Quenching and Partitioning: Science and Technology
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PassfederHüttenindustrieVorlesung/KonferenzDiagramm
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MaterialDiagrammVorlesung/Konferenz
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Airbus 300Satz <Drucktechnik>ComputeranimationDiagrammVorlesung/Konferenz
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MotorRutschungVorlesung/KonferenzDiagramm
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Band <Textilien>Satz <Drucktechnik>Vorlesung/Konferenz
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MotorLeisten
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SeitenleitwerkPatrone <Munition>Vorlesung/KonferenzTechnische ZeichnungDiagramm
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Schlichte <Textiltechnik>ErsatzteilBerg <Bergbau>EisenbahnwagenVorlesung/Konferenz
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Transkript: Englisch(automatisch erzeugt)
00:06
John Speer, who comes from the Colorado School of Mines,
00:34
which is the main center for steel research in the USA. And John Speer is one of the leaders there.
00:40
He's going to talk about the quench and partitioning process, which started fairly recently and has taken off all over the world. The number of papers you see and the quench and partitioning process being tried out in many different ways is really impressive. Thank you, Harri, for your kind introduction. Can everyone hear me OK?
01:02
Well, it's really a great pleasure to be here today with all of you. And I'm going to try to talk a little. In the short time we have a little bit of science and a little technology, I want to acknowledge my collaborators and co-authors, David Edmonds, who's in the room here.
01:26
Our acknowledgments were supposed to be on the first slide. But as I thought back on the development of this concept and all the process, one of the great pleasures, I think, was we often don't have the time or take the time to read as much as we should.
01:43
But I did have an opportunity to read a lot of the literature from the giants. And that was quite an enjoyable process. And I really want to acknowledge those people who have influenced probably all of us in the room here. And I also want to thank the many collaborators and students
02:01
that have worked on this over the years. A little bit of background going back to the beginning. So this quenching and partitioning process was really designed as a new concept to control retained austenite. And the original process concept was
02:21
that we interrupt a quench. So normally, you would quench austenite, perhaps to room temperature and form martensite. But we interrupt the quench here at a temperature where the martensitic transformation is incomplete. And the idea then is some subsequent thermal treatment,
02:41
either at the same temperature as the quenching temperature that we called one step or at some different temperature, two step, that carbon would go from the martensite into the untransformed austenite and would therefore stabilize it so that when we then completed the quench
03:01
to back down to room temperature, we would have more retained austenite. So that's the basic concept. We have extended that concept more recently to the case where we may have industrial processing concepts where the partitioning process would be non-isothermal.
03:21
And if we have time at the end, I might make a comment or two about that. This is a little bit more complicated. But for example, in hot rolled sheet production, you would use the run-out table cooling process to complete the partial martensitic transformation. And then the coiling temperature where you wind the coil up would
03:42
serve to control basically the time and temperature of the cooling profile that would define the extent of partitioning. So these are the sort of process concepts that we're thinking about. In the beginning, we started by trying to understand what the thermodynamics would tell us
04:03
about what kind of carbon partitioning could happen and how it could affect the microstructure and properties of the material. And so this is some of the simple analysis from our early papers. And again, if you think about a metastable equilibrium
04:21
between martensite and austenite, carbon would partition to this point in equilibrium. In the case where we have varying non-equilibrium fractions of austenite and martensite, though, if we analyze this and if the interface is immobile, what would happen
04:45
is the carbon would partition until its chemical potential is uniform in the two phases. That's why we have not a common tangent construction, but a point where the tangent intercepts on the carbon axis are the same.
05:01
And the interesting thing about this then is depending on the phase fractions, we could have multiple different conditions where the carbon potentials are equal in the phases. And so we could have very carbon-enriched austenite, much more than at equilibrium or less carbon-enriched austenite. So depending on the phase fractions,
05:20
we could have some interesting carbon enrichments. And that went into the early development. And then the next step of the process was to try to understand how we would control microstructure. And in that regard, this is an important diagram from our early literature. And so I want to just walk you through it for a moment.
