This interview was with Craig Zimmerman and Rich Shapiro from Bluewater Thermal Solutions. Craig is Technical Director for all of Bluewater and Rich is General Manager of Plant 1. Both are metallurgists.

During the interview we discussed:

Interview transcript

Michael: Good afternoon. Today I am with Rich Shapiro and Craig Zimmerman, from Bluewater Thermal. Bluewater Thermal provides heat-treating services for a variety of industries.

Rich Shapiro is the General Manager of Plant 1, in the Chicago area, and Craig Zimmerman is the Technical Director for all of the Bluewater Thermal plants, and both of them are very seasoned metallurgists. Welcome!

Both: Thank you.

Michael: Tell me briefly, what are the capabilities of Bluewater Thermal?

Craig: I would say that we’re a full service heat-treat provider. 90% of our business is probably ferrous steels or irons, although we do have one plant where we do exclusive heat-treatment of aluminum alloys there.

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(0:48) Michael: Do you ever run into situations where someone, they send you a component and they send you a drawing, and the specifications on the drawing are either incomplete or confusing?

Both: Yes.

(1:05) Michael: What happens in a situation like that?

Craig: Really, the main thing is to call whoever sent that drawing in. Let’s say it’s a buyer, and they don’t have the answers. It’s really a matter of getting to the technical people, and again, clarifying what their requirements are.

We get a lot of drawings that have the wrong name for processes on them, where it will say like “soft nitride” or things like that. There’s a lot of clarifying questions that happen when you get a drawing in, especially older drawings that may refer to some old terminology that’s not currently used anymore. There’s usually a lot of questions that go on, if that doesn’t make sense.

When you’re carburizing, there’s always a total case and effective case depth. A lot of times, the drawing will just say “carburize to the case depth.” We’ll have to ask the question “Do you want a total case depth or an effective case depth?”, which is how it’s measured and inspected. It gives you two different values. That’s another thing that happens, that’s confusing at times.

Rich: It may be, in a case where it’s total or effective case depth, sometimes the customer may not know the difference. So, you have to explain the difference in how you measure it, and what that difference is, between a total and effective case depth.

(2:24) Michael: In a case where they’re specifying the case depth, or even specifying the hardness, do you ever see situations where someone would specify something that you realize this may not work in what they’re trying to do, or it might not be the best thing in what they’re trying to do?

Craig: There’s that, where you wonder if the case is too thick on a thin part, is the entire part going to be brittle and want to crack, because there’s not enough core present in the part?

The other difference we see is distortion. A lot of times we’ll get parts, you’ll see a drawing of a thin, long part, and it will have a certain heat-treat listed on it where you know that you’re going to have to heat this part up to a very high temperature and maybe quench it rapidly, and you know that the part is going to warp or curl up or change size, to where it won’t be usable when it goes back to the customer again, because all the dimensional tolerances could be ruined by that particular heat treat.

You want to kind of alert your customer that “Hey, we can meet these requirements, but it’s really going to cause havoc to your dimensions,” and maybe discuss other options or other things you can do that might make your manufacturing a lot easier on it.

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(3:31) Michael: That makes sense. When you realize that there might be a problem, or you need more clarification, how does that affect the timing for how the project will go?

Craig: You usually try and catch all of that at the quoting stage, before they’re even making the parts or before the parts are even close to arriving in our facility. It’s when we’re coming up with the pricing estimates and those types of things, that’s when we kind of do our contractor deal. Hopefully, you can catch those things up front, the first time you see the drawing and the first time you’re maybe giving them a price for the heat treat, you can raise a lot of those questions.

Rich: It can delay a process, if you’re talking strictly on how long does that take. It really depends on how much back and forth between us and the customer. If they’re very knowledgeable, it may take a very short period of time. If they’re very knowledgeable, but we have to run some trials or something like that, it may extend the time.

You could resolve an issue like that in a week. It could take six months. It depends on the technical knowledge of who we’re dealing with. We feel pretty good at Bluewater, with the amount of metallurgists that we have, and the experience we have, that we are pretty good about knowing the outcomes. Sometimes, it’s a matter of convincing the customer of what those outcomes may be, good or bad.

Craig: A lot of times, our customer has to go back to their customer, or to the OEM, even. Sometimes you have to work your way backward up that chain with our questions, because our customer, they may be a stamper or a machine shop or a fine blanker. They may not have any idea, so they’ll have to work their way back up the chain to the OEM and ask those questions to the OEM. Sometimes that can take days, to get responses.

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(5:17) Michael: In thinking about the information on drawings, for a steel part that’s going to be through-hardened, what do you want to see on a print?

