EFFECT OF TEMPERING TEMPERATURE ON HARDNESS AND MICROSTRUCTURE OF LASER SURFACE REMELTED AISI H13 TOOL STEEL

Surface hardening was performed by laser surface remelting of AISI H13 tool steel samples using a high power fiber laser. The surface hardened samples were exposed to different tempering temperature of 500 degrees C, 700 degrees C and 900 degrees C in a furnace for one hour and brought back to room temperature in still air and by water quenching. Changes of the laser remelted and hardened layer were investigated in terms of microstructure and hardness before and after exposure to different tempering temperatures. Laser remelting caused mainly dendritic microstructure at the top layer but the dendritic structure of the remelted layer got altered after tempering at high temperatures. Air and water quenching caused almost similar result during tempering of laser remelted layer. The microhardness variations along depth after tempering at different temperatures indicates that the surface hardening imparted by laser remelting remains almost intact up to 700 degrees C but gets destroyed at 900 degrees C. Although the experimental temperature limits gives approximate threshold values, but it provides a clear indication of a safe limit for laser surface hardened components in high temperature applications like hot-forging dies and friction stir welding tool, etc.