Modern Steel Products (2014) - Fe-C phase diagram: lecture 3
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Modern Steel Products5 / 31
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Transcript: English(auto-generated)
00:04
OK. So let us start. First of all, I want to remind you that on Thursday, there will be a quiz. And so there's a quiz every Thursday from now on. And that's basically where you're
00:26
going to get your grades from. All right. And the quiz is, I remind you it's a yes and no thing. You just basically have to go through what you've been hearing from me in the last few lectures.
00:42
And you should be fine with the material I've given to you. And the no trick questions, OK? So the questions are very straightforward, OK?
01:01
You will see also that in the eClass, you get lots of material, OK? Because it's a technology course, and I want to make sure that at some level, graduate and work in the industry. You can go back to this course
01:21
and get information from it that's in some way useful to you in your activities at that time. So good. So let's, again, continue with what we had started,
01:40
namely the introduction. You're all familiar, I assume, with face diagrams. So face diagrams are very useful. We'll use a number of diagrams quite often, such as face diagrams. And so what do we use for? First of all, we use them as tools, right? In our course, we're not going into how you calculate them,
02:04
what they means in terms of thermodynamics. I assume you know these things. And we'll use them as tool in this course. And so what does it tell us? Is that any point on this diagram
02:23
has a temperature and a composition, for instance, carbon content here. And there's also information about the phases involved in the microstructure at that time. So in this particular case, I will
02:42
have gamma phase and cementite, if I have three points, say 2%. So I will not only have information about the number of phases, the type of phases, also the composition of each phase. It will tell me that at this temperature, the austenite contains this much, 1.5% of carbon,
03:03
and that the cementite contains 6.7% of carbon. So there are a number of points that are of interest here. For instance, this structure, the microstructure I get here, is a very well-known lamellar microstructure
03:23
that we call the eutectoid. Yes? So if we add the information here on the graph, I want to tell you that when we look at steels,
03:46
we typically define them as iron carbon and alloys with less than 2% of carbon. And you can see where this point, where
04:02
this definition comes from. It's due to the maximum solubility of carbon in austenite at around a temperature close to 1,200 degrees C. Many steels that we're dealing with, of course,
04:21
do not have this level of carbon. As a matter of fact, most steels will have up to 1% of carbon. Then you have close to 99% of the steels that you will encounter.
04:43
So we'll go up to about a percent of carbon. Yes? And a lot of steels are produced with even less carbon, with about 0.5% of carbon. So of all this huge, big, complex phase diagram,
05:02
the most important part of it is this little piece here, is this little piece, from 0 to 0.5% of carbon. And so no wonder that a lot of research is being done on what goes on here. But it's pretty simple in terms of the phase diagram
05:24
itself. Right. Let's move on. Now, this is a binary phase diagram. So any time we talk about steels, if you remember last lecture, where I wasn't there,
05:41
but I hope you listened to the video, we talked about silicon, and manganese, and chrome, et cetera. So we have a multi-component system. Steels are multi-component alloys. And so this phase diagram doesn't look like this
06:05
anymore, because it's not a pure binary iron carbon. And in particular, you will have this austenite region here. We call the austenite stability range. So this range here will be influenced,
06:23
will become larger or smaller, depending on the alloying. One of the things by which you can describe the effect of alloying elements is by looking at this eutectoid temperature. And what happens when you add elements to the austenite
06:47
here is, well, first of all, the eutectoid composition, the eutectoid composition, that is this composition here, goes from 0.8 to a lower value.
07:02
So that means eutectoid composition moves to lower carbon. And this is independent of whether or not the alloying element you add is austenite stabilizer or ferrite stabilizer.
07:20
You can see here nickel and austenite stabilizer has this effect in titanium also. All right. However, the eutectoid temperature moves in a very specific direction.
07:41
If the element, so you can see here, if this is the eutectoid temperature, this is the temperature here, it's around 720 degrees C. If I add a ferrite stabilizing element,
08:06
this point will move up in temperature. If I add austenite stabilizing elements, such as nickel and manganese, it will move down. And that is in line with the increase in the stability
08:22
range of the austenite. Good. So in this diagram, there are a number of things. Because you're in steel research, you should know. And so in particular, you should
08:44
know that in this system, there is a eutectic here. Again, it's not really important when you're dealing with steels at 1,148 degrees. This important here, this point here at 0.77% of carbon.