05:40
So if we think about quenching austenite, so once the quench temperature goes below the martensite start temperature, the amount of austenite that remains is diminishing. The amount of martensite that forms during the quenching is increasing with undercooling. So we stop at that quench temperature,
06:02
and then we partition the carbon. And most of the carbon would like to partition back into the austenite. And so depending on how much austenite and martensite are present, that defines the amount of carbon that can partition to the austenite.
06:21
And so this line right here tells you the carbon concentration of the austenite if the martensite gives up all its carbon to that austenite. And so if we have a lot of martensite and just a little bit of austenite, the carbon enrichment of that austenite is very great
06:41
and diminishes with reduced martensite content and increased austenite. And so this tells you about the stability of the austenite at the quench temperature after partitioning before we go through the final quench to room temperature.
07:02
And then depending on that stability, this line tells you how much of that austenite that existed after partitioning will remain after final quenching. So this is the amount of that austenite that transforms to new martensite. And so if we subtract that from the austenite that
07:21
was present at the quench temperature, we get this red line. And that's the end result of this that tells us how much austenite we could retain at room temperature. And so this functional behavior was very important, helping to guide our processing histories as we tried to verify that this concept would work.
07:41
So we have this peak in the behavior associated with particular quenching temperatures. Embedded in this fairly simple model were some assumptions that are pretty important from a physical metallurgy standpoint. First of all, that we had ideal partitioning,
08:02
all the carbon would like to go into the austenite. That we've completely suppressed the precipitation of carbides or conventional tempering reactions. That once you form the martensite during quenching, that you don't change the phase fractions anymore. That is, that the interfaces are immobile.
08:24
And that the austenite doesn't decompose in other ways like bainite formation. So these assumptions are not always correct. And so they're the source of a lot of interesting follow up that we can do as a metallurgical community. But still, the model is very helpful to guide us.
08:43
Now, this particular model that I described here was actually applied more recently in a completely different class of steels, the so-called medium manganese steels, that are very fine-grained, inter-critically annealed materials.
09:02
So fine ferrite, austenite mixtures, where the austenite is really stabilized by high manganese concentrations. And I'll just show you how this diagram was applied. In that case, it's basically the same kind of behavior, except in this case, the ferrite austenite fraction
09:23
is controlled by the annealing temperature. So the greater is the annealing temperature, the more austenite we have. And then if manganese can partition into that austenite, the austenite is enriched with manganese, but it really depends on the phase fraction.
09:40
So the more austenite that you have, the less manganese enrichment that you have. And so the less stable is that austenite in terms of remaining at room temperature. And so depending on its stability, it can transform to martensite during the final quenching. If we again subtract this from the austenite curve,
10:02
we end up with this black and blue function that tells us how much austenite would remain at room temperature. So a very similar application of the same fundamentals to a completely different class of steels. And this is actually how it worked out. So this is a seven manganese, I think 0.1 carbon steel.
10:24
This is the predicted austenite fraction as a function of annealing temperature. Again, assuming full manganese partitioning between the phases. This was a long annealing time. And these are the experimental data showing that this model was helpful to understand
10:41
the behavior of this other class of materials. So it's a very exciting time in steel development now. I think I'll go on the record to say this might be one of the most exciting times for steel development ever. In the automotive community, the need for increased fuel economy is dramatic.
11:03
And at least in the United States, there's a tremendous need for steel development to reduce weight and enhance vehicle performance or maintain vehicle performance. So in terms of application of crunching and partitioning, the automotive industry right now
11:21
is driving that interest in application. But there are other interests in ball bearings and high toughness plate steels. So there's much opportunity that hasn't been explored yet in crunching and partitioning. But in the automotive industry, so we saw this so-called banana diagram before
11:41
where we have this range of tensile ductilities and tensile strength for a whole variety of different kinds of steel materials. And I heard this described interestingly very recently by Anil Sachdev of General Motors as the miracle of steel. So we look at this somewhat mundanely as experts in steel.
12:04
But the fact is by small variations in composition and processing, we can create this tremendous window of different steel products with very different properties, so the miracle of steel. But we're trying to push the properties up into higher strengths and higher ductilities.
12:22
So that's very challenging and yet very exciting for the community. How are we going to get there? So this is a simple model developed by my colleague David Matlock, really looking at composite models for predicting uniform ductility based on phase properties.
