Features of Steel Structure Formation in Areas of High-Speed Laser Hardening from Liquid State

Features of structure organization during pulsed laser melting of steel surface layers are studied. Based on analyzing crystallization process rate kinetics it is established that temperature gradients and thermal stresses developing in irradiated metal areas during melting contribute to convective mixing of liquid at a rate of 10³ cm/sec, and also, despite an extremely short laser pulse time (10^–3 sec) they contribute to partial or complete dissolution of the original structure carbides. As a result, the chemical composition of melted metal changes, martensitic transformation points are lowered and a significant amount of textured metastable residual austenite (40–60%) is fixed that has a very fine structure (dendrite cross-section 5–9 nm). This has a favorable effect on irradiated material surface layer quality and mechanical properties. It is established by experiment that with a change in laser processing regime it becomes possible to achieve a wide range of cooling rates during crystallization, i.e., to control the degree of carbide dissolution, structural characteristics, and melted metal area properties. It is demonstrated that in order to achieve the maximum possible degree of hardening for irradiated steels having different chemical composition, it is necessary to dissolve 30–60% of carbides in the initial metal. As a result of this after laser irradiation material heat resistance increases by 50–100°C, and wear resistance increases by a factor of 2–4.