Isotropy and fatigue
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Ölund, Patrik
A lecture given by Peter Olund, at the Adventures in the Physical Metallurgy of Steels (APMS) conference held in Cambridge University. With the focus on steels for bearings, he talks about the role of inclusions in determining the anisotropy of fatigue properties, and of sampling volume effects. The demands of high cleanliness steel constantly increases due to the fact that higher loads are being introduced in the applications. Today there are cases where the elongated sulphide inclusions present in steel with "normal" sulphur content (around 80ppm) will cause premature fatigue failures. A clear trend is therefore reduced sulphur contents in air-melted steels. However, only increasing the desulphurization within the current process window, to produce low sulphur contents (less than 20ppm), will due to thermodynamics drastically increase the number and size of detrimental globular calcium aluminates. Re-melting process has therefore dominated the market for low sulphur steel. Recently, air-melt steel making processes have been developed to produce more isotropic low sulphur steel where the formation of large globular calcium alumina is thermodynamically suppressed. Consequently, steels produced with this process will exhibit more isotropic fatigue properties. In this work fatigue properties has been assessed for steels loaded in different directions with reference to the rolling direction for steels with different level of isotropy. Furthermore, the effect on operation temperature has been taking into account showing that moderate increases in temperature will affect the properties. This influence can be correlated to the microstructural stability, i.e. composition and structure, of the steel.
. I would like to introduce Patrick Erlund, who works at OVACA group. He will talk to us about factors that influence fatigue of materials. Thank you very much. It is a pleasure being here. Thank you for inviting. Considering that it is 50,000 people looking at you, eventually, not directly, but in the future, I will beat Bruce Springsteen, who was in Stockholm a month ago. It is only 30,000 there. I will talk about isotropy and fatigue. It is a bit different from what we have heard before. We have heard very much about nano-banite. This also, I think, will tie in quite well, because this is another aspect that we also need to be considered. I will sort of tell you a bit about what we think about, and what we think about what we could do in the future, also looking into more of these materials when it comes to fatigue. Well, say fatigue, I mean high strength, high cycle fatigue. It is also something. I have not written any other authors' co-authors here, because it is so many, I do not want to write, but it is all the people in my group, I will also acknowledge to this work. I will start to talk about fatigue a bit in general, when it comes to the influence of both the matrix and the defects. I think this is really something that we have experienced. It is very important that we really think about those two different mechanisms. What are we really controlling the fatigue? Is it the matrix, or is it defects? Could we change the matrix in order to accommodate a feature better? Is one microstructure better than the other? I think there are, I know there are differences, because we have seen that, and those kind of discussions, I think, are very important that we start to raise and start to put more effort and investigate things. But also, of course, the defects, because they are there, and we need to consider them. We have been working quite much with very complex loaded components, but we realize that a lot of the fatigue data, high strength steel that we see today that you find in the literature, is normally obtained from the most beneficial from a steel producer point of view direction, while a lot of components today are loaded in much more complex directions, which means this isotropy, thinking about isotropic properties, becomes more and more important. Another one is the influence of loaded volume. I will briefly go into that as well, because many want to have a fatigue limit for that material, and I want it now, and I can say something, pick a number, because it depends on also the loaded volume. It's very important when it comes to defect controlled fatigue, especially in the high strength steel. Last, we have also looked a bit in the influence of temperature, and all of these goes together, because the influence of temperature is also very interesting. A lot of components today are not run in room temperature, very few bar, to be honest, and the influence of the temperature is becoming more important. And I think one reason is because we want to have used less material, we want to put more power in it, I think that's the general, when I heard the other presenters as well, a lot of it is going through higher strength, because the requirements for higher strength, more ductile, more sustainable material is out there. So I made this picture, which I think is somewhat of the essence. I mean, we have to consider the matrix and the defects, and we see that they will also work together. I had a look at it a couple of times, and I think perhaps it's not right. It gives the impression, but what we've seen also is that, actually, the matrix may influence how a defect are generating a fatigue crack. So it's a bit more complex, but I think it gives at least an impression we have to take those into account. When I talk about fatigue, I normally try to correlate it. We need to have a baseline, we need to know what we are comparing with. And I think one extremely good thing is that what Murakami made, he made all these theories about the size of an inclusion and the hardness of material, and related quite simply a lot of good rule of thumbs that you could use. And one rule of thumb is that the fatigue strength here, looking at the very simple rotating bending fatigue, is proportional to the hardness, which is the way of putting the strength. And of course, you see this correlation is quite good, but it