The Enhancement of Wear Resistance of AISI-316 Austenitic Stainless Steel by Gas Nitriding: Response Surface Analysis and Optimization

Abstract

   Austenitic stainless steels are widely used in industrial applications, mainly due to their good corrosion resistance. However, its low hardness and poor wear performance impose strong limitations in many cases where tribological properties are of prior significance. Nitriding processes has gain acceptance as a technology for surface modification that seems to overcome these problems. In this regard, significant improvements on wear characteristics of stainless steels by gas nitriding have been reported by several workers..
    However, previous research attempts on studying the effects of the gas nitriding process parameters on the material performance were made with the old classical method of changing one parameter at a time. This approach is time consuming and did not investigate the interaction effects between the various nitriding parameters.

  Furthermore, optimization particularly multi-variable type was very difficult to perform. These shortcomings are tackled in the present work by using the Response Surface Methodology (RSM) in the design of experiments (DOE) statistical method. Therefore, this work investigates the wear performance of AISI 316 stainless steel in a designed statistical approach. An austenitic stainless steel type AISI 316 was subjected to gas nitriding processes throughout temperature range of 400 – 600 Co, nitriding time of 10- 50 hrs and flow rate of ammonia (NH3) of 100 - 600 liter/hr. Experimental design is first created using composite design method in the response surface methodology. Then gas nitriding and wear tests were performed accordingly. Wear tests were made with an Amsler-Disc- Machine A135 under rolling and sliding conditions. The nitrided layers were investigated using optical microscopy (OM) technique. The phases built-up at the nitrided surfaces were investigated using X-ray diffraction (XRD) method. The microhardness of the nitrided layers was also measured and incorporated in the analysis. Results showed that maximum wear resistance improvement was 93% as compared with the un-nitrided specimen, which is achieved at 528 oC for 33 hrs using 260 liter/hr ammonia flow rate. It is also concluded that the RSM is a powerful tool to perform gas nitriding analysis and optimization.