Evolution of Steel Surface Layer Structure and Properties during Pulsed Laser Processing

It is established that temperature gradients and thermal stresses appearing in metal zones irradiated with surface melting contribute to liquid convective mixing at a rate of 10 3 cm/sec and also to partial or complete carbide dissolution. As a result, a significant amount of textured metastable retained austenite (40–60%) is fixed, which has high dispersion of the structure (dendrite cross section is 5–9 nm). This leads to anisotropy of the main operating properties of irradiated products and increases wear resistance if the irradiated layer is under compressive stresses during operation. It is established that in the case of thermal deformation laser treatment without melting a steel surface the effects of local plastic deformation appear within irradiated zones, which leads to austenite dynamic polygonization, and after rapid cooling to formation of hereditary fine needle martensite. The physical nature and structural organization of the “white zone” formed around carbides in steel under the influence of pulsed laser radiation is studied. A “white zone” is a fine austenite-martensite structure with a martensite lath size of ~150 nm. Existence within laser-hardened steel of large amount of fine carbides (more than 40%), surrounded by “white zones”, contributes to creation of a special product working surface structural state. This has high microhardness values and is indifferent to external temperature and force loading during operation. Quantitative multifractal evaluation of the structure parameters of irradiated steels is conducted. This makes it possible to designate laser processing regimes in order to obtain structures within product surface layers that are resistant to external loads or adaptable to them.