3. Steels, Cast irons, phase diagram,

The speaker starts by mentioning material science and how they couldn't complete it last time. They discuss phase diagrams, Curie point, paramagnetic and diamagnetic materials, and the iron-carbon diagram. They talk about eutectic and eutectoid points, different phases, and the composition of iron. They mention hot shortness and the addition of manganese to remove impurities. They discuss binary isomorphous systems, grain boundaries, and embrittlement. They mention the lever rule, types of steel, and cast iron. They end by briefly mentioning different types of cast iron. Good morning folks, so now what I will start is material science and in material science last time I was just having this dinner and I could not complete it and I could not start material science, take care, so what I will do is this is one chapter and all of its theory part I will be covering all of its theory part and it's not much but hopefully I will be able to complete it to within I don't know how much time but only one chapter and it gives like 12 questions, think about it, the weightage of it, so now I start, so change of phase means either there is a change in microstructure or there is a change in lattice structure, so a phase diagram has temperature at its y axis and alloy composition on its x axis, Curie point is 768 degree centigrade, there is no change in phase and only magnetic properties change at 768 degree centigrade, what we see is 725 degree centigrade that is phase change temperature that is not the Curie temperature, Curie temperature is where magnetic properties will be changing, now paramagnetic materials, electrons are unpaired and such alloys exhibit color, okay, diamagnetic materials, electrons are paired and such alloys are colorless, what is that, okay, ferromagnetic iron behaves as paramagnetic as well as diamagnetic that is ferromagnetic, so okay, iron carbon diagram, so in this iron carbon diagram, I won't be talking much about it, because visually, okay, I started, so left side 100% iron is there, right side 6.67% carbon is there and there the structure is formed and I forgot which structure we call it, okay, eutectic, no, no, no, I don't know which one, not eutectic of course, okay, so at 2.1%, the important one is the eutectic point, okay, eutectic point is where liquid converts into two solids, so that is a eutectic point and eutectoid point is where a solid converts into two solid phases, so that is a eutectic, eutectoid like it's a toid and something different, eutectic is like and liquidish and like that, so that way I can remember, okay, so okay, so eutectic point of iron comes at 1150 degree centigrade and at a composition of 4.3%, whereas eutectoid composition is 0.8% and it will be at 725 degree centigrade and there are so many phases are there, so this alpha phase is there, then gamma phase is there and at eutectic point gamma phase into alpha plus Fe3C, that is ferrite, I forgot here, I have to remember, ferrite, I need to see here, okay, Fe3C, and at eutectic point which is at 1150 degree centigrade, there liquid changes into gamma phase and Fe3C, okay, that's all, 0.8% above it is hyper-eutectoid and 0.8% below it will be hypo-eutectoid steels and because at lower percentage, this iron is called steel, lower carbon percentage iron is called steel and at higher carbon percentage it is called cast iron, so simply that at eutectic point, at eutectic composition which is 4.3%, above 4.3% the iron will be called hyper-eutectic cast iron, it's not steel, it's not like some iron, it's cast iron and below it is hypo-eutectic cast iron and same at 0.8% it is hypo-eutectoid steels, hypo or hyper, whichever, okay, so this eutectic, I was talking about eutectic reaction and eutectoid reaction, eutectic liquid converts into gamma plus Fe3C, liquid into two solids and when solid into two solids, that is eutectoid reaction, so gamma is turning into alpha phase plus Fe3C, there are name of these phases, austenite is gamma, alpha is ferrite, Fe3C is, why don't I remember, I need to open my notebook, okay, peritectic reaction, let's see, this peritectic reaction which is at 1495 degree centigrade on the bird's peak, there it, we can see it on the bird's peak, it is delta phase plus liquid is changing into gamma phase, so that is called peritectic reaction, so some question might be coming over there, 0.18%, I don't care where it is, but it's okay, then low carbon steel, mild steel is less than 0.3% of carbon and steels are at 0.8% of carbon, mild steels are at mild carbon percentage, which is 0.3%, which is very less here and high carbon steels are greater than 0.7% of carbon, now the phenomena, okay, so iron carbon diagram is finished, main highlights were eutectic, eutectoid and peritectic reaction and after that, that's all, steel