Structured Approach to Material Testing Ensures Reliable Introduction of New Technologies: Advances in Rolling Contact Fatigue Strength Testing and Related Substitute Technologies

Rolling Bearing Steel: Design, Technology, Testing and Measurements

2022

Steels, and in particular bearing steel technologies, have gone through profound developments and some of these are reviewed in this book. Improved metallurgical cleanliness and a requirement for improved composition consistency have necessitated developments in both air-melt and remelt steelmaking. Carbon steels, through hardening, surface hardening and highly alloyed corrosion resistant high-speed steels are used in rolling bearings. Compositions are specified together with the development history of the standard 52100 bearing steel. The virtue in the 52100 (1C-1.5Cr) steel composition is described and information given as to why it is still used more than a century after its introduction. Case carburization bearing steel technologies are widely used in line contact rolling bearings and the development is described. The developments in the use of selective surface hardening in angular contact automotive wheel ball bearings is introduced. Air-melt steelmaking, casting and rolling and remelt steelmaking such as vacuum induction melting, electroslag remelting and vacuum arc remelting are reviewed for special requirements such as precision aerospace. Clean-steel powder metallurgy in combination with hot isostatic pressing is introduced as an upgrade to the properties of high alloy steels with hot-hardness characteristics. Knowledge of rolling bearing loading and the appropriate metallurgical design is fundamental to bearing steel technologies. Bearing steels are only suitable for rolling bearing manufacture once appropriate soft forming, prior soft treatments, subsequent final hardening heat treatments and damage-free hard machining have been applied. Different rolling bearing types can require different steel technologies and damage, failure terms and characteristics are reviewed. Accurate failure characterization, metallurgical and functional property testing are key topics. Metallurgical quality testing with respect to macro-, meso-, and microinclusion testing is reviewed and developments continue. Rolling contact fatigue and substitute fatigue testing methodologies have become difficult as bearing steels have improved and this is covered in detail later in the book.

Chapter 1 | History and Introduction to Rolling Bearing Steel Technologies

Rolling Bearing Steel: Design, Technology, Testing and Measurements

2022

Chapter 8 | Heat Treatment Process Technologies—Hardening and Tempering Treatments

Rolling Bearing Steel: Design, Technology, Testing and Measurements

2022

The choice of appropriate hardening heat treatments profoundly influences the functional properties of rolling bearings. Through hardening heat treatments of 52100 type steels include martensitic or bainitic treatments and such heat treatments are applied when the static capacity needs to be maximized in point contact rolling bearings. Surface hardening is used when toughness is a primary consideration with case-carburizing, surface induction hardening and laser heating being the most familiar surface hardening technologies. The formation of near-surface compressive stresses is a beneficial factor in the selection of case carburization heat treatments. Case carburization can be combined with nitrogen enrichment to enhance the raceway tolerance to contaminated lubrication. Microstructures developed by appropriate austenitization, quenching and lower bainite transformation are essential considerations with respect to functional properties. Hardness is an overriding property consideration but the amount of austenite is an issue with respect to bearing component dimensional stability. Increased volume percentages of retained austenite goes hand-in-hand with high carbon plate martensitic surfaces zones of case carburized bearing components. Tempering after martensitic quenching is applied to transform austenite retained in the microstructure and in certain cases subzero treatments are employed. Surface induction heating and quenching is an efficient methodology for hardening of rolling bearing raceways. Hub unit bearing raceways are selectively austenitized and quenched in a few seconds whereas large-size rolling bearing rings are progressively heated and quenched using lengthier cycle times. Secondary hardening is applied to higher alloyed bearing steels such as M50 when high temperature hardness retention is required. Nitriding of the surface in combination with through or case carburization is a methodology for improving performance in hybrid bearings, i.e., steel rings and silicon nitride rolling elements. Distortion of rolling bearing rings is the curse of the hardening operation and, if not controlled, can have a negative influence on grinding costs.

Chapter 17 | Super-Precision Aerospace Bearing Steel Technologies

Rolling Bearing Steel: Design, Technology, Testing and Measurements

2022

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.