Effects of the Combined Laser-Ultrasonic Surface Hardening Induced Microstructure and Phase State on Mechanical Properties of AISI D2 Tool Steel

The surface layers of AISI D2 tool steel were hardened by a laser heat treatment (LHT), by an ultrasonic impact treatment (UIT) and by a combined laser-ultrasonic treatment (LHT + UIT). The peculiarities of microstructure and phase formations in the surface layers were analyzed after the above-mentioned surface treatments performed in the optimum regimes. Microstructural changes were studied using an optical and a transmission electron microscopy to corroborate the results of XRD analysis. Based on the experimentally obtained data regarding the grain/subgrain size, the dislocation density, and the volume fraction and size of carbides, the differentiated contributions of various hardening mechanisms to the mechanical characteristics (sigma(0.2), HV) were theoretically assessed. The results indicated that the contribution of the grain boundary hardening is the most influential one among the hardening mechanisms in the cases of the LHT (similar to 47%) and combined LHT + UIT (similar to 51%) processes. Conversely, the dislocation hardening (similar to 34%) mainly contributes to the UIT induced hardening. The yield strength values calculated based on the microstructural studies correlate well with the experimental data describing the surface microhardness.