Structure Formation under Plasma-Assisted Hardening of Thin-Walled Low-Weight Parts

Abstract

The processes of structure formation during plasma hardening of samples made of 65G grade low-alloy structural steel have been studied using optical metallography. It is shown that the structure formation during quenching after exposure to a highly concentrated energy flux obeys general structure formation features described by the iron–carbon phase diagram. The depth of the hardened structural zones in occurring under plasma quenching depends on the parameters of heating and cooling and is determined by the mechanisms and kinetics of α → γ phase transformations under nonequilibrium conditions. By changing the heating parameters (heating rate, holding time), one can control the level of homogeneity and the size of austenite grains at the beginning of the γ → α transformation, and, by changing the cooling rate, one can control the dispersion level of martensite in the reverse γ → α transformation, therefore, the properties of steel. The study results show the fact that in order to obtain the maximum possible hardening, it is necessary to carry out a preliminary heat treatment, which could provide fine austenite grains by the beginning of the α → γ transformation with a more stable structural state of the initial phase in combination with the crystal structure defects.