Glazing of tool dies for semi-solid steel forming

This paper describes an analysis conducted into the effect of CO2 laser processing parameters on the surface modification and heat treatment of steels. The CO2 laser and in house developed apparatus for movement of the samples allowed for high speed surface heat treatment. The operation of these components in conjunction allowed a wide variation in the heat treatments and sample properties to be obtained. In particular, the mechanism to pass the steel samples at high speeds (700–1200 mm/s), kept the focus spot exposure times below 1 × 10−3 s. Laser power from 825 to 1050 W and continuous beam mode were used. Both laser hardened and laser glazed samples were analysed using optical microscopy, Vickers and micro hardness testing, and XRD. The samples showed functionally graded surface morphologies and corresponding hardness and stiffness responses. For more rapidly process samples three layers of distinct hardness were noted. In all cases crystal and property modification to approximately 500 μm was found. Results show how the laser treatment is directly related to the hardness profile through the surface and resultant microstructures. The results from all the samples show increased hardness due to (a) the formation of martensite and ferrite in the laser hardened samples and (b) a complete microstructural transformation to an amorphous state in the glazed samples. It is noted that the higher stiffness and hardness of the glazed samples can expect to result in a more wear resistant coating for the semi-steel forming process. The amorphous layer may also be expected to act as a thermal barrier to the higher temperatures encountered in semi-solid steel forming.