Study On Microstructure Of Nitrided Layer Of Stainless Steel With Dual Phase Microstructure

The nitrided layer produced by low temperature (400℃) and high temperature (500℃) plasma nitriding on the hot-rolled304stainless steel composed of alternating stripe-like α-ferrite and γ-austenite grain aligned in the rolling direction was investigated by SEM and EDS, OPM, TEM and XRD.The following conclusions are reached:400℃plasma nitriding:1. The nitrided layer is composed of the supersaturated nitrogen austenite (γN) and the supersaturated nitrogen ferrite (αN). The micro-hardness of αN and yN in the nitrided layer is six times higher than that of a-ferrite and y-austenite of matrix. They have better corrosion resistance than a-ferrite and y-austenite of matrix.2. The boundary line between the nitrided layer and matrix is wave-like rather than straight, following the distribution of a and y phases to form a continuous variation layer thickness. The formation of wave-like boundary is attributed to the difference of nitrogen’s diffusion speed between in the ferrite and in austenite and that between along the phase boundary and through the grains.3. Discontinuous precipitations of γNâ†'αCrN and αNâ†'αCrN occurs at the phase boundary of α-ferrite and y-austenite and y-austenite boundary to form lamellae structure consisting of CrN and α-ferrite. The orientation relationship of CrN and α-ferrite corresponds to Baker-Nutting relationship:(100)crN(100)α;[011]crN//[001]α.4. Intense diffuse scattering occurring on electron diffraction spots of aN and γN indicates that continuous precipitation occurs in αN and γN with short-range order, clusters of alloy elements during the nitriding process5. XRD patterns in different depth of nitrided layers indicate that the peaks of γN(111) and (200) shift to higher Bragg angles with the removal depth and the broadening of the peaks becomes weaker and weaker.500℃plasma nitriding:1. The nitrided layer is composed of y’-Fe4N, aN and CrN in surface, yN in subsurface and stripe-like aN aligned in the rolling direction. 2. The boundary line between the nitrided layer and matrix is generally straight, which indicates that the difference of nitrogen’s diffusion speed between in the ferrite and in austenite and that between along the phase boundary and through the grains during nitriding process at high temperature are much smaller than that at low temperature.3. CrN is formed in the austenite in the surface of nitriding layer via continuous precipitations, which contributes to worse corrosion resistance of nitrided layer.