Physical Metallurgy of Steels - Part 4
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Bhadeshia, Harry
A series of 12 lectures on the physical metallurgy of steels by Professor H. K. D. H. Bhadeshia. Part 4 deals with the design of carbide-free bainitic steels.
 Okay, welcome to the third lecture, fourth lecture. Okay, and today we're going to actually apply some of the theory that we've learned to design new steel. So just to remind you of the mechanism that we came up with for the transformation to bainite, imagine that bainite forms exactly like martensite where the plate of ferrite is fully supersaturated with carbon, but because we are transforming at a relatively high temperature, the carbon has an opportunity to escape into the austenite. If we are at a relatively high temperature then the carbon escapes completely leaving the plate of ferrite free from carbon and eventually this excess carbon will precipitate as cementite between the plates of ferrite. If we transform to lower bainite, that means a relatively lower transformation temperature, then there is an opportunity for carbides to precipitate inside the plate rather like tempering of martensite. And because we precipitate some carbides inside the ferrite, we get less carbon partitioning into the remaining austenite and therefore we have finer precipitates of cementite in a lower bainite microstructure. So I explained that this is actually tougher than this microstructure because of the fine scale of the carbides here. Even though lower bainite is stronger. Now of course when we go to even higher strength steels, these are like inclusions, they cause fracture. Cementite particles are harder than brittle and if you apply large enough stress then they will crack. And when we make high strength steels, we make them extremely clean, you know, very free from non-metallic inclusions. So then the cementite particles become the particles which initiate fracture. So bainitic steels of this kind containing fairly coarse carbides have not really succeeded in achieving the same toughness and strength combinations that you can get with tempered martensite because in tempered martensite you can control the tempering process and achieve fine carbides. So typically these are the sort of carbide particles that we see inside normal bainitic steels which have a high carbon concentration. So when I say a high carbon concentration of the order of 0.3, 0.4 weight percent carbon, you see fairly coarse cementite particles. And in the case of cleavage fractures, these particles initiate cleavage cracks and in the case of ductile fracture initiate the nucleation of voids. So inside the dimples that you see on the fracture surface you'll be able to find cementite particles. And the consequence of those particles is that you get a reduction in toughness. So here is the test temperature for a CHARPY test and the CHARPY impact energy and supposing we create a steel with a bainitic microstructure and coarse carbides then you get a reduction in toughness and an increase in the impact transition temperature. So these carbides are bad in the context of very high strength steels and when you have a medium carbon concentration. Obviously if you reduce the carbon concentration to less than 0.05 weight percent then the carbides will be fine. So in that case they are not the initiating sites for fracture. So how can I stop these carbides from precipitating? Any ideas? What elements can I add to the steel which stops cementite from precipitating? Titanium will capture the carbon and form titanium carbides which are cubic in shape and coarse because they form at a very high temperature. So we don't want that either. Silicon and anything else? You are right, you are quite right. How does silicon and aluminium stop the precipitation of cementite? What is the mechanism? We know very well even from history if you take cast iron, cast iron contains something like 3 weight percent of carbon, if you add silicon to the cast iron then instead of a white cast iron you get a grey cast iron. White cast iron is full of cementite, grey cast iron is full of graphite. So we have known this for more than a century that if you add elements like silicon to cast iron then we stop primary cementite from forming and you end up with cast iron containing graphite instead of very hard cast iron full of cementite. So how does silicon stop the precipitation of cementite? What is the mechanism? Any ideas? I think you all know about this because tripped steels are designed like this but do you know why these elements stop the precipitation of cementite from austenite? Let me give you a clue. So the mechanism involves partitioning of carbon from the super saturated plate into the remaining austenite. So it's this precipitation from austenite that we have to stop the formation of cementite from austenite. Now if you look at the solubility of carbon in cementite it's almost zero. It has virtually zero solubility in cementite but when you form cementite at a low temperature there is no opportunity for the carbon to escape while the cementite is growing. So when we talk about low temperature I'm saying 400 degrees centigrade the cementite is forced to absorb any silicon which is in your steel even though it does not want it. So once you force silicon into cementite its energy increases. It becomes less stable and here are some first principles calculations done by Jaehoon Jang and Indi Kim in GIFT where when you substitute an ion atom with a silicon atom so here is Fe3C and this is Fe3C but one of the ion atoms is substituted with silicon you get a dramatic increase in energy and here two silicon atoms have been substituted in different sites within the unit cell of cementite and again you get a large increase in the energy of the cementite. So if you force the silicon to go into the cementite then it becomes much less stable and in some circumstances if you've added enough silicon then it doesn't form at all and the same should apply to aluminium. Now why did we need to do this analysis by first principles calculations? Because if you have virtually zero solubility of silicon in cementite then you cannot actually make measurements. You cannot make thermodynamic measurements. So if you try to do a phase diagram calculation you don't have the thermodynamic data. So these thermodynamic data were created using first principles calculations and this is a wonderful application of first principles calculations because here you cannot even in principle do an experiment. So that's where you can only do calculations. So just to summarize that the mechanism by which cementite forms, cementite precipitation is prevented. So cementite precipitation is prevented. Let me just make this pen finer. Prevented because one the solubility of silicon in cementite is negligible. So this is important. Solubility of silicon, I'll use the same symbol theta for cementite throughout. Alpha is ferrite, gamma is austenite and theta is cementite. So solubility of silicon in cementite is negligible. And the second crucial thing is that during the bay night reaction you are forming cementite at a low temperature. So during bay night reaction theta forms at a low temperature. That means that the silicon cannot partition during the growth of the cementite. Therefore silicon cannot partition during cementite growth. It is basically forced to be in the cementite. It is forced into theta and therefore theta becomes unstable. So the important point is that if the cementite formed at a high temperature, you