Effects of Heat Treatment on Structures and Properties in Wrought and Cast High-speed Steels

摘要

The influence of heat treatment on the microstructure and properties of AISI M2 type high-speed steels obtained using conventional metallurgy and foundry technology has been studied. The primary focus was on the effects of austenitising temperatures on the behaviour of both the steels during quenching and tempering. The influence of austenitising temperature on the austenite grain size and size distribution, volume fraction and morphology of the primary carbides, MC and M6C, as well as the character of the secondary precipitation in the wrought and as-cast steels was investigated. It was shown that austenitising significantly affects the changes in the chemical composition, morphology, size and character of distribution of the primary carbides in both the steels, as well as the volume fraction of retained austenite in the matrix and its grain size. These changes in carbides seemed to be induced by a diffusion-induced redistribution of alloying elements between the steel matrix and carbide phases due to their mutual interaction at high temperatures, which is accompanied by carbide coagulations and dissolutions and, consequently, saturation of solid solution by alloying elements. Relationships between the structural parameters and mechanical properties for the steels have been established. It was found that as-cast high-speed steel heat treated in comparison with wrought one has more stable impact toughness and wear resistance in the whole range of austenitising temperatures studied in the experiments.