Craig: Pretty much the only thing you need for through-hardening is what hardness do you want, because it should be the same hardness pretty much through the entire cross-section of the part.

You may also get like an industrial specification where they see “heat treat for a General Motors spec,” or a John Deere spec or a Caterpillar spec, and they’ll have requirements on what they want to see out of us for furnace uniformity and how we’re calibrating our equipment, and things like that that aren’t really directly related to the heat treat, but that kind of create rules for us to work under.

So, quench and tempered, harden-tempered type of things, it’s usually “heat treat per this specification,” to a certain hardness level or a certain tensile strength, which we can convert back to hardness for ourselves.

Rich: One of things that I’ve always never really been a great fan of is you tell us the process and the outcome. Tell us the process, and if you’re telling us the process, okay, you’re going to get that outcome. If you tell us the outcome, we’ll figure out the process. But if you tell us the outcome and the process, then it can be a real challenge sometimes.

Typically, what I would like to see is one or the other. “Here is the process that you need to follow.” Okay, we’ll follow that process. Or tell us the outcome. We, as the metallurgists, will figure out the process.

(6:57) Michael: I agree with you.  If it’s a trusted supplier, a competent supplier, let them figure out how to do it. In the end, as a designer, all they should care about is the outcome. The mechanical properties of the part and the dimensions of the part.

When someone dictates things like the process, or other things that they have to monitor, what does that do to the complexity of the project, and what does that do to the cost of the project?

Craig: I would say one of the things where our hands are tied the most is a lot of times, we’re not allowed to choose the material or the grade of steel that’s being used. A lot of customers will pick certain grades because it’s easy to weld or it’s easy to stamp, or it’s easy to form, but a lot of those steels don’t have great hardenability.

So, when we go to heat treat them, especially thicker or larger parts, we may not be able to reach the hardness levels they’re looking for, with whatever quench we’re using, because they’ve kind of tied our hands by using a plain carbon steel instead of a low alloy steel, or a low hardenability steel, instead of something with a little higher hardenability.

So that, we always run into that kind of tug-of-war between us and maybe a stamping company, where the stamping company wants a low carbon steel that forms easy and stamps easy. But then it comes to the heat treat, and it’s much more difficult for us to heat treat, because it’s such a low hardenability grade. Maybe we’re not able to quench the core of that part up to the hardness levels that they’re looking for, if the part has got some thickness to it, where it can’t cool very rapidly in the quench.

We get things all the time where someone might say “I read that 1045 steel can be hardened to 55 Rockwell C.” But what they don’t always understand is that maybe a ½” thick piece of 1045 can be oil quenched up to 55 Rockwell C, but if you send us in a 4” diameter bar, we might only be able to get a 30 Rockwell C out of that same piece of 1045, just because the larger size can’t cool as fast in the quench. It doesn’t have great hardenability, so you don’t get the full hardness out of it, that your steel data sheet might tell you that you’re able to get.

Then you have to suggest “Maybe you should make this out of 4140 or 4340,” that has higher hardenability, and you’re able to through-harden it more easily, with a slower cooling rate. It might be necessary, because the part’s a massive part.

(9:06) Michael: You make some great points. Considering all the requirements for alloy selection is important if design teams want to have fewer component fabrication problems and decrease development time. In many cases, design engineers are selecting the same alloy used in previous designs, not realizing that, compared to previous designs, the differences in component shape or dimensions are enough to require using a different alloy. Or the design engineer selects an alloy used in another application and isn’t aware of all the metallurgical considerations for using the alloy.

(9:45) Michael: Why do the OEMs have specifications that call out the process? Heat treaters have a lot of experience. Why does the OEM do that?

Craig: I can tell you one story. We have a part that we heat treat where I know that the part used to be heat-treated inside that company’s captive heat treat department. They have their own recipes, they ran these parts in their own furnaces, and they knew exactly how to run them.

So, when they went to outsource that work and sent it to a commercial heat treater like us, they pretty much wrote a specification that said run it exactly the same way that we do it. Although furnaces can differ, so maybe we don’t always get the same results as them. But a lot of times, when it’s specified that highly, sometimes it is things that were heat treated captively, and now they’re outsourcing it, and they want us to follow their recipe right to the letter of exactly how they ran it in their shop.

Rich: We had a great example of that, as well, in this plant. It was a normalize job. Our forging customer shut down a furnace. They sent the parts here for normalizing. They said “Here’s the process that we want you to follow.” We tried for quite a long time to follow that process, tweak it here, tweak it there, tweak temperatures here, tweak some time there, to follow their process, but we just couldn’t quite do it.

