Effect of preheat-treatment during laser hardening on residual stress formation in high-chromium irons

Abstract

The distribution of residual stresses formed on the surface of high-chromium iron with 16% Cr under the influence of cw laser radiation is studied as the function of the initial state of the iron and the laser treatment parameters. It is revealed that on the surface of both cast and heat-treated iron under the influence of the laser action, residual stresses of a tensile nature are formed. The stresses are greatest at the edges of the solidified melt zone and, in the hardened zone, they are greatest in the central part. The distribution pattern of the residual stresses corresponds to the structure phase transformations taking place. The highest level of the residual stresses is in the hardened zone consisting of martensite, austenite and carbides (Cr,Fe)7C3. It reaches the value of 2140 MPa in the cast iron and 450 MPa in the heat-treated iron. In the latter, a great number of cracks are observed. Their formation is accompanied by a significant relaxation of stresses. Preheating up to 400 ‡C at the optimum structure of the hardened zone avoids crack formation under laser influence. In this case, the level of residual tress is about 440 MPa, which is comparable with the case of crack-formation with no preheating.