starts to deviate. And what we would like to do is we want to develop and work with material up at this point, where it starts to have a deviation. So, of course, the reason is, as you all know, the matrix will control this part, whilst the defects will control this part. And this challenge is really, how do we move up above this line here, which is a typical thing that we would like to challenge this nanobanitic material and have more tests about them, because what we've seen is that it is possible to go over this sort of theoretical, which is an empirical, theoretical simplified model. It is possible. I think that's something that will be very interesting for the future that we will work with. But also, if we want to go up in this high strength levels, we also need to consider the defects, because they are there. And regardless then, if we do a material with a beautiful matrix, we may not form it for 17 months and get a very nice structure. But we might fail from an inclusionist that, which would be very sad. There are a lot of different fatigue mechanisms. Sometimes you differentiate between contact fatigue and structural fatigue, some call it. I think, in principle, there are different test methods, but in the general mechanism, I think it's fairly much the same if you look in the material, from the material point of view. I think contact fatigue is a good way of really looking at the matrix material, what happens with the steel matrix, because you can see that quite good, because it's a rather slow process that we see. We can run it for a long time. And what do you see when you make a rolling contact fatigue test? Now we see something underneath the rolling raceway, we develop what we call the dark etching region, which is well accepted. We know it's there, but what is it? I mean, this is really the starting point for a fatigue. And I think we have been very, very much relying on fracture mechanics when it comes to fatigue. And that means we are trying to calculate the fatigue without knowing the cause of the initiation. And I think what we should investigate more in is the really initiation stage. So this process doesn't happen just for fun. It happens because you put in stress there, you have a certain temperature, and it develops. So it is something that happens, the process that happens in the material. We have done this test, which is actually then a test looking at the very much loaded zone. Compare that to the same sample, but away from that. So if you tested this rolling bearing material, you have a 100 chromium-6 material at a high load, at a high temperature, at a very long time. And then you go take the scan electron microscopy, and you go and have a look where it's not loaded, but it experiences the same sample. Now you see the retained austenite, you see the residual carbides, and you see the martensitic structure. If you move then into the dark etching region, what you see, you have much coarser structure, something has happened. This is not a continuum, this is not a fracture mechanics process that the crack has formed here. You see the residual carbides, and you see much coarser microstructure, and you see the absence of the retained austenite. So the material is gradually changing. My professor, when I was at the Royal Institute of Technology, he always said, you can always answer that it's more energetically beneficial to go to that. I mean, that was a standard phrase, but it's quite good. The material wants to go to its lowest possible energy, and that's caused here by the stress and the temperature going. You see, this is fatigue, this is starting of the fatigue mechanism. And of course then you are very interested in how does different structures respond to this. Of course, we also have defects, and these ones will enhance this, because they are some strain racers putting much more strain, and putting more energy into the material, and promoting this material decay process. So this is the mechanism of fatigue that I think is extremely interesting, trying to understand this. There are other ways of fatigue testing. You can make structural fatigue, I think it's the same mechanism. It looks differently, because here you have another. But if you have a good eye, or if you have a microscope, or sitting in the microscope, you can see the same kind of phenomenon. You have some kind of work down of the material in the close vicinity of the defect, making it fractured. So I see you decay the material until you need the fracture strength, where you create a crack. And then you can start to play around with the fracture mechanics. So usually, we made a lot of testing when it comes to fatigue, especially rotating bending fatigue, because it's a fairly simple way of producing samples. You can generate a lot of data in a short time. Now this is what we come up with, or one thing that we come up with, looking at. And it's just to support the Murakama theory, the larger the defect, the more detrimental for fatigue. And it works quite well. And all these curves are established from actually experimental tests, making or rotating bending fatigue. And then you see how you can increase the strength or fatigue strength, if you reduce the critical defect size. But of course, all the up to some certain point where you have the intrinsic fatigue limit that puts the cap on. For high strength material that we are interested in, we are talking about, the potential is dramatic. So there are still a lot of things to do to go up there. For the oxides, we made a lot of tests. We have looked at all, assessed all the fracture surfaces, calculating from what was the smallest possible inclusion at the slowest possible stress that ever caused fatigue failure. So it's thousands of samples that created. For the carbides and the titanium, we should have a bit more careful, they are more fitted. But that one is the oxide feel quite comfortable with. So I make a half stop here. Just so I would like to send a message because I think that this is what I think is very important. We should put more effort into looking into the fatigue as a process happening in the material. Right now, I