Finally, we met with them and said “Okay, let’s review this. Let’s review what kind of furnace you had, what kind of furnace we have.” We ultimately reached a process that worked, that met theirs. It didn’t exactly follow their process, but it got to the end result, because we had to adapt their process to our furnace.

(11:35) Michael: It seems to me that the simpler process for them would have been to start off with just saying “We want a certain microstructure. We want certain properties, and you figure out what you need to do, to do that.”

Rich: Let me follow up on that same example, then. As Craig said earlier, we were dealing with a forging shop. The forging shop, this was an automotive part, so they had to go back then and deal with the OEM.

It becomes more difficult for them to then have to have to re-PPAP a part, or resubmit documentation. If they were to just say “They followed our exact process,” then it makes that whole qualification effort, on their behalf, easier. There’s a lot of that that drives it. Once a process is approved, and then you go from captive to commercial, that customer really wants to make their life simple, and not change the process.

(12:32) Michael: It sounds like it added a whole lot of time to the development cycle.

Rich: In this plant, what you think is a simple, normalized process, it’s quite a lot.

(12:44) Michael: Do you ever get design engineers calling you up to talk to you about a part they’re working on, and getting input, asking you for input for the heat treatment that was possible?

Craig: Yes, we’ll get that all time, where they’ll say “Hey, we’re making a part this way. We’re having failures, or the part is wearing out too quickly,” or things like that are happening, and they’re looking for suggestions of a different process they could do or a different type of heat treat they could do, to maybe get higher wear resistance or higher strength, or maybe a different material, looking for ideas on what kind of different materials we think might be possible for them to use. They’re just looking for a better way to make their part last longer.

Michael: It sounds like that’s a situation where they’re looking to make improvements, whether it’s in quality or reliability.

(13:30) Michael: If it’s going to be a case-hardened part, let’s say a case-hardened steel, what information should be included on a drawing?

Craig: For those, we’ll typically have – now you’ll have a case on the outside of the part and a core on the inside of the part, that are going to have different properties. The first thing is they’ll want to specify case depth, so that we know how deep into the steel we’re supposed to treat it. Then, they’ll have a case surface hardness, where they specify how hard is that case supposed to be.

Then, they’ll also have a lower value for what is the core hardness or the core yield strength supposed to be, or core tensile strength.

There are also – sometimes there will be some microstructure requirements, as well, on case hardening, where they’ll say the case needs to be 100% hardened. They’ll have requirements for maximum percentage of retained austenite that may be present, how much inner granular oxidation may be present from the surface, and any carbides or nitride networks forming on the grain boundaries.

So yeah, there’s often a lot of microstructural requirements, as well. Even for the core, as well, you can have a customer say “We want the core to be fully martensitic, or we want the core to show some ferrite.” So yeah, it gets a little more complex with the case-hardening jobs.

Rich: Yeah, and I was going to go back to my gear days of exactly that. A gear in an aerospace application may have a certain amount of retained austenite requirement, because you don’t want that retained austenite to convert to untempered martensite. In a lawnmower application, it may not be as critical. A lot of that all depends on the criticality of what the component is being used for.

In a case like that, then the heat treater does need to know if you need X amount of percent retained austenite, or intergranular oxidation, because obviously in aerospace gearing, you don’t want things like that. You don’t want a bad failure, due to some intergranular oxidation.

(15:36) Michael: What you’re saying is that the detail that’s required in certain parts may be more than in other parts, depending upon the criticalness of the part, and the reliability required for the part.

Rich: That also goes then into the type of thing with the furnace control or the furnace capability, as well. Typically, the aerospace parts are going to require a higher level of uniformity, or certain things on your furnaces, or temperature uniformity surveys. So, the criticality of the part, not only of the heat treat process, but then that also helps drive what the requirements of the furnace are, itself.

Craig: A lot of what goes with that is the frequency, how often you’re doing those tests. You might be doing temperature uniformity surveys weekly, instead of quarterly, or instead of annually, for different industries. System accuracy tests might be done more frequently for aerospace, versus general industrial type parts.

That actually affects the cost of the part. That is, if we have to do more and more of this periodic testing and qualification and calibration of all of our instruments, our furnaces, there’s a cost to that. Doing heat treatment for those critical industries that require us to do increased frequency of inspections and calibrations, it definitely drives the cost up for those parts, compared to customers that use furnaces where we don’t have to do those inspections quite so frequently.

(17:05) Michael: Have there been cases where you’ve talked to someone about their drawing, and you said “No, do it this way,” and you do it for them the way they tell you, after you’ve questioned their specifications, and you give them the part, and it meets their specifications, but it ends up not working. Has that happened?

