Effects of laser heat treatment combined with ultrasonic impact treatment on the surface topography and hardness of carbon steel AISI 1045

Abstract

The surface layers of medium-carbon steel AISI 1045 were hardened by laser heat treatment (LHT) and by ultrasonic impact treatment (UIT) applied separately and in different sequences. The advantages of the two latter combined laser-ultrasonic hardening and finishing methods (the LHT + UIT and UIT + LHT combined treatments) are also analyzed in this work. The laser transformation hardening process was implemented using a 1 kW solid-state fiber laser with scanning optics and a proportional integral derivative (PID) closed-loop temperature control. The ultrasonic strain hardening process was carried out by means of a 0.3 kW ultrasonic generator and an ultrasonic oscillatory system, which contained a piezoceramic transducer, step-like horn and the impact head with seven cylindrical pins. The single-path processes were studied. The separately applied LHT and UIT process were firstly analyzed using a response surface method (RSM) and the analysis of variance (ANOVA). The effects of the heating temperature (1050–1300 °C) and the specimen feed rate (40–140 mm/min) used at the LHT process on the features of the hardened zone and surface hardness/microhardness of subsurface layer were assessed. The effects of the vibration amplitude of ultrasonic horn (15–18 μm) and treatment duration (60–240 s) at UIT on the surface roughness/waviness and hardness were also investigated. The experimental plan is based on a miscellaneous design matrix method. The quadratic regression equations for predicting the studied output parameters were developed and the optimum regimes of the LHT and UIT processes were determined based on the highest surface hardness and minimum roughness/waviness. Then, the influences of the combined treatments on the surface topography and hardness were analyzed and compared to those of single LHT and single UIT processes. Results show that the combined treatments provide more than triple increase in the surface hardness in comparison with that of the initial state, as well as formation a regular microrelief with minimum surface roughness.