think we are using fracture mechanics too early. And we should move up there and we should help the fracture mechanic people because they are really good at calculating cracks when they have a crack. But we have to guide them here and the physical metallurgist, I think, has a good knowledge and base to drive these kind of investigations and understand this. And then don't forget the defects because at the end of the day, we also have to work with this one. Okay, that was the general part. Then I'm going into more, into the isotropic. What we see today is that more and more, as I said, products are higher stressed in very many different, difficult directions. This is an example for fuel injection part, where you have a whole drill there, you have a whole drill there, you have a chamber there, which is subjected to a pressure of around 2000 bars. Which means that in these kind of holes in all directions, you will have a very high stress generated. Which means if you have elongated defects in that region, going out from that, they will be potential causing a fencing failure. That's why we are trying to get away from the conventional steel and developing more and more steels that are cleaner and with less elongated and smaller inclusion sizes. These are typically sulphides deformed in the rolling directions and the round one more represent in the oxides. If we can make those to another shape, they will be more symmetrical. And that's possible. And then we need also going from being physical metallurgists also talk to the process metallurgists. Because I think they also very need to be fed with this kind of information. What is really important for them. So today we have a possibility to choose by controlling the process a good way or different ways. Going from different types of inclusion sizes or shapes or morphology and compositions. We sometimes represent that we can choose how should we set the inclusions in order to be as harmless as ever possible. Going from just low sulphur steel, just lowering the sulphur content might not be sufficient because then we know we will end up with quite big calcium illuminates. We need to do something else and then we can start to make isotropic steel that we call isotropic quality steel which change their compositions into more deformable. And then understanding this we also have to connect this to the degree of reduction. Because if we also look at that because that has another aspect. If you start to deform the material, the area fraction of the large size inclusions or the ones that we cause to fatigue will reduce because they will be smaller and not detrimental anymore. Whereas if you have conventional steel, they are less prone. So we have to work with that. And it's possible to do quite a significant improvements. For instance, if you look at the fatigue. In this case, just from taking bending fatigue, we see that longitudinal wise, the best, there is not a big difference between different steel, cleanliness levels. But if you go into the transfer and start to have this complex, then you can really benefit. Isotropy should be considered when it's needed. We made a quite fun test which was very simple but quite illustrative, I think. We know that it is different depending on how big volume that you test, what fatigue data you get. And sometimes when people ask me what is the fatigue limit, as I said, I just pick a number and you see the reason. We took the same heat, we just machined from the same bar, different size samples in different dimensions. And then we tested different volumes and this is the different result. So it's quite important to, you can refine this even more. I made one consideration when we look at both direction and volume. And this is quite interesting to look at. If you have the most beneficial direction and you test the 100 cubic millimeter, you will run up and you can almost have a 15%, that's around 50%, which is the fatigue limit at 1300. And if you run that in a different direction, the 50% will be there, which is 350. That's a dramatic difference. And you also have a quite dramatic shift if you go and test the bigger volume. If you go up and test 1000, for a 50%, it drops down to somewhere around 900. So pick a number. The last is the influence of temperature, which I think is new and what we think is quite interesting. We have retrofitted machines making fatigue testing at elevated temperatures. So I will more or less end with this one, which is really looking at what happens. And the interesting thing is not the absolute number here, because the absolute number here in many cases represents the steel cleanliness. But the slope that we see here. And I think different kind of material, different kind of microstructure will respond differently. And this is one way of looking at the material. How could you make a material more prone or more resistant to temperature? I think one interesting feature here, which is not outwritten, but I will tell you if you can keep a secret. No, kidding. If you look at those two materials, they are from the same heat, same place really, just heated, heat treated differently. And this shows that you do also influence, you have the same inclusion population, but you have a difference in fatigue limit, which means also the heat treatment and the microstructure will affect it. And I will stop there. I got the one minute sign. I started nervous. APPLAUSE OK, very interesting talk there, Patrick. Thank you very much. And I will throw it open to questions. Got one straight away in the middle. Working my way up. Oh, Lucy's got a question. I just wanted to know, you said that the austenite in the contact fatigue decayed due to stress and temperature. Is it that it's decaying into ferrite and carbides, I presume, rather than tripping to martensite? That's what we assume when we look into the other micro-op. We didn't see anything like the martensite or nothing. OK, so it's literally just decomposing. I mean, it's a long test, so it has transformed and then it continued to do something. And so how does the stress influence that transformation, rather than it just being a temperature-driven