Physical Metallurgy of Steels - Part 4

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Physical Metallurgy of Steels - Part 4
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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.
Keywords Bhadeshia, Harshad Kumar Dharamshi Hansraj

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D OK but concluded that lecture look lecture and production going to actually apply some of the theory that we've learned to design a new study of the history remind you of the mechanism that we came up with for the consummation debate I imagined that they might forms exactly like Mike inside where the plate of characters is fully supersaturated with covered but because we're transforming at a relatively high temperature the Coburn has an opportunity to escape into the US 93 at relatively high temperature the carbon escaped completely leaving the plate of far-right Freedom from carbon and eventually this access :colon precipitate a seaman died between the plates of rice if we transformed to lower big nite that means a relatively lower transformation that then there is an opportunity for car buyers to proceed inside the place rather like tempering of modern and because the principle some Carbide's inside the fire that we get less carbon petitioning into the remaining Austinite and therefore we have final president of seamen died in an overnight might show that explained that this is actually a tougher than this might rastructure because of the fine scale of the provides here even the of Bay nite is stronger now not when we go to even higher strength steel these are like intrusions the corals fracture assuming that particles those are hard and brittle and if you apply the large enough stress than there crack and when make constant stages we make them extremely clean enough that free from nonmetallic inclusions so then the semen that particles become particles which initiates fracture so but it takes the use of this kind containing fairly coarse Carbide's I have not really succeeded in achieving the same toughness and strength combinations that you can get rid them but markets because in them but might inside you can control the tempering process and achieve fine
so particularly these are the sort of carbide particles that we see inside non-Albanian 60 years which have a high carbon concentration so when I say a high-carbon concentration of Europe .period 3 . 4 weight carbon you see fairly across that and in the case of polluted fractures these particles initiate carriage cracks and in the case of ductile fracture initiated the nutrition of rights so inside the dimples that you see on the fracture surface you'll be able to find seem that part of it and the consequences of
those particles is that you get a reduction in toughness so there is a best temperature for a choppy destined Joppy impact energy and supporting the creator steel the thing they did Mike structure and cost then you get a reduction in toughness and an increase in the impact transition back so those Carbide's of bad In the context of red and steals and then you have a medium carbon concentration obviously if you reduce to govern conservation to less than a point 0 5 or 8 per cent than the Carbide refined so in the end that place them not initiating sites for Fletcher so how can I stop these Carbide's from facilitating any ideas what elements can add to this deal reached stops seamen died from sorry the Itanium will capture the carbon yeah and form Palestinian Carbide's reach out to make in shape and cost because the format of very high temperatures so we don't want that either yeah In silicone pans or anything else that you're right you're quite right and how the silicon and Armenian stock the preservation of Simone Beck what is the mechanism there you know very well you know you from his chief UK cast-iron cast-iron contains something like the late Percent of problems if you add soaked into the and then instead of a white cast and you get a great cast that process and it's full of seamen died Rick Austin is full of graphite so know this for more than a century that he had elements like Silicon cast iron then we stopped primary seamen died from forming and you end up with a cast iron containing graphite instead of very hard cast iron will of so how does silicon stop the preservation of semen what is the mechanism any ideas I think you're going about this because the steals are designed like this but do you know why these elements stopped the presentation of the entire from Boston but let me give you a clue so the mechanisms
are in worlds partitioning of Harbin from this supersaturated played into the remaining last nite so is this presentation from Austin Night at the Opera stuff the formation of seamen died from loss now if Europe look at the solubility of carbon in seamen died it's almost 0 and it has worked in 0 solubility in seamen died but when new forms added the low temperature there is no opportunity for the carbon to escape while the seem intent is growing so when we talk about damage and 400 degrees centigrade the seeming bad is 1st the observed and silicon which is in new scheme even tho it does not want so once you 1st silicon into Siemens died its energy increases becomes less stable here are some 1st principles calculations done by June during an indication rejected there when substitute and iron atom with the silicon atoms here is an effort to see and this is nothing to see but 1 of the iron Athens is substituted with silicon you get a dramatic increase in energy and here do silicon atoms have been such not silicon has been substituted a different site within the that and again you get a large increase in the energy of the semen so if you 1st the silicon to grow into the statement added that becomes much less stable and in some circumstances if you've added enough silicon than it's been informed at all and the survey should apply to to Aluminium not why did we need to do this analysis by 1st principles calculations because if you have virtually 0 solubility of silicon instrument died then you cannot actually make measurements don't make thermodynamic measurements the strategy of phase diagram calculation you don't have the thermodynamic data so many stellar Dynamic Data Book created using 1st principles of nations and this is a wonderful application of 1st principles speculation because here you cannot even in principle do an experiment that's where you can only do calculation so just to summarize that the
