Were now in a position because we understand how the decomposition of Austenite works. How we can use isothermal transformation diagrams to predict ahead of time the type of microstructure that we would expect to see if we quench from the Austenite temperature and hold down at a lower temperature? So, here is our isothermal transformation diagram for the Eutectoid Steel composition. We have plotted along the x-axis, the time and it's in the scale of logarithms and we're looking at the temperature on the y-axis. And again, the upper portion is associated above the Eutectoid temperature, is associated with the austenite. And as we go down lower in temperature, we see that the phases that begin to form will be different. So, we get the Pearlite at high temperatures. We go down to a lower temperature and we get Bainite. So, we're going to quench and we're going to hold. So we first start out by holding above 727. Follow the arrow, and quench down to a temperature of 600 C. Now what we do is, if we can imagine looking at the structure at that temperature, what we will see is 100% austenite in the microscope. Now what we're going do is we're going to hold it for a certain period of time at that temperature, hence developing the isothermal transformation. And when we look at the diagram, we estimate that the structure now consist of 75% Austenite and 25% Pearlite. We quenched and hold for a longer period of time and, this time, at the dotted line. That dotted line represents 50%, so that means that 50% of the Austenite is gone and 50% of that Austenite is now transformed into Pearlite. We continue the process and now what we get is 25% Austenite, 75% Pearlite, and then eventually what'll happen is, we hold it all the way out to that final dot. Now what we're going to do is to start out again at above 727 and this time what we're going to do is to quench following the blue line. Now here we've gone to a higher temperature. So what will happen is, again we're going to get Pearlite, but we'll describe what the Pearlite's going to look like as a result of that transformation temperature. So, we hold it for this period of time, indicated by the blue circle. We look at the microstructure at that temperature, and we see 100% Austenite. And now what we're going to do is we're going to hold it until we reach that dotted line inside of that region between the start and the finish, which is the 50% line. So now what we have is 50% Austenite, 50% Pearlite. And what we'll see here is after holding it beyond 10 to the 2 seconds, the structure is completely Pearlite. Now if we make a comparison to the structures that we saw by heat treating at this last temperature at the higher temperature, the Pearlite colony, as we described previously, should be coarser than it is with respect to the spacings at 600C. Now, let's go back and look at this diagram again but this time, what we're going to do is we're going to quench it down to a lower temperature. So we start out again at 727, we quench down to 300 degrees. When we exam to 300 degrees, what we see is 100% Austenite. And now, we come over to this dotted line again, and we're in between the start and the finish of the Bainite. And so what we're going to have is a microstructure that's composed of 50% Austenite and 50% Bainite. Now if we hold it over here to 10 to the 4 seconds, what now happens is all of the Austenite has transformed and we have 100% Bainite. So this is the way we can develop these different microstructures by holding, by quenching to a temperature, and holding it for various periods of time. I want to emphasize that when we began each one of these experiments, we started out with Austinite. So if you need to reheat treat a steel, you always have to take that steel composition back to the Ostenite temperature range to get rid of any other microstructures that you might have as a result of the material that was shipped to you. You go back, ostenitize it and then you go through and do your quenches so you produce the type of microstructure that you're interested in. If we were to take that micro structure where we had 50% Astenite and 50% Bainite and we quenched that material down to room temperature, that is we would be below the Martinsite start and Martinsite finish temperatures. What would happen is, we would have a microstructure that is made up of 50% Bainite. That 50% Bainite is what formed at 300 degrees for the period of time where the quench line is. Now, what we're assuming is that when we quench it down to the low temperature, what will happen is we will take the remaining Austinite and we will decompose that Austinite into Martensite. So our structure was 50% Bainite and then the Austinite that transforms gives us a microstructure of 50% Bainite and 50% Martensite. Now we can begin to understand the type of microstructure that would develop as a result of different quench paths that we use where we quench from the Austenite down into different regions, and hold for various periods of time. And we can then predict what the microstructure would be based upon the use of these isothermal diagrams. Thank you.