12:43
And you can see if we look at combinations of ferrite and martensite, we get property combinations that you would expect that are fairly parallel to the different grades on the banana diagram, so ferrite by steels. If instead we look at combinations of a particular stable austenite mixed with martensite,
13:04
we get much better strength ductility combinations in this future need area of steels. And actually if instead of using stable austenite, we can control the austenite stability. I don't think we really know how to tune the austenite stability in practice
13:23
as well as we would like to, but we can move this curve around as well. And so it's a very interesting opportunity. But in terms of how we get to the future, our philosophy in automotive steel development is pretty much we need to have retained austenite in the microstructure. And so that's driving interest in Q and P steels,
13:44
in carbide free bainite steels, in medium manganese steels, as I mentioned. So very exciting time. There are a number of things on this slide that I want to mention. First of all, this is all, this data is all experimental data
14:02
from quench and partition steels. And some of the people who generated it are in the room here. And this green shaded region over here, this represents the original target that we set out to achieve in 2003. And so we were pretty happy
14:21
that our development was fairly successful in getting us high strength materials with quite good ductility. The dashed line and the solid line here are the same predicted curves that I showed on the last slide. So we were quite happy. But what's happened in the meantime is that the property targets
14:41
keep getting more challenging. So one of the large automotive companies in 2010 defined a target up here, which is way up in the future band of desired properties. And then the next year put some targets up here.
15:00
The United States Department of Energy has funded a integrated computational materials engineering program with the industry. And they've set some even greater targets than the industry has set. So when we think about how successful we are in meeting property targets, we also have to recognize that the challenge is increasing.
15:23
So the targets are moving. I promise this is my last banana diagram for this presentation. Our original models looked at transformation behavior and subsequent models incorporated
15:45
some partitioning kinetics. And so these are some dictum models. You can see that the ferrite gives up its carbon rather quickly. It takes longer to equilibrate the austenite.
16:00
So what's interesting about that is under certain partitioning scenarios, the austenite might contain most of the carbon but with a non-uniform chemical composition gradient. We get some interesting effects then when we try to understand the stability of that austenite. So we calculate it using the Koesten and Marburg equation locally.
16:24
But in fact, we don't know how good that assumption is. So one of the questions for the community then is what is the stability of austenite in the case where we have a local concentration gradient that's on the same scale as the martensite microstructure.
16:42
These are some examples. So quenching partitioning has been applied now in commercial steels, first by Bao Steel in China. These are commercial applications. There are other companies around the world though who I believe have a real interest
17:00
in considering this technology. So in the few minutes that remain for my presentation, I thought I'd present some curiosities, challenges and opportunities. So we've learned a lot over the last 10 years but there's still a lot of things that we don't understand and hopefully this community will come back
17:21
at some future time and help. So here are Q&P property data. So this is the product of tensile strength and elongation versus the amount of retained austenite. And so our desire was to produce high amounts
17:40
of retained austenite, but you can see from this diagram that in fact the properties are not highly correlated with the fraction of retained austenite. So I think that we still don't completely understand what controls the work hardening behavior and the property combinations in these steels.
18:01
We've looked a lot at partitioning mechanisms and some of the physical metallurgy and the group that leads has led us in this regard. One of the questions is about carbide precipitation, which generally speaking we don't want because it takes carbon from the microstructure that we would otherwise use for austenite stabilization.
18:23
I'm showing a particular steel that's partitioned at two different temperatures. In one case, we get a lot of epsilon transition carbides in the microstructure. In the other case, at higher temperature, we get austenite stabilization. So for the community, I think one of the challenges then
18:41
is how do we control the stability of transition carbides other than perhaps temperature, according to the models that we think we understand, but if we could turn on and turn off transition carbide formation using other means than we understand now, it would be a powerful alloy design tool.
19:02
The last comment I want to make, we've had a lot of discussion about whether the martensite-austenite interface is stationary or mobile. There's been some interesting modeling work that's been done at Delft in particular.