mechanism by which cemented Fulton's assumed that presentation is prevented so similar that
presentation is parental this makes this fence foreigners prevented because run the solubility of Stephen of silicon Consumer Guide is negligible so this is this is important solubility of silicon I use the sun symbol the duffel seamen died throughout offer is fair gamma-rays Austinite and the testimony that the solubility of silicon is assuming died is negligible and the 2nd crucial thing is that during the May 9th reaction seamen died at the low temperature so during the reaction the dough forms at the a low-temperature that means that the silicon cannot buy patient during the course of all the 4 Silicon cannot but mission during assuming died growth it is it is basically 1st to be In the same it is forwards the the Dow becomes unstable so the important point is that if the seem that formed at a high temperature know about let's say 600 degrees centigrade then the silicon could partition and there is no problem in precipitating statement saying the same speed if I fall further light on form that the relatively high temperature of around 600 degrees centigrade slightly regards the transformation but it doesn't stop the presentation of the so you have to have your conditions 1st that the silicon is forced into the cement that that it's in other words that have performed at the scene and that at the temperature in order to use silicon to suppress the presentation of the report the 100 that so that is the mechanism by which silicon stops the preservation of seamen died from Austin and probably and Union works in the same way but we don't have the same kind of evidence that actually you here for the effect of Aluminium on assuming that station so it's possible now
you know given this kind of thermodynamic data To do calculations on the growth rate of seamen died in which you 1st the silicon and therefore to calculate how much silicon you need in order to suppress the presentation of human roughly speaking over the kind of thing is that we are interested in you need in access of about 1 Percent of silicon to stop the preservation of Symantec's OK so let's on let's imagine that the
study to different allies very simple advice but all of them contain a certain level of silicon to stop the preservation of and you've got carbon concentrations that I'll explain to you later and some elements to give you a hard nobility because you want to stop the preservation of other kinds of transformation products such as polite and so forth not many really transformed the size of thermally into Bay late-night we get the mike
structure which have shown you previously extremely fine plates of the need of remember this game is like a meager when you have a thin plate shape the grain size is approximately twice the thickness of the play because the grain size is mean linear intercept you measure grain sizes Amenia insects and when you have a thin plate shape the probability of slip happening in the plane of the lake is negligible so the means intercepted about twice the thickness that would be a during a grain size which is a fraction of a mike just face transformation you don't need any complex tho mechanical processing 2nd is Musa suppress typist station so in between the plates we've got these regions of Austinite and Austinite does not have a duck brittle transition temperature so if I if
I let stand diagram drew in the Last Lecture where am during the give stressed that the function of temperature this so here we have temperature and this is the stress and this is the stress required to clean the crystal arrange and if I now play out on this the temperature dependence on the strength of Austinite then fall temperatures it's easier to fly by plastic deformation this is Austinite and therefore you do not get Austinite breaking by cleavage mechanisms do not have a brittle transition of Austin In the case of fraud by a pointed out to you that you get an intersection between the locals and the previous status and that's why you get doctor British transitions so 1 major advantage of having Austinite indeed might structure Is that we have no doubt that brittle transition temperature for all
these reasons of regained Austinite not is there any other and he had the advantage of having the Austinite there that you can think of why do we often Austinite universities the effects of transformation induced assisted the real going to do this in a bit more detail later but when you know the material the consummation of the US nite into modern actually adds to the plasticity obviously but even tho the material strong you can get a Haida Billy and the solubility all hydrogen is is high but the diffusion rate of hydrogen into the UST is orders of magnitude smaller than that in fact what that means is that hydrogen will have difficulty in getting into your steel that's a good thing because when you put your skin into service you don't really want hydrogen to get into the deal by corrosion reactions accepted hydrogen and recommend becomes less of a problem if you have sufficient Austinite surrounding the far-right region so there are many advantages to having this beautiful Mike structure which is extremely fine I'm just going to list the advantages forgotten how to
