For the great part of machines and devices (e.g., machine-tools, cars, railway cars, aircraft engines etc.) service properties depend significantly on precision, durability and reliability of bearings.
Parts produced of bearing steel are balls, rollers, bearing rings, forcing valves, pushing and other parts requiring high hardness, wear resistance, contact strength.
Rolling-contact bearings that serve in normal conditions are made of ØÕ15 and ØÕ15ÑÃ steel grades.
Microstructure, non-metallic inclusions, hardness and decarburization depth are controlled strictly for bearing steel.
Decarburization of finished bearings is not permitted. If ball surface is decarburized (even slightly), friction in bearing increases because of ball surface crushing under load, bearings warm up and fail.
Hot rolled steel is usually used in ring manufacture.
Structure of an incoming billet greatly influences the quality of finished bearings. The billet shall have homogeneous globular perlite structure. When bearing steel is heated over 800°C, fine carbide grains dissolve and coarse grains remain. During further slow cooling coarse carbides grow. As a result, steel structure looks like inhomogeneous coarse-grained perlite. Coarse carbides affect quenched ball quality. Hardenability of bearing steel can be increased by adding 0.40-0.65% of silicon and 0.9-1.2% of manganese.
Carbide network or its remains sharply decrease viscosity of quenched balls that causes fracture under impact load.