decomposition of austenite? That's a really good question, and that's what I would like to know, what we would like to know, because it is not purely the temperature and they work together. That's what we see. I mean, if we, for instance, if you just temper it before and make the test, it's not the same as testing at the temperature, for instance, what we tested. So one way or another, if it's dislocation movements, if it's diffusion-aided, probably some diffusion-aided, we are not fully aware of that. That's what we would like to look more into. So it could feasibly be the stress state of the austenite is kind of a really toned-down version of the kind of applied stresses that provoke it into a BCT transformation, maybe also making it slightly more favorable for it to transform into BCC alongside the temperature. I don't know. Could be whatever. That's really interesting anyway. Thank you. Have you attempted to compare the mechanical energy going in during a contact fatigue test with the thermodynamic energy required for those changes in the microstructure? Yeah. Shit, I struggled with that. Yeah. Not on the rolling contact fatigue. I made quite a lot of work looking into the number of cycles, the temperature, the stress, and trying to see what is actually the energy that we have put in here, and then trying to correlate that to, and I think that's something that we will continue to do, because I like that idea, looking at the more energy. At that point, I didn't get any clear when I made a quick and dirty. I didn't make a very good relation. I think you have to, but I think you should dig more into it. Okay. I was, well, concerning that question, we actually published a paper on computational material science early this year on that very topic. But my question is, have you considered what artifacts of hydrogen ingress during processing in fatigue life during processing? Because this is a hot topic in other steel amylase, and I wonder if you have studied that. No, we haven't really studied. I mean, that's a very hot topic. Of course, you see that if you charge it with hydrogen, you've seen certain effect. What we have done, we are testing on material with fairly low hydrogen content. We are below 0.5 ppm normal, and we haven't been in any fancy heat treatment, so we have not really looked into that at all. Thank you. Okay, Will. One question, why do you choose the temperature you use for the rolling contact fatigue test 100 degrees, and have you performed any rolling contact fatigue testing at other temperatures, including room temperature, and see what is the difference in the... Yeah, we have prepared a simple rolling contact fatigue of a flat washer test machine, and we have looked at the temperature, and you can clearly see an influence of temperature on the material decay. On the rolling contact fatigue. And the other question was... Why did you choose that temperature? 100. I don't know. We start to see more effects within reasonable time, I would say. And it's not a very high temperature, which makes it crazy either. I have a question. So if I have well understood, you would like to improve the isotropy of the material by reducing the number of elongated inclusion in the steel, so by lowering the sulfur content. But what about... First of all, in any case, the oxide content will remain a source for failures during fatigue testing. And in any case, what about the machinability of the pieces then? Because sulfur is often considered as improving the machinability. You're absolutely right. And what we're doing to reduce the sulfur content, we do, and also the oxide content. And then you will end up with a much poorer machinability. But fortunately, we have very good companies working with machining tools. I know that this is a method of discussion. And they are working quite much, because today you can find these low sulfur steels, and you can machine it. But you need to develop it. Yes, sir. I have a question. So your attributes, if I have well understood you, the anisotropy of the fatigue properties to the elongated shape of the inclusions. So is this right in your analysis? Yes. But what about... Is there any effect of crystallographic anisotropy? You know that the metal matrix also has a... Also maybe anisotropy. Not what we've seen. I mean, not of any significant... No. Okay. Thank you. Okay. My question is about your slide on the relationship between fatigue life and the size of the sample. Yeah. You know, if you can comment something on that. And also whether you have checked what is the particle that actually caused the fatigue in relationship with your relationship between oxide, sulfides, carbides. Which slide did you mean? Size of the sample. Size of the samples, yes. Oh. Basically what you... Yeah. That's the one. That one. If we have... Yeah, well, I mean, we have looked at all the samples that we test, and we ensure that there is an inclusion in the cause of the fatigue. If it's not an inclusion causing the fatigue, then if it's a surface scratch, we have to make a retest because then we call it a failure. That was... So is this related to the probability of finding a critical inclusion when you increase the size? Purely. Okay. Okay. Ladies and gentlemen, I think we'll throw it over to people asking questions from around the world now. We got questions from Turnerist, Argentina. In the second slide, there seemed to be no data point from the metrics in the transverse direction. Are there any of these available? How would they compare with our picture? There was no data point from the metrics in the transverse direction. I think we mean the... Okay. No. There is... No, because we haven't tested... We have concentrated on that strength level. So we haven't so far made any transversal test in the hardest less than 750, I think. So that's still something that we would like to move on. We've developed a quite good method of testing transfer by splitting up a tube. We make a tube, by the way, to be split and gently forge into a flat and then we take samples. So we have very good control. Thank you. Another question is, have you observed any failure from a dark etching region? If you did, how does it happen? Does the crack form from this? Not that I can say from a dark etching region by itself. No. There is... A lot of peculiarities, but no, I can't say that. Okay, thank you. Go on then, Harry. I'm very surprised nobody has asked the question, how did you produce isotropic inclusions? That's a good question. No, it's a matter of working with the pros and the metallurgy. It's not something that we talk about. I see a lot of other steel producers are doing the same. It's not that we are extremely unique. We are doing it the good way, but we cannot tell you. I cannot tell you. That's the simple answer. Okay. Thank you, Patrick.
