The Effect of Initial Microstructure and Hardenability on Diode Laser Surface Hardening of Medium-Carbon Steels

This study systematically investigates the laser surface hardening (LSH) behavior of two medium carbon steels-the low alloy 42CrMo4 and the plain carbon C45-using a 4 kW high power diode laser (HPDL). The influence of laser parameters (power: 3.0-3.8 kW; scanning speed: 10-16 mm/s), post-laser quenching medium (oil vs. air), and, critically, the initial material condition (normalized "raw" vs. quenched and tempered "Q&T") on the case hardening depth (CHD) was evaluated. Hardness profiles defined the CHD at a threshold of 392 HV1, and microstructural analysis was conducted via optical microscopy. The results demonstrate that prior conventional Q&T heat treatment of 42CrMo4 enhances the subsequent laser-hardened depth by approximately 27% compared to laser treatment of the normalized material under identical parameters, providing a quantitative basis for process optimization. For Q&T 42CrMo4, the quenching medium had an insignificant effect on CHD, with air cooling proving equally effective as oil across the tested parameter range, offering an empirically validated route for sustainable processing. In contrast, C45 exhibited a substantially lower and less parameter-sensitive CHD, constrained by its inherent low hardenability. This comparative analysis underscores that hardening depth in 42CrMo4 is linearly controllable via energy input, whereas for C45 it is hardenability-limited. This work establishes that an integrated approach combining conventional bulk heat treatment with diode laser hardening using air cooling offers a highly effective, controllable, and sustainable surface engineering route for high-performance alloy steels.