NITRIDING RESPONSE OF CHROMIUM-ALLOYED STEELS.

Hardness-increasing chromium-nitride particles of submicroscopic size develop from chromium initially dissolved in the ferritic matrix of FeCr and FeCrC alloys. Transformation of chromium-carbide particles leads to coarse incoherent chromium-nitride particles and precipitation of cementite at grain boundaries. Due to the long-range strain fields surrounding the precipitates, the ferritic matrix can be supersaturated with nitrogen. The tendency of the case toward volume expansion leads to compressive residual surface macrostress. The misfit between second-phase particles and the matrix induces residual microstresses. At latter stages of nitriding discontinuous precipitation, loss of excess nitrogen leads to void development and the relaxation of compressive residual surface macrostress.