and cast iron, now coming to hot shortness, so what is this hot shortness, when Fe comes with sulfur, Fe plus S, so FeS forms and this FeS iron sulfide has very low melting point, okay, low melting point, because of low melting point upon warm working, it melts out, if it will be melting out, manganese is added, so manganese is added to remove FeS, because MnS form, MnS is very, very dangerous thing, manganese sulfide, manganese sulfide is so hard, they call it the army steel or some steel is there, I forgot, I have to remember, okay, binary isomorphous system, now I am going on to the next topic, hot shortness is just, if FeS is melting point is less than, what is the point of being steel or cast iron, if it cannot sustain, if cracks will be forming like that, I don't want to get into it, binary isomorphous system, okay, why binary isomorphous system, what other systems are there, okay, this doesn't mention the other systems, take care, it's okay, materials which are completely soluble in liquid as well as, as well as, I have to subtract it here, okay, so copper-nickel phase diagram, that is a binary isomorphous, isomorphous is like both liquid and solid state, there will be, there will be a homogeneous mixture present, so copper and nickel phase diagram, nickel of course, its melting point is very high, which is 1452 degree centigrade, I thought it will be more, but who cares, okay, copper's melting point is 1085 degree centigrade, and there is a message on this that, and here the, one line rule was there, so to get the percentage of liquid or the mass, so if I want to know the percentage of liquid, I will just see the other side, the side which is towards solid, and divide it, simple, and I will get it, now Hume-Rothery's condition has to be satisfied in order to, in order for alloys formation, and these three conditions he sets, atomic size difference should be less than 15 percent, size difference, that I can say radius, radius difference should be less than 15 percent, where length C of both materials should be same, and electron negativity difference should be minimum, almost same electron negativity should be there, okay, now I talk about grain boundaries, so what are the grain boundaries, now I know about them, what is written here, the reason where two solidification fronts meet, there will be orientation mismatch, and a boundary will be found, yes, that is grain boundary, so two solidification fronts meet, but their orientation are different, so that is grain boundary, wow, nice, some common grain boundaries defect, and their hot shortness, local melting of impurity at grain boundary causes crack, already you know, now why this is at this grain boundary, because any foreign material will be pushed out of the normal lattice structure, normal crystal structure, and pushed to the boundary, so FeS will be at the boundary, so hot shortness will happen temper embrittlement, segregation of metal impurity at grain boundary, again the same thing, which I just explained temper embrittlement, grain boundary embrittlement, ductile metal when brought into contact with a low melting point metal, it undergoes crack at low stress, I don't know, all this seems to be same to me, but okay, now I talk about binary eutectic system, means eutectic system will be there, only one eutectic point will be there, two faces will be there, like lead antenna, lead comes first, antenna comes second, and as I remember that there was this, what I say, malleability, yeah, malleability, we were talking about malleability, so Aurum malleability is very high, then silver, then copper, then tin, then lead, and then iron, like that it was, okay, I remember it, that's cool, okay, so lead-tin diagram, we talk like copper-nickel-lead-tin diagram, we don't talk tin-lead diagram, we talk lead-tin diagram, so lead is of course on left, and 62% we get this eutectic point, and this, that, I don't care, 183 or what, this diagram is not at all important, so I am leaving it, and I just need to know that what is a eutectic point, eutectic point is again where liquid converts to two solids, so the alpha-beta are two solids here, and what was the lever rule was there, according to lever rule I can get the percentage of both phases, what is the percentage of alpha phase and beta phase, otherwise eutectic point, eutectic point is where solid turns into two solids, solid phase, two solid phase, two solid phases solution, okay, then peritectic reaction was there, in peritectic reaction two, sorry, one solid, one liquid, they convert to one solid, so that is peritectic reaction, okay, now types of steel, head-filled steel, 12% manganese, again I told this hot shortness, FPS is there, it's low melting point to remove that, we put manganese, and if you put 12% of manganese inside the steel, it is head-filled steel, very, very