EFFECT OF UNDISSOLVED CEMENTITE AND GRAIN -SIZE ON BENDING STRENGTH OF THE CARBURIZED AND INDUCTION HARDENED STEEL

A new heat treatment, where induction hardening is carried out after c arburizing, was studied to increase the bending strength of automotive differential gears. The modified JIS SCM822 grade was prepared and wa s cold-forged to form the differential gears, which were plasma-carbur ized, induction-hardened and low-temperature-tempered. The bending tes t was carried out using an automotive differential unit. In the harden ed 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 wa s refined by induction heating, bending strength decreased with an inc rease in the amount of undissolved cementite. At the fracture origin, an intergranular fracture containing undissolved cementite on its surf ace was observed. These results indicate that the increase of the mass ive 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 thi s cementite cannot be fully dissolved by induction heating.