Abstract
AISI 316 and AISI 316L austenitic stainless steels have been extensively used (to replace the more common AISI 304) in applications requiring enhanced corrosion resistance. It is well known that these types have good corrosion resistance and strength, but poor tribological properties. Gas nitriding is a well established process for surface hardening that can be applied to austenitic stainless steels with the aim of enhancing its surface hardness and wear resistance. In this work, the response surface methodology (RSM) in the design of experiment (DOE) statistical method was used to study the effect of gas nitriding conditions on the sensitization to intergranular corrosion (IGC) of AISI 304, AISI 316, and AISI 3l6L stainless steels by applying the standard oxalic acid etch test. Optical microscope and X-ray diffraction (XRD) were used to determine the effect of processing parameters on microstructure and phase-composition of the developed nitrided layers. In addition, using the same RSM method, surface hardness, and erosion resistance analysis were carried out to the stainless steel which passed the IGC test. Results of this study showed that gas nitriding of AISI 304, AISI 316, and AISI 316L stainless steels produces similar nitrided layers in the three types, which varies in thickness and composition according to the different nitriding conditions. Furthermore, under the experimental conditions carried out in this study, it is found that nitrided AISI 304 and AISI 316 are prone to localized intergranular corrosion (IGC) more than the nitrided AISI 316L. This behavior in AISI 304 and AISI 316 stainless steels is mainly due to the grain boundary precipitation of chromium carbides during nitriding processing. Finally, it is found that gas nitriding of AISI 316L stainless steel increased the surface hardness by 547% and also improved the erosion resistance by 93% and 54%, for the specimens tested at 30° and 90° impact angles, respectively.
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