A computer simulation study of the effects of temperature change rate on austenite kinetics in laser hardening

The paper presents a modelling of the laser hardening process by a high-power diode laser (HPDL). Through numerical implementation into the finite element method (FEM) code ABAQUS, the model is used in the computer simulation of two case studies of laser hardening selected for experimental validation. In the experiment, 100 x 100 x 15 mm cuboid samples made of 50CrV4 steel were subjected to laser hardening with significantly different sets of applied technological parameters (laser beam power, laser beam velocity) but still aiming at attaining a comparable maximum temperature on the sample surface. The simulation considers two alternative approaches to microstructure evolution and subsequent material hardness determination: one relying on the heating rate dependent austenitisation temperatures (Ac-1 and Ac-3) governing microstructure transformation kinetics and the other neglecting heating rate dependence. Physical objectivity of the computed results is verified based on the corresponding temperature field measurements on the sample surface during heat treatment process and hardness measurements through the thickness of the laser-hardened sample. The experimental validation clearly proves that considering austenite kinetics at a high temperature change rate in computer simulation is definitely more physically congruent. In the study of the applied process parameters impact, the effect of a higher temperature change rate on austenite kinetics is shown by the temperature shift of austenite and ferrite to austenite start formations. From the investigation of the effect of different heat inputs providing the same maximum temperature on the sample surface it results that deeper area of increased hardness is established when less laser beam power and velocity are applied. (C) 2015 Elsevier B.V. All rights reserved.