Chapter 17 | Super-Precision Aerospace Bearing Steel Technologies

Abstract

Steel technologies used in precision aerobearings are unlike those used in industrial machines or automotive rolling bearings, etc. Quality assurance, with respect to metallurgical cleanliness, is achieved by the use of multiple vacuum steelmaking and a revised ASTM E45 micro-inclusion rating method. Vacuum induction melting (VIM) in combination with vacuum arc remelting (VAR) is the mostly applied steelmaking method. Aeroengine rolling bearings typically run at elevated temperatures. In order to achieve the required high temperature, hot hardness requirements, secondary hardening steels such as M50 (through hardening) and M50NiL (case carburizing) steels are employed. In addition to M50 and M50 NiL, other aerobearing steel grades and heat treatments are applied especially when resistance to corrosion is required. Advanced high-speed steel compositions such as 7-7-7-11 (Aerospace Materials Specifications, AMS 6560), using powder metallurgy manufacturing methods, are increasingly being applied. The super precision aerobearing market requires numerous types, and sizes, of relatively exotic bearing rings and are produced in small batches. Application loads and lifecycle requirements means that forging with rolling bearing raceway conforming material flow conformance are specified in the design. Due to the use of VIM-VAR or other remelt steelmaking, “classical” subsurface inclusion initiated spalling is usually not observed. Aerobearing ring and rolling elements often have to tolerate lubricant contamination. The failure mode is thus surface distress and for this reason surface strengthening heat treatments have been developed and used. Nitriding of the steel ring working contact surfaces is often applied especially in combination with silicon nitride rolling elements. Corrosion resistance is recognized as a requirement in aeroengine bearings especially when engines are intermittently used (i.e., standstill corrosion conditions). Corrosion resistant bearing steel development and testing is a demanding topic (see Chapter 10). Functional test results on various “standard” and newly developed aerobearing steel technologies are published in ASTM STPs.