Thermal stability of expanded austenite formed on a DC plasma nitrided 316L austenitic stainless steel

Abstract

Expanded austenite formed during low temperature plasma nitriding of austenitic stainless steels is known for its excellent wear and corrosion resistance. These wear and corrosion properties may degrade by exposure of the surface hardened steel to high temperatures, between 400°C and 700°C. DC low temperature plasma nitrided 316L austenitic stainless steel (400°C for 20h) was heated up to investigate the stability of the 3μm thick expanded austenite layer in the range 400°C<T<700°C. Time-resolved X-ray diffraction experiments were undertaken in a thermomechanical simulator coupled to a synchrotron light source. Two series of experiments were carried out: a) isothermal treatments and b) a continuous heating experiment from room temperature up to 700°C. The results show that during the isothermal heating experiments, expanded austenite remained stable up to 400°C. Although no precipitated phases (ferrite or nitrides) were detected at this temperature, nitrogen diffusion towards the matrix occurred, promoting thickening of the expanded austenite layer. The coefficients of thermal expansion of expanded austenite, determined by linear regression, in the range 80 to 400°C, resulted in different values for different crystallographic directions. These differences can be attributed to an anisotropic relief of residual stresses in different directions, which compensate a “non-stressed” thermal expansion of expanded austenite. Very fine and dispersed CrN nanosized precipitates are formed in the nitrogen rich region of the expanded austenite layer, after exposure to temperatures higher than 400°C. A lamellar product – called nitrogen pearlite – forms, in the nitrogen poor region of the expanded austenite layer, near the matrix, after exposure to temperatures higher than 500°C. The higher is the temperature, the coarser is the nitrogen pearlite formed. Nitrogen diffuses to the matrix along austenite grain boundaries, leading to nucleation and growth of nitrogen pearlite in regions relatively far from the nitrided layer. Thermodynamic modeling of the Fe-16.4Cr-9.8Ni-2.0Mo system, taking into account the incorporation of nitrogen during nitriding, shows that the eutectoid temperature decreases steadily with increasing nitrogen content. Based on this result it is possible to explain different precipitation behaviors in the nitrogen rich and nitrogen poor regions of the expanded austenite layer.