know above let's say 600 degree centigrade, then the silicon could partition and there is no problem in precipitating cementite. So in the same steel, if I form pearlite, pearlite forms at a relatively high temperature around 600 degree centigrade, silicon slightly retards the transformation but it doesn't stop the precipitation of cementite. So you have to have two conditions. First, that the silicon is forced into the cementite lattice. In other words, that you have to form it at the cementite at a low temperature in order to use silicon to suppress the precipitation of cementite. Is everyone happy with that? So that is the mechanism by which silicon stops the precipitation of cementite from austenite and probably aluminium works in the same way but we don't have the same kind of evidence that I've shown you here for the effect of aluminium on cementite precipitation. So it's possible now, you know, given this kind of thermodynamic data to do calculations on the growth rate of cementite in which you force the silicon and therefore to calculate how much silicon you need in order to suppress the precipitation of cementite. Roughly speaking, you know, for the kind of steels that we are interested in, you need in excess of about one weight percent of silicon to stop the precipitation of cementite. Okay, so let's carry on. Let's imagine that we study three different alloys, very simple alloys. All of them contain a certain level of silicon to stop the precipitation of cementite and we've got carbon concentrations that I'll explain to you later and some elements to give you hard nability because you want to stop the precipitation of other kinds of transformation products such as pearlite and so forth. Okay. Now when we transform these isothermally into bainite, we get the microstructure which I've shown you previously. Extremely fine plates of bainitic ferrite. So remember this scale is one micrometer. So when you have a thin plate shape, the grain size is approximately twice the thickness of the plate. Okay, because the grain size is a mean linear intercept. Right? You measure grain size as a mean linear intercept and when you have a thin plate shape, the probability of slip happening in the plane of the plate is negligible. So the mean linear intercept is about twice the thickness. So here we are achieving a grain size which is a fraction of a micrometer just by phase transformation. You don't need any complex thermomechanical processing. Second is we've suppressed cementite precipitation. So in between these plates, we've got these regions of austenite and austenite does not have a ductile brittle transition temperature. So if I plot that diagram which I drew in the last lecture where I'm doing the cleavage stress as a function of temperature. Okay, so here we have temperature and this is the stress and this is the stress required to cleave the crystal, cleavage. And if I now plot on this the temperature dependence of the strength of austenite, then for all temperatures it's easier to flow by plastic deformation. So this is austenite and therefore you do not get austenite breaking by a cleavage mechanism. You do not have a ductile brittle transition for austenite. In the case of ferrite, I pointed out to you that you get an intersection between the flow curve and the cleavage stress and that's why you get a ductile brittle transition. So one major advantage of having austenite in the microstructure is that we have no ductile brittle transition temperature for these regions of retained austenite. Now is there any other advantage of having the austenite there that you can think of? Why do we often want austenite in our steel? Trim effect, so transformation induced plasticity. Now we are going to do this in a bit more detail later but when you pull the material the transformation of the austenite into martensite actually adds to the plasticity of your steel. So even though the material is strong you can get a high ductility and the solubility of hydrogen in austenite is high but the diffusion rate of hydrogen into the austenite is orders of magnitude smaller than that in ferrite. What that means is that hydrogen will have difficulty in getting into your steel. That's a good thing because when you put your steel into service you don't really want hydrogen to get into the steel by corrosion reactions etc. So hydrogen embrittlement becomes less of a problem if you have sufficient austenite surrounding the ferrite regions. So there are many advantages to having this beautiful microstructure which is extremely fine and I am just going to list the advantages. Forgotten how to change this page? Okay let me clear it. Okay so let's list the advantages. The first one is that we achieve an incredibly fine structure just by phase transformation. So we achieve incredibly fine structure by phase transformation. Now we've got a mixture of ferrite and austenite and this austenite is created using an extremely cheap alloying element. It's basically stabilized by the partitioning of carbon and you cannot get a cheaper alloying element than carbon. So the austenite is stabilized by an extremely cheap alloying element. So gamma is stabilized by cheap solute which is carbon whereas you know the original trip steels were actually with very large concentrations of nickel something in excess of 28%. Here we are just using carbon to stabilize the austenite. Average carbon concentration is quite low about 0.4 but when you partition it the austenite ends up with a carbon concentration in excess of 1 weight percent and therefore it becomes stable. So we are using extremely cheap solute. Number 3 is that hydrogen has a very low diffusivity in austenite and that is always a good thing when you are dealing with high strength steels. So hydrogen has very low diffusivity in gamma. So you should reduce hydrogen embrittlement problems. Because we have removed the carbon from the bainite and partitioned it into the austenite that makes the bainitic ferrite ductile as well. Because if ferrite contains carbon in solid solution that definitely embrittles it. That's why you have to temper martensite when you use martensite. So number 4 is that the carbon concentration of the bainitic ferrite is much less than the average value. So carbon concentration of bainitic ferrite is much lower than average of steel. So that makes it tough. Makes the ferrite. And we are also getting our strength from the very fine grain structure. And grain size is one of the only mechanisms by which you can increase both the strength and toughness. Very often when you try to increase the strength the toughness will deteriorate. But with grain size refinement you increase both strength and toughness. So we have a very fine grain size. Really the strength is produced by grain size effect. Strength due to fine grain structure. And grain refinement is the only mechanism for both increasing the strength and toughness. For increasing strength and toughness. So the very simple addition of silicon to our steel together with transformation into a mixture of bainitic ferrite and austenite gives us what appears to be an ideal microstructure. So this appears to be ideal microstructure. One happy sofa. And of course if you remember each plate is only about 10 micrometers in length because the plasticity associated with the shape deformation stops it from growing once it reaches about that length. So it's actually finer than martensite. Okay let's have a look at the toughness of this wonderful microstructure. Right. When we actually do