09:05
That's the eutectoid composition. And the eutectoid temperature is 727 degrees C in the system. The other thing you should know is that the carbon content of the cementite
09:23
is about 6.7% of carbon. And that we call all the steels with carbon contents less than the eutectoid composition. We call them hypo-eutectoid.
09:41
And the ones that are above this, we call them hyper-eutectoid. And we'll see what the big difference is. So if we are in steel compositions very close to this point, very iron-rich composition,
10:03
then we see ferrite. And many steels actually look like this. Many carbon steels, carbon steel looks like this. There's nothing much to see. Small grains, equiaxed grains, and it's ferritic. And there may be lots going on, but the microstructures
10:22
are pretty simple. So good. Now, once we add carbon, we'll start seeing things happening. In particular, there will be some reactions.
10:40
And again, we will focus on this part of the diagram in this lecture. So let's see what happens. So at high temperature, we have the austenite. And at low temperature, we have ferrite. We know if it's pure iron. But in the case of the iron-carbon diagram, we have ferrite and cementite at room temperature.
11:04
So what happens? Well, if we have the composition, this eutectoid composition, we know that the austenite transforms into ferrite and cementite in a laminar fashion. And this happens at this 720. But for many steels, we're actually
11:22
dealing with part of the diagram that you can't actually see here. So I blow this up. I blow this part up. And this is what I see. I see, of course, ferrite stability range up to 910 degrees C. So above this, pure iron is stable.
11:45
And then as I add carbon, the austenite stability range increases. So this temperature here decreases. So this temperature here is called the AE3 temperature.
12:04
It is the eighth. Many of these lines here, because they're used so often, we don't, we just call them by, we label them. So this is the AE3 temperature.
12:23
The E stands for equilibrium. And so this is this line here, this line. This is the AE3 line. This line here, the eutectoid temperature, this horizontal line here, is the AE1 temperature.
12:43
And while we're at it, the name of this line is a CM line, a CM line for the precipitation of cementite. And when it has a little e, it means
13:01
it's the equilibrium temperature. So here, the e means it's equilibrium temperature. Why do we specify this? Because when we heat steels or we cool steels, the temperature at which these transformations, transformation
13:21
reaction happen, are not exactly AE3. When you heat up, the transformation tends to be at a higher temperature. And when you cool down, this transformation tends to be at a lower temperature. So we give them other names. When we heat up, we call them AC3 line.
13:46
And when we cool down, we call them the AR3 line. And so the C and the R come from French.
14:01
The E also comes from equilibrium. AC comes from chauffeage, meaning heating. The C of chauffeage. And R comes from rue fradisme, which means cooling in French. And the same for AE1. You have an AR1, and you have an AC1, and an AR1 temperature.
14:30
And those are the things you generally measure that are of importance to you if you're running a industrial line that involves heat
14:44
treatments. That's the temperature at which the transformations will happen. Let's have a look at structures here. So if you have the eutectoid composition, the microstructure looks at this.
15:01
So this microstructure you know is called perlite. And it's alternating layers of ferrite and cementite. If you've never seen perlite, there are a number of people that do research on pearlitic steels. And it polishes. When it polishes, it makes a very nice silvery shine.
15:24
And people call it, it looks like pearl surface. And so that's where the name comes from. It's not somebody's name. Or it just looks like this nice gray surface.
15:43
All right, good. So now, just a few words about what are these compositions above this 2%? Do we use these alloys? Yes, we do. We use them actually quite a lot.
16:03
And we call them cast irons. And again, very different metallurgies. And microstructure controls will not talk about cast irons in the course of our lecture.
16:21
This is an example here where you can see one of the big differences that you can have in cast irons. This is what's called a gray cast iron. And instead of having cementite, Fe3C, in the microstructure, we have graphite.
16:43
And in this particular cast iron, this graphite is nodular, forms little nodules. Now let's go back to our steels.
17:05
And so in steels, we don't see graphite. We see cementite and carbide. So what happens now if you have compositions
17:22
which are hypo-eutectoid and hyper-eutectoid? How do they compare in microstructure? Well, you can see it here. So here, the composition is very close to 0.77, but a little bit less than.
17:42
So here, the carbon is less than 0.77%. And here, the carbon is larger than 0.77%.
18:01
And so what you get is you still see the pearlite in the microstructure. And here, you see the pearlite also. But in this case, you have ferrite, these ferrite grains that surround these pearlite islands.
