Resistance of laser hardened steel structures to softening during heating

Abstract

Metallophysical study of heat resistance of laser irradiated steels in comparison with their heat resistance after bulk hardening under standard conditions is presented. It is shown that the main conditions for increasing the heat resistance and operational properties of surface layers in laser irradiated materials are related to features of their structural state. These features include a fragmented texture of basic phases and an increased density of defects in the crystal structure of solid solutions. The impact of laser hardening on the heat resistance of steels is characterized by a reduction in the rate of decomposition of martensite. The main reason for this is the formation of segregations (clusters, atmospheres) of carbon atoms and alloying elements on dislocations in the α‑solid solution lattice. Additionally, these segregations exhibit stable existence at higher heating temperatures in comparison to structures obtained by bulk hardening. It is shown that after laser treatment, additional possibilities for improving the set of properties of the irradiated metal are provided by multiple formation of nano-sized carbide precipitates on dislocations during thermal action, i.e. nanoprecipitation. Laser irradiation also reduces the tendency of carbide phase precipitates to coagulate and slows down the rate of hardness decrease with the increase of heating (tempering) temperature. The texture effects observed in austenite and martensite in laser hardened steels persist when heated to high tempering temperatures. These effects lead to anisotropy of properties, particularly a significant reduction in the coefficient of friction in tribocouplings. Pulsed laser treatment of steels allows the hardness of irradiated areas to increase to 8–11.5 GPa and heat resistance to improve by 50–120 °C. This contributes to the enhanced performance of irradiated products.