Patent ID: 7641742

Claim:
A heat treatment method for steel, comprising the steps of: carbonitriding or nitriding at 810-950° C. a part formed of a steel containing 0.8-1.5 wt % of carbon, 0.4-1.2 wt % of Si, 0.8-1.5 wt % of Mn, and 0.5-1.8 wt % of Cr, the steel having a position exhibiting HRC50 in a hardenability test (JISG0561) apart from a quenched end by at least 12.7 mm; subsequently cooling the part to a temperature range lower than a transformation point A 1 of said steel to provide a quenched part; tempering the quenched part a first time; subsequently heating the part again after tempering to a quenching temperature range not lower than the transformation point A 1 and lower than a temperature used for said carbonitriding or nitriding, to quench the part a second time, and tempering the part a second time after quenching a second time.