Formation of a gradient-layer structure in structural steel during plasma hardening

Abstract

The structural features of structural steel in the process of surface plasma hardening have been studied. It is shown that during rapid heating and cooling in the surface zone, a gradient-layered structure is formed, determined by various mechanisms of austenite transformation (γ→α) in the section of the hardened zone. It has been established that a diffusion-free martensitic transformation develops in the surface zone, which leads to the formation of acicular martensite. In the lower layers, the decomposition of austenite is accompanied by the process of diffusion and the further formation of a lamellar ferrite-carbide mixture of varying degrees of dispersion. It is shown that the higher the cooling rate, the more disperse the resulting ferrite-carbide mixture, the smaller the interplate distance. This leads to the fact that with an increase in the degree of dispersion, the strength characteristics (strength - σ in hardness - HB) increase, and the plastic characteristics (relative elongation - δ and contraction - ψ) decrease.

It is noted that the formation of a gradient-layered structure in the surface layer of steel makes it possible to eliminate the formation of a sharp transition boundary from martensite structures to trooste-martensitic and mixed lamellar structures (troostite, sorbite), which is one of the important factors by which contact fatigue strength can be increased steel will continue to contribute to its crack resistance.