Stainless Bearing Steel Extends Life and Corrosion Resistance

Stainless Bearing Steel Extends Life and Corrosion Resistance Until recently, engineers designing bearings for use in corrosive environments were forced to sacrifice a significant amount of service wear life. This is because traditional martensitic, or hardenable, stainless materials used in bearing applications, such as AISI440C, have a much shorter fatigue life in rolling element bearings than standard non stainless high carbon bearing steels. While AISI440C can be hardened to 60Rc, it only yields about 60percent of the fatigue life when compared with standard non stainless bearing steels such as AISI52100. Additionally, the martensitic stainless steel offers only marginal corrosion resistance when compared to other grades. Stainless steels gain their corrosion resistance by the addition of a minimum of 13percent chromium. The chromium allows the steel to form a passive chromium oxide layer on the surface of the steel that prevents the iron in the steel from oxidising. There are several classification groups of stainless steel. Austenitic and martensitic stainless steels are among the most widely used. Austenitic stainless steels have moderate strength and excellent corrosion resistance. Martensitic stainless steels are hardenable and have excellent strength but marginal corrosion resistance. Martensitic AISI440C has a chromium content of 16 to 18percent and a carbon content of 1percent. Despite the high chromium content, this steel is less corrosion resistant than the austenitic stainless steel grade AISI304 which has similar chromium content (17.5to 20percent) but a much lower carbon content (.08percent max). The high carbon content in 440C (1percent) reduces the beneficial effect of the chromium. The introduction of high nitrogen martensitic stainless steel allows for improved fatigue life and corrosion resistance when compared to AISI 440C. Nitrogen in the steel allows the steel to contain lower carbon levels and still achieve a hardness of up to 60Rc after heat treatment. The chromium content is similar to AISI440C but the carbon content is less than half. The combination of high chromium and reduced carbon allows the steel to have improved corrosion resistance when compared with AISI440C. Salt spray testing shows the high nitrogen stainless steel performs slightly better during low hour testing, and significantly better at longer hour testing. After 200 hours of salt spray testing, the 440C steel¡¯s surface is typically 80percent affected by oxidation and the high nitrogen stainless steel is only 30percent affected. Additionally, the results show that the 440C material starts subsurface pitting early on in the testing, while the nitrogen stainless steel shows only signs of mild surface corrosion, even after 200+hours of testing. A significant gain in fatigue life can be achieved by moving to the high nitrogen martensitic steel over AISI 440C. Heat treated AISI440C has a large, course and non-homogeneous carbide structure, which reduces the expected fatigue life when used in bearing applications. The large and course carbides structures are prone to the initiation of fatigue cracks and corrosion. The average size of carbides in heat treated AISI40¡ãC is in the range of 80microns as compared to 15microns for the high nitrogen stainless steel. This reduction in carbide size in the nitrogen stainless is due to the lower carbon content. The lower carbon per centage leads to fine and more evenly distributed carbides. Rolling element bearing testing shows high nitrogen stainless steel offers a significant improvement over AISI440C and similar to or increased fatigue life when compared with AISI52100. John Normandin is with Schatz Bearing Corporation, Poughkeepsie, NY, USA. www.schatzbearing.com