Residual stress distributions in laser transformation hardened steels

Smooth and notched fatigue specimens, made of steels equivalent to AISI 414 0 and AISI 4130, respectively, were surface hardened at the gauge section b y laser scanning. The hardening results were evaluated by metallurgical stu dies, mainly microstructure classifications and hardness test. The martensi tic phase was found to be predominant in the hardened zone. The residual st ress field induced by laser hardening was studied by the X-ray diffraction method. Compressive stresses were found in the hardened zone of all specime ns, while tensile stresses in the core. The results clearly showed that the characteristics of the residual stress field depended on the material, the specimen type, and the laser-hardening condition including the processing parameters and the scanning pattern. By carefully selecting the laser scann ing pattern, a large surface area may be hardened by multi-pass scan withou t substantial tempering in the overlapped zone and with high compressive st resses as demonstrated by hardening of the smooth specimens with a spiral t rack.