Microstructural Changes in Aerospace Bearing Steels under Accelerated Rolling Contact Fatigue Life Testing

Abstract

Operating in the extreme environment of a turbine engine, aerospace bearings must endure high thrust loads and very high rotational speeds. Subsurface fatigue damage is often observed as stressed zones, dark etching regions, light etching regions, and white etching regions. American Iron and Steel Institute (AISI) 52100 bearing steel is widely studied for microstructural changes and has been reported in the literature. Microstructural changes in a large test matrix of advanced materials for aerospace applications have not been studied. In this study, one through-hardened (VIM-VAR AISI M50) and four case-hardened materials (M50 NiL, M50 NiL Nitrided, Pyrowear® 675 High Temperature Temper, and Pyrowear® 675 Low Temperature Temper) were tested with silicon nitride balls under accelerated life conditions on a ball-on-rod rolling contact fatigue bench tester. Microstructural changes were characterized by light optical microscopy and scanning electron microscopy in radial and axial cross-section views. White etching cracks and white etching regions were observed in M50, M50-NiL, and M50-NiL (N) with shorter average fatigue lives as compared to either version of Pyrowear® 675.

Corrosion Initiation and Propagation on Carburized Martensitic Stainless Steel Surfaces Studied via Advanced Scanning Probe Microscopy

“…In corrosive environments, the overall wear lifetime may be controlled by resistance to corrosion initiation, in which case LTT and CN could provide greater benefit than HTT. Previously conducted wear studies are in agreement with the recommendations given, and the results of this study provide nanoscale insight to help understand why HTT outperformed both CN and LTT during rolling contact fatigue testing even though it had significantly lower corrosion resistance [29,30]. Based on this work, P675 HTT would be recommended over the other two tempering procedures for use in aerospace bearings where corrosion is not a primary concern.…”

“…For all steels studied, the carbides showed variable shades of purple/blue in the MFM maps (~1–3° phase shift), indicating slightly different magnetic properties within the individual phases (Figure 3), likely due to different carbide compositions in terms of the relative amounts of chromium and molybdenum, which influence the magnetic properties of phases [61,62,63,64]. Sample CN had a much less homogenous matrix that showed considerable variation in magnetic properties and is likely an effect of the relatively high surface retained austenite (18–22%) found within the matrix phases compared to LTT (10–13%) and HTT (1–2%) [9,30]. The bulk magnetism of the steels will change with tempering temperature and heat treatment process, following changes to the microstructural phases formed [9,30].…”

“…Sample CN had a much less homogenous matrix that showed considerable variation in magnetic properties and is likely an effect of the relatively high surface retained austenite (18–22%) found within the matrix phases compared to LTT (10–13%) and HTT (1–2%) [9,30]. The bulk magnetism of the steels will change with tempering temperature and heat treatment process, following changes to the microstructural phases formed [9,30]. Further work is currently underway to investigate the implications of local magnetism and magnetic domains on resulting wear and corrosion mechanisms.…”

“…Samples differed in the final tempering temperature and carburization atmosphere: high-temperature tempering (HTT) at 496 °C, low-temperature tempering (LTT) at 315 °C, and carbo-nitrided (CN) where the case was obtained through a carburizing cycle followed by nitriding cycle during heat treating. Further details on the processing routes are discussed in previous works [9,10,29,30]. Prior to SPM characterization, samples were mechanically ground with SiC paper (to 2000 grit) in deionized (DI) water, followed by sequential polishing to 0.02 µm with a colloidal silica aqueous slurry.…”

“…Compared to M50, surface hardened P675 can be significantly more corrosion-resistant, and higher processing temperatures typically increased susceptibility to general corrosion damage, while lower temperatures exhibited more localized corrosion relative to untreated P675 [7]. The influence of processing on P675 wear performance for the same steels in non-corrosive wear testing has also been reported, where higher processing temperatures (HTT) yielded longer bearing lifetimes compared to low-temperature temper (LTT) [29,30]. However, there remains a need for research into the interdependency between simultaneously balancing corrosion resistance and surface hardness for bearing applications, since wear resistance (i.e., bearing performance) in corrosive environments is ultimately limited by corrosion [11].…”