18:22
And here, you see this white streak here. It's actually cementite. And this is called primary cementite. And in this case, we have primary ferrite. That means when you transform the austenite
18:41
to ferrite and cementite, first you form ferrite in the hypo-eutectoid composition steels, and you form primary cementite in the hyper-eutectoid situation. So let's just illustrate again.
19:01
Let's say you have 0.2% of carbon. So when you cool this material to room temperature like this, the first phase to form from the austenite will be ferrite. You come in this region here, ferrite plus austenite.
19:22
So the first phase to form are these primary ferrite grains. You know that at this temperature, you will have two phases. One will be a very low carbon ferrite.
19:44
That would be this grain here. And you will have this high carbon austenite. These are the two phases. And as you cool down, the composition of the austenite will move to this.
20:00
And the composition of the ferrite will move to this. Eventually, you reach 727 degrees. And the remaining austenite transforms to ferrite. And this is what you see here. This used to be this is this austenite that transforms.
20:23
So of course, if I have very little, less carbon, I will have more ferrite in the microstructure. And as I increase the carbon, I get more and more perlite in the microstructure. When I have a composition that's, for instance,
20:41
1.2% of carbon, what happens here? When I cool, let's say in this region here, at this point here, I will start forming primary cementite at these grain boundaries. The composition of the austenite
21:00
will move along this till it hits this point. And there, the remaining austenite transforms to perlite. So let me just go back to remind you of. Right? So in this particular case, for instance,
21:24
yes, I'll do it a little bit later just to remind you of. OK, so you just seal this. OK, so what you have, for instance, if you go from, say, 0.1 to about 0.77% of carbon,
21:47
you get the amount of ferrite decreases and the amount of perlite increases. So at about 80, 0.77% of carbon,
22:01
the structure is fully, we say, fully pearlitic. So how do we know how much of each phase we have? Well, we use what's called the lever rule.
22:25
So for instance, if at this temperature, and this carbon content, 0.2% of carbon, I want to know how much I know I have alpha with this composition and gamma with this composition.
22:44
I want to know what's the phase fraction. Well, I measure this piece of length, and I call it A. And I measure this piece, and I call this C. Yes? So then the phase fraction of alpha
23:03
is equal to A divided by C plus A, yes? And the phase fraction of gamma is C divided by C plus A.
23:25
All right? So I can always, also from the phase diagram, determine the phase fractions to expect. So here, for instance, for perlite,
23:43
I see ferrite and cementite. So if I want to know what is the phase fraction of each phase in this case, well, I have its perlite. So say it's about 0.77%.
24:02
What phases do I have? Well, I have ferrite plus cementite. The cementite is here, yes? So let's get rid of this here.
24:25
And so what is the amount of ferrite and cementite? Well, so this let here, from here to here,
24:42
I call A. And from here to here, yes, I call C. So the fraction of ferrite, for instance, is equal to A divided by C plus A.
25:10
And I can just measure this because C plus A is 6.69, yes?
25:20
And A is 6.69 minus 0.77, yeah? If you calculate this, you will find the phase fraction of ferrite.
25:47
So now one of the things we're very interested in when we make steels, when we process steels, is we want to know what happens when we process steels
26:03
in conditions which are non-equilibrium, yes? Because when you heat up your steel, you cool it down, you deform it at high temperature, you use a certain heating rate, a certain cooling rate, and the information you have from the equilibrium phase
26:24
diagram is of limited use. So we need to have information about the transformations, yes, and their dependence on the undercooling most of the time.
26:41
So we'll see what happens. We'll first go through this analysis of the effect of the cooling rate or the undercooling first, and we'll look at the isothermal eutectoid transformation.
27:00
So what we do here is we go from austenite with 0.77% and we cool down to below 727. And we do this at different temperatures. So I go suddenly to this temperature or to this
27:23
temperature, and then I keep the temperature constant. So I do the transformation isothermally, I keep the temperature constant, and I do it at a certain amount of undercooling. So for instance here, the undercooling being the temperature below the AE1 temperature.
27:48
So let's see what we have. So what we see is that the speed,
28:01
the kinetics of the transformation are very sensitive to the undercooling. So for instance, if you measure the amount of perlite formed as a function of time, it doesn't go like this. There is no instantaneous transformation.
