Carbo-nitriding process for martensitic stainless steel and stainless steel article having improved corrosion resistance

A method comprising:

• (a) providing a bearing ring comprised, at least in part, of a martensitic stainless steel,
• (b) carburizing the bearing ring within a temperature range of 1625° F.-1680° F., and
• (c) then carbo-nitriding the bearing ring within a temperature range of 1575° F.-1625° F., wherein carbon and nitrogen are simultaneously introduced into the case of the stainless steel, thereby producing a case-hardened martensitic stainless steel bearing ring,
• wherein steps (b) and (c) are performed such that the bearing ring exhibits:
• a hard carburized case up to a depth of about 900-1125 μm,
• a presence of nitrogen or nitrides up to depth of 250 μm,
• 0.3-0.7 wt % nitrogen at the surface and 0.1-0.3 wt % nitrogen at a depth of about 250 μm, and
• carbon in the range of 1.5-2 wt % in the hardened case, and
• wherein the martensitic stainless steel prior to steps (b) and (c) consists of:
• 0.03-0.10 wt % C, 0.5-1.0 wt % Mn, 0.20-0.6 wt % Si, 12.0-15.0 wt % Cr, 2.40-3.0 wt % Ni, 1.50-2.0 wt % Mo, 0.40-0.80 wt % V, 5.0-6.0 wt % Co, the balance being Fe and unavoidable impurities, and the core of the bearing ring contains the following weight percentages of elements:
• Mo+V=0.8 to 4.0 wt % and
• Co+Ni=2.0 to 12.0 wt %.