Abstract
The carburizing time is expected to be short at high temperatures, where grains of the dual-phase steel are fine and hardly grow. If it can be used for the elevated temperature case hardening steel, the processing time might become short by reducing carburizing time and by skipping the first step quenching for grain refinement of the un-carburized core. In the present study, the applicability of the dual-phase steel for the elevated temperature case hardening steel has been investigated.
The 2.0 mass% Si-(0.01-0.08) mass% C-(0.1 mass% V, 0.05 mass% Nb and 0.05 mass% Ti) steel were carburized at 1323 K for 10.8 ks, quenched from various temperatures and then tempered at 423 K. The microstructural observation, hardness measurement and tensile test of the core were carried out. When the microstructure was controlled to be dual-phase with α+γ at the carburizing temperature, the grains were fine and no grain growth was observed during the carburizing process. Therefore, the first step quenching for grain refinement might be skipped. The core showed the excellent strength-ductility combination compared with the conventional case hardening steel. The distribution of hardness in the carburized layer showed a gentle gradient due to the elevated temperature carburizing. The carburizing time was reduced to be 1/ 3 compared with the conventional carburizing. The grains in the carburized layer were also refined by additions of V, Nb and Ti.