28:23
This does not occur. The transformation is not instantaneous. Why is it not instantaneous? Because the perlite needs to grow from the austenite. And this requires that the carbon be redistributed
28:45
in addition to the phase transformation. So you need to form ferrite. You also need to redistribute the ferrite between the cementite and the ferrite.
29:00
So what happens actually? So if we have a small undercooling, for instance, we're close to the AE1 temperature. What we see is that if we plot the transformation
29:22
in a log diagram, with a log time scale, we see an S-shaped curve. If we increase the undercooling, we go lower and lower, we see that the transformation actually goes faster.
29:43
I want to point out that in your undergraduate years, very often people say, oh, this curve is S-shaped. Yes? The shape of the curve depends very much on what you use as a x-axis.
30:06
If I were to plot this on a regular linear time plot,
30:20
you wouldn't see much of this S-shape. This is a log diagram. So at 600 degrees C, you say the transformation is faster. Actually, it's extremely much faster. Because here, it takes maybe five seconds to be fully done.
30:45
Here, it takes 20 at 100 seconds. This is probably 500 seconds. So this is not faster. It's much, much faster. So it's very sensitive.
31:01
But when you plot the log time scale, you expand the short time scale. So now, why does this curve have this shape?
31:26
Well, and so why isn't this transformation, the formation of perlite instantaneous? Well, it's because this particular transformation and many transformations we are dealing
31:41
with in high temperature transformation are the result of nucleation and growth. So in this S-curve, here in the region where nothing much seems to happen, you're in the nucleation stage.
32:01
So we make small nuclei that becomes larger and larger. So in this nucleation rate, this is important. We see that the nucleation is higher when delta T is larger.
32:20
So you get more nucleation when the undercooling is larger. And the reason is because you have a higher driving force for the transformations. And if you want to have it in terms of thermodynamics,
32:43
the free energy change is larger. So lots of nucleation. Now, in the growth stage, you see the particle or whatever, the perlite islands grow.
33:01
And in order to grow, you have to have diffusion and long-range diffusion. So the growth rate, the rate of growth, is higher at high temperature, at small delta T.
33:23
And the reason is simply because at higher temperature, the diffusion. So in the nucleation stage, we have the free energy that plays a big role, free energy differences. In the growth stage, we have the diffusion is important.
33:41
And why does the curve not continue like this, but goes like this, gives you this S shape, doesn't go like this? That's because as these islands of perlite start to grow, they will start impinging on each other.
34:03
And they will prevent each other's growth in certain directions where they touch each other. And that gives me this lowering of the kinetics at the later stages. So let's view this process of nucleation and growth.
34:26
And it's important for you to realize that nucleation and growth are not separate stages. They're happening at the same time. So you have, from the very beginning,
34:40
you have a nucleation rate at a certain temperature, and you have a growth at a certain temperature. So if I start at T0, there's nothing. And delta T, I have two particles. So what is my nucleation rate?
35:03
This is a unit volume. So my nucleation rate is two particles per unit time, per unit volume. So that means that after two delta T, three delta T, every time I will see two new nuclei will be formed.
35:28
But of course, there is also growth. That means these nuclei will grow at a certain rate. So the change in their radius per unit time
35:42
is a certain value. So here, for instance, they're one nanometer in size. Here, they're two nanometers in size, et cetera. So their growth rate is, and you see this,
36:03
at the same time, you get growth and nucleation at the same time. It's not separate stages. It's not like a stage where you form nuclei, and then that stops. And then you have a growth stage. It happens at the same time.
36:23
So let's go back now to what happens to perlite. So in the nucleation stage, we still talk about the nucleation state. It's because the rate of the nucleation is higher. Of course, there's nothing, so the main thing that happens
36:42
is nucleation. Yes? And we have already told you that if you do the formation of perlite at lower temperatures, there's lots of driving force, free energy difference. So we will have higher nucleation rate.
37:04
In the growth process, what is important there is diffusion. Diffusion of atoms towards the growing phase, and that is higher at high T or at small undercooling. So if we look now at the perlite situation.
37:27
So I have austenite, and I look at the perlite islands, or we call them colonies, packs of parallel lamellas. The temperature just below the eutectoid temperature,
37:44
the nucleation rate will be low, and the growth rate will be high. So I get fewer larger colonies. If the temperature is very far below TE,
38:01
I get a very high nucleation rate. But these nuclei don't grow very large, because the diffusivity is low. And in between, I have an intermediate nucleation rate and an intermediate growth rate.
