(V) 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 is the aim of this work. Concerned parts are stopper (SUS303, 0.25 wt. % Molybdenum) and position ring (1.4305, 0.4 % Mo). Current parts had carburized thickness of 21.3-24.1 μm, hardness HV 914-959.

New carburizing and pickling are developed to reduce process time 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 resolve easily by hydrogen during carburizing and subsequently enable activated carburizing. In conclusion, 1.4305 showed excellent properties, hardness of 911-1059 HV, thickness of 20-25 um incl. satisfaction of weldability.