.  I would like to introduce Patrick Erlund, who works at OVACA group.  He will talk to us about factors that influence fatigue of materials.  Thank you very much. It is a pleasure being here.  Thank you for inviting.  Considering that it is 50,000 people looking at you,  eventually, not directly, but in the future,  I will beat Bruce Springsteen, who was in Stockholm a month ago.  It is only 30,000 there.  I will talk about isotropy and fatigue.  It is a bit different from what we have heard before.  We have heard very much about nano-banite.  This also, I think, will tie in quite well,  because this is another aspect that we also need to be considered.  I will sort of tell you a bit about what we think about,  and what we think about what we could do in the future,  also looking into more of these materials when it comes to fatigue.  Well, say fatigue, I mean high strength, high cycle fatigue.  It is also something.  I have not written any other authors' co-authors here,  because it is so many, I do not want to write,  but it is all the people in my group,  I will also acknowledge to this work.  I will start to talk about fatigue a bit in general,  when it comes to the influence of both the matrix and the defects.  I think this is really something that we have experienced.  It is very important that we really think about those two different mechanisms.  What are we really controlling the fatigue?  Is it the matrix, or is it defects?  Could we change the matrix in order to accommodate a feature better?  Is one microstructure better than the other?  I think there are, I know there are differences,  because we have seen that, and those kind of discussions,  I think, are very important that we start to raise  and start to put more effort and investigate things.  But also, of course, the defects, because they are there,  and we need to consider them.  We have been working quite much with very complex loaded components,  but we realize that a lot of the fatigue data,  high strength steel that we see today that you find in the literature,  is normally obtained from the most beneficial from a steel producer point of view direction,  while a lot of components today are loaded in much more complex directions,  which means this isotropy, thinking about isotropic properties,  becomes more and more important.  Another one is the influence of loaded volume.  I will briefly go into that as well,  because many want to have a fatigue limit for that material,  and I want it now, and I can say something, pick a number,  because it depends on also the loaded volume.  It's very important when it comes to defect controlled fatigue,  especially in the high strength steel.  Last, we have also looked a bit in the influence of temperature,  and all of these goes together,  because the influence of temperature is also very interesting.  A lot of components today are not run in room temperature,  very few bar, to be honest,  and the influence of the temperature is becoming more important.  And I think one reason is because we want to have used less material,  we want to put more power in it,  I think that's the general, when I heard the other presenters as well,  a lot of it is going through higher strength,  because the requirements for higher strength, more ductile,  more sustainable material is out there.  So I made this picture, which I think is somewhat of the essence.  I mean, we have to consider the matrix and the defects,  and we see that they will also work together.  I had a look at it a couple of times,  and I think perhaps it's not right.  It gives the impression, but what we've seen also is that,  actually, the matrix may influence how a defect are generating a fatigue crack.  So it's a bit more complex,  but I think it gives at least an impression we have to take those into account.  When I talk about fatigue, I normally try to correlate it.  We need to have a baseline, we need to know what we are comparing with.  And I think one extremely good thing is that what Murakami made,  he made all these theories about the size of an inclusion  and the hardness of material,  and related quite simply a lot of good rule of thumbs that you could use.  And one rule of thumb is that the fatigue strength here,  looking at the very simple rotating bending fatigue,  is proportional to the hardness,  which is the way of putting the strength.  And of course, you see this correlation is quite good,  but it starts to deviate.  And what we would like to do is we want to develop and work with material up at this point,  where it starts to have a deviation.  So, of course, the reason is, as you all know,  the matrix will control this part,  whilst the defects will control this part.  And this challenge is really, how do we move up above this line here,  which is a typical thing that we would like to challenge this nanobanitic material  and have more tests about them,  because what we've seen is that it is possible to go over this sort of theoretical,  which is an empirical, theoretical simplified model.  It is possible.  I think that's something that will be very interesting for the future that we will work with.  But also, if we want to go up in this high strength levels,  we also need to consider the defects, because they are there.  And regardless then, if we do a material with a beautiful matrix,  we may not form it for 17 months and get a very nice structure.  But we might fail from an inclusionist that, which would be very sad.  