Craig: Yeah. Probably one of the biggest issues I run into is people want to try a certain process of ours when they’re prototyping, when they’re saying “Let’s try this process on this part, because I think it would work well.” But a lot of times, I’ll run into situations where we’ll heat treat that part, but we don’t know if we can keep it straight enough or keep it round enough, or keep it flat enough.

I run into issues with that quite a bit. We’ll try a process for a company, and say “Hey, we’re going to hope for the best on your prototypes, and see if it works.” Sometimes you get done running those prototypes, and they’ll come back to you and say “Yeah, but the threads on this part changed size too much,” or the diameter grew too much, or the part shrank too much.

Sometimes you can still go back and say “Okay, what if you compensate for that in your incoming sizes? Maybe if you machine the threads a little oversized, or if you make the part a little undersized coming in to us, then we’ll know that it’s going to grow into those dimensions.”

So, sometimes on a second go-around on prototypes, you can fix things that didn’t work in the first go-around, because you saw after the first run, how things changed. Then, they’re able to compensate in the second round.

But there are some things that I’ve run, that I’ve done sample parts for people, and it just doesn’t work. They just can’t, because we heat parts up to such a high temperature, and things want to move around. You’ve got phase transformations going on, that causes the material to shift. Sometimes, you just can’t hold peoples’ dimensions perfectly, that they’re looking for, and for whatever reason, they can’t machine it afterwards.

(18:59) Michael: It sounds like sometimes, especially for a new part, a new design, that people might have to build in some time and expense for doing some experimentation.

Craig: Yeah, and sometimes you into customers that just have parts that we feasibly just can’t run through our furnaces, for whatever reason, or you run into strange geometry parts.

(19:20) Michael: It sounds like the best thing is for them to just call you up and talk to you about it and discuss it, and discuss what options are available.

Rich: To me, it’s communication, communication between the customer and us, or a heat treater. The more communication, the better. That resolves a lot of problems right up front. A lot of times, you work things out before you even touch a piece.

(19:43) Michael: It’s good advice. I think it applies to all aspects of component fabrication.

Thanks for meeting with me to do this interview. It’s been very informative.

Both: Thank you.

Abstract: Annealing is a heat treating process used to modify the properties of cold-worked metal. This article discusses the reasons for annealing, the metallurgical changes that take place within a metal during cold working and annealing, the effects of these metallurgical changes on the properties of metals, and the effects of annealing temperature and time on the final microstructure and properties of annealed metals.

Many metal fabrication processes involve cold-working, such as cold rolling sheet and plate, wire drawing, and deep drawing. Due to metallurgical changes that occur to a metal during cold working, the ductility of a metal decreases as the amount of cold-working increases. There comes a point when additional cold working is not possible without causing the metal to crack. At this point, it is necessary to anneal the metal if continued cold-working is required.

Need metals engineering help with annealed components? We provide metallurgy consulting. info@imetllc.com

During an anneal, metallurgical changes occur that returns the metal to its pre-cold-worked state. These changes result in a reduction of the metal’s yield and tensile strength and an increase in its ductility, enabling further cold working. In order for these changes to occur, the metal must be heated above its recrystallization temperature. The recrystallization temperature for a particular metal depends on the metal’s composition. This specific annealing process is sometimes called a recrystallization anneal, though other names like process anneal are also used.

Metallurgical effects of cold working
During cold-working there is an increase in the number of dislocations in a metal compared to its pre-cold-worked condition. Dislocations are defects in the arrangement of atoms in a metal (discussed in Principles of Metallurgy).

The increase in the number of dislocations causes a metal’s yield and tensile strength to increase and its ductility to decrease.  After a certain amount of cold work, a metal cannot be cold worked further without cracking.  The amount of cold working that a particular metal can withstand before cracking depends on its composition and microstructure.

Want to learn more about annealing and other heat treatments? See our metallurgy courses. Questions? info@imetllc.com

Metallurgical effects of recrystallization anneal
During a recrystallization anneal, new grains form in a cold-worked metal.  These new grains have a greatly reduced number of dislocations compared to the cold-worked metal.  This change returns the metal to its pre-cold-worked state, with lower strength and increased ductility.

With continued time at the annealing temperature, some of the newly formed grains grow at the expense of neighboring grains.  There is some further decrease in strength and increase in ductility as the average grain size increases during the grain growth phase of the annealing process.

The final grain size depends on the annealing temperature and annealing time.  For a particular annealing temperature, as the time at the temperature increases the grain size increases.  For a particular annealing time, as the temperature increases the grain size increases.  A piece of metal with large grains has lower strength and more ductility than a piece of metal of the same alloy with smaller grains.

