We're going to continue our discussion on how to heat treat and form various microstructures that result as a consequence of the decomposition of. The first thing that forms at the high temperature, that structure that forms of the morphology of that structure called pearl light. Develops in a very particular way. So lets focus again on the region of the Eutectoid diagram of interest to us. So here is our composition range. We start off in the austenite field at the upper dot. We quench down to below this transformation temperature of the Eutectoid and hold it for various periods of time. What happens is, as we hold it for various periods of time at that temperature, the austentite begins to decompose. And what happens as a result of holding at time and temperature, we'd begin to produce progressively More and more pearlite as we go through the process. Now, we can actually calculate using the equilibrium diagrams we can actually calculate what fraction of the microstructure is ferrite. And what fraction of the microstructure is cementite. So, we'll use the compositions along the eutectoid temperature. And we see that we can calculate the fraction of alpha. It turns out that that fraction is on the order of about 88%. Again, if we calculate how much we have of the cementite that is present is on the order of about 12%, and you're going to see why I'm rounding these things off to about 88 and 12% in just a moment. The way the structure forms is by diffusion along the interface that separates the three phases, that is the austinite, the ferrite. And the cementite and the way the phase forms is by the diffusion of carbon and iron in the austenite adjacent to the prolate structure that's black, white, black, white, as indicated in the diagram. A standard keyboard on a piano contains 88 keys. And this was the way that I remembered. That I was able to recall how much of these various components are in the pearlite. And in the bainite structure by remembering the keys on the piano. There are 88 keys. There are 88 ferrite percentages and they're 12% Fe3C, so that's our standard keyboard. And it helps you kind of remember the structure that you get because it does look like this La Mer structure of a piano. Plus it gives you a sense of how much there are of the phases present. Okay. When we take the material that's been produced and we etch it and look at it in a scanning electron microscope, we see this pattern that looks very much like the piano keyboard, where we have alternating white and black. And the alternating white and black in this image represents the black is the ferrite phase and the white is the FE3C phase. And it turns out that that phase is a lamellar phase in that it goes down into the microstructure and it looks like a plate. So, here's the way it forms. The structure is to the left, austenite, and to the right, the material that has been transforming, as a function of time. And so we get the two phases. Cementite and ferrite. Then, we describe them as alternate lamellaes. So, pearlite is an lamellar structure and we have an advancing interface where as we continue to transform, the austenite recedes and it's replaced by, This alternate lamellar structure that we call pearlite. Now, it turns out there's an interesting reason why pearlite has the name pearlite. Because of the spacings between the cementite phase, when the material is polished and given a light etch, it has almost a pearlescence. Glow to the surface primarily because of the spacings that you have in the pearlite. So it's scattering light, much likely a CD or a disc does and so you're seeing this kind of pearlescent structure on the surface and so hence the name pearlite. So, our carbon is diffusing along the interface and counter to it is the diffusion of iron. So we're trying to produce carbon rich fe3c and we're trying to reduce the iron so that we get down to about 0.02 weight percent carbon. So we're enriching that phase with iron. And so the process is occurring along the interface and we refer to this then as a short range diffusion process. Now we can look at a structure and see how the pearlite begins to form. I have illustrated up here this chicken wire structure. And those chicken wire structure represents the ostenite grain boundaries. And what you can see are all their net lamely of pearlite colonies that have nucleated at the boundaries of the austenite. And that all occurs in a particular unit of time. So we have the austenite grains, the pearlite colonies that nucleated and they've done in a time step t1. Now, if I continue the process In a second time step, what I begin to see is additional pearlite colonies continue to nucleate and the ones that formed in the first time step, they continues to grow. And then as time goes on, and we go to our third time step, we're beginning to see the austenite is beginning to get. Completely filled with the pearlite structure. So, this structure is beginning to take over and eventually what we'll see is all of the austenite is gone, and what we only have left is the pearlite. Now, when we look at the temperature at which the pearlite forms, what we see here is. A scanning electron micrograph image of the pearlite of the cementite that forms the pearlite and the ferrite. And this was done at 655C. If I take that same material, and I quench it to a lower temperature and hold it and allow all of the austinite to decompose, what I find is, these are all taken at the same magnification we'll see. And what it's telling me is that the spacing of the cementite is decreasing. Now I go to a slightly lower temperature and I can see finer spacing of the FE3C, and as we go down even further in temperature. So we're decreasing the temperature at which the pearlite is formed and consequently the structure is becoming finer. Now that we have some idea of how the pearlite colonies begin to grow and continue to nucleate and continue to grow, we can actually look at how the data might be presented when we look at it in terms of the isothermal transformation. The data might look something like this where what we've done is looked at the fraction transformed as a function of time. And what we've done is to identify the start of their action which is on the order of about 0.01. And that's when the time starts and the time of the finished reaction is up at .99 so that's the end of the reaction. So we have the beginning and we have the end of the reaction. And we're going to plot these and when we plot them for this particular temperature, we're going to see the start and the finish line where the austinite has began to decompose once we reached the start line. And as we progressively go through that two phase region where the austenite is beginning to be replaced while the cementite and the ferrite in the form of pearlite until we get all of the pearlite formed, and we have no austenite left. Then what we would do is to choose another temperature, go through, plot the data, and then we would incorporate this isothermal data on the curve as indicated to the right. So this is the way in which we can begin to produce these isothermal transformation diagrams. Thank you.