argue change this page of the electric chair OK so let's unless list but wanted to use the first one is that we achieved an incredibly fast structure just by phase consummation during change fine structure part phase transformations now means we that a mixture of and Austinite and this arsenide is created using an an extremely cheap following element there is basically stabilized by the partitioning of carbon and you cannot get cheaper allowing element that carbon so the Austinite is stabilized by an extremely cheap Allen government it's a gamble stabilized she's some of these companies there's not the original clips deals were actually with very large concentrations of nickel something in excess of 20 rapist Percent here we are just using carbon to stabilize the OS Abbas carbon constitution is quite know about . 4 but when you petitioning the US nite ends up with a carbon concentration in excess of 1 way cent and therefore it becomes stable the music extremely cheap art cellular number trade Is that hydrogen has a very low diffuser in Austinite and that it's always a good thing when you're dealing with high-strength steels of hydrogen has very low the facility Indiana so you should read hydrogen embezzlement problems because we have removed the carbon from the bay nite and petition it into the last night that makes that they need to ferret out as well because if contains carbon in solid solution that definitely inventions and that's why you have to Temple might incite when use much insight so number before it is that the carbon concentration of debated variety is much less than the average value concentration they need different is much lower the average of steel that makes it tough next and we're also getting our strand france the very fine-grained structure and grain size is the 1 of the only mechanisms by which you can increase brought the strength and toughness very often many tried to increase the strength the government through the period between grain size refinement increase both strength and toughness the very fine grain size the media stent is produced by grain size effect strength fine grain structure and grain refinement is the only mechanism for vault increasing the strength and toughness for increasing strength and toughness so the very simple addition of silicon pure steel together with transformation into a mixture of many people Austinite gives us what appears to be an an ideal Microsoft this appears to be idea might structure I wasn't unhappy so and of course if you
remember each Blake is only about 10 micrometres in Lens because the classes the Associated Press the shape deformations stops it from growing once it reaches about that is actually finer than Martin said appellants have a look at the toughness of this wonderful my construction right but we
actually do an experiment to measure the toughness it's terrible yeah you can see here that the impact transition temperature it is more than 100 degrees centigrade so that's completely unacceptable for any engineering material that 100 degrees centigrade you get affected by clearance and this is the ally that I listed in 1 of those tables earlier not 1 for Coventry manganese into silicon so something is very wrong with our science here that we have an idea like a structure and it's giving extremely poor properties the let's lesser stamping what is the problem somebody show you an optical microgram of destruction give me up so look
at the Scalia 50 micrometres so this is an optical might and develop that need is a nite she In a transmission electron micrograph then there would look exactly like the ones that I showed you earlier which are extremely fine and the firms up Austinite in between them but not it's 1 more thing we have these large regions Our on transformed high-carbon Austinite you considers large white regions which have the scale of the order of 50 micrometres which on transformed I govern tonight not when you increase the size of Austinite it becomes easier for it to transform into my so as soon as I stress students material the 1st regions which kids are these regions and their firms 50 micrometres sized regions of on pampered high-carbon modern so it's like creating a beautiful fine structure and then throwing a huge rock indeed the structure which is brittle not the library have these large regions of Austinite left you know material even tho we have transformed I suppose for a long time to generate data and why does this material refused to transform further because I will I don't want those large regions of Austin and I want them to transform into the fine mixture of the Friday and firms operating just 9 why does the reactions .period this is because we have a thermodynamic limit that as soon as the composition of the US nite which that boundary it cannot transform further so here is