The figure shows micrographs of a brass alloy that was cold-rolled to 50% of its original thickness and annealed at two different temperatures.  Figure (a) shows the microstructure of the cold rolled sample.  Figure (b) shows the microstructure of a sample that was cold rolled and then annealed at 1022 °F (550 °C) for 1 hour.  Figure (c) shows the microstructure of a sample that was cold rolled and then annealed at 1202 °F (650 °C) for 1 hour.

The cold-rolled sample had a yield strength of 80 ksi (550 MPa).  The sample that was annealed at 1022 °F (550 °C) for 1 hour had yield strength of 11 ksi (75 MPa).  Many small grains are present in this sample.  The sample that was annealed at 1202 °F (650 °C) for 1 hour had yield strength of 9 ksi (60 MPa).  Fewer, large grains were present in this sample compared to the sample in Figure (b).

Need help with a failure analysis? We can help. See our failure analysis page for information. Questions? info@imetllc.com

Other reason for recrystallization anneal
In addition to enabling additional cold-working, recrystallization annealing is also used as a final processing step to produce metal sheet, plate, wire, or bar with specific mechanical properties.  Control of the annealing temperature and time, heating rate up to the annealing temperature, and amount of cold-working prior to anneal is important for obtaining the desired grain size, and therefore the desired mechanical properties.

Imagine you’re a warrior during the middle ages and it’s time to get a new sword. So, you go to a blacksmith to buy a sharp, shiny long sword. A few weeks later you’re in a battle, fighting at the front of the shield wall. You take a huge swing at the enemy, who meets your blow with his sword, and your sword shatters into several pieces. Unfortunately for you, your blacksmith outsourced a batch of swords to a blacksmith on the other side of town who didn’t have time to temper the swords. As a result, the swords were strong, but brittle. Their lack of toughness meant that they could not absorb much of an impact before fracturing.

Want to learn more about steel metallurgy? See our metallurgy courses page.

Tempered martesite

Tempered martensite

Purpose of tempering

Tempering is used to improve toughness in steel that has been through hardened by heating it to form austenite and then quenching it to form martensite. During the tempering process the steel is heated to a temperature between 125 °C (255°F) and 700 °C (1,292 °F). At these temperatures the martensite decomposes to form iron carbide particles. The higher the temperature, the faster the decomposition for any given period of time. The micrograph shows a steel after substantial tempering. The black particles are iron carbide.

Untempered martensite

Untempered martensite is a strong, hard, brittle material. The stronger and harder it is, the more brittle it is. The strength and hardness is a due to elastic strain within the martensite, which is a result of too many carbon atoms being in the spaces between the iron atoms in the martensite. As the amount of carbon in a steel increases (up to about 0.8 weight percent carbon) the martensite strength and hardness increases.

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What happens during tempering

During the tempering process, the carbon atoms move out of the spaces between the iron atoms in the martensite to form the iron carbide particles. The strain within the martensite is relieved as the carbon atoms move out from between the iron atoms in the martensite. This results in an improvement in the steel toughness, at the expense of reduced strength.

Amount of tempering required

The amount of tempering required depends on the particular application in which the steel will be used. In some cases, toughness is not important, so tempering at a low temperature for a short period of time is acceptable. In cases where very strong and tough steel is required a high carbon steel tempered at a high temperature might be used.

More information about steel heat treating is in our online, on-demand courses Principles of Metallurgy or Metallurgy of Steel Heat Treating. The book Steels: Processing, Structure, and Performance by George Krauss provides a comprehensive discussion of steel heat treating.

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Abstract: Recrystallization annealing is a heat treating process used to modify the properties of cold-worked metal.  This article discusses the reasons for preforming a recrystallization anneal, the metallurgical changes that take place within a metal during cold working and annealing, the effects of these metallurgical changes on the properties of metals, and the effects of annealing temperature and time on the final microstructure and properties of annealed metals.

Many metal fabrication processes involve cold-working, such as cold rolling sheet and plate, wire drawing, and deep drawing.  Due to metallurgical changes that occur to a metal during cold working, the ductility of a metal decreases as the amount of cold-working increases.  There comes a point when additional cold working is not possible without causing the metal to crack.  At this point, it is necessary to anneal the metal if continued cold-working is required.

Need help figuring out a component failure or quality problem? We can help. See our failure analysis page. info@imetllc.com

The specific annealing process used is called recrystallization anneal.  During this annealing process, metallurgical changes occur that returns the metal to its pre-cold-worked state.  These changes result in a reduction of the metal’s yield and tensile strength and an increase in its ductility, enabling further cold working.  In order for these changes to occur, the metal must be heated above its recrystallization temperature.  The recrystallization temperature for a particular metal depends on its composition.

