeth, the root of the gear was about a half inch, just slightly more; and if we go to this same chart, go to “Center” of “Mild oil quench,” and then go to a “0.5 inch,” and when you go straight down, that’s the J3.

Is a gear necessarily a round? Of course, the answer to that is no. So, in complex shapes you can use this data, but you have to interpolate it in order to understand it.

To some extent, the first time you run this, you’re going to say, “I have a gear, and the root is a half inch across. And I know that the J3 is 40. And I’ll run this part, and I’ll section it and I’ll measure it and it’s 40. And I’ll say that’s a good approximation of that.” And experientially, you build confidence in this, that is, it’s your operation, your quenching operation, and your components. It allows you to interpolate these, and they become extremely useful.

So, is it definitive? No. Is it useful? Yes.

Doug Glenn: It gives you a ballpark, right? I mean, it’s giving you something, maybe guardrails.

Michael Mouilleseaux: It gives you a ballpark; it gives you guardrails. And I can tell you that after having run gear product in the same equipment for ten years, I can say that it’s definitive. I can say that if I have this hardenability, and I get this hardenability number for this heat, and these gears are made from this heat of steel, and it has a J3 of 42. If I’m at 38, I know something is going on other than just hardenability. And, at that point, I would suspect my heat treat operation.

Doug Glenn: Yeah. I have one more question about this chart: On the bottom right part of the graph there are two plot lines on there. What do those represent? I was thinking one represented the water quench and the bottom one represents the oil quench.

Plot lines representing maximum hardenability and minimum hardenability
Source: Erie Steel, Ltd.

Michael Mouilleseaux: The top one represents the maximum hardenability. And the lower the lower one represents the minimum hardenability.

Doug Glenn: That’s your band. Okay. Those are basically your values over on the left-hand side then. Very good.

I don’t know about you, but I found that helpful. I really didn’t ever know how to read these tables. So, maybe someone else will find that useful. Thanks, Michael. I appreciate your expertise.

Michael Mouilleseaux: It’s been my pleasure.