steel bearing material

STEEL

440 Grade Martensitic Stainless Steel

Good corrosion resistance to water and many weak chemicals

Good for temperatures from -70°C up to 250°C constant or 300°C intermittent

Slightly softer than chrome steel so lower load ratings

Will corrode in salt water or salt spray and poor resistance to acids/alkalis

More expensive than chrome steel


More resistant to corrosion due to the greater chromium content and the addition of nickel, 440 grade stainless steel is the most commonly used for corrosion resistant ball bearings. The chromium reacts with oxygen in the air to form a chromium oxide layer, known as the passive film, on the surface of the steel. It is hardened by heat treatment and gives a good combination of strength and corrosion resistance. It is magnetic unlike 300 grade austenitic steel.


The load capacity of AISI440 grade is approximately 20% less than chrome steel so life ratings will be slightly reduced. This grade exhibits good corrosion resistant when exposed to fresh water and some weaker chemicals but will corrode in seawater environments or in contact with many aggressive chemicals. 440 grade stainless steel will also withstand higher temperatures than chrome steel, coping with up to 250°C constant and up to 300°C intermittent with reduced load capacity. Above 300°C, bearing life can be considerably shortened.


AISI316 Austenitic Stainless Steel

Very good corrosion resistance to water, salt water and many chemicals

Good for temperatures up to 500°C as full complement type

Suitable for cryogenic applications down to -250°C

Non magnetic

More expensive than 440 grade due to low production quantities.

Suitable for very low load and low speed only

Not suitable for low noise applications


316 grade stainless steel bearings are used for greater corrosion resistance to seawater, salt spray and some acids/alkalis. They are suitable for very high temperature applications as the steel is useful in temperatures of up to 500°C. They can also be used in cryogenic applications as the steel retains its toughness down to -250°C.  316 stainless steel bearings are also non-magnetic.


316 grade stainless steel cannot be hardened by heat treatment and will only support low loads and low speeds. The load and speed ratings of 316 grade bearings are significantly less than the equivalent 440 grade bearings. 316 grade stainless steel exhibits good corrosion resistance in marine environments when used above the waterline or when temporarily submerged if washed down with clean water. It is less suitable when permanently submerged unless there is a regular high rate flow of water over the bearing. This is because the passive film on the surface of stainless steel relies on the presence of oxygen to regenerate itself. In a low oxygen underwater marine environment (e.g stagnant seawater or under mud/silt) the steel may be prone to pitting or crevice corrosion. 316 stainless steel is less resistant to warm seawater. Pitting corrosion is a risk in seawater over 30°C whereas crevice corrosion can occur in as little as 10-15°C. 316 grade is still much more resistant to corrosion than 440 grade.


Bearings made from 316 grade stainless steel can be used at high temperatures provided a suitable cage material is used or the bearings are full complement. Polyethylene, PEEK or PTFE are often used for retainers in 316 stainless steel bearings.