an experiment to measure the toughness it's terrible. You can see here that the impact transition temperature is more than 100 degrees centigrade. So that's completely unacceptable for any engineering material that below 100 degrees centigrade you get fracture by cleavage. And this is the alloy that I listed in one of those tables earlier. No.4 carbon, 3 manganese and 2 silicon. So something is very wrong with our science here that we have an ideal microstructure and it's giving extremely poor properties. So let's just try and think what is the problem. So I'm going to show you an optical micrograph of this structure. Here we are. So look at the scale over here. It's 50 micrometers. So this is an optical micrograph. And if I looked at these bainite sheaves in a transmission electron micrograph then they would look exactly like the ones that I showed you earlier which are extremely fine and with films of austenite in between them. But notice one more thing. We have these large regions of untransformed high carbon austenite. You can see these large white regions which have a scale of the order of 50 micrometers which are untransformed high carbon austenite. Now when you increase the size of austenite it becomes easier for it to transform into martensite. So as soon as I apply a stress to this material the first regions which trip are these regions and therefore 50 micrometer sized regions of untempered high carbon martensite. So it's like creating a beautiful fine structure and then throwing a huge rock into the structure which is brittle. Now why do we have these large regions of austenite left in our material even though we have transformed isothermally for a long time to generate bainite? Any ideas? Why does this material refuse to transform further? Because I don't want those large regions of austenite. I want them to transform into the fine mixture of bainitic ferrite and films of retained austenite. Why does the reaction stop? T0 curve. We have a thermodynamic limit that as soon as the composition of the austenite reaches the T0 boundary it cannot transform further. So here is the T0 curve and this is the average carbon concentration of the steel and if I'm transforming at this temperature then I can only get bainite until the austenite composition reaches the red line. So if I apply the lever rule to the T0 curve then I should be able to calculate the maximum amount of bainitic ferrite that I can obtain. The lever rule is basically that the amount of bainitic ferrite is given by this distance here divided by this distance assuming that the ferrite has zero carbon concentration. So everyone happy with the lever rule? You apply it to a phase diagram to work out the volume fraction of a phase, the maximum volume fraction of a phase. So it's this distance here divided by this distance which if I express mathematically is the carbon concentration given by T0 minus the average carbon concentration of the steel divided by the carbon concentration given by the T0 curve and the carbon concentration of ferrite which we are going to assume is zero. So I want you to give me three ways, three ways in which I can increase the volume fraction of bainite so that I can get rid of the large regions of austenite. If you look at this equation there should be three ways of doing this. So I am waiting for you to give me the answer. What are the three ways in which I can increase the volume fraction of bainite? Because if I can't then we are stuck. This is a thermodynamic limit to the amount of transformation. Even if you can't think metallurgically, think mathematically. What are the three ways I could increase the volume fraction of bainitic ferrite? This is the volume fraction of bainitic ferrite, right? This is the symbol for bainitic ferrite. Yeah, I think you have an idea. How can I increase this? Come on guys. Guys is refers to both men and women. It's an asexual word. Yeah, so if I reduce the average concentration of carbon, that means I shift this line towards the vertical axis, then I can increase the volume fraction of bainite. So what this says is if I actually reduce the carbon concentration of the steel, I should get more bainite, get rid of those large regions of austenite, and I would not compromise strength because I am increasing the amount of bainite. So it goes against our normal thinking that if I reduce the carbon concentration, I actually get an increase in strength because I am increasing the amount of bainite. Second method? See this curve depends on the substitutional solids in our material. So if you modify the substitutional solids, for example manganese and so forth, then we can shift this curve to higher carbon concentrations. So that's the second method. And the third method is why not lower the transformation temperature? There is of course a limit. What is that limit? How low can I go? Martensize temperature. So the three ways of improving the toughness of increasing the volume fraction of bainitic ferrite. One is reduce x bar, the average carbon concentration of the material. Two is modify substitutional solids to move the t0 curve to higher carbon. And the third is to reduce the transformation temperature. But this is limited by the fact that we mustn't produce martensite. So the lowest I can go is close to MS. So but limited by the martensite start temperature. I remember we started with a steel which was Fe 0.4 carbon, three manganese and two silicon weight percent. So the first thing I'm going to do is I'm going to cut the carbon concentration by a factor of 2. So the prediction from this very simple theory is that if I make a new steel which is Fe 0.2 carbon, three manganese, two silicon, I should get an improvement in toughness without a loss of strength. And I know and I'll illustrate to you later that if I substitute manganese by nickel, then the t0 curve moves to a higher carbon concentration. So the second modification that we make, t0 curve you can calculate very easily. Just download some software from our website. So we will keep the carbon concentration the same because we are trying to prove a point here. And instead of manganese, we will use nickel and two silicon. So these are the two steels which are predicted from our simple theory that if we do as we expect, we will improve the toughness without a loss of strength. So here are those three alloys that I just wrote about. In one case, we are going to cut the carbon concentration by a factor of 2. And in the second case, we are going to change the substitutional alloying addition to shift the t0 curve to the right hand side to higher carbon concentrations. So we made these alloys and we achieved the required refinement of the structure in terms of getting rid of the large regions of austenite. So this is the one with half the carbon concentration. And we have this beautiful microstructure without the large regions of austenite. And this is the high carbon, 0.4 carbon, but with a higher nickel concentration. The next slide that I will show you is absolutely amazing because this is the original impact transition curve with, you know, transition temperature being more than 100 degrees centigrade. And these are the two new alloys where we have shifted the transition temperature by 200 degrees centigrade just by a simple equation. So the toughness has improved dramatically in one case, maintaining the carbon concentration in the other case by reducing the carbon concentration by a factor of 2. And without compromising the strength, anybody can improve the toughness by reducing the