It is a precipitate-free metastable supersaturated solid solution of nitrogen or carbon or both.
The compound layer was γ′-Fe4N and the diffusion layer was a solid solution of nitrogen in iron.
The solution treatment after nitriding (STAN) was performed to improve the fatigue strength in type 304 steel by means of the solid solution of nitrogen.
To get solid state solution of nitrogen in the diffusion zone, instead of precipitates of nitride phases, the cooling rate should be higher than a critical value of about 0.95 °C/s.
As a result of the nitrogen interaction, a nitrided layer was formed that consists of titanium nitrides, followed by an interstitial solution of nitrogen in the hcp α titanium phase.
When the treatments are carried out at temperatures up to 673 K, the outer modified layer consists mainly of the S phase and a nitrogen induced h.c.p. martensite; in the inner modified layer a solid solution of nitrogen in γ-Fe is present.
In the STAN specimen, the strain-induced martensitic transformation in the run-out specimen at a stress level of fatigue limit was not detected by X-ray diffraction method, indicating that the γ-phase was stabilized by the solid solution of nitrogen.
But high temperature (1273 K) treatment induced the transformation of β-Ti to α-Ti due to solid solution of nitrogen, which promoted the formation of nitrogen-enriched α-grains (α-Ti(N)) layer on the top of NDZ and sequentially increased the Re up to 6.72 folds.
