Measurement of Flow Stress in Supercooled Austenite for High Hardenability Steel
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CCORDING to the Fe-C binary alloy phase diagram, we can identify the phases of steel at any temperature. When considering the cooling process from a high temperature, it is common knowledge that the phase is austenite over the A3 temperature [for example, 1473 K (1200 °C)], austenite and ferrite between the A3 and the A1 temperatures, and ferrite and cementite below the A1 temperature. In the case of a high cooling rate, the phases existing at these temperatures are different from the phases of the Fe-C binary alloy phase diagram. For example, austenite exists below the A1 temperature. Austenite that exists below the A1 temperature during fast cooling is called supercooled austenite. In the process of hot stamping[1,2] and ausforming,[3,4] deformation usually takes place in supercooled austenite. Deformation conditions, such as the deformation temperature, the die dimensions, and the load capacity of the press, are determined based on the flow stress of the materials. Therefore, it is important to measure the flow stress accurately in supercooled austenite. The tensile test is a common method for measuring flow stress. When flow stress is measured at high temperatures, the tensile test is conducted in a furnace. By using conventional types of furnaces, it is possible to control both the specimen and the atmospheric temperature. However, it is not possible to change the specimen temperature quickly. As mentioned previously, supercooled austenite forms at the nonequilibrium phase, and
it exists only during fast cooling. This means that supercooled austenite transforms to other phases when the specimen temperature is controlled by conventional types of furnaces. In recent studies, flow stress of supercooled austenite was measured by modern systems for hot tensile tests.[5,6] As also noted in these articles, the incubation time for phase transformation from supercooled austenite to other phases is very short at lower temperatures, especially in high hardenability steel. Therefore, it is implied in these studies that phase transformation occurs during hot tensile tests. To avoid this phase transformation during hot tensile tests, a unique testing procedure was proposed in another study.[7] The tensile test was conducted during continuous cooling to avoid bainitic transformation. In that procedure, the temperature of the specimen varied during the tensile test. Therefore, isothermal data of the flow stress could not be evaluated. The purpose of this study was to develop a method for measuring the isothermal flow stress in supercooled austenite for high hardenability steel. An isothermal tensile test was first conducted. Second, the tensile test during continuous cooling was improved to measure the isothermal flow stress. Finally, the difference of both tensile tests was examined. Microscopy was also conducted to observe the phases during the tensile tests to show the validity of the developed measurement method.
II. YASUHIRO YOGO, MICHIAKI KAMIYAMA, TAKAMICHI IWATA, and NORITOSHI IWATA, Research Scientists, are with Toyota Central
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