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Unformatted text preview: 8.4 Continuous Cooling T ransformation Diagrams Objectives • Predict microstructures that arise from a given heat treatment. • Design a heat treatment to produce a desired microstructure. In the previous section we looked at isothermal transformations. However, instead of holding a constant temperature it is often easier to just heat the steel up above the eutectoid temperature to “erase” the previous microstructure and then just let it cool down at some constant rate. If we do this we can not use the isothermal transformation diagram to predict the structure. Instead we need to use a continuous cooling transformation diagram . The continuous cooling transformation diagram for plain carbon eutectoid steel is shown in Figure 8.4.1. You should immediately notice one big difference between this diagram and the isothermal transformation diagram: there is no bainite on the continuous cooling diagram. This is because of the kinetics of bainite formation. To get bainite we have to cool to below 550° C. But let’s say we use a constant cooling rate fast enough to avoid forming pearlite. If we do that it turns out the cooling rate is so fast that the material doesn’t spend enough time below 550° C for bainite to form. The result is that neither pearlite or bainite can form, and we just get martensite. Figure 8.4.1: Continuous cooling transformation diagram for plain carbon eutectoid steel. Unfortunately it is not possible to use the continuous cooling diagram directly to get cooling rates. This is because the time axis is in a log scale, and even more importantly because the rate you would calculate off of the graph depends on what temperature you start at. There rate you would calculate off of the graph depends on what temperature you start at....
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- Spring '08
- Austenite, pearli te, fine pearli te