Effect of Tempering Temperature on Laser Transformation Hardened 8620 Steel
P. G. Ranaware
Laser transformation hardening (LTH) is the efficient method of surface hardening used for synthesizing a hard, wear resistant surface layer on parts through the ultrafast heating action of the high energy scanned laser beam. Initial microstructure plays a very significant role on nucleation and growth kinetics of austenite formation during ultrafast heating of laser treatment. The conventional hardening and tempering heat treatment can produce very interesting microstructure as initial microstructure for LTH. In this work the effect of the tempered structure as the initial structure on LTH 8620 steel was investigated. It was observed that initial tempered structure significantly affects the surface hardness and hardening depth after LTH. It was observed that microstructure containing more martensitic features and lesser carbides produces coarser martensitic structure with shallow hardened case depths. On the other hand, microstructure containing more ferritic structure and high-volume fraction of carbides showed finer martensite with deeper hardened case depths. LTH of all tempered specimens shows that with increase in tempering temperature surface hardness decreases. As compared to conventional hardening by quenching, it was observed that surface hardness increased by 52 to 128% for LTH. After LTH about 15 to 22% drop in surface hardness was observed with increase in tempering temperature for different scanning speeds. On the other hand, laser transformation hardened depth and laser transformation hardened width are observed to rise for all scanning velocities with tempering temperature. After LTH, laser transformation hardened depth is observed to increase by 18 to 24% whereas laser transformation hardened width is observed to rise by 12 to 43% when tempering temperature is raised from 205 to 705oC.
Keywords: Fibre laser, 8620 steel, laser transformation hardening (LTH), microstructure, tempered structure, ultrafast heating