section%2080.3%20-%20Isothermal%20Transformation%20Diagrams

section%2080.3%20-%20Isothermal%20Transformation%20Diagrams...

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Unformatted text preview: 8.3 Isothermal T ransformation Diagrams Objectives Predict microstructures that arise from a given heat treatment. Design a heat treatment to produce a desired microstructure. We will now apply the concepts of kinetics to the practical issue of microstructure in steel. Steel is an alloy of primarily iron, plus carbon and sometimes other elements. Carbon increases the yield strength of the steel, while the other elements also change the strength as well as other properties. For example, stainless steel has chromium, which provides protection against corrosion. For this section we are going to only consider a specific type of steel: plain carbon steel that consists of 99.24 wt% iron and 0.76 wt% carbon. Before we can get to microstructure development, we need to know what the different microstructures of steel are. Figure 8.3.1 shows part of the iron-carbon phase diagram. This looks like a eutectic phase diagram, but is actually called eutectoid because the high temperature phase is solid instead of liquid. The key microstructures on this phase diagram are: Austenite: The high temperature phase that exists above the eutectoid temperature. Ferrite: The phase. Cementite: A line compound with the chemical formula Fe 3 C. Pearlite: The eutectoid microstructure of ferrite and cementite lamellae. It is important to remember that pearlite is not a phase; it is a microstructure. Figure 8.3.1: I ron-carbon phase diagram. Figure 8.3.2: Isothermal transformation diagram for eutectoid plain carbon steel. Figure 8.3.2 shows the isothermal transformation diagram for plain carbon steel of eutectoid composition. There is a lot of information on this figure that you have not learned yet; we will be going through this figure step-by-step. The first thing to notice is the overall U-shape of the curve. This is the typical phase transformation curve that we saw in Figure 8.1.4. At high temperatures pearlite is formed, as would be expected based on the phase diagram. The thickness of the pearlite lamellae depends on the transformation temperature. Go back for a moment and look at Figure 6.3.3. This figure shows how the temperature....
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section%2080.3%20-%20Isothermal%20Transformation%20Diagrams...

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