Effects of silicon and boron additions on the susceptibility to quench embrittlement and the bending fatigue performance of vacuum carburized modified 4320 steel

The effect of B and Si additions on fracture and fatigue performance of vacuum carburized 4320 steel and modifications of 4320 steel containing additions of Si (1.0 and 2.0 wt pct) and B (0 and 17 ppm) was evaluated by bending fatigue testing. Three rates of gas quenching, in 10 bar nitrogen and 15 and 20 bar helium, were used to cool specimens after carburizing. The B, protected by Ti additions, together with the Si additions, increased core hardenability. The B/Si modified steels showed no improvement in fatigue resistance, as measured by endurance limits established by 10 million cycle runouts without fracture. However, scanning electron microscopy showed that Si reduced sensitivity to intergranular fracture or quench embrittlement, a major cause of bending fatigue crack initiation, and contributed to variable fatigue performance, with both low-cycle failures and runout performance at applied stresses significantly above measured endurance limits. The vacuum carburized steels containing 2 wt pct Si showed appreciable grain growth relative to the other steels, a result that contributed to low fatigue resistance. Results are presented and discussed relative to the effects of steel chemistry and gas quench rate during vacuum carburizing.