19:20
One of my former students, Grant Thomas, looked at some higher-alloyed steels, not intended for commercial applications, but intended to study the partitioning mechanisms. And one of the things that we looked at is the change in the austenite fraction during partitioning in these steels where austenite,
19:42
you could quench to room temperature and then partition subsequently. In a high-nickel-containing steel, we found that the austenite fraction was stable. It did enrich in carbon. So the assumption of a stationary interface was pretty good. In the case of a high-manganese steel, though, we actually increased the austenite fraction
20:02
during partitioning. So clearly we think the interface was not immobile. EVSD results. So the green is the austenite phase. The change from left to right is with partitioning. And you could see, again, nickel-steel increase in austenite fraction,
20:22
nickel-steel approximately constant austenite fraction. And some inverse pole figure diagrams, really looking at what happens to the microstructure during partitioning. This is in the manganese steel. These colors represent austenite orientations.
20:42
And so these we think are probably the original austenite grain orientations. I'm confirming that we think the austenite is growing during partitioning rather than nucleating. But we don't completely understand why that happens in one steel and not another.
21:00
I think I'll bypass this slide and go to the conclusion. Clenching and partitioning science and technology continues to advance. The process has been commercialized and hopefully there will be other applications besides automotive sheet steels and growth in those applications.
21:23
But challenges and opportunities remain both in terms of science as well as in technology and I end in closing here showing some QNP microstructures. This is one that we obtained in the laboratory, intercritically annealed. So we have ferrite here. This is actually a commercially produced QNP steel.
21:44
And then we have a mixture of austenite and thick, excuse me, of martensite and thick austenite films. So with that, I conclude my presentation. And again, thank you for being here.
22:00
Thank you very much, John, for creating one of the modern concepts of automotive steels. Very interesting talk. We are open for questions. Very short question. In fact, you mentioned that bainite is something that we don't want in QNP steels. But could you comment a little bit more about that?
22:20
I understand that we don't want bainite because it takes part of the carbon. Well, to your point of mechanical properties, I expect that maybe people should go in this direction to sort of combining some bainite with, yeah, could you? So I don't remember saying that you don't want bainite. But I think that when bainite forms
22:44
from the austenite at the partitioning temperature, then you really have a mixed microstructure. So you have a QNP mechanism, but you also have a, you know, austempering bainite formation mechanism. So you get a mixed microstructure.
23:00
And I think there are some interesting properties that people are getting in cases where they do have those kinds of mixed microstructures, so hybrid mechanisms. So I think they might actually be fairly important industrially. So, yes.
23:22
Thank you, it's a very nice talk. You mentioned these medium manganese steels. So that's probably a very hot topic at the moment. And apart from manganese, what's your opinion on the range of other elements, such like aluminum or silicon?
23:40
Well, I think there are some, aluminum and silicon wouldn't be strong austenite stabilizers so you'd be looking at different concepts with those elements. So manganese is interesting because it allows us to really increase the retained austenite fractions to quite high levels.
24:01
There are some other interesting concepts, particularly with high aluminum, where aluminum is being used to reduce the density of the steel. So those are different concepts that are a little bit outside the scope of the design concepts that I discussed today, but are also of considerable interest right now.
24:25
Okay, thank you for your talk. I have a question regarding the resource from Thomas on the high nickel and high manganese steels, in which he observes an increase in the fraction of austenite during the partitioning step.
24:43
And I wonder if, I know that at those temperatures, the diffusion of manganese is extremely small, but I also saw some articles from another researchers in which they think that the diffusion of manganese is underestimated at low temperatures.
25:00
So do you think this increase in the austenite fraction can be due to some manganese partitioning actually? So I don't know the answer to that exactly. This is a, I mean, this is a very low temperature to be thinking about manganese partitioning, but I do agree, not necessarily in this temperature regime,
25:23
but actually in the temperature regime of annealing of the medium manganese steels. So in the inner critical regime, we're getting much more manganese partitioning than you would expect from the sort of published diffusivity data. So I agree that manganese diffusion seems to be much faster
25:44
than we thought it was. I don't know that that's a contributor at the temperatures that Grant Thomas was looking at here though. Because it's interesting when you have lower manganese levels, you don't see those, or at least we don't observe such an increase in the fraction of austenite,
26:01
but at those levels it's observed. So that's maybe related, but. Yeah, so what's happening in the nickel steel then? Yeah. Okay, well, thank you. Okay, thank you. We have a question from the rest of the world. I'm still from Netherlands, they have a question. Okay.