the fact 0 Co yeah and this is the average carbon concentration the and if I'm transforming at this temperature then I can only guess late-night until the Austinite composition reaches the red line if I had planned the room the busier Co then I should be able to calculate the maximum amount of they needed but that kind of thing that little is basically that the amount of being fired is given by this distance here divided by systems you that the fact that 0 carbon concentrations they were unhappy with the liver you know you have played to a phase diagram to work out the volume fraction of a phase that you the maximum volume fraction of things so it's this distance here derided by businesses which if I expressed mathematically is the carbon concentration given by the 0 minus the average carbon concentration of the steel divided by the carbon concentration given by the the 0 growth and the carbon concentration of power which we are going to resume is 0 so I wanted to give it to him the 2 in which I can increase the volume fraction of so that I can get rid of the larger regions of Austinite you look at this equation there should be 3 ways of doing this so I'm waiting for you to give me the answer again whether the 3 ways in which I can increase the volume fraction of they know because if I can't than Estep there this is a thermodynamic limit to the amount of transformation even if you can't think metallurgical think mathematically but other 3 minutes I could increase the volume fraction of the thorough this is the volume impression of a naked characteristics of the single already yeah I think you have an idea How can I increase please come on guys guys is 1st what men and women this actual 1 the yeah they fight the fight reduced the average concentration of carbon that means I hit this line the roads that you would see a political access then I can increase the volume fraction of being so what this says is if I actually reduce the carbon concentration of this deal I should get mad they nite get rid of those large regions of course tonight and I would not compromise strand because I'm increasing the amount of data they goes against our knowledge thinking that if I reduce the carbon concentrations I actually get an increasing strength because I'm increasing the amount of pain and 2nd method the biggest disco depends on the substitution of solid In no material so you modified the substitution of solid of for example manganese and so forth then we can shift this scope to higher carbon concentrations that's the 2nd method and the Fed method is why not lower the transformation temperature there is a person limit what is that limit hello can I go mountainsides of temperature because so that the days well improving the
toughness it therefore increasing the
volume fraction of the immediate what is news text by the of the average carbon concentration of material 2 is modified substitution to move that date 0 to higher cover and deterred it is to reduce the transmission temperature I'm sure that this is limited by the fact that we must introduce modern the lowest 2nd goal is close to mn Ltd might incite start temperature remember we started with a steel which was FEE 0 . 4 0 carbon 3 manganese and do silicon rate per cent so the 1st thing I'm going to do is I'm going to cut the carbon concentration by a factor of 2 2 so the prediction from this very simple theory is that if I make the a new steel which is at the 0 . 2 carbon 3 manganese he silicon I should get an improvement in the office without a loss of strength and I know and state to you later that if I substitute manganese by Nicole then the 0 moves to a higher carbon concentrations so the 2nd modification that we make this year you can calculate very easily just download some software from our website yet so we will keep the carbon concentration the same because they're trying to prove a point here and instead of manganese reviews and to Seneca so I believe that too 4 steals which are predicted From our simple theory but if the move editors as we the we improve the toughness without loss of strength looking so here those
2 yellow instead of just short about In 1 case that would cut the problem consternation by a factor of 2 and in the 2nd case we are going to show me the substitution allowing addition to share but is do
the right hand side to higher carbon concentrations so we made these elements and
achieved the required refinement of the structure in terms of getting rid of the large regions of Boston I'm so this is the 1 with half the carbon concentrations and yet this beautiful Mike structure without the large regions of last nite and this is the high problems . 4 carbon but with a higher nickel concentrations at the next leg that I show you absolutely amazing
because this is the original impact condition because of it you know what transition temperature being more than 100 degrees centigrade and these too new allies there shifted the transition gentrified 200 in recent that this by a simple equation so the has improved dramatically In 1 case mandating the carbon concentrations in the other case by reducing the carbon concentration by a factor of 2 and without compromising the standard anybody can improve the toughness by reducing this trend but here we have maintaining the strength and dramatically improving he got so lads regions of Austinite began unstable to apply to stress and the speaker tumbled into high carbon site on pampered mountainside obviously are going to be bad for your steel what we want is a nice films of Austinite between the plates all there were happy with that but as of now I'm going to show you some products which come out of this very simple theory so Is anybody recognize
reveals that sections of road and rail lines yet and there made in different sizes depending on applications but where is the normal structure they've alliance like here and somebody else not only what is the structure of the violence so yeah the described to me how how is the mixture the spherical particles of seamen died in fact Lamela which means that alternating layers of semen that in fact yet here
so this is the typical structure that you would see in an optical micrograph of life where it appears that you got the separate layers of seamen and 5 and for the past at 60 or so years the performance of Graham alliances increased enormously by refining the spacing of the polite difficult if you find the distance between the seamen died fire then you improved trend the hardness of the material but actually the real bad I described the structure is wrong it doesn't consist of alternating layers of seamen died and fire let me show you what it really looks like I really enjoyed this life so this