Metallurgical effects of cold working

During cold-working there is an increase in the number of dislocations in a metal compared to its pre-cold-worked condition.  Dislocations are defects in the arrangement of atoms in a metal (discussed in Principles of Metallurgy).  The increase in the number of dislocations causes a metal’s yield and tensile strength to increase and its ductility to decrease.  After a certain amount of cold work, a metal cannot be cold worked further without cracking.  The amount of cold working that a particular metal can withstand before cracking depends on its composition and microstructure.

Metallurgical effects of recrystallization anneal

During a recrystallization anneal, new grains form in a cold-worked metal.  These new grains have a greatly reduced number of dislocations compared to the cold-worked metal.  This change returns the metal to its pre-cold-worked state, with lower strength and increased ductility.

With continued time at the annealing temperature, some of the newly formed grains grow at the expense of neighboring grains.  There is some further decrease in strength and increase in ductility as the average grain size increases during the grain growth phase of the annealing process.

Want to improve your metallurgy and metals engineering knowledge? See our metallurgy courses page for training options.

The final grain size depends on the annealing temperature and annealing time.  For a particular annealing temperature, as the time at the temperature increases the grain size increases.  For a particular annealing time, as the temperature increases the grain size increases.  A piece of metal with large grains has lower strength and more ductility than a piece of metal of the same alloy with smaller grains.

The figure shows micrographs of a brass alloy that was cold-rolled to 50% of its original thickness and annealed at two different temperatures.  The figure on the left shows the microstructure of the cold rolled sample.  The center figure shows the microstructure of a sample that was cold rolled and then annealed at 1022 °F (550 °C) for 1 hour.  The figure on the right shows the microstructure of a sample that was cold rolled and then annealed at 1202 °F (650 °C) for 1 hour.

Annealed brass grains

The cold-rolled sample had a yield strength of 80 ksi (550 MPa).  The sample that was annealed at 1022 °F (550 °C) for 1 hour had yield strength of 11 ksi (75 MPa).  Many small grains are present in this sample.  The sample that was annealed at 1202 °F (650 °C) for 1 hour had yield strength of 9 ksi (60 MPa).  Fewer, large grains were present in this sample compared to the center sample.

Need help selecting an alloy for a component? We can help. See our metallurgy consulting page for information.

Other reason for recrystallization anneal

In addition to enabling additional cold-working, recrystallization annealing is also used as a final processing step to produce metal sheet, plate, wire, or bar with specific mechanical properties.  Control of the annealing temperature and time, heating rate up to the annealing temperature, and amount of cold-working prior to anneal is important for obtaining the desired grain size, and therefore the desired mechanical properties.

Whether you are through hardening steel, annealing a cold-worked metal, or solution treating an aluminum alloy, the heat treating temperature is critical for obtaining the desired microstructure, and therefore, the desired metal properties.  Microstructure refers to such things as the metallurgical phases present in a metal and the grain size.

Using a temperature that is too hot can result in a metallurgical transformation that proceeds too quickly or the formation of undesired phases.  Using a temperature that is too low can result in incomplete metallurgical transformations, cold worked metals that do not soften sufficiently, or insufficient stress relief.

Want to learn more about steel and aluminum heat treating and metallurgy? See our metallurgy courses page for training options.

Steel heat treating

For example, during the through hardening heat treatment of a carbon steel, the steel is heated to transform all the ferrite and cementite to austenite and then quenched to form martensite.  If the steel is not heated to a high enough temperature, then there is the risk that all the ferrite and cementite does not transform to austenite.  If this occurs, then when the steel is quenched, the remaining ferrite and cementite will be present along with the martensite.  These ferrite and pearlite remnants can weaken the steel.

Annealing

Another example is cold-rolled sheet metal that is annealed to improve its ductility, and reduce its strength and hardness.  If the annealing temperature is too high, then excessive grain growth will occur.  This will result in the metal having lower strength and hardness than intended.  Also, if the metal is to be formed, there is the risk of orange peel, a cosmetic defect in heavily formed metals with grains that are too large.

Why temperature might be too high or low

So, why might a heat treater use a heat treating temperature that is too high or too low?  To save money, to save time, or just sloppy.  To reduce energy costs a heat treater might try to run its furnaces at the low end of the required temperature range.  However, normal temperature variations throughout a load and normal composition variations within the metal can result in the temperature being too low to cause the desired metallurgical transformations.

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To save time, a heat treater might operate a furnace at the high end of the specified temperature range to try to move the metallurgical transformations along as fast as possible.  Again, with normal temperature and composition variations, the temperature may end up being too high, resulting in excessive or undesired changes in the metal’s microstructure.

As for a sloppy heat treater, who knows what you will get from batch to batch of metal stock or components.