strength. But here we are both maintaining the strength and dramatically improving the toughness. So large regions of austenite which are unstable to applied stress and which decompose into high carbon martensite, untempered martensite, obviously are going to be bad for your steel. What we want is the nice thin films of austenite between the plates of ferrite. Everyone happy with that? Okay, so now I am going to show you some products which come out of this very simple theory. So does anybody recognize what these are? Sections of railway lines, yeah, and they are made in different sizes depending on applications. What is the normal structure of railway lines? Pearlite, yeah. And somebody else now tell me what is the structure of pearlite? Sorry? Yeah, but describe to me how is the mixture. Is it spherical particles of cementite in ferrite or lamella which means, you know, alternating layers of cementite and ferrite, yeah? Okay. Here, so this is the typical structure that you would see in an optical micrograph of pearlite where it appears that you have got these separate layers of cementite and ferrite. And for the past 60 or so years the performance of railway lines has increased enormously by refining the spacing of the pearlite. Yeah, because if you refine the distance between the cementite and ferrite then you improve the strength, the hardness of your material. But actually the way that I describe the structure is wrong. It doesn't consist of alternating layers of cementite and ferrite. Let me show you what it really looks like. I really enjoy this slide. Okay. So this is a cabbage. And a cabbage has leaves which are all connected in three dimensions, right? So think of that as the crystal of cementite. It's a single crystal of cementite. And inside this I've got water and the water is a single crystal of ferrite. So when I put the cabbage inside the bucket, I've got a bicrystal of cementite and ferrite. And the toughness is determined by the size of that bicrystal, not by the spacing between the planes because the crystallography is identical within a colony. You've got a single crystal of cementite which is intimately mixed with a single crystal of ferrite. So if you have a crack then it propagates across the whole colony of pearlite. But the strength depends on the distance between the leaves of this cabbage. We section that cabbage, it looks like pearlite, right? So although rails have improved in terms of their hardness, their toughness remains quite low. Yeah? So if we replace this by the structure that we've just created from a mixture of very fine plates of ferrite and carbon enriched films of retained austenite, we have no carbide particles in the material. And both the toughness and the strength depend on the scale of the bainitic ferrite and films of austenite. So we made railway lines out of this material, the bainitic ferrite and austenite. And there are two main characteristics that are required from railway lines. One is called rolling contact fatigue. So when you have a wheel going over a surface, it induces a stress under the surface, the cyclic stress every time a wheel goes. And that causes fatigue. And then you get a bit of the steel just coming off. And of course, once you've damaged the surface, that damage grows every time something goes over it. And it can ultimately lead to fracture. This is the normal, polytic rail. These are full scale tests. Normal, polytic rail. This is a rail which is hardened on the surface. And this is the new bainitic rail which didn't break. So we stopped the test because these are extremely expensive tests. Now the second characteristics that you require is wear resistance. Because you cause a little bit of damage every time a wheel goes over the surface and that causes wear of the rail. And certainly if you're going around a curve, then you also have specific areas from which you get wear. This is a rail which has been in service, but one part of it is ordinary rail and the other part is the new bainitic rail. So here is the carbide free bainitic rail. And you can see that the damage here from wear processes is much bigger. And why is the wear resistance better? Well, if you have hard particles in your microstructure and they come off, then they will act like an abrasive. But we don't have these hard particles in the carbide free bainite. It's just a mixture of austenite and ferrite. So if you look at the damage that you cause at the surface, it's more like plastic flow rather than wear mechanism. And here are some quantitative results. The wear process is quite complicated. You know, it's not just rolling but also slip. So the tests involve a certain amount of slip deformation between the two objects in contact. And we are looking both at the wheel and the railway line. So this is the wear rate on the wheel caused by moving on an ordinary, pearlitic rail. And this is for martensite. And this is the only structure which actually reduces the wear rate on the wheel as well. And this is the tunnel which goes under the sea between Britain and France. And these are the rails made from carbide free bainitic ferrite. So just from three lectures, okay, you have the theory to be able to design a steel which can go into service. So if ever you come to Britain or to France, make sure you go through the Euro tunnel and you will be travelling on carbide free bainitic rail. Okay. Right. I can show you some more examples. This is a torpedo car which carries molten steel going over a bainitic rail and so on. Now, of course, there are many other applications that you can think of of this particular structure. And here I'm plotting the toughness, fracture toughness versus the ultimate tensile strength. And there's a whole collection of data from many different studies. And what I'd like you to see is this is maraging steel. Maraging steel is a martensitic steel, okay, a very highly alloyed martensitic steel. And it has almost zero carbon and you quench it and then you heat treat it at around 500 degrees centigrade to precipitate intermetallic compounds and that makes it hard. But because it doesn't have the carbon, it's very tough. And it's used for example for rocket motor casings. In some cases, we are matching the properties of maraging steels using much cheaper alloying addition. So here you're looking at incredible toughness, you know, about 130 megapascal root meters of toughness at a strength level of about 1600 megapascals. And these are quenched and tempered martensitic steels and these are all of these are carbide free bainitic steels. There is much more potential with this structure because of the absence of carbides. And for example, in GIFT itself, we have worked on producing these steels by continuous annealing. You know, just like you produce the trip assistive steels, we can produce without the electromorphic ferrite and therefore get a much stronger trip steel as opposed to a trip assisted steel means there are other phases, you know, just like equiaxed ferrite in the structure. So any questions? The most important point I want to get over to you is that if you understand the mechanisms of phase transformations, then you can find a logical way of creating a better steel instead of just making many, many different chemistries and looking at their properties. The simple equation here and the recognition that large regions of austenite are no good for the toughness helps to create actual products which go into service. Okay, that's all for today.
