The core program was set to heat the samples up from room temperature to 1000 ◦ C and held at that temperature for 120 seconds until a fully austenitic microstructure was achieved. After high frequency induction heating, the samples were consequently cooled with helium gas. Different cooling rates were applied to the samples to measure its effect on the microstructure of the dual-phase steel and can be seen in Table 1. Furhermore, an isothermal heat treatment was applied to one of the samples. A cooling rate of 20 ◦ C/s was applied in this treatment followed by a cooling rate of 50 ◦ C/s. Finally, the samples were cooled down to room temperature and the container is set back to atmospheric pressure. The tungsten wires connected to the thermocouple were cut and the steel samples were stored in separate plastic sample bags. Using WinTA, the output data were processed and the phase transformations diagrams of the dual-phase steel were acquired. 2
MS43010 - N OVEMBER 25, 2019 Table 1: Samples with their corresponding cooling rates Sample Length (mm) Width (mm) Thickness (mm) Cooling rate ( ◦ C/s) CR250 9.88 4.42 1.18 250 CR15 9.80 4.42 1.18 15 CR10 9.81 4.43 1.18 10 CR5 10.05 9.91 1.18 5 CR1 9.91 4.46 1.18 1 ISO 9.90 4.41 1.18 20 & 50 3 Results As mentioned in section 2, to study the phase changes, all samples were heated to 1000 ◦ C and the temperature was maintained for 2 minutes to ensure a full austenite microstructure. To acquire the different phase transformation temperatures for different cooling rates, tangents are drawn on the S-curve surfaces. 3.1 CR250 From Figure 1, it can be observed that the change in length (green) of sample CR250 shows a curvature at around 730 o C where austenite is starting to form. The phase transformation to a full austenite microstructure is completed shortly after the curvature when the line is straight. This phenomenon occurs in all the samples because they are all heated up to 1000 ◦ C. However, during cooling, between temperatures 453 o C and 326 o C a discontinuity is observed, reflecting an additional phase transformation. In this case, it is the transformation of austenite into martensite. Figure 1: Graph Temperature ( o C) - Time (s) - Change in length ( μ m) for fast quenching. 3.2 CR15 The significantly lower cooling rate on sample CR15 resulted in a different starting temperature for phase transformation. According to Figure 2, the phase transformation starts at 595 o C and the process finishes at around 330 o C. The expected microstructure is ferrite, bainite and martensite. 3
MS43010 - N OVEMBER 25, 2019 Figure 2: Graph Temperature ( o C) - Time (s) - Change in length ( μ m) for a rate of 15 o C/s. 3.3 CR10 For sample CR10, a phase transformation can be observed in Figure 3 between 615 o C and 334 o C. The expected microstructure is ferrite and bainite. Figure 3: Graph Temperature ( o C) - Time (s) - Change in length ( μ m) for a rate of 10 o C/s. 3.4 CR5 In Figure 4, sample CR5 showed a peculiar behaviour during cooling, two phase transformations were presented, the first one at 699 o C finishing at 400 o C with the expected microstructure of ferrite and pearlite, and the second transformation at 185 o C, in which the expected microstructure is martensite.
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- Spring '18
- Dr. maarten bakker