Effects of nitriding at low nitrogen partial pressures on yield and fatigue in some stainless steels

Abstract

Nitriding stainless steel at nitrogen partial pressures from 0·25 to 0·95 atm and temperatures from 980 to 1160 C resulted in selective internal nitridation of the strong nitride-forming elements, Ti and Cr. Titanium nitride ahrays formed in Tibearing steels, but chromium nitride only for nitriding conditions helmr the Cr₂N Solvus. A nitrided case could, therefore, consist of up to three zones, one of nitrogen in solid solution, one of precipitated TiN, and one of Cr₂N or mixed rhromium-titanium nitride, depending on nitriding conditions and composition. The case was the shallmrer the greater the intervention of internal nitridation in the diffusion process. Swiace hardness, strength, and fatigue endurance were usefully increased by nitriding. The strengthening was wholly attributed to nitrogen in solid solution, the precipitates being too coarse to contribute. Thus, tensile andfatigue strengthening were directly related to case depth, which, in turn, was optimized by nitriding Ti-free steels without Cr₂N precipitation.