strong, tough, by the addition of silicon, if the removal of oxygen is complete, it is called killed steel, okay, great, designation of steel, how do we represent steel, so something is given here, x multiplied by, x is by 100% of carbon, so 2 into may, x is nickel, 31 xx head-filled, nickel, chromium, then 4 triple x, molybdenum, I am not kidding what they are talking about, stainless steel, 3213, x multiplied by, some designation is there here, I am still not kidding, okay, I need to go ahead, gray cast iron, okay, if it starts, the steel starts with 3, that is nickel chromium alloy, if it starts from 5, only chromium alloy, if it starts from 2, nickel alloy, okay, now let us talk about some cast iron, so this cast iron, there are 5 types of cast iron, so gray cast iron, white cast iron, chilled cast iron, ductile cast iron, there are 2 types of ductile cast iron, nodular, spherical, and one more there, segregated is like malleable cast iron, okay, so gray cast iron is where carbon is present in free or flake forms, and the carbon should be 2.5 to 4% of carbon should be there, in white cast iron, the carbon is present in combined form, okay, chilled cast iron normally freezes gray, but forced to freezes white, so I have to read about this white cast iron, what about this, I have this notes, so I will tell you, so while I was at my coaching, so I made these notes, and I really love them, and sometime, if I will be something, and I will have something, I will be uploading these notes for publicly available, and wait, so I was talking about this in my library, I will find it, just a second, my notes, material science, I found it, I think, are we talking about defects test, 3D diagram, heat treatment, gray cast iron and all, so this will come under, I think, iron phase diagram, it will come, oh no, it is not coming under this, this defects are there, just to give me some time, triple D diagram is there, then heat treatment starts, corrosion is there, corrosion, three questions came from corrosion in 2022, which is very high actually, if I see, the number of questions, okay, crystallography is there, oh, okay, phase diagram is there, let me see, let me see, here we should be seeing this, okay, oh, iron carbon phase diagram, what I am waiting for, iron carbon phase diagram, okay, in that, we should be finding this thing, classifications of cast iron, so in cast iron, gray cast iron is there, where in a pure iron carbon system, you have the percentage of carbon is more than 6.66% of a portion of, then, then what will happen, the portion of carbon will come out in the free or flakes form, that is called gray cast iron, okay, this material is used in the manufacturing of machine beds, engine blocks, so on, these graphite pockets absorb a portion of vibration, okay, then white cast iron is in pure iron carbon system, if the percentage of carbon is less than or equal to 6.67%, entire amount of carbon will appear in the combined form, what is the combined form, in the interstitial gap in structure of this iron or you can say, I mean, between somewhere carbon, some carbon iron bond will be there or something like that, okay, so that is white cast iron, and then chilled cast iron, cast iron of such composition in which normally it is freeze to its gray, but forced to appear as white by rapid cooling, if rapid cooling will be there, then this, if rapid cooling will be there, then it will be, the carbon, if it will be trying to come out, it won't be able to come out and it will freeze there, so it will be, it will make the cast iron brittle, brittle in nature, so these materials are not used directly in any engineering application, because of course, this is brittle and we don't want like brittleness most of the time, these materials are used to make tactile cast iron, okay, so we have freezed it, now after that we can make a tactile cast iron, how do we make tactile cast iron, let's see, spheroidal cast iron is there, so the speed at which we cool it down, that is what matters for this ductile cast iron, so whether it is nodular or it is, notice the white cast iron, simply freezing it, chilled cast iron, white cast iron is normal, less than or equal to, so it is not, the iron won't be coming out, so that is white cast iron, okay, so spheroidal gray cast iron, what is a spheroidal gray cast iron, with the magnesium and cerium, we put magnesium and cerium, so if you put magnesium and cerium and cool it at, from 1150 degree centigrade, if you cool it at extremely low speed, which is 20 degree centigrade per hour, like it is literally very low, if you know, like I will compare it to the extreme example, which is quenching, quenching within millisecond, it drops from like let's say it was 900 or 1000 to just room temperature, so that is like literally too much, but here 20 degree