 Okay, welcome to the third lecture, fourth lecture. Okay, and today we're going to actually  apply some of the theory that we've learned to design new steel. So just to remind you  of the mechanism that we came up with for the transformation to bainite, imagine that  bainite forms exactly like martensite where the plate of ferrite is fully supersaturated  with carbon, but because we are transforming at a relatively high temperature, the carbon  has an opportunity to escape into the austenite. If we are at a relatively high temperature  then the carbon escapes completely leaving the plate of ferrite free from carbon and  eventually this excess carbon will precipitate as cementite between the plates of ferrite.  If we transform to lower bainite, that means a relatively lower transformation temperature,  then there is an opportunity for carbides to precipitate inside the plate rather like  tempering of martensite. And because we precipitate some carbides inside the ferrite, we get less  carbon partitioning into the remaining austenite and therefore we have finer precipitates of  cementite in a lower bainite microstructure. So I explained that this is actually tougher  than this microstructure because of the fine scale of the carbides here. Even though lower  bainite is stronger. Now of course when we go to even higher strength  steels, these are like inclusions, they cause fracture. Cementite particles are harder than  brittle and if you apply large enough stress then they will crack. And when we make high  strength steels, we make them extremely clean, you know, very free from non-metallic inclusions.  So then the cementite particles become the particles which initiate fracture. So bainitic  steels of this kind containing fairly coarse carbides have not really succeeded in achieving  the same toughness and strength combinations that you can get with tempered martensite because  in tempered martensite you can control the tempering process and achieve fine carbides.  So typically these are the sort of carbide particles that we see inside normal bainitic  steels which have a high carbon concentration. So when I say a high carbon concentration  of the order of 0.3, 0.4 weight percent carbon, you see fairly coarse cementite particles.  And in the case of cleavage fractures, these particles initiate cleavage cracks and in  the case of ductile fracture initiate the nucleation of voids. So inside the dimples  that you see on the fracture surface you'll be able to find cementite particles.  And the consequence of those particles is that you get a reduction in toughness. So here  is the test temperature for a CHARPY test and the CHARPY impact energy and supposing  we create a steel with a bainitic microstructure and coarse carbides then you get a reduction  in toughness and an increase in the impact transition temperature. So these carbides  are bad in the context of very high strength steels and when you have a medium carbon concentration.  Obviously if you reduce the carbon concentration to less than 0.05 weight percent then the  carbides will be fine. So in that case they are not the initiating sites for fracture.  So how can I stop these carbides from precipitating? Any ideas? What elements can I add to the  steel which stops cementite from precipitating? Titanium will capture the carbon and form  titanium carbides which are cubic in shape and coarse because they form at a very high  temperature. So we don't want that either. Silicon and anything else? You are right,  you are quite right. How does silicon and aluminium stop the precipitation of cementite?  What is the mechanism? We know very well even from history if you take cast iron, cast iron  contains something like 3 weight percent of carbon, if you add silicon to the cast iron  then instead of a white cast iron you get a grey cast iron. White cast iron is full  of cementite, grey cast iron is full of graphite. So we have known this for more than a century  that if you add elements like silicon to cast iron then we stop primary cementite from  forming and you end up with cast iron containing graphite instead of very hard cast iron full  of cementite. So how does silicon stop the precipitation of cementite? What is the mechanism?  Any ideas? I think you all know about this because tripped steels are designed like this  but do you know why these elements stop the precipitation of cementite from austenite?  Let me give you a clue. So the mechanism involves partitioning of carbon from the super saturated  plate into the remaining austenite. So it's this precipitation from austenite that we  have to stop the formation of cementite from austenite. Now if you look at the solubility  of carbon in cementite it's almost zero. It has virtually zero solubility in cementite  but when you form cementite at a low temperature there is no opportunity for the carbon to  escape while the cementite is growing. So when we talk about low temperature I'm saying  400 degrees centigrade the cementite is forced to absorb any silicon which is in your steel  even though it does not want it. So once you force silicon into cementite its energy increases.  It becomes less stable and here are some first principles calculations done by Jaehoon Jang  and Indi Kim in GIFT where when you substitute an ion atom with a silicon atom so here is  Fe3C and this is Fe3C but one of the ion atoms is substituted with silicon you get a dramatic  increase in energy and here two silicon atoms have been substituted in different sites within  the unit cell of cementite and again you get a large increase in the energy of the cementite.  So if you force the silicon to go into the cementite then it becomes much less stable  and in some circumstances if you've added enough silicon then it doesn't form at all  and the same should apply to aluminium. Now why did we need to do this analysis by first  principles calculations? Because if you have virtually zero solubility of silicon in cementite  then you cannot actually make measurements. You cannot make thermodynamic measurements.  So if you try to do a phase diagram calculation you don't have the thermodynamic data. So  these thermodynamic data were created using first principles calculations and this is  a wonderful application of first principles calculations because here you cannot even in  principle do an experiment. So that's where you can only do calculations. So just to summarize  that the mechanism by which cementite forms, cementite precipitation is prevented. So cementite  precipitation is prevented. Let me just make this pen finer. Prevented because one the  solubility of silicon in cementite is negligible. So this is important. Solubility of silicon,  I'll use the same symbol theta for cementite throughout. Alpha is ferrite, gamma is austenite  and theta is cementite. So solubility of silicon in cementite is negligible. And the second  crucial thing is that during the bay night reaction you are forming cementite at a low  temperature. So during bay night reaction theta forms at a low temperature. That means  that the silicon cannot partition during the growth of the cementite. Therefore silicon  cannot partition during cementite growth. It is basically forced to be in the cementite.  It is forced into theta and therefore theta becomes unstable. So the important point is  that if the cementite formed at a high temperature, you know above let's say 600 degree centigrade,  then the silicon could partition and there is no problem in precipitating cementite.  