is a cabbage and that cabbage has news which are connected in 3 dimensions the think of that as the crystal of assuming that it's a single crystal of that an insider's about water and the world is a single crystals of the rights of an put the carnage inside the bucket about a abide crystal of semen that and the toughness is determined by the size of that by Microsoft not by the spacing between the because the crystallography is identical to win a colony got a single Pressler seamen died which is intimately mixed with a single crystal affair so if you have a crack they propagates across the whole colony of polite but the stance dependent the distance between the needs of his cabinet section that averages looks like well like that so and the retailers have improved in terms of the hardness the toughness remains quite low yeah so if you blow status by the structure that you just created from a mixture of ratifying plates all of them and carbon its films of retain just now we have no carbide particles in the material and both the toughness and the strength depend on the scale 3 Benedict far-right and films of Austin we made really
airlines out of this material the Benedict and Austin and that main characteristics let out a cry from river lines 1 is called Rolling contact repeat the when you have a meal going there surface it induces a stress under the surface the psychic stress every time real go and that causes fatigue and then you get a bit of steel this coming off and of course want to damage the surface that damage grows every time something goes forward and can ultimately lead to fracture this is the normal political ended up full-scale tests nobody parade This is raid which is hard on the surface and this is the new they need which didn't break so we stopped the task because the extremely expensive tests not the 2nd characteristics that you require Israel resistance because you "quotation mark a little bit of damage every time I will go toward the surface and that was where all the rail and suddenly if you're going on because then yourself hello specific areas from which you get there
this is which has been in service but 1 part of it is ordinarily and the other part is a new beginning to create the world's the carbide preventing great and you can see that the damage here From the processes used much bigger and why Riley is the rare resistance better well if you have hot particles in you might structure and they come out of then but like and abrasive but we don't have these hard by those in the Coventry it's just a mixture of Boston and so if you look at the damage that call that the service is more like plastic flow rather than the McCanns
and some quantitative results at the the red process is quite complicated you know it's not just a rolling but also slipped so the tests involve a certain amount of slip deformation between the 2 objects in contact and we're looking had the vehicle and the daily so but this is the way the raid on the real cost by on an ordinary politically and this is for modern and this is the only structure which actually reduces the wary on the village and this
Is the tunnel which goes under the sea between Britain and France and these are the areas made from private then the dust from 3 lectures you have the theory to reality designers going to but if I you come to Britain on 2 fronts next thing you go to the European and you will be traveling on commentary being integrated right I can show you some more
examples this is a torpedo car which carries more than steel going over a Benedict later and so on now Of course there are many other applications that you can think of all of this particular structure and
here I'm letting the deafness fracture toughness as well as the ultimate tensile strength and as a whole collection of data from many different studies and what I'd like you to see that this is my sphere mired in fear is a modern civic steel many highly alloyed modern considered extinct and it has always zero-carbon and granted and then you he treated at around 500 degrees centigrade precipitating the metallic compounds and that makes it hard but because it doesn't have the problem and it's very tough and is used for example for a rocket mortar casings In some cases matching the properties of moderating steel using much cheaper following editions that here you're looking at incredible toughness in about 130 maggot bicycle route meters of toughness at the staff level of about 16 hundred megabytes growth and these are quenched and tempered modern city and or all of these are covered 3 Benedict said there is much more potential for destruction because of the absence of Carbide's and for example in India the itself we have worked on producing steered by continuous annealing you know just like you produced the trip deals we can produce without the election will be fair and therefore will get a much stronger get steel as opposed to a just assistance still means there are other phases notice that nuclear experiment in the structure so many questions the most important point want to get a to you is that if you understand the mechanisms of phase transformations then you can find a logical way of creating a better instead of just making many many different chemistry and looking at their properties the simple equation here and the recognition that large regions of Boston I know for or the government helped to create action products which are going to say but that's all for today the
that the the the the the the the


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