Research into the patterns of formation of the structure of tool high-speed steels during surface electron beam treatment

Abstract

The objective of the research was to study the modes of electron beam hardening on the structure and properties of surface layers of tool high-speed steels. The limiting modes of electron beam hardening were determined experimentally, which allow obtaining the maximum microhardness of the hardened layer without melting the surface being treated. Based on the results of experimental studies, a comprehensive parameter of the treatment mode was proposed, according to which the intensity of heating and cooling of the surface layer of the tool can be determined - the power density of the beam. This parameter includes all other parameters of the hardening treatment mode: diameter, power, and speed of beam movement relative to the tool surface. It has been established that the maximum hardness of the hardened layer during electron beam treatment can be achieved by high-temperature hardening without melting the treated surface. Hardening without melting also ensures the formation of a highly dispersed structure throughout the entire depth of the hardened layer. Melting of the surface of a tool undergoing electron beam hardening should be considered an extremely undesirable processing option. In the case of melting of the surface layer in a tool made of high-speed steels, a significant decrease in microhardness is observed. At the same time, the surface layer contains a significant amount of residual austenite. Hardening with melting, in which a significant amount of residual austenite is formed, is the main reason for the sharp decrease in the content of carbide phases in the surface layer. Together, all this leads to a decrease in the wear resistance of tool high-speed steels. In the case of hardening with melting, there is also a deterioration in the tool's resistance to large plastic deformations at elevated temperatures in the cutting zone. It has been established that the depth of the hardened layer significantly depends on the initial structure of the steels. The maximum depth of the hardened layer in tool high-speed steels can be obtained by their preliminary heat treatment in the form of volumetric hardening and tempering. Within the framework of the experimental studies, a range of optimal values of the overlap coefficient was established, which corresponds to the minimum values of the tempering zone width.