38:22
So we can track the change, different ways to do this, this transformation. For instance, we look at the transformation at 675. We track the percent that's transformed
38:40
as a function of the log of the time, and we measure where the transformation starts, where it's 50% transform, and where it reaches 100%. And we use this data, and we put it on a diagram of temperature versus the same time. And of course, at one temperature,
39:01
one point for the start of the transformation, 50% transformed and 100% transformed. And I can do this at different times, yes? Excuse me, at different temperatures, different isotopes. And I will get these curves, these typical C curves, which
39:22
are characteristic of nucleation and growth transformations in alloys, yes? So what we see is that at very close to 727,
39:46
it takes a very long time for the perlite reaction to start, because I have very little nucleation rate. There's very little driving force for the formation of perlite.
40:01
But then as I reduce the temperature, around 550, the kinetics are very high. It takes seconds for the transformation to be complete. And as I continue reducing the temperature, the kinetics are slower again.
40:23
And so if we would look at it picturally, at this point here, from all this region here, austenite grains are stable. Then they start to transform to perlite. And then at the red, the microstructure is fully transformed, because that's what happens.
40:43
There's another thing that happens, is that as we increase the undercooling, you can refine the perlite. You can make it finer.
41:00
The reason is that at lower temperature, diffusion is slower, perlite is finer, and the colonies are smaller. So you can refine the microstructure by choosing the temperature. So you get the same phases, pretty much the same composition, but the microstructure is refined. And this is a trick that is very often used
41:24
to do transformations at lower temperatures to refine the microstructure. And one of the reasons why you would refine the microstructure is to make it more homogeneous. And if you're concerned about mechanical properties, you also refine microstructure, and we'll
41:41
see why that is, has a higher strength. And at higher temperature, we have faster diffuser, coarser perlite, and larger colonies. And again, the reason is because the nucleation rates are so small.
42:01
OK, good. Let's say we're doing this transformation. We form perlite. And what happens if we continue dropping the temperature? We get finer and finer, progressively finer
42:25
microstructure. No, as you reduce the temperatures, you get into a situation where the diffusion becomes very almost impossible.
42:40
At what temperature is that? Well, below 550, 550 to 500 degrees C, you can assume that the diffusion of substitutional solutes, so elements such as manganese, silicon, moly, chrome, et cetera,
43:01
in steels, substitutional solute, is finished. There will not be any movement of these species anymore. What about interstitials such as carbon? They could still move over long distances. And that has an impact on the transformation.
43:23
So below the 550 to 500 temperature range, you stop making perlite. And you make another, what we call a decomposition product. And that's bainite.
43:40
And so what's specific about bainite is that the growth is a combination of diffusionless processes and diffusion controlled processes. So how can something be, at the same time,
44:02
diffusionless and diffusion controlled? Well, there is no diffusion of substitutional elements. And there is diffusion of interstitials. That's basically what the definition means. So at low temperatures, if you look at the microstructure, you don't get these lamellas anymore.
44:21
What you get is bainite. This is a much finer microstructure, as you can see here. And we'll say a few more words about this as we go along. So you have made either perlite or bainite.
44:46
But you have a structure. And it's a lamellar. And is that the equilibrium structure that the material will have?
45:03
Well, no. If you would say, I'll just continue holding on at this temperature, let's say at 600. I'll wait for a long time, hours and hours, yes. Do the same thing with bainite. I see that the microstructure will continually evolve.
45:24
And you'll end up having what's called spheroidite. Spheroidite, there, instead of a lamellar microstructure, you get these cementite nodules. So why does this happen? Why do I get?
45:41
That is simply because in the perlite, you have lamellas of alternating ferrite and cementite. And so you have lots of interfaces, lots of interfaces.
46:01
And they contribute energy, interfacial energy. And it's very high in the perlite. So in comparison, this situation where you have the spherical cementite nodule,
46:23
it has a much lower surface energy. And so long-term, cementite will tend to become spherical and form spheroidite. This is not of academic interest only.
46:43
Spheroidization is very common in certain products, steel products. Very many, in particular, wire products go through this stage of spheroidization
47:02
in order to make them very soft. So this is an example here of this spheroidization. And we call this globular cementite. Of course, you've all heard of martensite.
