High-grade stainless steel improves bearing life

Paul Boughton

NSK has developed ES1 (Excellent Stainless Steel) for use in high-humidity, chemical and hygienic environments where corrosion can impair the operating life of rolling bearings. ES1 is claimed to provide superior corrosion resistance and rolling contact fatigue life compared with conventional AISI 440C steel in wet and aggressive environments; this improvement helps to reduce maintenance, downtime and replacement bearing costs.
Prior to the development of ES1, AISI 440C had been the general choice for use in bearing applications where corrosion resistance was a primary concern. Because the steel contains coarse eutectic carbides that can act as crack initiators under rolling contact stress, the performance of AISI 440C had not always proven satisfactory.
ES1 steel, however, has no eutectic carbides; rather it consists of fine carbides, nitrides and strong martensite, resulting in improved resistance to corrosion and fatigue. ES1 also has optimised nitrogen content and lower carbon content to suppress the eutectic carbides generated in conventional steelmaking processes. The operating benefits of this new material specification have been proven following extensive testing by NSK; ES1 outperformed other stainless steels in a 5 per cent aqueous sodium chloride immersion test, a salt-water spray test, 5-N sulphuric acid and 5-N hydrochloride solution immersion tests, and during life tests in water.
In the aqueous sodium chloride immersion test, test pieces of ES1, 13Cr stainless steel and AISI 440C were immersed in a 5 per cent aqueous sodium chloride solution at room temperature for eight hours. Before immersion in the solution, all surfaces of the test pieces were polished, then passivation treatment was applied. The test revealed that the conventional AISI 440C and 13Cr stainless steels rusted, whereas ES1 did not. Similar results, were also recorded in the saltwater spray test, with both AISI 440C and 13Cr rusting considerably, while ES1 did not.
Additional performance advantages were recorded for ES1 in the water immersion test. The test bearings were 51305 thrust ball bearings with fluororesin cages and silicon nitride balls. They were subjected to a load of 980N (maximum contact pressure 1470MPa) and rotating speed of 1000rpm until the bearing vibration level reached about five times its initial value Under these conditions the test results indicated that ES1 lasts approximately five times longer than AISI 440C.
Because bearings manufactured from stainless steel are routinely exposed to cleaning solutions of sulphuric acid and hydrochloric acid, a further corrosion test was undertaken. This evaluated the corrosion resistance of ES1, 13Cr stainless steel and AISI 440C in a 5-N aqueous sulphuric acid solution and 5-N hydrochloric acid solution. In the test, samples of each type of steel were immersed in the solutions at room temperature for 20 hours. The amount of corrosion was then measured; this indicated that ES1 is superior to both conventional stainless steels in its resistance to these acids.
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