26:20
The question is, if the inner critical annealing temperature has been done in, say, 800 to 900, is it possible for the manganese to partition in, say, 100 seconds? That's the question. So are we talking about a medium manganese steel?
26:45
I guess so. So I guess it doesn't really matter. But I think there's some data in the recent literature that shows that you can get significant manganese partitioning at inner critical temperatures in relatively short times.
27:04
Another question from AK Steel. Is it possible for manganese carbon interaction is different compared to carbon interaction to nickel? In fact, these are. Okay, so this author is actually at AK Steel,
27:22
so I might ask him to answer this. But so is it possible that there is a carbon nickel interaction that's different than a carbon manganese interaction? And I haven't thought about that, and this might not be a good venue to be thinking out loud.
27:45
John, I wonder if it doesn't explain it, experimental evidence that you saw, may be related to carbon trap in some places that you have no mention and should be considered, such as dislocations, twins, or other interfaces,
28:03
because quenching the steel before that isothermal agent, you create a high dislocation densities. And I wonder if the carbon will be really comfortable on those spaces before partitioning to the austenite.
28:22
Then maybe you are losing carbon in the process of carbon partitioning that cannot rule out that carbon balance that you are considering for the way of retaining the austenite at room temperature at the end. And also, what we have observed
28:41
in carbide-free bainitic steels is that for subsequent low temperature temperance that around the temperature that we get the epsilon carbide that you mentioned before,
29:02
those dislocations segregated in carbon can be a key issue. Because with time and temperature, cultural atmospheres will evolve, what we have seen in clusters. And after that, that will be the perfect nucleation site for epsilon carbide. Then I wonder if that you investigate your dislocation
29:24
densities and the carbon trap at those dislocation, you can explain the appearance or not of epsilon carbide or more or less retaining austenite at the end in your microstructure. So that was a complicated question. But I think part of the core of that question
29:45
was what's the role of carbon trapping in Q&P. And I think carbon trapping should be important. And we've seen a lot of when we have done carbon balances, we have seen instances where we're obviously
30:03
not getting all the carbon out into the austenite. And if we could reach the kinds of carbon enrichments that we predict from our thermodynamics, that is if we could turn off the competing mechanisms, we could really get some interesting carbon enrichment.
30:21
So carbon trapping is part of that. Although in order to study that, we've done some atom probe. But to study it, you also have to understand the tempering reaction and the carbide precipitation. So that's all challenging. And we haven't completed that kind of work yet. According to your dictro calculation, the diffusion of carbon is still slow.
30:42
And the diffusion distance is limited. Is there any desirable size of retained austenite? How can we control the size of the retained austenite? So how do we control the size of the retained austenite? Well, the austenite. Maybe the size should be limited to the standardized
31:00
the retained austenite. So we haven't studied experimentally ourselves yet the influence of the starting austenite grain size and morphology extensively. But I have heard that there are some people who are working on some micro-alloyed versions of quenching and partitioning
31:22
where they've maybe refined the austenite and gotten some better mechanical properties. The size of retained austenite between the mountain side. Right. But we really, once you make the austenite and then quench it, then the austenite size and morphology
31:41
is controlled at that point. And you're moving the carbon around. Carbon moves rather fast. So you can decarburize the marten site pretty quickly. But if you want to change the austenite size and morphology, you need to change the starting microstructure then. And that part we haven't gotten to yet.
32:05
Yeah, in the video Manganese asked here, you mentioned that partitioning of manganese is quite critical to stabilize the retained austenite. But I think that there is also contribution from the refined grain. So what do you think about?
32:21
So what's the importance of grain refinement in those steels? So grain refinement is going to help stabilize the austenite and help increase the strength as well. Yeah, besides the manganese. So that's a critical aspect of the microstructure in those medium manganese steels
32:40
that you have very fine ferrite and austenite. OK, thank you very much. Thank you.
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