47:21
In the case of martensite, the transformation is fully fusionless. So there's no substitutional diffusion, no interstitial diffusion. And depending on the carbon content, your lattice,
47:40
your BCC lattice, can be distorted. Good, yes. You can have different types of martensite, from lathe martensite to plate martensite, yes. What is interesting about the martensite transformation
48:03
is it's a, in low carbon steels, it's a thermal. A thermal means it's not time dependent, yes. So if you, as you know, there is a mass start temperature.
48:23
So if you go to a certain undercooling, you will get a certain amount of martensite. And if you keep it there, nothing will happen. Nothing, nothing. So that's the definition of a thermal.
48:42
So if you want to make more martensite, you have to decrease the temperature, yes. If MS is below room temperature, it's not going to transform to martensite, unless you cool it below room temperature. So very important about a thermal transformations,
49:01
they're not time dependent. They're driven by undercooling. You want more martensite, you have to go below. You want to have 100% martensite, you have to go below the MF temperature. MF temperature is the temperature at which the transformation is done. And this is actually a very common form of martensite,
49:26
which we call lath martensite. Very complex microstructures. You can't say really much about them unless you look into this microstructure with a TM. You see actually the microstructure.
49:43
It's very difficult to, unless you're working with bainite and martensite for your research, very difficult to tell them apart, yes. But this is the very common form of martensite
50:02
that you get in low carbon steels. So there are many, so we call engineering steels. Steels we build machines with, have this microstructure.
50:21
But if you have high carbon martensite, this is the microstructure you get. Very different, very different. And so actually there are two phases here. These feathers are martensite.
50:40
And you can see there's still this white background here. That's austenite that has not transformed. This is austenite, so these feathers here are martensite. Usually we say alpha prime.
51:01
It's here, it's called, it hasn't transformed. So why would that happen? How can you have these two phases at room temperature? Well, it's very simple. This is the temperature, and this is the time. And say this is room temperature, room temperature.
51:24
And your transformation, so this is the MS temperature and this is the MF temperature. And you cool down to room temperature. Yes? Then the amount of transformation
51:40
will be such that it's not fully transformed. It's not fully transformed because MF is below room temperature. The transformation is only partial, and it stays like this. So that's an interesting thing to do
52:03
to control the range of MF and MS. Because that allows you to keep austenite stable at room temperature. You have to realize, for instance, this steel,
52:23
the temperature, according to equilibrium diagram, the temperature at which the austenite should transform is about 700 degrees C. This is at room temperature. You've got austenite. It's stable at room temperature.
52:41
Not for five minutes, but forever. Because the transformation at thermal. So very interesting property. So you can actually, by doing some clever things in steels, you can have high temperature phases at room temperature.
53:03
Or stable at room temperature. And of course, the presence of this gamma phase, you're going to find this on a phase diagram.
53:23
Because the phase diagram tells me, at this temperature, the stable phases are ferrite and cementite. And so the martensic transformation, as I said, it's not diffusion transformation.
53:44
It's called a shear transformation. So it basically means that the austenite is transformed by very specific shear. So all the atoms before and after the transformation
54:00
have a specific position. That's not the case when you have a diffusion and, excuse me, a nucleation and growth process. The position of the atoms after the transformation are due to random diffusional jumps. So for instance, here, you can
54:24
see that for this specific shear here, this is the austenite piece of the austenite microscope. You can see here, I've shown a 1-1-1 plane. And you can see here this nice hexagonal shape of the 1-1-1 plane. When we shear it in this specific direction, exactly
54:43
this amount, this phase is turned into a 1-1-0 phase of a PCC or martensite. OK. So again, very different shapes of martensite.
55:08
So here, again, this was what we call lenticular. This martensite here is called lenticular
55:22
because the shape is like a lens, yes? The reason why it's formed is because of strain energy associated with the transformation. But you can have plate. Here, you have the martensites are plates. So here, dark gray is what we call epsilon martensite.
55:41
And this here is just austenite. And this plate-type martensite is very common in the iron-manganese alloys. So studying this translation to martensite's importance,
56:06
we have different ways to look at this transformation martensite. But one of the things that is important in connection
56:22
with martensite, and certainly for engineering steel, is the idea that's called hardenability concept. Hardenability concept. So in engineering steels, natural people don't talk much about hardenability because they don't very often make martensite
56:40
as a microstructure. The people that make motor parts, for instance, use this microstructure, this martensite. So they want to know, how easy can I make martensite? Can I make this special microstructure?
