Carburizing Steel for High Temperature Service.

A prime concern in airborne equipment is to avoid a brittle fracture in gearing or bearings which could lead to catastrophic engine or propulsion system failure. Projected requirements for advanced aircraft and helicopters suggest that currently employed through-hardened materials will no longer be adequate for these applications due to their low fracture toughness. Hence there is an interest in carburized materials which have somewhat similar surface characteristics to the through-hardened materials but inherently higher core toughness due to the lower core carbon levels. Five steels similar in composition to CBS1000 and a low carbon M50 composition were evaluated with respect to carburizing characteristics, temper resistance, hot hardness and carburized fracture toughness. Si, Mo and Ni levels were varied in an effort to identify a composition that would maintain a surface hardness of 58 HRC minimum at 315 C 600 F without a deterioration in fracture toughness properties. Si and Ni were both shown to retard carburization but have little influence on hardness retention or fracture toughness. A composition with 2.3 Ni was shown to have optimum carburizing, hardness and fracture toughness properties. The modified steel showed an improvement over CBS1000 in case fracture toughness but the core fracture toughness was lower than that of CBS1000. This steel was tested in rolling contact fatigue and found to be similar in performance to through-hardened M50.