EFFECT OF P AND C SEGREGATION AND GRAIN-S IZE ON BENDING STRENGTH OF THE CARBURIZED AND INDUCTION HARDENED STEEL

In order to increase the bending strength of automotive differential g ears, a new heat treatment, where induction hardening is carried out a fter carburizing, was studied. The bending test specimens of a modifie d JIS SCM822 grade were gas-carburized, followed by quenching, inducti on 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 boundari es, which is the cause of the case intergranular embrittlement of carb urized steels. The calculation of the diffusion distance indicated tha t the duration of the induction heating was too short for phosphorus a nd 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 indu ction hardened steel increased the bending strength by the amount of 3 6 per cent.