Microstructural Characteristics of Plasma Nitrided Layer on Hot-Rolled 304 Stainless Steel with a Small Amount of α-Ferrite

Abstract

The hot-rolled 304 stainless steel with γ-austenite and approximately 5 pct α-ferrite elongated along the rolling direction was plasma-nitrided at a low temperature of 693 K (420 °C). X-ray diffraction results revealed that the nitrided layer was mainly composed of the supersaturated solid solution of nitrogen in austenite (γN). Transmission electron microscopy (TEM) observations showed that the microstructure of the γN phase exhibited “fracture factor contrast” reflective of the occurrence of fine pre-precipitations in γN by the continuous precipitation. The occurrence of a diffuse scattering effect on the electron diffraction spots of γN indicated that the pre-precipitation took place in γN in the form of strongly bonded Cr-N clusters or pairs due to a strong attractive interaction of nitrogen with chromium. Scanning electron microscopy and TEM observations indicated that the discontinuous precipitation initiated from the γ/α interfaces and grew from the austenite boundaries into austenite grains to form a lamellar structure consisting of CrN and ferrite. The orientation relationship between CrN and ferrite corresponded to a Baker-Nutting relationship: (100)CrN//(100)α; [011]CrN//[001]α. A zigzag boundary line following the banded structure of alternating γ-austenite and elongated α-ferrite was presented between the nitrided layer and the substrate to form a continuous varying layer thickness, which resulted from the difference in diffusivities of nitrogen in α-ferrite and γ-austenite, along the γ/α interfaces and through the lattice. Microstructural features similar to the γN were also revealed in the ferrite of the nitrided layer by TEM. It was not excluded that a supersaturated solid solution of nitrogen in ferrite (αN) formed in the nitrided layer.