Effects of laser process parameters on the hardness profile of AISI 4340 cylindrical samples: statistical and experimental analyses

In the present paper, continuous Ytterbium Laser Systems (YLS) fiber laser hardening of cylindrical AISI 4340 steel specimens was studied using experimental and statistical analyses. Three laser parameters, namely laser power, laser feed speed, and sample rotation speed were selected to evaluate their influence on the depth of the hardened zone and the maximum surface hardness. Mathematical models were developed as a function of these three parameters and the analysis of variance (ANOVA) was used to conduct the statistical study. Microhardness measurements revealed three distinct regions in the heat-affected zone (HAZ) of all samples: the hardened zone (Z1) near the surface, with the highest value of hardness, the hardness loss zone (Z2), where hardness had started to decrease, and the overheated zone (Z3) adjacent to the core, with hardness values that were less than those of the base metal. Based on experimental measurements, a maximum surface hardness of 60.8 HRC was attained. Furthermore, the maximum depth of the hardened zone was observed as 500 µm. The microstructures of laser-hardened samples were studied using optical and scanning electron (SEM) microscopes. The hardened region appeared to have a hard martensitic microstructure. By comparing the predicted and measured data for maximum microhardness values, it was revealed that the models represent the experimental values with correlations close to 100%.