57:04
And in particular, they want to know, because they make parts which have a certain section, they want to know, if I cool down at a certain cooling rate, yes, how easy is it to get a fully martensitic microstructure?
57:23
So first of all, we need to know, why are these people interested in martensite? So why martensite for engineering steels? Well, for a simple reason is because martensite,
57:41
what we call ferrous martensite, can have very high strengths. The typical ferrous martensite can give you 1.5 gigapascal to 2 gigapascal in UTS,
58:01
ultimate tensile strength. That's very high. And that's mostly achieved through carbon. Carbon solute solution, solute in solid solution. In solid solution.
58:24
So you remember, I said that the solubility of ferrite, of carbon, excuse me, in ferrites, almost nothing. Yes? Yes? So in order to keep the carbon in solid solution,
58:41
I need to make martensite. In fact, martensite is what you get as a result of a transformation.
59:03
By adding carbon in such solution, yes, I make a very hard transformation product that is interesting for many engineering applications.
59:21
OK, very important here. When we talk about martensite or martensitic transformation, the result, the product, does not have to be hard. So martensite is very common in the world of material science. Any material system that has transformation,
59:43
you can probably make martensite. Ceramics, other alloys, semiconductors. It's not something that's on steel. And the result.
01:00:00
The martensite is the result of a military shear transformation. That's what martensite is. So it's not necessarily hard and strong and brittle, like you may think. The reason why it's hard and strong and brittle, in the case of ferrous martensite, is because we have carbon in solid solution.
01:00:22
That makes it hard and brittle. In fact, there are steels. Some of the highest strength steels available, which are called Marr aging steels, have a very soft martensite, very soft martensite.
01:00:40
So you can actually make martensite and shape parts with this steel. And then after that, you do a heat treatment to make it very hard. But martensite does not have to be hard and brittle. So they want this phase.
01:01:03
And I'm going to simplify things, temperature and time. So what do you do to make martensite? You need to just, these are, for instance,
01:01:27
perlite or bainite. So log t, I'm going to make log t. And I'll say in a moment why I use log t here. If you have a thin sheet of material
01:01:46
and you put it into a quenching medium, which can be oil, special quenching oils, or other water,
01:02:01
for instance, if I put this down here, I will achieve a certain quenching rate. And so for instance, if I go from 1,000 degrees C, I will be able to do this. Yes? Yes? Now, this looks like a curve, yes?
01:02:23
It's because it's in a log diagram, yes? If I actually make a linear t, it's a line, OK? But because it's logarithmic, it looks like a curve, yeah? OK? So but if I have a part, for instance, that's a crankshaft,
01:02:43
yes, yeah, like this, yeah, a crankshaft, which has a certain section, yes? At the surface, I will be able to achieve this, yes?
01:03:02
But not in the center, yes? Not in the center, and so I'll get this, yes? So I'll be forming something else before I make martensite, yes? OK. So in this case, if this is the transformation
01:03:20
behavior of this particular steel, I say this is not a very hardenable steel, yes? I can only harden it if I increase the cooling rate somehow. Cooling rate, yes, must be higher
01:03:42
so that in the middle, the cooling rate is such, yeah? So this would be in the middle, and this would be at the surface, OK? Or the other thing I can do, yes, is I will add alloying elements, hm? I will add alloying elements.
01:04:01
I'll just use another steel which has this transformation behavior, yes? So then with this cooling rate, I can still make it 100% martensite and achieve a fully martensitic microstructure
01:04:21
with this cooling rate. So there are different ways in which we study in practice this concept of hardenability. So we have the so-called Jominy test, and the other one is the quench test. So in the Jominy test, what do we do?
01:04:42
We take a bar of this material, yes? And we quench it, yeah? We quench it. You see a picture here. This is the bar. You can see this is not paint. It's just in the austenitizing temperature, so it's red. And of course, the end here is being cooled by this water
01:05:01
so it looks dark, right? And as this cools, yes, you can see the part getting darker and darker, right? The one seconds, four seconds, five seconds. OK, so you do this. At this end here, of course, this end is quenched very quickly, but the cooling rate here
01:05:23
and here and here and here becomes progressively less. So in general, if you then measure the hardness, you see that you only have these very high hardnesses very close to the quench end and then it decreases, yes? You can do this kind of test or you
01:05:42
can do a quench test where you basically, again, take a bar, yes? And now you cool it from the side, yes? And you cut it and then you measure the hardness profile, yes? The more martensite you have, the higher the hardness, yes?
