Effects Of Working Deformation In Room Temperature On The Nitriding Behavior Of 304 Stainless Steel

AISI 304 stainless steel specimens with three kinds of microstructure are prepared to be nitrided:(1) Austenite (by solution treatment); (2) Austenite+2～5% ferrite in rolling direction (hot rolled plate); (3) Austenite+ 2～5% ferrite in rolling direction+ 50% martensite (produced by the tensile deformation of hot rolled plate). In this paper, the phase present, phase distribution in the nitrided layer and the microstructure features of the nitrided layer on the different matrix under different processing temperatures (400 ℃、450℃、500℃、550℃) were investigated by OPM, SEM, XRD, TEM and EDS. The influence of microstructure characteristics of matrix on the microstructure characteristics of nitrided layer were investigated comparatively.The conclusions are listed as followed:1. The nitriding layers on three kinds of matrix at low temperature (400℃) are mainly composed of nitrogen supersaturated solid solution:the layer on the specimen in solution is mainly composed of nitrogen supersaturated austenite (γN); the layers on the two specimens in hot-rolled and hot-rolled with tensile deformed are mainly composed of nitrogen supersaturated austenite (γN) and nitrogen supersaturated ferrite (αN).The hardness of the nitrided layer is 6 times greater than that of the substrate. The nitrided layers exhibit a better corrosion resistance, compared with the substrate.2. The XRD peeling experiment shows that the nitrided layers of the outer-surface on the three kinds of specimens at the nitriding temperature of 450-550℃ are mainly composed of γ’-Fe4N、CrN and αN. And CrN and γ’-Fe4N are only presented in within 10 μm depth from the most-surface. The expanded austenite(γN) was still found in the sub-surface at the nitriding temperature of 450-550℃, but the extent of lattice expansion is relatively low.3. There is no significant difference in the extent of lattice expansion of γN in the nitrided layers on the three kinds of specimens. It is suggested that the presence of deformed martensite and the increase of lattice defects by tensile deformation are not enough to affect the nitrogen content and stress station in the layer.4. The thickness of the nitrided layer on three kinds specimens are various at different nitriding temperature.The thickness of nitrided layer decreases in an order from specimens with tensile-deformed, specimens hot-rolled to specimens solutioned. It is suggested that the increase of lattice defects and the presence of deformation-induced martensite facilitates the diffusion of atom N.5. In the 400℃ nitrided layer on the specimen in hot-rolled with tensile deformed,the atom N existed in the supersaturated austenite(γN) and supersaturated ferrite(αN) in the form of Cr-N segregation group which is in long-range disorder and short-range order.6. The 450℃ nitrided layer consists of pearlite-like tissue which are γ+CrN and α+CrN in interleaved distribution.This tissue origins from the decomposition reaction of supersaturated austenite(γN) and supersaturated ferrite(αN):γN�'γ+CrN and αN�'αN+CrN.7. The 500℃ nitrided layer consists of pearlite-like tissue which is α+CrN in interleaved distribution.This tissue origins from the decomposition reaction of supersaturated austenite(γN) and supersaturated ferrite(αN):γN�'α+CrN and αN�'α+CrN.