Monitoring of Tempering Behavior in Fe-C-Mn Alloys by Precise Measurement of Electrical Resistivity

Abstract

Mechanical properties in metallic materials are generally determined by microstructure, which is controlled by thermo-mechanical treatment in addition to its composition. Tempering is the conventional process of heat-treated steel. Therefore, understanding microstructural change during the process is especially important. As is well known, electrical resistivity measurement gives useful information regarding microstructures and lattice defects. In this technique, average information can be obtained from larger volume of specimens than TEM specimens as 108 times. So the purpose of this study was to monitor electrical resistivity changes related to microstructural changes during tempering in Fe-0.19 C-1.5 Mn, Fe-0.35 C-1.5 Mn alloys. Bar shaped specimens were cut from the sheets of steels. The specimens were heat-treated in austenite region and then quenched with ice-water. Then tempering was cumulatively carried out by direct dipping in a nitrate bath held at 423-673 K. Electrical resistivities at 77 K (in liquid nitrogen) and 300 K were measured in each state by a direct current (DC) four-point method. The resistivities abruptly decreased and then gradually decreased by the tempering. First drop and following decreasing of resistivities are corresponding to vanishing vacancies in martensite and precipitation of carbide, respectively. The organized resistivites by Hollomon-Jaffe parameter showed infections during the carbide precipitation. The points must be related to transition of precipitation mode.