Stress corrosion study of carburized AISI 9310 and carburized M-50 NiL steels

摘要

A stress corrosion cracking (SCC) of carburized AISI 9310 and carburized M-50 NiL steels was undertaken. AISI 9310 is a substitute to 440 C for the bearing inner race in the Space Shuttle Main Engine Alternate Turbopump Development Program. M-50 NiL may also find applications as a bearing alloy. Round tensile specimens of these alloys at several stress levels, as well as corrosion samples of AISI 9310, were exposed to 100 percent relative humidity at 38 C. The maximum tensile stresses that produced no SCC failures in one year of exposure were 172 MPa (25 ksi) for AISI 9310 and 345 Mpa (50 ksi) for M-50 NiL. Each AISI 9310 steel fracture showed a circumferential crack between the case and the core. Both alloys developed rust on the surface and pitting. However, AISI 9310 rusted to a greater extent than M-50 NiL. Thin dense chrome (TDC) was also evaluated in this program as a corrosion barrier alternative; however, its protection was minimal. Corrosion spots visible to the naked eye appeared in less than 5 days of exposure. Although TDC plating on test specimens prevented the intensive corrosion attack which occurred in bare samples after a long time exposure, the TDC plating did not offer sufficient protection to avoid failures. The results obtained here must be carefully considered when designing bearing components made of these materials.