01:06:05
And you see here the hardness drops, yes? At the center where the cooling rate is the lowest, you have the softest material, yes? This profile here, you have to realize,
01:06:21
is a function of the steel's hardenability. And what is that? That's basically the composition, yes? So you have steels that will give you this hardness profile, right? This one is a very hardenable steel, yes?
01:06:40
Or you will have steels that will give you the same steel will then give you this hardness profile, yes? So this is a very hardenable steel, OK? And this hardenability is, as I said, function of the composition, yes? In other words, if you have added alloying elements which
01:07:04
influence these kinetics by influencing the stability of these phases of the ferrite or influence the kinetics of the transformation,
01:07:20
then that's the way you engineer the hardenability. So let's have a look a little bit more now with certain cases. So let's have a look at a 0.4 carbon steel. 0.4 is very common engineering steels, yes? To make parts, for instance, motor parts, OK?
01:07:43
So well, let's have, first we start with a steel called 1,040. It has, this is a, we'll talk about standards after this introduction class here.
01:08:01
This 40 here, first of all, let me backtrack here. This is a standard which is in AISI SAE standard, North American standard, very common, very commonly used, And it means that you have 0.4% of carbon.
01:08:22
The last two digits give you the carbon content, yeah? And so it doesn't have much of alloying elements. So this is what you measure as a function of the quashed end, So you see a very quick drop off of the hardness
01:08:44
as a function of, yeah? Now I add alloying elements. Here in this case, 0.5% Crowe and 0.2% of moly, yes? You can see now the hardness is here, yes? I can continue adding alloying elements, Crowe,
01:09:04
and 1.8% of manganese. And now the steel is extremely hardenable, yes, OK? So from this hardness, yes, I can
01:09:23
get to know what is the hardness here that corresponds to the amount of martensite that corresponds to this hardness.
01:09:42
So if I have here at, say, 45, a Rockwell hardness of 45, yes, so what I show here is the hardness as a function of the carbon content, yes, for different amounts of martensite in the martensite.
01:10:01
So the more martensite I have, the harder, yes? The more carbon I have in the same amount of martensite, the higher the hardness, yes? So say, for instance, this is close to 100% of martensite. The more carbon I add in the martensite,
01:10:22
the harder it gets, yeah? So right, so this point is important, 45 Rockwell C, because this corresponds to, so I know in this particular steel I have 0.4% of carbon, yes?
01:10:44
So 0.4%, I have this hardness, yes? So how much martensite do I have? 50%, in this steel microstructure, OK? So for instance, in this case, where do I
01:11:02
have 50% of martensite? Here, right? It's very, very close to the, it's already 50%, not 100%, 50%. Now, at this carbon content, what's the hardness for 100%?
01:11:23
Everything is turned into martensite. Martensite, that corresponds to this point, yeah? So this point corresponds to what? Well, that's what we have at the surface, OK? So surface, I have, just for measuring the hardness and using this diagram here, I
01:11:41
know that I have 100% of martensite, and the martensite contains 0.4% of carbon. So it's, from this diagram, I cannot have 65 hardness, because that would require 100% of martensite,
01:12:06
but a lot more carbon. This is the maximum I can get. All right. So now the question is, how do I
01:12:22
work with this in practice? The Germany test, we have a flat end, yes? And we cool this end. But there are not many parts that we treat this way, yes?
01:12:42
Usually what we have are bars, or something that's cylindrical, yes? And then you need to have something to, because most of the producers of steels, steel companies, they will give you this kind of tests, test results, yes?
01:13:02
And so you want to be able to know, OK, well, if I have a bar with a certain diameter, where can I expect, how much diameter should the bar be, or can the bar be, to make sure we still
01:13:21
have 50% of martensite in the center, for instance, yes? OK. All right, well, we'll answer this question on Thursday, because it'll take a little bit too long. And I'd rather have you understand it well. So we'll break here, and we'll talk about the parameters
01:13:46
that come into play. And in particular, the coolant, as you will see, comes into play in this approach.
01:14:04
All right, so again, for those who were not here this morning, this quiz Thursday.