Studies on Wear Behaviour and DBTT in Sub-zero Regimes of Cryo-Treated High Nitrogen Martensitic Stainless Steel (HNMS)

The development of space engines with liquid propellants require use of highly engineered bearings. High nitrogen martensitic stainless steel (HNMS) is the newly developed steel and is the potential candidate for anti-friction bearings in cryogenic engines in launch vehicles. The present paper aimed to assess the effect of cryogenic treatment on HNMS steel at sub-zero temperature regimes. HNMS steel specimens were treated using optimised heat treatment cycle comprising of hardening at 1075 degrees C, cryogenic treatment at - 196 degrees C for 8 h followed by soft tempering at 100 degrees C for 1 h. The specimens were tested for hardness, wear loss (test load 20-80 N), surface roughness, and impact toughness from room temperature (RT) to the cryogenic temperatures. Worn surfaces, subsurface features, and fractured surfaces were analysed using FE-SEM. It was established that the HNMS steel showed reduced wear rate by 28% and 60% for 20 N and 80 N wear test load, respectively, at - 196 degrees C as compared to the RT. Severe delaminated wear mechanism was observed at RT as compared to mild adhesive wear at - 196 degrees C. Impact testing revealed that the DBTT for HNMS steel lied between - 30 and - 60 degrees C.