Gradient Layer Structure Formation during Plasma Treatment of Wheel Steel

Mechanism and kinetics of structure formation across the section of the hardened zone under surface plasma hardening of wheel steel are considered. It is shown that depending on the cooling rate and temperature conditions of austenite decomposition perlite and martensite, transformation takes place with formation of gradient layered structure leading to the surface steel layer structure upgrading at constant chemical composition, structure and properties of the central layers of the processed product. Diffusion-free martensite transformation develops in the surface zone, which leads to the formation of needle martensite. In the underlying layers, austenite decomposition flows by diffusion way and is followed by the formation of a plate ferrite-carbide mixture of different degrees of dispersion. The increase in cooling rate leads to a strong crushing of the structure characterized by increased dispersibility of ferrite-carbide mixture, which results in increased strength and reduced plasticity of the steel. The formation of a gradient-layer structure of plasma-strengthened wheel steel in the surface layer makes it possible to eliminate the formation of a sharp transfer boundary from martensite structures to tropostite-martensite a