Optimization of Cryogenic Process for Enhancing the Wear Resistance of Novel HNMS Steel for Cryogenic Bearings of Launch Vehicle

Abstract

Bearings are highly engineered components that have to tolerate vibratory stresses, bending moments, and high rotational speeds in cryogenic applications. AISI 440C is most widely used for antifriction bearings due to its good strength and high hardness but is prone to carbide banding and failure during service. High-nitrogen martensitic stainless steel (HNMS) was recently developed by partially substituting carbon with nitrogen. This article explores the possible improvement in wear properties by cryotreating HNMS steel. The hardening temperature was varied from 1050 to 1100 °C, cryosoaking period from 0 to 12 h at −196 °C, and tempering temperature from 100 to 300 °C for 1 h. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis revealed the presence of carbides and carbonitrides. The phase fraction and size of the precipitates was found to be influenced by the cryosoaking period. The carbide density variation with the cryosoaking period followed a sigmoidal curve. Eight hours of cryogenic treatment at −196 °C was observed to be the optimal cryosoaking period for HNMS steel. Maximum hardness (671 HV) and minimum wear rate (0.00714 mm3/m) and adhesive wear were observed for the optimum heat treatment condition. Compared to conventional hardening-tempering (HT) treatment, the optimized hardening-cryogenic-tempering (HCT) treatment cycle resulted in a 38% increase in hardness and a 31% decrease in the wear rate. The improved properties can be attributed to the presence of fine and uniform distribution of precipitates in the matrix and transformation of retained austenite to martensite.