To learn more about the effects of temperature control on steel microstructure and properties, take our Metallurgy of Steel Heat Treating course or read Practical Heat Treating by J.L. Dorsett and H.E. Boyer or Steels: Processing, Structure,and Performance  by George Krauss.  Also, the two courses mentioned in the introduction above will discuss the effects of temperature control on precipitation strengthening and annealing cold-worked metals.

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The strength of metals is improved by impeding the motion of dislocations through the metals. One approach to achieving this improvement is to form a uniform distribution of closely spaced sub-micron sized particles throughout an alloy. The particles, which are called precipitates, impede dislocation motion through the alloy. Not every alloy can be precipitation strengthened. Alloys that can be precipitation strengthened include Al-Cu, Al-Mg-Si, Cu-Be, and 17-8 PH steel. The figure shows precipitates in a Al-Cu alloy.

Al2Cu precipitates in a Al-4% Cu alloy. © DoITPoMS Micrograph Library, Univ. of Cambridge

Al2Cu precipitates in a Al-4% Cu alloy. © DoITPoMS Micrograph Library, Univ. of Cambridge

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The particles are formed by precipitation, which involves a series of heat treatment steps. The first step is solution heat treatment. This involves heating the alloy up to a temperature that results in the atoms of the alloying element being dissolved within the solid structure formed by the array of atoms of the main element. For Al-Cu alloys, the copper atoms dissolve into the array of aluminum atoms. The dissolved structure is then retained at ambient temperatures by cooling the alloy rapidly, such as by water quenching.

After cooling, precipitates are formed either by natural aging or artificial aging.  With natural aging, the precipitates form at room temperature. With artificial aging, the precipitates form when an alloy is heated to a temperature lower than the solution heat treatment temperature. Only certain alloys will undergo natural aging. The other alloys must be artificially aged.

Regardless of the aging process, as the precipitation process proceeds the precipitates go through a series of stages, with changes in the size, form, and composition of the precipitates. The particular stage of the precipitates has a direct influence on the strength of the alloy. For artificially aged alloys, this is controlled by the aging temperature and time. At any particular aging temperature, there is an aging time at which the alloy will reach its maximum strength. This maximum strength corresponds to a specific stage of the form and composition of the precipitates. Aging for a time that is too short or too long will result in less than maximum alloy strength.

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For artificially aged alloys, the aging temperature affects the maximum strength that can be obtained, and the time required to reach maximum strength. For naturally aged alloys, the strength increases over time. The time required to reach maximum strength depends on the alloy.

Finally, precipitation strengthening can be combined with cold-working to give even greater alloy strength.

More information about the metallurgy of precipitation strengthening and precipitation strengthening heat treatment is in our Precipitation Strengthening course. Also, Heat Treatment: Structure and Properties of Nonferrous Alloys by C. R. Brooks, Precipitation Hardening by J.W. Martin, and ASM Handbook, Volume 4: Heat Treating discusses precipitation strengthening.

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In this article, I’m going to take a step back to consider the basic science of metallurgy.  On a microscopic level, there are many things going on inside of a metal.  Metals consist of numerous microscopic structures that have a direct and large influence on the properties of metals.  Through composition, mechanical treatment, and thermal treatment these microscopic structures can be modified to impart specific properties.  Whether the desired structures, and resulting properties, are obtained in a completed component or joint between components depends on the knowledge and skill of designers and manufacturers.

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One set of major structures within a metal are the crystal lattice, grains, and phases.  The crystal lattice is the arrangement of the atoms within the metal.  Grains are individual crystals within a metal.  Figure 1 shows grains in a brass alloy.  Phases are different combinations of the elements present in an alloy.  Figure 2 shows pearlite in steel.  The light colored material is the ferrite phase, which is comprised of iron with a little bit of carbon mixed in.  The dark colored phase is cementite, which is comprised of the compound Fe3C.  It is also referred to as iron carbide.  The properties of a metal are affected by the size of the grains and the phases present.

Grains in brass

Grains in brass

Pearlite in steel

Pearlite in steel

Defects in the metal crystal lattice make it possible to form alloys and deform metals with the metals cracking.  These defects are not the same as manufacturing defects such as voids, inclusions, seams, and cracks.  Instead, without crystal lattice defects we would only have pure, brittle metals.

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Various mechanical (e.g. cold rolling) and thermal (e.g. through hardening and precipitation strengthening) processes take advantage of these crystal defects in order to bring about modification of the grains and phases present in a metal, to obtain the desired properties.

Also, the number of crystal defects in the metal can be modified to obtain desired properties.  For example, cold rolling results in an increase in the number of dislocations in a metal, resulting in increased strength.  The, annealing a cold-rolled metal results in a reduction in the number of dislocations and modification of the grains, resulting in a decrease in the metal strength.