centigrade per hour we are doing it, then if you are doing it a little fast, okay and magnesium cerium is already there, then if let's say the speed we have increased to 100 degree centigrade per hour, then so nodular nodules will be there and like that, simple, simple thing is that ductility will be there, but not as much as spheroidal cast irons have, because of the spheroids they will be forming, spheroidal are the points where carbon is allowed like to gather together, so carbon will be there and gather together, in nodular it won't be able to come, till it will be coming it will be fridged like that, so and now I go ahead with this thing, okay, so wait, I have this, I am a system, please just a second, okay, okay, okay, okay, so what I was doing, what I was saying, I forgot, okay we were at, what is kish effect, not kish, it's kish, so graphite is having lower density, okay yes, due to which it comes over the surface of liquid iron in the red hot condition and it sparkles, so that is called kish effect, okay, whatever, simple, graphite is like this carbon's one, like how do I say it, three structures of carbon are there now, so one is this diamond, other is full ring and one is this graphite, the most popular is the graphite only, carbon is expensive and full ring also making is expensive, but whatsoever, so this graphite will come up on top and it will be, it will look like it is sparkling, so that is simple, effect of alloying in steel element, steel, in the steel, if I will be adding something, what effect it will bring, so this seems to be very important, let's say this question comes up and some matching question comes up, what if I put sulphur, sulphur will be, machinability will increase as this iron sulphide will be there, that will like, as soon as it is, some hot work we will try to do, it will melt and we can easily cut it through, molybdenum forms abrasion resisting particles, improve creep properties, okay, phosphorus improves machinability in free cutting steel, now I have to see this free cutting steel, okay, now let's go for cobalt, cobalt contributes to red hardness by hardening ferrite, ha, now this ferrite is, I was right, ferrite Fe3C, I have to see how this cobalt effect, now chromium, chromium corrosion resistance increases hardness, simple, nickel hardenability, tungsten heat resistance, silicon is for magnetic permeability, increases resistance to high temperature oxidation, okay, I think silicon oxide will be forming at high temperatures, vanadium is hardenability, nickel and vanadium are same, increases strength, manganese is for hardness, manganese, we have already talked about manganese, manganese, okay, I go ahead, addition of silicon to cast iron, silicon, I have already talked about this silicon, silicon magnetic permeability, it will increase or it will increase resistance to high temperature oxidation, two things it can do, now let's see addition of silicon to cast iron, promotes graphite flakes formation, okay, increases fluidity of molten metal, that is very good thing, graphite flakes are there, so fish effect we can see, okay, now addition of magnesium to cast iron, manganese we have seen, head filled steel, but now what will happen if I add magnesium to cast iron, it increases ductility and strength in tension, whereas head filled steel is very hard, so two different things are there, okay, spheroidal graphites will be there, magnesium helps it form more strength, I don't know, wait, I will confirm whether magnesium, spheroidal cast iron, no, there is no such writing here, so if I talk about it, spheroidal graphites is there, spheroidal ductile cast iron is there, okay, just magnesium will increase ductility, monel metal, okay, some of the alloys I have given, babbitt alloy which is bearing, for making bearing, babbitt alloy is used, copper based alloy having lead and tin, okay, low carbon steel, wire nails, wire nails, crane hooks, rolled steel section, okay, medium carbon steel, high carbon steel, what are the purposes of these applications of these, so now I think I, half an hour is over and I am also not able to focus as much as I could have been doing, so what I will do is, we will finish this session here itself and next time I will come up and I will start again from this, what I say, okay, application of the alloys, so this like, every like point here can rise, give rise to a question and we might be, we might be getting one question out of this, I don't know, I have to see, but there are a lot of questions and I can prepare a lot, but whatever, next time we will do it, triple decal and heat treatment, two things are remaining, three things are remaining according to this magnesium, sorry, alloys and their application, iron alloys and their applications and then triple decal and heat treatment, so I will see you, bye.