Effects of Molybdenum on Hardening Properties of Stainless Steels By Low Temperature Vacuum Carburizing and Pre-Treatment

To develop low temperature vacuum carburizing for next generation of injector parts was the aim of this work. Concerned parts were stopper (SUS303) and position ring (1.4305) with current properties of carburized thickness of av. 22.7 μm and hardness of HV0.05 av. 935. New carburizing and pickling were developed to reduce the cost and improve product quality. Especially, both steels showed different pitting and oxide regeneration behavior during pickling due to the composition difference. In the case of SUS 303, MnS inclusions at the surface led to severe MnS pitting,_in contrast, 1.4305 formed Mo-oxides on the surface_during pickling, which the excessive pitting by MnS inclusion_site was prevented, confirmed by OM, SEM-EDX_and XPS. Furthermore, these oxides resolved easily by_hydrogen during carburizing and subsequently enabled activated carburizing. In conclusion, 1.4305 showed excellent properties, hardness of 985 HV0.05, thickness of 23 μm_incl. satisfaction of weldability.