Throughout our civilization, periods of time have been denoted by materials. We had the Stone Age, the Bronze Age, the Iron Age and thus we've had the semiconductor or Silicon Age. But that Iron Age was facilitated by steel. The iron horse locomotive opened up the American West. That was because of cast iron and steel. One of the most important phase diagrams associated with civilization is the iron cementite phase diagram. With that, we want to make sure we have an understanding of its usefulness, its meaning, the phases and the microstructure. How do we correlate certain mechanical properties to the microstructure of steels and cast irons. The advent of steel promoted large-scale buildings. If you look that the 1891, you see we had 200 foot high building that was enabled by steel and in 2010, now we have one that's almost 3,000 feet high. Again, enabled by steel and cast iron. We look at the iron cementite phase diagram, here we have iron, here we have cementite. Cast irons, typically are going to be in this range and here we will have our steels. Let's take a moment for inquiry. Well, we know that cast iron is going to be brittle, so in that case, we will have cast iron in service, we want to use it in compression. Whereas steels are going to be ductile hence, they can be loaded in either compression or tension. Now what are some of the important characteristics of the cementite phase diagram? Well, first, let's look at the important phases. We start off with the high temperature FCC austenite phase. It has up to 2.14 weight percent carbon in it. Now it has carbon dissolved in, it has a nice FCC type structure and it resides in this region. Next on the hit parade, we have Ferrite. Ferrite or referred to as Alpha iron has a little bit of carbon dissolved in it, 0.02 weight percent carbon, so almost no carbon, but there is some. It has a BCC structure. Finally, the hard iron carbide is cementite. Fe_3C, is at 6.67 weight percent. It's a very hard and brittle compound. We vary between the very soft Ferrite, which is essentially iron. Austenite, the FCC high temperature version that has up to 2.14 weight percent carbon in it. We're going to look a little bit of the still terminology. We just said that Ferrite, which is Alpha iron BCC, with a little carbon dissolved in it, is essentially soft and very ductile. Austenite in high-temperature phase is FCC, is ductile and cementite, which is a ceramic hence, it's going to be hard and brittle. When we talk about the terminology, often you hear people refer to it as 1020 steel or 4140 steel, so we have an ideal. The first two numbers will tell you the alloy type. If I'm talking about a plain carbon, meaning no alloying, that's going to be a tin, so a tin 20 or a Tin 60. If you have alloy, you're going to have a non tin number out front, like a 4140 or 4340 alloy steel. The second number tells you the weight percent. A tin 80 is going to have 0.8 weight percent carbon in it. If I had a tin 100, that's going to have one weight percent carbon in it. Now, sometimes we hear terms low-carbon or mild steel, it'll be in this range up to our high carbon steels in this range. After that, we have our cast irons. Let's take a moment for inquiry. Ferrite, the soft guy, is going to have a BCC structure, Austenite, which has FCC structure, cementite is hard, Ferrite is indeed soft. Now how does the carbon concentration affect the mechanical properties? Well, if we compare the tensile strength, which correlates to hardness, we know that as I increase the carbon concentration, the hardness increases and the tensile strength and yield strength correlate with hardness. If hardness goes up, the tensile strength is going to go up as well as the yield strength. Now, we know that the ductility has to be the inverse. As carbon concentration goes up, the ductility is going to go down. Whether we look at that in terms of reduction in cross sectional area, elongation, as well as the toughness will go down, the energy absorbed at fracture. Again, as the carbon concentration increases, tensile strength hardness increases. As carbon concentration increases, the ductility in terms of elongation and reduction cross-sectional area reduces as well, and the toughness is reduced. Now, one of the most important compositions is going to be the eutectoid composition, which is 0.78 weight percent. Sometimes you'll see it just written as 0.8 or 0.76. Fine. As long as we're looking in this regime. The eutectoid steel is based on the eutectoid reaction. If I'm at 0.78 weight percent carbon 727 degrees C, I have at the eutectoid point, I can have three phases in thermal equilibrium. Gamma goes to Alpha plus cementite. Now, if we look at the microstructure above the eutectoid temperature, at the eutectoid composition, I'm going to be all Gamma or austenite. If I cool just below the eutectoid temperature or isotherm. Now, I will drive this reaction through the eutectoid reaction so all the austenite will transform into pearlite. If we recall, the pearlite forms by co-operative growth, hence we're going to get this laminate type structure and there's a pearlite. Now we look at the pearlite microstructure so we're clear. We say the term pearlite microstructure is not the phase. Ferrite and cementite are the phases, so just make sure. The pearlite describes this microstructure often we don't refer to grains of pearlite, it's referred to as colonies of pearlite. We have this high temperature austenite, the FCC undergoes a eutectoid reaction. It has to expel carbon at the interface to form the BCC or the Alpha phase, which has essentially 0.02 weight percent carbon in it. To transform the austenite into cementite, you have to nucleate cementite and then it becomes a sink for that carbon. It has to gather up as much carbon to undergo the transformation. Regardless throughout this, the overall composition does not change. If we go back to the previous guy here, the composition does not change. Let's take a moment for inquiry. We're looking at the amount of weight percent carbon. Well, austenite has up to 2.1 weight percent, cementite, 6.7, and ferrite essentially, almost no carbon. We make sure we have those numbers in our head. Hopefully, with this particular lecture, we became more familiar with the iron cementite phase diagram. We have an ideal about the phases present. The important ones, austenite, which is the Gamma phase, ferrite, which is the Alpha phase, and cementite, the ceramic phase. Then we looked at the eutectoid composition and the microstructure that we obtained upon cooling. Also, don't forget, we looked at how the mechanical properties, tensile strength, yield strength, toughness, ductility varies with carbon concentration. Thank you.