So in the same steel, if I form pearlite, pearlite forms at a relatively high temperature  around 600 degree centigrade, silicon slightly retards the transformation but it doesn't  stop the precipitation of cementite. So you have to have two conditions. First, that the  silicon is forced into the cementite lattice. In other words, that you have to form it at  the cementite at a low temperature in order to use silicon to suppress the precipitation  of cementite. Is everyone happy with that? So that is the mechanism by which silicon  stops the precipitation of cementite from austenite and probably aluminium works in  the same way but we don't have the same kind of evidence that I've shown you here for the  effect of aluminium on cementite precipitation. So it's possible now, you know, given this  kind of thermodynamic data to do calculations on the growth rate of cementite in which you  force the silicon and therefore to calculate how much silicon you need in order to suppress  the precipitation of cementite. Roughly speaking, you know, for the kind of steels that we are  interested in, you need in excess of about one weight percent of silicon to stop the  precipitation of cementite. Okay, so let's carry on. Let's imagine that we study three  different alloys, very simple alloys. All of them contain a certain level of silicon  to stop the precipitation of cementite and we've got carbon concentrations that I'll  explain to you later and some elements to give you hard nability because you want to  stop the precipitation of other kinds of transformation products such as pearlite and so forth. Okay.  Now when we transform these isothermally into bainite, we get the microstructure which I've  shown you previously. Extremely fine plates of bainitic ferrite. So remember this scale  is one micrometer. So when you have a thin plate shape, the grain size is approximately  twice the thickness of the plate. Okay, because the grain size is a mean linear intercept.  Right? You measure grain size as a mean linear intercept and when you have a thin plate shape,  the probability of slip happening in the plane of the plate is negligible. So the mean linear  intercept is about twice the thickness. So here we are achieving a grain size which is  a fraction of a micrometer just by phase transformation. You don't need any complex thermomechanical  processing. Second is we've suppressed cementite precipitation. So in between these plates,  we've got these regions of austenite and austenite does not have a ductile brittle transition  temperature. So if I plot that diagram which I drew in the last lecture where I'm doing  the cleavage stress as a function of temperature. Okay, so here we have temperature and this  is the stress and this is the stress required to cleave the crystal, cleavage. And if I  now plot on this the temperature dependence of the strength of austenite, then for all  temperatures it's easier to flow by plastic deformation. So this is austenite and therefore  you do not get austenite breaking by a cleavage mechanism. You do not have a ductile brittle  transition for austenite. In the case of ferrite, I pointed out to you that you get an intersection  between the flow curve and the cleavage stress and that's why you get a ductile brittle transition.  So one major advantage of having austenite in the microstructure is that we have no ductile  brittle transition temperature for these regions of retained austenite. Now is there any other  advantage of having the austenite there that you can think of? Why do we often want austenite  in our steel?  Trim effect, so transformation induced plasticity. Now we are going to do this in a bit more  detail later but when you pull the material the transformation of the austenite into martensite  actually adds to the plasticity of your steel. So even though the material is strong you  can get a high ductility and the solubility of hydrogen in austenite is high but the diffusion  rate of hydrogen into the austenite is orders of magnitude smaller than that in ferrite.  What that means is that hydrogen will have difficulty in getting into your steel. That's  a good thing because when you put your steel into service you don't really want hydrogen  to get into the steel by corrosion reactions etc. So hydrogen embrittlement becomes less  of a problem if you have sufficient austenite surrounding the ferrite regions. So there  are many advantages to having this beautiful microstructure which is extremely fine and  I am just going to list the advantages. Forgotten how to change this page? Okay let me clear  it. Okay so let's list the advantages. The first one is that we achieve an incredibly  fine structure just by phase transformation. So we achieve incredibly fine structure by  phase transformation. Now we've got a mixture of ferrite and austenite and this austenite  is created using an extremely cheap alloying element. It's basically stabilized by the  partitioning of carbon and you cannot get a cheaper alloying element than carbon. So  the austenite is stabilized by an extremely cheap alloying element. So gamma is stabilized  by cheap solute which is carbon whereas you know the original trip steels were actually  with very large concentrations of nickel something in excess of 28%. Here we are just using carbon  to stabilize the austenite. Average carbon concentration is quite low about 0.4 but when  you partition it the austenite ends up with a carbon concentration in excess of 1 weight  percent and therefore it becomes stable. So we are using extremely cheap solute. Number  3 is that hydrogen has a very low diffusivity in austenite and that is always a good thing  when you are dealing with high strength steels. So hydrogen has very low diffusivity in gamma.  So you should reduce hydrogen embrittlement problems. Because we have removed the carbon  from the bainite and partitioned it into the austenite that makes the bainitic ferrite  ductile as well. Because if ferrite contains carbon in solid solution that definitely embrittles  it. That's why you have to temper martensite when you use martensite. So number 4 is that  the carbon concentration of the bainitic ferrite is much less than the average value. So carbon  concentration of bainitic ferrite is much lower than average of steel.  So that makes it tough. Makes the ferrite. And we are also getting our strength from  the very fine grain structure. And grain size is one of the only mechanisms by which you  can increase both the strength and toughness. Very often when you try to increase the strength  the toughness will deteriorate. But with grain size refinement you increase both strength  and toughness. So we have a very fine grain size. Really the strength is produced by grain  size effect. Strength due to fine grain structure. And grain refinement is the only mechanism  for both increasing the strength and toughness. For increasing strength and toughness.  So the very simple addition of silicon to our steel together with transformation into  a mixture of bainitic ferrite and austenite gives us what appears to be an ideal microstructure.  So this appears to be ideal microstructure.  One happy sofa. And of course if you remember each plate is only about 10 micrometers in  length because the plasticity associated with the shape deformation stops it from growing  once it reaches about that length. So it's actually finer than martensite. Okay let's  have a look at the toughness of this wonderful microstructure. Right. When we actually do  an experiment to measure the toughness it's terrible. You can see here that the impact  transition temperature is more than 100 degrees centigrade. So that's completely unacceptable  for any engineering material that below 100 degrees centigrade you get fracture by cleavage.  