FCC phase expanded by nitrogen atoms is obtained.

The literature has shown the possibility of austenitic stainless steels nitride processing, which is the occurrence of nitrogen in solid solution, increasing the hardness of the surface and the wear resistance without loss of corrosion resistance. The nitride process, independent of the method, increases the superficial hardness of austenitic stainless steels and promotes higher wear resistance. One alternative is the thermochemical heat treatment, the well-known nitriding process that basically can run by one of the three methods: gaseous, salt bath, or plasma. To surpass this problem there are alternatives in the surface engineering to improve surface properties, without hindering all the other properties like the corrosion resistance. The austenitic stainless steels, despite the elevated corrosion resistance, have low hardness that can only be partially improved by cold deformation reducing its application in components submitted to severe wear conditions. These steels are also employed in orthopedic implant due to its biocompatibility. The austenitic stainless steels are plenty utilized in chemical processes equipment in pharmaceutical, foodstuff, textile, petroleum, and cellulose industries, where these components are exposed at aggressive ambient conditions and low temperatures. The mechanical strength of both steels after nitriding processes reveals significant hardness values, almost 1100 HV, on the nitrided layers.

An enhanced diffusion can be observed on AISI 304 steel comparing with AISI 316 steel (a nitrided layer thicker can be noticed in the AISI 304 steel). The nitriding processes have formed two layers, one external layer formed by expanded austenite with high nitrogen content, followed by another thinner layer just below formed by expanded austenite with a high presence of carbon (back diffusion). The initial cold work (previous plastic deformations) in both AISI 304 and 306 austenitic stainless steels does not show special influence in all applied nitriding kinetics (in layer thicknesses). The microstructure, layer thickness, hardness, and chemical microcomposition were evaluated employing optical microscopy, Vickers hardness, and scanning electron microscopy techniques (WDS microanalysis). The nitriding behavior of austenitic stainless steels (AISI 304 and 316) was studied by different cold work degree (0% (after heat treated), 10%, 20%, 30%, and 40%) before nitride processing.

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