There are a lot of different fatigue mechanisms.  Sometimes you differentiate between contact fatigue and structural fatigue, some call it.  I think, in principle, there are different test methods,  but in the general mechanism, I think it's fairly much the same  if you look in the material, from the material point of view.  I think contact fatigue is a good way of really looking at the matrix material,  what happens with the steel matrix, because you can see that quite good,  because it's a rather slow process that we see.  We can run it for a long time.  And what do you see when you make a rolling contact fatigue test?  Now we see something underneath the rolling raceway,  we develop what we call the dark etching region, which is well accepted.  We know it's there, but what is it?  I mean, this is really the starting point for a fatigue.  And I think we have been very, very much relying on fracture mechanics  when it comes to fatigue.  And that means we are trying to calculate the fatigue  without knowing the cause of the initiation.  And I think what we should investigate more in is the really initiation stage.  So this process doesn't happen just for fun.  It happens because you put in stress there, you have a certain temperature,  and it develops.  So it is something that happens, the process that happens in the material.  We have done this test, which is actually then a test looking at the very much loaded zone.  Compare that to the same sample, but away from that.  So if you tested this rolling bearing material,  you have a 100 chromium-6 material at a high load,  at a high temperature, at a very long time.  And then you go take the scan electron microscopy,  and you go and have a look where it's not loaded,  but it experiences the same sample.  Now you see the retained austenite, you see the residual carbides,  and you see the martensitic structure.  If you move then into the dark etching region,  what you see, you have much coarser structure, something has happened.  This is not a continuum, this is not a fracture mechanics process  that the crack has formed here.  You see the residual carbides, and you see much coarser microstructure,  and you see the absence of the retained austenite.  So the material is gradually changing.  My professor, when I was at the Royal Institute of Technology,  he always said, you can always answer that it's more energetically beneficial to go to that.  I mean, that was a standard phrase, but it's quite good.  The material wants to go to its lowest possible energy,  and that's caused here by the stress and the temperature going.  You see, this is fatigue, this is starting of the fatigue mechanism.  And of course then you are very interested in how does different structures respond to this.  Of course, we also have defects, and these ones will enhance this,  because they are some strain racers putting much more strain,  and putting more energy into the material, and promoting this material decay process.  So this is the mechanism of fatigue that I think is extremely interesting, trying to understand this.  There are other ways of fatigue testing.  You can make structural fatigue, I think it's the same mechanism.  It looks differently, because here you have another.  But if you have a good eye, or if you have a microscope, or sitting in the microscope,  you can see the same kind of phenomenon.  You have some kind of work down of the material in the close vicinity of the defect, making it fractured.  So I see you decay the material until you need the fracture strength, where you create a crack.  And then you can start to play around with the fracture mechanics.  So usually, we made a lot of testing when it comes to fatigue,  especially rotating bending fatigue, because it's a fairly simple way of producing samples.  You can generate a lot of data in a short time.  Now this is what we come up with, or one thing that we come up with, looking at.  And it's just to support the Murakama theory, the larger the defect, the more detrimental for fatigue.  And it works quite well.  And all these curves are established from actually experimental tests, making or rotating bending fatigue.  And then you see how you can increase the strength or fatigue strength, if you reduce the critical defect size.  But of course, all the up to some certain point where you have the intrinsic fatigue limit that puts the cap on.  For high strength material that we are interested in, we are talking about, the potential is dramatic.  So there are still a lot of things to do to go up there.  For the oxides, we made a lot of tests.  We have looked at all, assessed all the fracture surfaces, calculating from what was the smallest possible inclusion  at the slowest possible stress that ever caused fatigue failure.  So it's thousands of samples that created.  For the carbides and the titanium, we should have a bit more careful, they are more fitted.  But that one is the oxide feel quite comfortable with.  So I make a half stop here.  Just so I would like to send a message because I think that this is what I think is very important.  We should put more effort into looking into the fatigue as a process happening in the material.  Right now, I think we are using fracture mechanics too early.  And we should move up there and we should help the fracture mechanic people because they are really good at calculating cracks when they have a crack.  But we have to guide them here and the physical metallurgist, I think, has a good knowledge and base to drive these kind of investigations and understand this.  And then don't forget the defects because at the end of the day, we also have to work with this one.  Okay, that was the general part.  Then I'm going into more, into the isotropic.  What we see today is that more and more, as I said, products are higher stressed in very many different, difficult directions.  This is an example for fuel injection part, where you have a whole drill there, you have a whole drill there, you have a chamber there, which is subjected to a pressure of around 2000 bars.  Which means that in these kind of holes in all directions, you will have a very high stress generated.  