Measurement and analysis of residual stresses after laser hardening and laser surface melt-hardening on flat specimens

The measurement of residual stresses is very important in exacting dynamically loaded machine parts which have been subjected to different kinds of heat treatment. Designers in designing parts very frequently demand the presence of compressive residual stresses after heat treatment and finish grinding of the surface, since this increases the fatigue strength of the material and reduced the danger of fracture. To investigate the residual stresses after laser surface hardening and laser surface melt-hardening, the size and variation of residual stresses were measured as a function of the hardened layer depth on flat specimens from different kinds of steels, gray and nodular irons. Optimal laser hardening conditions were chosen, while only the way of guiding the laser beam over the surface of flat specimens was varied. To measure the residual stresses, the relaxation method was used, involving gradual electro-chemical removal of the hardened layer in which the deformation of the specimen was measured by resistance strain gauges. On the basis of the measured deformation at a specific depth of material removal, it is possible to calculate the size or variation of residual stresses depending on the depth of the hardened layer. The results of the measured residual stresses confirm that the stresses strongly depend on the laser heat treatment procedure (hardening, surface melt-hardening) and on the way the laser beam in guided over the surface of flat specimens (overlapped hardened traces/single hardened traces).