Effects of Optimized Laser-Ultrasonic Surface Hardening Parameters on Residual Stress and Structure-Phase State of Medium-Carbon Steel

AISI 1045 medium-carbon steel workpieces were selectively hardened by a laser heat treatment (LHT) followed by a multi-pin ultrasonic impact treatment (UIT) to enhance the surface properties. The laser surface hardening was done using a high-power fiber laser and scanning optics mounted in the computer numerical control (CNC) machine. The LHT tests were implemented using a constant temperature strategy. The LHT-processed flat specimens were subsequently peened by the CNC ultrasonic processing with a seven-pin impact head. The paper focuses on the effects of combined laser-ultrasonic surface hardening technology on the hardness, structure-phase, and residual stress state of AISI 1045 steel. Particular attention was paid to analyzing the grain size and residual stress magnitudes under various LHT + UIT conditions. Results have shown that the combined LHT + UIT technique provides a high surface hardness (~60 HRC5) due to the formation of the fine-grained martensitic microstructure. After UIT treatment, compressive residual stress is formed.