Patent ID: 8608870

Claim:
A vehicle workpiece carburized for surface hardening, comprising: a surface having a mixture of martensite and residual austenite; and a core having a mixture of 30˜50 wt % of ferrite and a balance of martensite, in which the ferrite contains at least 30% of initial ferrite not subjected to phase transformation during carburization heat treatment; wherein the carburization heat treatment includes using a vacuum carburizing furnace carburization within a temperature range of A 1 ˜A 3 during carbonizing and then quenching, and wherein the vehicle workpiece is an annulus gear for a transmission made of a steel material selected from the group consisting of: SCr420H, SCM420H, SNCM420H, ASTM 4120, ASTM5120, and ASTM8620.