Carburizing of Dual-Phase Steel

Carburized parts have been usually quenched twice; from above Ac3 of the core for grain refinement, then from above Ac1 for full hardening of the case. This two-step quenching often causes cracking and distortion. In the present work, the applicability of dual-phase steel to case hardening material has been investigated, because these kinds of steels generally have a fine grain size at high temperatures, so that it has been expected to skip the 1st step quenching.1-2%Si-0.3%Cr-0.01-0.25%C steels were prepared for examining the grain size and the mechanical properties after carburizing at 950°C, quenching being made from 850°C and tempering at 150°C. It has been found that the grain coarsening in the core scarcely occurs if the steel has the two-phase structure of α+γ at the carburizing temperature. While the depth and hardness of the case are somewhat reduced by the addition of 2%Si, the core shows the outstanding strength-ductility combination characteristic of dual-phase steel.