Tuesday, January 7, 2014

Properties of Bearing Steel

Bearings are important mechanical components that find their primary application in continuously moving machine parts which may get subjected to high fatigue and stress conditions. These bearings facilitate in effectively transmitting dynamic loads within the machine body, as well as help in reducing friction between the moving parts, thereby ensuring a smooth motion. These could be of various types (such as ball bearings and tapered, cylindrical or needle roller bearings) and are manufactured using a grade of steel that has mechanical properties which are more suited for lowering the friction levels (compared to the other available carbon steel grades). Manufacturing of bearing steel requires melting the metal and degassing it, followed by certain metallurgical processes, and then tempering and quenching, with finally grinding or polishing the alloy.

Chemical Composition of Bearing Steel
Bearing steel has carbon content in the range of 0.55 to 1.10 %, manganese in the range of 0.10 to 1.15 %, silicon in the range of 0.15 to 2.0 %, phosphorus and sulphur up to a maximum of 0.03 %, chromium in the range of 0.5 to 2.0 %, as well as traces of titanium. The remaining % is iron.

Chemical properties determine the anti-corrosive properties of the alloy. The chemical composition of the steel also dictates its mechanical and physical properties (such as the strength and hardness, toughness and brittleness, ductility and malleability).

Mechanical Properties of Bearing Steel
Bearing steel has a bending strength of 2400 MPa, and can withstand high stress and centrifugal forces, although it has a low corrosion resistance.

Bearings materials can either be through-hardened or case-hardened, and may also require vacuum-processing to ensure purity. A minimum expected hardness for bearing components is 58 Rc, but bearing steel has generally higher hardness levels.

Carbon increases the strength of bearing steel, which ensures that parts made from it do not deform on the application of stress and load, while also increasing its hardness. Ductility and weldability however decrease with increasing carbon content, as is the case with bearing steel.

Bearing steel is designed to have high fatigue strength and life, needs to respond uniformly to the heat treatment process, and should have a compact structure with a consistent grain flow and a fine grain size that imparts high impact toughness to the alloy.

Physical Properties of Bearing Steel
Bearing steel has a density of 7.85 gm/cubic-cm, coefficient of linear expansion of 0.00001 /K, thermal conductivity of 30-40 W mK and is magnetic and a good thermal and electrical conductor.


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