Investigating the mechanisms of microstructural change in EN 31 bearing steel during cryogenic treatment

Roller element bearings (REB) commonly fabricated from EN 31 bearing steel are heat treated by hardening and tempering. These conventional heat treatment (CHT) processes produce a microstructure consisting primarily of martensite and undissolved carbides. Owing to the alloys high carbon content, a quantity of metastable austenite is usually retained after hardening. The presence of any retained austenite in a finished REB compromises dimensional stability, wear performance and even affects fatigue life. Cryogenic treatment is a bulk through-thickness heat treatment administered to engineering materials to improve their properties beyond those attainable by CHT processes alone, utilising temperatures < 193 K to cause permanent microstructural change. It has been known for almost a century that cooling to these temperatures converts some of the metastable austenite to the desired martensite phase, although the research community has failed to agree upon the fundamental nature of the transformation at these sub-zero temperatures and how the transformation is affected by prior thermal processing. Concurrent to this is the desire to understand the effects of deep cryogenic treatment (DCT) on tempering behaviour for differing thermal processing routes, with views as to how DCT may aid REB dimensional stability, making inferences to the phase transformations and mechanisms responsible. Therefore, a series of experiments utilising varied austenitising temperatures were conducted to detect (i) sub-zero martensite formation during thermal cycles representative of those performed in industry (as close as could be simulated through laboratory based testing) and (ii) to evaluate the effects of an industry standard DCT cycle on tempering behaviour with the kinetics of phase transformations calculated where possible. Sub-zero martensite formation progresses mainly during cooling with athermal characteristics for austenitising temperatures of 1223 K, causing compressive strains to build up in the untransformed austenite. Isothermal martensite formation occurs during holding at temperatures of 153 K, but no transformation was detected at temperatures of 93 K, where it is theorised that the thermal activation is too low. During tempering it was found that DCT reduces the activation energy on tempering for retained austenite decomposition for all austenitising temperatures investigated, and it is suggested that the defect density in austenite increased during DCT allowing for easier decomposition of austenite into cementite and ferrite.