Heat Treatment of Steel MSE 528（钢的热处理MSE 528）.pdfVIP

MSE 528 - HEAT TREATMENT OF STEEL Purpose The purposes of this experiment are to: Investigate the processes of heat treating of steel Study hardness testing and its limits Examine microstructures of steel in relation to hardness Background To understand heat treatment of steels requires an ability to understand the Fe-C phase diagram shown in Figure 6-1. Steel with a 0.78 wt% C is said to be a eutectoid steel. Steel with carbon content less than 0.78 wt% C is hypoeutectoid and greater than 0.78 wt% C is hypereutectoid. The region marked austenite is face-centered-cubic (FCC) and ferrite is body-centered-cubic (BCC). There are also regions that have two phases. If one cools a hypoeutectoid steel from a point in the austenite region, reaching the A3 line, ferrite will form from the austenite. This ferrite is called proeutectoid ferrite. When A1 is reached, a mixture of ferrite and iron carbide (cementite) forms from the remaining austenite. The microstructure of a hypoeutectoid steel upon cooling would contain proeutectoid ferrite plus pearlite (α+ Fe C). 3 The size, type and distribution of phases present can be altered by not waiting for thermodynamic equilibrium. Steels are often cooled so rapidly that metastable phases appear. One such phase is martensite, which is a body-centered tetragonal (BCT) phase and forms only by very rapid cooling. Much of the information on non-equilibrium distribution, size and type of phases has come from experiments. The results are presented in a time-temperature-transformation (TTT) diagram shown in Figure 6-2. As a sample is cooled, the temperature will decrease as shown in curve #1. At