D.A. Lesyk, S. Martinez, V.V. Dzhemelinskyy, A. Lamikiz, B.N. Mordyuk, G.I. Prokopenko

Abstract
This paper is focused on experimental analysis of the effects of laser surface hardening (LSH) combined with subsequent ultrasonic impact treatment (UIT) on the surface microrelief, hardness and microstructure in near-surface layers of AISI D2 high-chromium, cold worked tool steel. The LSH provides fast heating of the near-surface layers to the temperature above that of the phase transformation and temperature then rapidly cools by a self-quenching process. The formed heat affected zone is hardened thanks to the rapid heating/cooling process affecting microstructure, phase composition and carbide formation. Conversely, the UIT induces multiple impact loads providing severe plastic deformation of near-surface layers, and the hardening occurs by a dislocation mediated process. The optimal parameters of each process were determined to obtain maximum hardness and regular surface microrelief. Further, complete analysis of the effect of combined treatment (LSH + UIT) on the surface hardness, microhardness depth profile and surface microrelief was performed. Results show that the combined LSH + UIT process provides almost triple, double and a 10% increase in hardness in comparison with those of the initial, UIT-processed and LSH-treated states, respectively. The surface microrelief, waviness and roughness parameters were respectively diminished after LSH + UIT by approx. 50, 65, and 90%. XRD analysis was carried out after LSH and LSH + UIT processes, which showed essential α-Fe peak broadening due to the formation of microstrains (0.27% and 0.47%, respectively) and reduction in crystallite size (84 nm after LSH + UIT). Favorable compressive residual stresses (− 205 MPa and − 409 MPa, respectively) were also observed in the near-surface layers of ~ 350 and 80 μm thick, respectively. The obtained results demonstrate that the combined LSH + UIT process is a feasible surface treatment for the quality improvement of the tool steel components including both the surface microrelief and hardness characteristics.

Keywords
•    Laser surface hardening;
•    Ultrasonic impact treatment;
•    Surface roughness;
•    Surface waviness;
•    Hardness;
•    Tool steel

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