Nitride precipitation and ferrite to austenite phase transformation induced by nitrogen absorption in Fe-25%Cr-Ti alloy

Fe-25%Cr-Ti alloy with ferritic single structure was held at 1473K in a nitrogen gas atmosphere (solution nitriding). As a result, nitrogen atoms diffused into ferritic matrix from the material surface, and induced precipitation of nitride (TiN) and phase transformation to austenite. The austenite grains nucleated at the surface grew into the material with absorbing nitrogen, and the specimen was completely covered with austenitic phase after long time solution nitriding. The austenite contained about 1mass% nitrogen which is high enough to lower Ms temperature below ambient temperature; thus, stable austenitic structure was obtained after the solution nitriding. On the other hand, the TiN nitrides precipitated owing to the decrease in solubility of titanium by nitrogen absorption. Since the TiN precipitation preceded the ferrite-austenite phase transformation, the grain growth of austenite was effectively suppressed by the pinning effect due to the dispersed TiN particles. However, the austenite grains had anisotropic shape elongated to the direction from the specimen surface to the center. This is due to the pinning effect by TiN particles that is effective for the migration of (austenite/austenite) grain boundary in the minor axis direction, but not effective for the migration of (austenite/ferrite) interface in the major axis direction.