NSK Ltd. (Japan; Tokyo: NSK; 5251R9N7) announced it is commercializing its newest stainless steel, ES1, suited for use manufacturing bearings that will see installation in applications where corrosion resistance is paramount.

The most common stainless steel used to manufacture bearings is 440C. However, 440C has a key limitation, which is that is has concentrated chromium coarse eutectic carbide inclusions. Those inclusions lead to three key areas of weakness. First, those inclusions make it virtually impossible to get the best finish on the bearing races, so 440C bearings normally run loud, rough, and can run hot. Second, under load, cracks initiating at these inclusions are what usually lead to a 440C bearing failure through flaking and spalling. And third, those inclusions make 440C less corrosion resistant because the chromium is lost in the martensitic matrix.

ES1, originally discussed by NSK in 2000, is a new martensitic stainless that NSK developed with an eye toward carbide size and comparable Rc hardness to 52100 bearing steel. ES1 is essentially 440C with higher nitrogen and lower carbon and chromium. Reducing the chromium and carbon is what helps eliminate the eutectic inclusions. Alloying the nitrogen is what provides superior corrosion resistance.

NSK paper: Excellent Stainless Bearing Steel (ES1)
With no eutectic carbides, ES1 bearings are highly corrosion resistant, have low noise and vibration, and have a longer under-load rolling fatigue life.

Under corrosion testing, ES1 thrust bearings using silicon nitride balls last 5 times longer than 440C. Standard 6203 ES1 ball bearings using silicon nitride balls lasted 25 times longer than 440C.

Schaeffler Group (parent of INA and FAG bearings) has a similar specialty stainless steel, Cronitect (Chromium Nitrogen Protection), a martensitic hardening steel developed by refining Nirosteels. Cronitect is an affordable version of Cronidur30, which is only slightly superior but unaffordably expensive and difficult to produce. Cronitect takes a two-pronged approach: not only altering the composition of 440C but also surface treating the bearing races by controlling the surface nitrogen alloy in a type of case hardening. Ceramic hybrid Cronitect bearings with loose seals or no seals are being used in a number of competitive sports equipment and food processing applications where they are normally running dry or in light oil.