A common representation of the relationship between properties, composition, microscopic structures, and manufacturing defects is shown in the image below.  When the effects of the manufacturing processes on the microscopic structures are properly understood, it is possible to consistently produce metal components and joints that have the desired properties.  Essentially, the people in charge of the manufacturing processes are responsible for making sure that during the processes the atoms in metals move to where they need to be.  And designers are responsible for specifying where the atoms should be.

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For more information about the microscopic structures in metals take our online, on-demand Principles of Metallurgy course or read Metallurgy for the Non-Metallurgist, A.C Reardon, editor or Materials Science and Engineering, W.D. Callister.

The metallurgical phases present in an alloy have a huge impact on the properties of a metal component.  Phases are distinct materials that are comprised of the elements in the alloy.  These distinct materials have distinct properties that have an impact on the overall properties of the entire alloy.  Additionally, the size, shape, and location of the phases within the alloy also effect on the overall properties of an alloy.

Within many common alloys it is possible to alter the phases present with heat treatment.  Forming one or more phases from a different phase is called a phase transformation.   Phase transformations occur when in an alloy is heated or during cooling from an elevated temperature.

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There are several types of phase transformations that can occur when an alloy is cooled from an elevated temperature.  The type of transformation that can occur depends on the specific alloy.  Not all types of transformations occur in all alloys, and in some alloys no transformations are possible, other than the solid-liquid transformation.

Two of the most common phase transformations encountered with common alloys are eutectoid and precipitation.  For both types of transformation, the transformation involves the movement of atoms through the metal to rearrange themselves to form the new phase or phases.

Ferrite (white) and cementite (dark) in steel.

Ferrite (white) and cementite (dark) in steel.

A eutectoid transformation involves a change from a single phase to two other phases when the initial phase is cooled form an elevated temperature.  The most common alloy in which this transformation is encountered is steel.  The transformation occurs when steel is cooled from the austentizing temperature.  During slow to moderate cooling, the austenite transforms to ferrite and cementite.  The microstructure consists of cementite plates with ferrite between the plates.  This is commonly referred to as pearlite.  A micrograph of a steel alloy with 0.6% carbon is shown here.

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During faster cooling of some alloys, the ferrite bainiteforms in the shape of needles or plates and the cementite forms as particles.  This structure is referred to as bainite.  A micrograph of a steel with bainite is shown in Figure 2.

The reverse transformation occurs when steel with ferrite and cementite is heated.  When the temperature is high enough, the ferrite and cementite transform to austenite.  So, the austenite to ferrite + cementite phase transformation is reversible, and repeatable.

Precipitation transformations involve the formation of particles of one phase within an already existing phase.  These particles are called precipitates.  This transformation occurs when an alloy is cooled from an elevated temperature.  At the elevated temperature the phase present consists of the main element in the alloy with the alloying elements in solid solution.  When the alloy is cooled the solid solution is not able to hold all the atoms of the alloying elements in solution, so precipitates form that consist of the solute atoms and possibly the atoms of the main element in the alloy.

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Al2Cu precipitates in an aluminum matrix. © DoITPoMS Micrograph Library, Univ. of Cambridge

Al2Cu precipitates in an aluminum matrix. © DoITPoMS Micrograph Library, Univ. of Cambridge

For engineered metal components, precipitation  during cooling is undesirable because of the resulting size and location of the precipitates.  So, the process is modified by first quenching the alloy to room temperature to suppress the atom motion.  Then the alloy is either allowed to transformation at room temperature, if room temperature transformation is possible, or the alloy is reheated to an intermediate temperature to speed up the transformation.  An example of a common alloy system in which precipitation is used is the aluminum-copper system.  This figure shows a micrograph of Al2Cu precipitates in an aluminum matrix.

The precipitation transformation occurs in a number of alloys including aluminum alloys (Al-Cu, Al-Mg-Si, Al-Zn-Mg, and Al-Zn-Mg-Cu), precipitation hardened steels (e.g. 17-4 PH, 15-5 PH, and 13-8), some copper alloys (Cu-Be and Cu-Cr), and Zn-Al alloys.

Regardless of the particular transformation, control of the heating temperature, heating time, cooling rate, and, if necessary, reheating temperature and time are all important factors for controlling whether the desired transformation is complete and the shape, size, and location of the phases that form.  These in turn have a big impact on the properties of a metal component.  The relationship between heat treating process conditions, final microstructure, and properties is discussed in our Metallurgy of Steel, Metallurgy of Steel Heat Treating, and Precipitation Strengthening courses.

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