all right the last thing we're going to cover in this chapter on phase diagrams is an example of probably the most important phase diagram to material scientists which is the iron carbon phase diagram and that's to give that's the one that gives us steel so the steel phase diagram is fascinating first off let's just take a look at it here here i've shown it uh what do you see do you recognize any of the reactions we've learned about well what about eutectics do you see any eutectic reactions how about right here a liquid turning into two different solids gamma plus fe3c we call fe3c cementite by the way and gamma we call it austenite that's the austenite phase so right there at 4.3 weight percent carbon if you cool it down at a 1147c you get the eutectic formation of cementite and austenite there's one what else do we see well how about this reaction over here that one in that top left corner at 1493 celsius well as you cool it down you're going to have a mixture of liquid plus this delta phase so at that point you're going to have liquid plus delta turning into austenite so that's a liquid and a solid turning into another solid we call that a peritectic point right so it's a peritectic what else do we have how about right here cooling it down right there at .76 weight percent carbon we go from pure austenite into a mixture of ferrite and cementite where the alpha phase here is is the ferrite phase so one solid into two other solids is a eutectoid right gamma turning into alpha plus fe3c that's a eutectoid right so a lot of cool reactions happening in this diagram now um if you go just below this point from across that eutectoid let's say you were right here right you're going to end up with a structure that's a mixture of ferrite and cementite and just like before the eutectic structure forms a lamellar structure you're going to get the same thing so just above so this is that t above 727 degrees celsius you're just going to have grains of the gamma phase austenite right but then at t just below 727 celsius you're going to end up with a all of the austenite is going to be gone right because the eutectoid reaction took place and so now you're going to have in these regions a mixture of ferrite and cementite and it's going to be this lamellar structure because it happens so quickly okay so just like we saw before you're going to see this lamellar structure now to the left and the right of that point we end up with hyper hypereutectoid and hypoeutectoid compositions so hyper means you've got too much carbon so those are going to be greater than .768 carbon right so your hypereutectoid steels are going to be over here on the right hand side that's your hyper eutectoid and then to the left you've got your hypoeutectoid steels over here okay hyper eutectoid and hypoeutectoid compositions okay now iron when you go and you buy iron at like home depot it's going to have very small amounts of carbon in it we want it to be flexible we don't want steel wire sometimes sometimes you want iron wire right so that's going to have less than .008 weight percent carbon right because there's uh to get this pure ferrite phase you don't have very good solubility of carbon in that phase okay now steel is anywhere between this the iron region so point zero zero zero point zero zero eight and two point one four weight percent so your steels are going to be in this region right here that's where your steels are right all the different steels that you can buy and then your cast irons go all the way 2.14 up to technically 6.7 but usually around four and a half so this region through here these are your cast irons right so when you buy a cast iron dutch oven it has much more carbon than any of the steels that you're going to buy now there's there's consequences to that what do we know about these things well clear over here pure iron right this is iron with just a little bit of carbon that's going to be very ductile you can bend that wire really easily but over here this fe3c that's a ceramic it's a full on ceramic it's a carbide at that point so it's going to be very hard high melting point uh very tough not very tough very hard very um refractory not ductile at all and then in this region where you have both of these the ductility and the hardness of your alloy is largely a function of how much of these two phases you have present so if you're more on this side more on the low carbon steel side you're going to end up with a more ductile alloy where if you're on this side more on the high carbon steel side you're going to end up with something that has more of the hard brittle ceramic phase to it so it's going to be a high hardness low ductility alloy more prone to fracture but stronger okay so you can learn more about these and we will in a later chapter but you've got your low carbon steels they're mild steels low cost easy to shape they're not very hard but you can do things to the surface like carburize the surface we will talk more about that when we talk about diffusion you've got your medium carbon steels these are between 0.3 weight percent and 0.8 this is like a sort of good combination of ductility and strength and they have pretty good wear properties finally you've got your high carbon steels between point eight and maybe two weight percent very strong uh really cool things and then you've got ultra high carbon steels really these are cast irons extremely strong but they are brittle okay so we will talk a little bit more uh later in this in this semester about the different microstructures of steel that can happen because so far we've shown you that you've got you know alpha plus fe3 in this whole region this whole region here is just a mixture of alpha plus fe3c but in fact there's lots of different ways that you can get that to occur so we will come back to that and talk about the difference between say perlite versus bainite versus spheroidite we'll come back to that later once we talk about kinetics so hold just put a pin in that for now okay so what would the structures look like if you cool the structure down in the hypoeutectoid hypoeutectoid means that we're cooling it down to the left of the eutectic right so over here somewhere what will it look like well it's going to pass through this intermediate region right here right as it passes through that region it's going to form alpha plus austenite so some of your austenites going to turn into alpha ferrite right so if we cool it down it will look like this so it starts out fully austenite so everywhere is austenite and then as it cools down you're going to form ferrite now where will the ferrite form if possible it's going to want to form along these grain boundaries why why does it want to form along the grain boundaries it wants to form along the green boundary so this is the ferrite that's forming initially it wants to formalize the green boundaries because remember the grain boundaries cost us energy if you zoomed in on this you had atoms arranged in one way atoms arranged in another and then you had this nasty sort of interface where they couldn't form nice bonds so that surface was already costing this compound energy and so if you're going to form a new surface you might as well get rid of an old surface right so now you've got a surface on this side and on this side but you got rid of the old one anyways new phases like to form along grain boundaries if possible now in order for that to happen you have to diffuse atoms to or away from these regions from the bulk maybe you have to diffuse it away from there or to it and if your diffusion is not fast enough then you won't be able to do that instead what you'll form is this alpha phase forming as precipitates we'll talk more about precipitate hardening in a later chapter anyways as you continue to cool this thing down you're going to reach some point just above the eutectic eutectoid where you've reached the lever rule and you've got as much of the ferrite phase is as is thermodynamically favorable to form right so here's all of your ferrite phase as much as it wants to form is now formed along these green boundaries now the second so this is temperature just above the eutectoid okay when you go below that eutectoid right at temperatures below the eutectoid temperature you've still got those regions for the most part intact right so all of that pro-eutectoid ferrite remember pro-eutectoid is the eutectoid solid that's forming before the eutectoid reaction so the ferrite is part of the eutectoid reaction and this is the pro-eutectoid phase but then you end up with the lamellar in between okay so that would be an example of a hypoeutectoid alloy how would the hypereutectoid look different well instead of having alpha here and a mixture of alpha plus fe3c the only difference if it was hypereutectoid is that this would be fe3c instead of alpha for that one that would be the only difference so we've got some examples on youtube i'll put these links to the homework problem and you can see some nice examples in the book about how these would happen okay so that is phase diagrams and drawing these microstructures again we'll talk more about it in uh in a later chapter when we dive into the different kinetic impacts on the steel phase diagram