Chapter 19 | Bearing Ball Material Science

Abstract

A frequently posed bearing steel technology question is: “how are balls made”? The answer is surprisingly simple but significant details need to be addressed in state-of-the-art rolling bearing ball manufacture. ASTM F2215-15, Standard Specification for Balls, Bearings, Ferrous and Nonferrous for Use in Bearings, Valves, and Bearing Applications, covers the dimensional requirements. Bearing ball air melt steel quality can be made by both ingot and continuous casting provided adequate account is taken of the need for center and surface metallurgical integrity. Because ball blanks are formed by pressing, known as heading, the center of the steel remains on the surface on two sides of the ball blank. This region is known as the pole containing the metallurgical center material. The metallurgical center can be of poor quality if inadequate quality measures are taken during steelmaking. The general surface quality is a function of the billet, bar, and wire quality. Specialist bearing steel steelmakers and wire rod manufacturers (wire converters) produce the input steel for bearing ball production. The hardening operations are performed in, for example, shaker-type furnaces with appropriate tempering to reduce the quantity of retained austenite but retaining a very high hardness. The hardness of the surface is increased by a process known as tumbling (or scouring) which improves the ball rolling bearing functional properties. The tumbling produces surface compressive stresses and if the stresses are too high the ball rolling contact fatigue life can be degraded. Most bearing balls are produced from 52100 steel but aerospace quality balls are generally produced from re-melt processed M50 steel. Large primary carbides in the microstructure can produce significant surface quality issues in the form of raised carbide defects. These defects may be circumvented by the application of powder metallurgy and hot isostatic high speed steel steelmaking. Further improvements in M50 steel ball properties may be achieved by, for example, nitriding heat treatments.