We're now going to focus in this lesson, on the iron-carbon system. Now in order for us to do that, let's go back and take a look at the iron-carbon phase diagram. When we look at the phase diagram over a range of different temperatures, what we see is iron melts at a temperature of 1538 C. And as you cool down pure iron and some amount of carbon in the material what you get is a phase that is called delta iron. It's body center cubic. Now if you continue to cool, what you see up there is a peritectic reaction, where the two phase field goes into the single field, austenite phase which is forming at 1394. So we have this nice broad range of carbon that's soluble in the austenite and the maximum solubility for carbon and austenite is given over at the eutectic temperature and the composition is about 2.2 weight percent carbon. That's our FCC austenite. Now when we look at the eutectic, the eutectic is at 1148 C. Now it turns out we're not going to be talking about eutectic compositions in this class, but when you look at compositions that are in the vicinity of the eutectoid or the eutectic that's the region of the composition range where cast irons are produced. But we're going to be focusing on not the eutectic temperature, but a lower temperature that we call the eutectoid temperature. What we find is that as we continue to go down below two lower coolings in the vicinity of about 0.8% carbon. What we get is the phase alpha, and that alpha phase is body-center cubic. Now one of the things that I want you to note is the fact that the maximum solid solubility for carbon in ferrite is on the order of 0.02 weight percent carbon. Now that's a large difference between the solubility that you can have in ostenite with respect to carbon as opposed to ferrite. And there in lies where all the heat treating how and why all the heat treating is important for producing the microstructure. So the eutectoid temperature occurs at 727, the eutectoid composition, as I say, is on the order of about 0.8, and the maximum solid solubility is 22%. We have cementite, which is all the way across the diagram, and cementite is a metastable phase, and it is a compound of iron and carbon and the composition of carbon is on the order of about about 6.7 weight percent carbon. Now, what we'll do is we'll focus on the compositions of interest which are in the vicinity of the eutectoid. So we're going to be looking at the red line and those associated red and blue dots. Now it turns out that if we form pearlite, bainite or martensite, and what we do is we hold those phases at a temperature which his higher than the temperature at which those phases were formed what begins to happen is that the two phase region of ferrite and cementite begin to change their morphology. And they begin to develop something called spherodite, and that is a two phase microstructure that is a mixture of ferrite and cementite. And the idea of the spherical particle is what leads to the name. And it's the cementite particles that begin to spheroidize and the rate of the spheroidization process is going to be a function of the time and temperature that they are held at the elevated temperature. Time, temperature, starting material affects the morphology. So we know for example, that pearlite forms at a higher temperature. It's a coarser microstructure than does bainite that forms at a lower temperature than pearlite. So depending upon which of those two microstructures you get, you're going to wind up with a microstructure that will be finer at the lower temperature bainite. Now when you look at martensite, martensite is a single faze material, but ultimately what happens because the equilibrium diagram tells us that we should have alpha and ferrite. What begins to happen is that the carbon begins to leave the martensite, and it begins to diffuse away, forming ferrite and cementite particles. And so you start out with a saturated carbon single phase microstructure and you develop a two phase micro structure of ferrite and cementite, and of course because you start out with that martensite phase the phase is going to be quite small. Let's focus on the composition of the eutectoid, and we're going to divide the phase diagram into two portions, and we do this for historical reasons. And it makes our description of where we are on the phased diagram easier to understand when we speak the words hypo and hyper. So hypo are compositions that lie to the left of the utectoid. Where as hyper are the compositions which lie to the right and consequently, hypoeutectoid steels have the composition of less than 0.7. Hyper have compositions greater than eutectoid of 0.77, and those are the compositions that lie to the right. So again, we'll take a look at the diagram. And there are alloys whose compositions lay to the left or the compositions lay to the right. For the compositions that lay to the left or hypoutectoid alloys, what we get is a phase that forms as we come down out of the austenite toward the temperature of the eutectoid. And what we get is a phase that forms before the eutectoid, and we refer to this as proeutectoid ferrite, and it's because it's the face that forms prior to the eutectoid transformation. Now, if we look at the composition to the right where we're dealing with a hypereutectoid composition. Now, what we're looking at is a face that forms, coming out of the austenite before we get to the eutectoid temperature and what we will get is proeutectoid cementite. So we've looked at now three composition ranges, we've looked at eutectoid compositions, and we will also be describing later on how we can heat treat and look at the eutectoids, the hypo, and hyper eutectoid compositions. Thank you.
