Effect of P and C Segregation and Grain Size on Bending Strength of the Carburized and Induction Hardened Steel

Abstract

In order to increase the bending strength of automotive differential gears, a new heat treatment, where induction hardening is carried out after carburizing, was studied. The bending test specimens of a modified JIS SCM822 grade were gas-carburized, followed by quenching, induction hardening and low-temperature-tempering.
Induction hardening after carburizing prevented intergranular fracture of the case. The results of Auger electron spectroscopy showed that induction heating decreased the phosphorus and carbon segregation at the austenite grain boundaries, which is the cause of the case intergranular embrittlement of carburized steels. The calculation of the diffusion distance indicated that the duration of the induction heating was too short for phosphorus and carbon to resegregate fully to the austenite grain boundaries which were reproduced during the heating.
Such reduction in segregation and the austenite grain refinement in the case of the carburized and induction hardened steel increased the bending strength by the amount of 36 per cent.