And this is the alloy that I listed in one of those tables earlier. No.4 carbon, 3 manganese  and 2 silicon. So something is very wrong with our science here that we have an ideal  microstructure and it's giving extremely poor properties. So let's just try and think  what is the problem. So I'm going to show you an optical micrograph of this structure.  Here we are. So look at the scale over here. It's 50 micrometers. So this is an optical  micrograph. And if I looked at these bainite sheaves in a transmission electron micrograph  then they would look exactly like the ones that I showed you earlier which are extremely  fine and with films of austenite in between them. But notice one more thing. We have these  large regions of untransformed high carbon austenite. You can see these large white regions  which have a scale of the order of 50 micrometers which are untransformed high carbon austenite.  Now when you increase the size of austenite it becomes easier for it to transform into  martensite. So as soon as I apply a stress to this material the first regions which trip  are these regions and therefore 50 micrometer sized regions of untempered high carbon martensite.  So it's like creating a beautiful fine structure and then throwing a huge rock into the structure  which is brittle. Now why do we have these large regions of austenite left in our material  even though we have transformed isothermally for a long time to generate bainite? Any ideas?  Why does this material refuse to transform further? Because I don't want those large  regions of austenite. I want them to transform into the fine mixture of bainitic ferrite  and films of retained austenite. Why does the reaction stop? T0 curve. We have a thermodynamic  limit that as soon as the composition of the austenite reaches the T0 boundary it cannot  transform further. So here is the T0 curve and this is the average carbon concentration  of the steel and if I'm transforming at this temperature then I can only get bainite until  the austenite composition reaches the red line. So if I apply the lever rule to the  T0 curve then I should be able to calculate the maximum amount of bainitic ferrite that  I can obtain. The lever rule is basically that the amount of bainitic ferrite is given  by this distance here divided by this distance assuming that the ferrite has zero carbon  concentration. So everyone happy with the lever rule? You apply it to a phase diagram  to work out the volume fraction of a phase, the maximum volume fraction of a phase. So  it's this distance here divided by this distance which if I express mathematically is the carbon  concentration given by T0 minus the average carbon concentration of the steel divided  by the carbon concentration given by the T0 curve and the carbon concentration of ferrite  which we are going to assume is zero. So I want you to give me three ways, three ways  in which I can increase the volume fraction of bainite so that I can get rid of the large  regions of austenite. If you look at this equation there should be three ways of doing  this. So I am waiting for you to give me the answer. What are the three ways in which I  can increase the volume fraction of bainite? Because if I can't then we are stuck. This  is a thermodynamic limit to the amount of transformation. Even if you can't think metallurgically,  think mathematically. What are the three ways I could increase the volume fraction of bainitic  ferrite? This is the volume fraction of bainitic ferrite, right? This is the symbol for bainitic  ferrite. Yeah, I think you have an idea. How can I increase this? Come on guys. Guys  is refers to both men and women. It's an asexual word. Yeah, so if I reduce the average concentration  of carbon, that means I shift this line towards the vertical axis, then I can increase the  volume fraction of bainite. So what this says is if I actually reduce the carbon concentration  of the steel, I should get more bainite, get rid of those large regions of austenite, and  I would not compromise strength because I am increasing the amount of bainite. So it  goes against our normal thinking that if I reduce the carbon concentration, I actually  get an increase in strength because I am increasing the amount of bainite. Second method? See  this curve depends on the substitutional solids in our material. So if you modify the  substitutional solids, for example manganese and so forth, then we can shift this curve  to higher carbon concentrations. So that's the second method. And the third method is  why not lower the transformation temperature? There is of course a limit. What is that limit?  How low can I go? Martensize temperature. So the three ways of improving the toughness  of increasing the volume fraction of bainitic ferrite. One is reduce x bar, the average  carbon concentration of the material. Two is modify substitutional solids to move the  t0 curve to higher carbon. And the third is to reduce the transformation temperature.  But this is limited by the fact that we mustn't produce martensite. So the lowest I can go  is close to MS. So but limited by the martensite start temperature. I remember we started with  a steel which was Fe 0.4 carbon, three manganese and two silicon weight percent. So the first  thing I'm going to do is I'm going to cut the carbon concentration by a factor of 2.  So the prediction from this very simple theory is that if I make a new steel which is Fe  0.2 carbon, three manganese, two silicon, I should get an improvement in toughness without  a loss of strength. And I know and I'll illustrate to you later that if I substitute manganese  by nickel, then the t0 curve moves to a higher carbon concentration. So the second modification  that we make, t0 curve you can calculate very easily. Just download some software from our  website. So we will keep the carbon concentration the same because we are trying to prove a  point here. And instead of manganese, we will use nickel and two silicon. So these are the  two steels which are predicted from our simple theory that if we do as we expect, we will  improve the toughness without a loss of strength.  So here are those three alloys that I just wrote about. In one case, we are going to  cut the carbon concentration by a factor of 2. And in the second case, we are going  to change the substitutional alloying addition to shift the t0 curve to the right hand side  to higher carbon concentrations. So we made these alloys and we achieved the required  refinement of the structure in terms of getting rid of the large regions of austenite. So  this is the one with half the carbon concentration. And we have this beautiful microstructure  without the large regions of austenite. And this is the high carbon, 0.4 carbon, but with  a higher nickel concentration.  The next slide that I will show you is absolutely amazing because this is the original impact  transition curve with, you know, transition temperature being more than 100 degrees centigrade.  And these are the two new alloys where we have shifted the transition temperature by  200 degrees centigrade just by a simple equation. So the toughness has improved dramatically  in one case, maintaining the carbon concentration in the other case by reducing the carbon concentration  by a factor of 2. And without compromising the strength, anybody can improve the toughness  by reducing the strength. But here we are both maintaining the strength and dramatically  improving the toughness.  