Physical principles of simulation and optimization of laser-induced surface hardening of steels

Abstract

beam. This paper contains a theoretical analysis of the physical model of processes in laser steel hardening without melting the surface, and on its basis the problem of the proper choice of a l£ser thermohardening regime is discussed. The heat treatment of steels is based on different abilities of the high-temperature ('/-phase) and the lowtemperature (a-phase) modifications of iron to dissolve carbon and alloying elements. The carbon solubility in a-iron is of 0.01%. The upper limit of the carbon solubility in 7-iron is 2%. The constructional steels contain from 0.3 to 0.7% of carbon, while the carbon content in tool steels is from 0.7 to 1.3%. Excess carbon in these steels precipitates as iron carbide, so a homogeneous mixture of ferrite crystals (0.01% solid interstitial solution in a-iron) and plate cementite (iron carbide FeaC) is formed. When a steel with a carbon content, e.g., of 1%, is heated to the temperature of the a-- 7 transformation and then kept for some time at this temperature, all carbon will pass into solution. The state of solid carbon solution in the 7-phase is referred to as austenite. The process will be reversed on cooling, i.e., 7-Fe transforms to a-Fe, whereas the excess carbon precipitates as iron carbide. A different situation arises in the presence of fast cooling. The diffusion of atoms, which is necessary for the crystal lattice to be rearranged and for carbon to precipitate, has no time to occur. At a temperature below 200 °C an extended (tetragonal) crystal lattice is formed like in the a-phase, though it contains an amount of carbon possible only in the 7-phase. This basic component of the hardened steel is ca~ed martensite. The formation of martensite is accompanied by a reduction in the size of the original metal grains to smaller blocks having size <300 ~. The emergence of a great number of new interfaces prevents the motion of dislocations. This motion is also retarded by the carbon atoms in the supersaturated solution and also by the iron carbide particles being formed at block boundaries. This is the physical reason for the high hardness and strength of the martensite. Nevertheless, the martensite is not only hard, but brittle as well, because of the high internal stresses produced in the crystal lattice during the martensite transformation. For the tool steel to be free of excessive brittleness, it can be reheated to 150-200 ° C (tempering). Martensite partially decomposes during tempering, but the hardness of the metal remains sufficiently high, and brittleness is eliminated. As to the constructional steels, not only the hardness is of importance, but also the plasticity and ductility. In this case high tempering is used, i.e., heating to temperatures of 500-600°C at which the martensite decomposes completely. The