A new heat treatment, where induction hardening is carried out after carburizing, was studied to increase the bending strength of automotive differential gears. The modified JIS SCM822 grade was prepared and was cold-forged to form the differential gears, which were plasma-carburized, induction-hardened and low-temperature-tempered. The bending test was carried out using an automotive differential unit.
In the hardened case, massive hyper-eutectoid cementite, which was produced during the carburizing and remaind during the induction heating, was observed along the austenite grain boundaries. Although the austenite grain was refined by induction heating, bending strength decreased with an increase in the amount of undissolved cementite. At the fracture origin, an intergranular fracture containing undissolved cementite on its surface was observed.
These results indicate that the increase of the massive undissolved cementite decreased the strength between the austenite grain boundaries, and then diminished the contribution of grain size refinement to the increase of bending strength.
The formation of hyper-eutectoid cementite during carburizing should be minimized, since this cementite cannot be fully dissolved by induction heating.