Which means if you have elongated defects in that region, going out from that, they will be potential causing a fencing failure.  That's why we are trying to get away from the conventional steel and developing more and more steels that are cleaner and with less elongated and smaller inclusion sizes.  These are typically sulphides deformed in the rolling directions and the round one more represent in the oxides.  If we can make those to another shape, they will be more symmetrical.  And that's possible.  And then we need also going from being physical metallurgists also talk to the process metallurgists.  Because I think they also very need to be fed with this kind of information.  What is really important for them.  So today we have a possibility to choose by controlling the process a good way or different ways.  Going from different types of inclusion sizes or shapes or morphology and compositions.  We sometimes represent that we can choose how should we set the inclusions in order to be as harmless as ever possible.  Going from just low sulphur steel, just lowering the sulphur content might not be sufficient because then we know we will end up with quite big calcium illuminates.  We need to do something else and then we can start to make isotropic steel that we call isotropic quality steel which change their compositions into more deformable.  And then understanding this we also have to connect this to the degree of reduction.  Because if we also look at that because that has another aspect.  If you start to deform the material, the area fraction of the large size inclusions or the ones that we cause to fatigue will reduce because they will be smaller and not detrimental anymore.  Whereas if you have conventional steel, they are less prone.  So we have to work with that.  And it's possible to do quite a significant improvements. For instance, if you look at the fatigue.  In this case, just from taking bending fatigue, we see that longitudinal wise, the best, there is not a big difference between different steel, cleanliness levels.  But if you go into the transfer and start to have this complex, then you can really benefit.  Isotropy should be considered when it's needed.  We made a quite fun test which was very simple but quite illustrative, I think.  We know that it is different depending on how big volume that you test, what fatigue data you get.  And sometimes when people ask me what is the fatigue limit, as I said, I just pick a number and you see the reason.  We took the same heat, we just machined from the same bar, different size samples in different dimensions.  And then we tested different volumes and this is the different result.  So it's quite important to, you can refine this even more.  I made one consideration when we look at both direction and volume.  And this is quite interesting to look at. If you have the most beneficial direction and you test the 100 cubic millimeter,  you will run up and you can almost have a 15%, that's around 50%, which is the fatigue limit at 1300.  And if you run that in a different direction, the 50% will be there, which is 350.  That's a dramatic difference.  And you also have a quite dramatic shift if you go and test the bigger volume.  If you go up and test 1000, for a 50%, it drops down to somewhere around 900.  So pick a number. The last is the influence of temperature, which I think is new and what we think is quite interesting.  We have retrofitted machines making fatigue testing at elevated temperatures.  So I will more or less end with this one, which is really looking at what happens.  And the interesting thing is not the absolute number here, because the absolute number here in many cases represents the steel cleanliness.  But the slope that we see here.  And I think different kind of material, different kind of microstructure will respond differently.  And this is one way of looking at the material. How could you make a material more prone or more resistant to temperature?  I think one interesting feature here, which is not outwritten, but I will tell you if you can keep a secret.  No, kidding. If you look at those two materials, they are from the same heat, same place really, just heated, heat treated differently.  And this shows that you do also influence, you have the same inclusion population, but you have a difference in fatigue limit,  which means also the heat treatment and the microstructure will affect it.  And I will stop there. I got the one minute sign. I started nervous.  APPLAUSE  OK, very interesting talk there, Patrick. Thank you very much.  And I will throw it open to questions. Got one straight away in the middle.  Working my way up.  Oh, Lucy's got a question.  I just wanted to know, you said that the austenite in the contact fatigue decayed due to stress and temperature.  Is it that it's decaying into ferrite and carbides, I presume, rather than tripping to martensite?  That's what we assume when we look into the other micro-op. We didn't see anything like the martensite or nothing.  OK, so it's literally just decomposing.  I mean, it's a long test, so it has transformed and then it continued to do something.  And so how does the stress influence that transformation, rather than it just being a temperature-driven decomposition of austenite?  That's a really good question, and that's what I would like to know, what we would like to know,  because it is not purely the temperature and they work together. That's what we see.  I mean, if we, for instance, if you just temper it before and make the test,  it's not the same as testing at the temperature, for instance, what we tested.  So one way or another, if it's dislocation movements, if it's diffusion-aided,  probably some diffusion-aided, we are not fully aware of that.  That's what we would like to look more into.  So it could feasibly be the stress state of the austenite is kind of a really  toned-down version of the kind of applied stresses that provoke it into a BCT transformation,  maybe also making it slightly more favorable for it to transform into BCC alongside the temperature.  I don't know. Could be whatever.  