Influence of the initial structure on the efficiency of laser processing of steel

According to experiments, the initial martensitic structure with inherent carbon content is most suitable for high-speed laser hardening during preliminary volumetric heat treatment, releasing a suitable amount and dispersion of carbides during tempering. When choosing a volumetric heat treatment mode, it is necessary to consider that the α→γ transformation during laser heating occurs via a shear mechanism with the hereditary transfer of carbon atoms and alloying elements, as well as defects in the crystal structure, to austenite from the original martensite. High-temperature laser-hardened austenite has a fragmented structure with subgrains 200–300 nm in size. This feature is due to the dynamic polygonization of austenite, which occurs because of the superposition of dynamic thermostrictive effects in the laser processing zones on phase hardening during the α→γ transition. After high-speed cooling, the irradiated zones contain hereditary fine-needle martensite with high-strength properties, a certain amount of textured residual austenite, and a carbide phase. It has been established that the strengthening of irradiated steel, which occurs under conditions of thermal deformation of laser radiation, is associated, among other things, with the formation of nanosized zones at the carbide steel matrix composition interface, which have a dispersed structure and increased hardness (10–12 GPa). Metallophysical and urometric studies showed that the maximum hardness of laser-hardened steels is achieved if dispersed carbides occupy more than 40% of the irradiated area and have dimensions of 0.5–1.5 μm. A reasonable choice of the initial steel structure and laser processing mode enabled obtaining a structural state of surface layers on the working surfaces of products having high and predictable performance properties.