So large regions of austenite which are unstable to applied stress and which decompose into  high carbon martensite, untempered martensite, obviously are going to be bad for your steel.  What we want is the nice thin films of austenite between the plates of ferrite. Everyone happy  with that? Okay, so now I am going to show you some products which come out of this very  simple theory. So does anybody recognize what these are? Sections of railway lines, yeah,  and they are made in different sizes depending on applications. What is the normal structure  of railway lines? Pearlite, yeah. And somebody else now tell me what is the structure of  pearlite? Sorry? Yeah, but describe to me how is the mixture. Is it spherical particles  of cementite in ferrite or lamella which means, you know, alternating layers of cementite  and ferrite, yeah? Okay. Here, so this is the typical structure that you would see in  an optical micrograph of pearlite where it appears that you have got these separate layers  of cementite and ferrite. And for the past 60 or so years the performance of railway  lines has increased enormously by refining the spacing of the pearlite. Yeah, because  if you refine the distance between the cementite and ferrite then you improve the strength,  the hardness of your material. But actually the way that I describe the structure is wrong.  It doesn't consist of alternating layers of cementite and ferrite. Let me show you what  it really looks like. I really enjoy this slide. Okay. So this is a cabbage. And a cabbage  has leaves which are all connected in three dimensions, right? So think of that as the  crystal of cementite. It's a single crystal of cementite. And inside this I've got water  and the water is a single crystal of ferrite. So when I put the cabbage inside the bucket,  I've got a bicrystal of cementite and ferrite. And the toughness is determined by the size  of that bicrystal, not by the spacing between the planes because the crystallography is  identical within a colony. You've got a single crystal of cementite which is intimately mixed  with a single crystal of ferrite. So if you have a crack then it propagates across the  whole colony of pearlite. But the strength depends on the distance between the leaves  of this cabbage. We section that cabbage, it looks like pearlite, right? So although  rails have improved in terms of their hardness, their toughness remains quite low. Yeah? So  if we replace this by the structure that we've just created from a mixture of very fine plates  of ferrite and carbon enriched films of retained austenite, we have no carbide particles in  the material. And both the toughness and the strength depend on the scale of the bainitic  ferrite and films of austenite. So we made railway lines out of this material, the bainitic  ferrite and austenite. And there are two main characteristics that are required from railway  lines. One is called rolling contact fatigue. So when you have a wheel going over a surface,  it induces a stress under the surface, the cyclic stress every time a wheel goes. And  that causes fatigue. And then you get a bit of the steel just coming off. And of course,  once you've damaged the surface, that damage grows every time something goes over it. And  it can ultimately lead to fracture. This is the normal, polytic rail. These are full scale  tests. Normal, polytic rail. This is a rail which is hardened on the surface. And this  is the new bainitic rail which didn't break. So we stopped the test because these are extremely  expensive tests. Now the second characteristics that you require is wear resistance. Because  you cause a little bit of damage every time a wheel goes over the surface and that causes  wear of the rail. And certainly if you're going around a curve, then you also have specific  areas from which you get wear. This is a rail which has been in service, but one part of  it is ordinary rail and the other part is the new bainitic rail. So here is the carbide  free bainitic rail. And you can see that the damage here from wear processes is much bigger.  And why is the wear resistance better? Well, if you have hard particles in your microstructure  and they come off, then they will act like an abrasive. But we don't have these hard  particles in the carbide free bainite. It's just a mixture of austenite and ferrite. So  if you look at the damage that you cause at the surface, it's more like plastic flow  rather than wear mechanism. And here are some quantitative results. The wear process is  quite complicated. You know, it's not just rolling but also slip. So the tests involve  a certain amount of slip deformation between the two objects in contact. And we are looking  both at the wheel and the railway line. So this is the wear rate on the wheel caused  by moving on an ordinary, pearlitic rail. And this is for martensite. And this is the  only structure which actually reduces the wear rate on the wheel as well. And this is  the tunnel which goes under the sea between Britain and France. And these are the rails  made from carbide free bainitic ferrite. So just from three lectures, okay, you have  the theory to be able to design a steel which can go into service. So if ever you come to  Britain or to France, make sure you go through the Euro tunnel and you will be travelling  on carbide free bainitic rail. Okay. Right. I can show you some more examples. This is  a torpedo car which carries molten steel going over a bainitic rail and so on. Now, of course,  there are many other applications that you can think of of this particular structure.  And here I'm plotting the toughness, fracture toughness versus the ultimate tensile strength.  And there's a whole collection of data from many different studies. And what I'd like  you to see is this is maraging steel. Maraging steel is a martensitic steel, okay, a very  highly alloyed martensitic steel. And it has almost zero carbon and you quench it and then  you heat treat it at around 500 degrees centigrade to precipitate intermetallic compounds and  that makes it hard. But because it doesn't have the carbon, it's very tough. And it's  used for example for rocket motor casings. In some cases, we are matching the properties  of maraging steels using much cheaper alloying addition. So here you're looking at incredible  toughness, you know, about 130 megapascal root meters of toughness at a strength level  of about 1600 megapascals. And these are quenched and tempered martensitic steels and these  are all of these are carbide free bainitic steels. There is much more potential with  this structure because of the absence of carbides. And for example, in GIFT itself, we have worked  on producing these steels by continuous annealing. You know, just like you produce the trip assistive  steels, we can produce without the electromorphic ferrite and therefore get a much stronger trip  steel as opposed to a trip assisted steel means there are other phases, you know, just  like equiaxed ferrite in the structure. So any questions? The most important point I want  to get over to you is that if you understand the mechanisms of phase transformations, then  you can find a logical way of creating a better steel instead of just making many, many different  chemistries and looking at their properties. The simple equation here and the recognition  that large regions of austenite are no good for the toughness helps to create actual products  which go into service. Okay, that's all for today.
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