Formation of the Nitrided Layers on an Austenitic Stainless Steel with Different Grain Structures

In this work, the formation of nitrided layers on an austenitic stainless steel with very different grain structures is analyzed. Two different grain structures, that is, 1) nanotwined and 2) consisted of dislocation low‐angle grain boundaries are produced by hydrostatic extrusion either hot (with preheating at 1000 °C) or room temperature with a total true strain of 1.4. The coarse‐grained sample is used as a reference one. These three types of samples are nitrided using low‐temperature plasma‐assisted nitriding at 430 °C for 5 h. Nitrided layers consisting of S‐phase are produced on all the samples. However, only minor differences in their thickness are observed. The analysis reveals that the formation of nitrided layers is controlled by volume diffusion disregarding the grain structure of the substrates. This is attributed to very specific grain boundaries (twin and dislocation grain boundaries) dominating in hydrostatically extruded samples. Such special grain boundaries do not provide fast diffusion channel but act as trapping sites for diffusing atoms. As a consequence, much more nitrogen is stored in the layers formed on the samples previously subjected to hydrostatic extrusion but the layer thickness does not differ significantly.