That's really interesting anyway. Thank you.  Have you attempted to compare the mechanical energy going in during a contact fatigue test  with the thermodynamic energy required for those changes in the microstructure?  Yeah. Shit, I struggled with that. Yeah.  Not on the rolling contact fatigue.  I made quite a lot of work looking into the number of cycles, the temperature, the stress,  and trying to see what is actually the energy that we have put in here,  and then trying to correlate that to, and I think that's something that we will continue to do,  because I like that idea, looking at the more energy.  At that point, I didn't get any clear when I made a quick and dirty.  I didn't make a very good relation.  I think you have to, but I think you should dig more into it.  Okay.  I was, well, concerning that question, we actually published a paper on computational  material science early this year on that very topic.  But my question is, have you considered what artifacts of hydrogen ingress during processing  in fatigue life during processing?  Because this is a hot topic in other steel amylase, and I wonder if you have studied that.  No, we haven't really studied.  I mean, that's a very hot topic.  Of course, you see that if you charge it with hydrogen, you've seen certain effect.  What we have done, we are testing on material with fairly low hydrogen content.  We are below 0.5 ppm normal, and we haven't been in any fancy heat treatment,  so we have not really looked into that at all.  Thank you.  Okay, Will.  One question, why do you choose the temperature you use for the rolling contact fatigue test  100 degrees, and have you performed any rolling contact fatigue testing at other temperatures,  including room temperature, and see what is the difference in the...  Yeah, we have prepared a simple rolling contact fatigue of a flat washer test machine,  and we have looked at the temperature, and you can clearly see an influence of temperature on the material decay.  On the rolling contact fatigue.  And the other question was...  Why did you choose that temperature?  100. I don't know.  We start to see more effects within reasonable time, I would say.  And it's not a very high temperature, which makes it crazy either.  I have a question.  So if I have well understood, you would like to improve the isotropy of the material  by reducing the number of elongated inclusion in the steel, so by lowering the sulfur content.  But what about...  First of all, in any case, the oxide content will remain a source for failures during fatigue testing.  And in any case, what about the machinability of the pieces then?  Because sulfur is often considered as improving the machinability.  You're absolutely right.  And what we're doing to reduce the sulfur content, we do, and also the oxide content.  And then you will end up with a much poorer machinability.  But fortunately, we have very good companies working with machining tools.  I know that this is a method of discussion.  And they are working quite much, because today you can find these low sulfur steels,  and you can machine it.  But you need to develop it.  Yes, sir.  I have a question.  So your attributes, if I have well understood you, the anisotropy of the fatigue properties to the elongated shape of the inclusions.  So is this right in your analysis?  Yes.  But what about...  Is there any effect of crystallographic anisotropy?  You know that the metal matrix also has a...  Also maybe anisotropy.  Not what we've seen.  I mean, not of any significant...  No.  Okay.  Thank you.  Okay.  My question is about your slide on the relationship between fatigue life and the size of the sample.  Yeah.  You know, if you can comment something on that.  And also whether you have checked what is the particle that actually caused the fatigue in relationship with your relationship  between oxide, sulfides, carbides.  Which slide did you mean?  Size of the sample.  Size of the samples, yes.  Oh.  Basically what you...  Yeah.  That's the one.  That one.  If we have... Yeah, well, I mean, we have looked at all the samples that we test, and we ensure that there is an inclusion in the cause of the fatigue.  If it's not an inclusion causing the fatigue, then if it's a surface scratch, we have to make a retest because then we call it a failure.  That was...  So is this related to the probability of finding a critical inclusion when you increase the size?  Purely.  Okay.  Okay.  Ladies and gentlemen, I think we'll throw it over to people asking questions from around the world now.  We got questions from Turnerist, Argentina.  In the second slide, there seemed to be no data point from the metrics in the transverse direction.  Are there any of these available?  How would they compare with our picture?  There was no data point from the metrics in the transverse direction.  I think we mean the...  Okay.  No.  There is... No, because we haven't tested... We have concentrated on that strength level.  So we haven't so far made any transversal test in the hardest less than 750, I think.  So that's still something that we would like to move on.  We've developed a quite good method of testing transfer by splitting up a tube.  We make a tube, by the way, to be split and gently forge into a flat and then we take samples.  So we have very good control.  Thank you.  Another question is, have you observed any failure from a dark etching region?  If you did, how does it happen? Does the crack form from this?  Not that I can say from a dark etching region by itself. No.  There is...  A lot of peculiarities, but no, I can't say that.  Okay, thank you.  Go on then, Harry.  I'm very surprised nobody has asked the question, how did you produce isotropic inclusions?  That's a good question.  No, it's a matter of working with the pros and the metallurgy.  It's not something that we talk about.  I see a lot of other steel producers are doing the same.  It's not that we are extremely unique.  We are doing it the good way, but we cannot tell you.  I cannot tell you.  That's the simple answer.  Okay.  Thank you, Patrick.
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