Effect of processing parameters on the microstructure and hardness of laser transformation hardened tool steel

Laser transformation hardening of tool steel containing 0.64 wt% carbon and 4.48 wt% chromium was studied. The delivery condition of the steel was annealed, with a microstructure of M7C3 carbides in a matrix of ferrite. Hardening was performed with a direct diode laser. Surface temperature during hardening was measured with a dual-wavelength pyrometer and the traverse speed was varied between 1.1 and 33.3 m s−1. A set of samples was quenched in liquid nitrogen to reveal the effect of retained austenite on the resulting hardness. Ac1 isotherm depths were measured from all samples. Volume fraction of retained austenite and carbide dissolution were estimated from thermodynamic considerations.The surface hardness was greatly influenced by the dissolution of carbides during austenitization. The dissolution of chromium carbides requires a high temperature and is limited by chromium diffusion. Highest hardness values were achieved with slow traverse speeds and high surface temperatures for both normal and sub-zero hardened samples. Highest measured hardness in samples, in which the surface was cooled by self-quenching, was 822 HV and in samples quenched in liquid nitrogen 926 HV. A good agreement was found between the carbide dissolution and characteristic diffusion distance of chromium