The Physical Nature and Structural Organization of Steel Surface Layers Formed Under Thermal Deformational Effect Caused by Pulsed Laser Radiation

Abstract

 During pulsed laser processing and modification of the surface of steels and alloys, several still unresolved issues arise related to the structure formation and property development in the irradiated zones. The aim of this article is to study and scientifically analyze the features in phase transformations, structure and properties formation of the steels surface layers under variable modes of high-speed laser heat treatment. Metallophysical studies in the work were carried out on samples of carbon and alloy steels subjected to pulsed laser irradiation using a technological device Kvant-16 with a radiation power density of 50–250 MW/m2. The identification of the phase composition and the materials structure study were carried out by several methods: metallographic, X-ray, scanning probe microscopy, hardness measurement.It is shown that the locality of laser treatment, a high heating rate and high temperature gradients along the layer depth led to the thermoelastic stresses appearance, which play the main role in the steels surface hardening. The physical nature and structural organization of the “white zone” formed around carbide inclusions in steel have been established by metallophysical studies. The main part of the "white zone" is a nanosized austenite-martensite structure with a martensite lath size of ~150nm, which has anomalously high values of microhardness.It has been established that under the conditions of pulsed laser radiation thermal-deformation effect during the treatment of steels and alloys without surface melting, the main role in structure formation and formation of properties is played not by diffusion processes, but by processes of local plastic deformation. It is shown that the presence in laser-hardened steel of a dispersed carbides large amount (more than 40% of the irradiated volume of steel), surrounded by "white zones", contributes to the creation of a structural state on the working surfaces of products with high microhardness values.