Up until this point, we've been focused on alloys whose composition is at the eutectoid composition. But now what we're going to do is to turn our attention to off eutectoid alloys. Now here is the section of the iron carbon phase diagram, and I'm calling your attention to the fact that we're going to look at a couple of different compositions along the carbon editions from the on the order of about 0.4 up to about 1.0 carbon with the eutectoid in the middle. So what we have are the equilibrium phase boundaries and I have also indicated the martensite start temperature as a function of carbon content. We haven't talked about this, but as you change the composition of the alloy in terms of carbon, what you find is that the martensite start temperature depends upon what the carbon content is. And so we see that increasing the carbon content decreases the martensite's start temperature. It will also decrease the 50% lines, and the martensite finish lines as well. So, when the material quenches from the austenite phase field down through the two phase field of ferrite and cementite, because we are in a section of the diagram where we have the proeutectoid ferride form, that alpha is forming as we cool down when we reach the eutectoid line. Then structure that's left over of austenite is the material that will ultimately transform depending upon the temperature to either perlite, bainite, or martensite. And what we will see is that the temperature at which the martensite forms is higher than the temperature at which the martensite would form if we were looking at a eutectoid steel. And if we looked at a composition that was to the right, or hypereutectoid, the structure that we would see would be proeutectoid cementite coming down through that range. And what we would find is that there is a continued decrease and in the martensite temperature with the increased carbon content. Now what we'll do is make a comparison between the equilibrium diagram and the isothermal transformation diagram which is given to the right. So up until this point, our IT diagrams are diagrams that were associated with the eutectoid composition. But now, because we are hypoeutectoid, we have to consider the development of the proeutectoid phase. And that's what happens in the upper portion of the IT diagram. So as we cool down, we produce the proeutectoid phase ferrite, and it's in this region where the ferrite forms. Then I want to draw your attention as we go down to lower temperatures and we look at lower temperatures, we see that proeutectoid region narrowing down to a point. And we'll discuss this in a bit more detail. So, our proeutectoid ferrite forms, and now we have the martensite start temperature. So we know where the martensite lines are going to be. And again, it's in comparison to the martensite temperature we have for the eutectoid steel. Now, one of the things that I want to point out is, we look at how narrow that proeutectoid region gets and what it's telling us is if we can quench our micro structure down below where that point is, the micro structure that we will get will be 100% bainite. There will be no proeutectoid phase. Whereas if we go to a temperature above that critical point, what we find is we will get austenite decomposing into proeutectoid ferrite, followed by the austenite that transforms into perlite. Okay. So let's look at this in a bit more detail. So what we're going to do is we're going to come along this line and we're going to quench and hold.. And when we hold it, what we're going to see is austenite, 100%. And now we're going to hold it for a slightly longer period of time in that region where the proeutectoid ferrite forms. And so now we get austenite and the proeutectoid ferrite. Now we can't really tell exactly how much of this phase that we have, because we're cooling down to different temperatures. And so consequently, all we can really say from this diagram is that we will have, at this particular ccomposition will have austenite and we'll have some eutectoid alpha that has formed. Generally speaking that eutectoid alpha is going to form at the grain boundaries of the austenite. If we hold it for a longer period of time, any of the austenite that have left over, that was not consumed by the formation of the proeutectoid alpha, winds up going into being able to transform from austenite into perlite at this particular temperature. So, what we get is proeutectoid alpha. We're still in that region, so all of the sustenite has not decomposed and we have perlite. And we get over to this final point, and now what we get is a microstructure that contains proeutectoid alpha and perlite. Now let's go down to a lower temperature and consider what happens if we happen to quench in the region that we refer to as bainite. So now we hold it for the amount of time. We get 100% austenite because I haven't started the transformation into the bainite two phase region. And so we will wind up with, at our next point, 50% austenite and 50% bainite. And if we continue on holding it, we wind up getting 100% bainite, and so at those three structures, we have austenite, we have 50% austenite, 50% bainite, and finally all the austenite is gone, and we have only the bainite phase present. Thank you.
