The formation of hard, wear and corrosion resistant surface layers on austenitic stainless steel using nitrogen insertion at elevated temperatures is nowadays a well established process. Here, a lattice expansion of up to 12 % is observed, incorporating nitrogen up to 38 at.%. However, for more demanding applications, more modern classes of stainless steel, such as super austenitic, duplex or precipitation hardening steels are employed in industry. There, nitriding processes are less investigated with sparse data found in the literature.
The main problem with conventional investigations of nitrogen content, nitriding depth and lattice expansion is that they are measured only after the nitriding process. Thus, details may either be missed or require a large number of samples.
In this presentation the dynamic nitriding as function of time and depth for an austenitic stainless steel (1.4571), super austenitic steel (1.4539), a duplex stainless steel (1.4462) and precipitation hardening steel (1.4545) is compared for a range of process temperatures.
The investigated austenitic steels show a similar evolution of layer thickness with time and temperature: however the super austenitic steel presents a much higher lattice expansion for (200) oriented grains at a similar nitrogen content. Furthermore, a similar thermal stability is found for both alloys with the formation of CrN starting around 475 – 500 °C.
For a duplex steel a faster diffusion is observed for the ferritic grains compared to austenitic ones while no formation of expanded ferrite is observed by XRD. As it is well known that nitrogen stabilized the austenitic phase, a transition from ferrite towards austenite with increasing nitrogen incorporation is postulated. The onset of CrN formation is slightly shifted to lower temperatures by about 20 K with increased nitrogen incorporation.
In contrast, a very peculiar behaviour is found for precipitation hardening steel. A very fast diffusion is observed estimated from the disappearing of base material peaks in a very short time. Even at 350°C – much below the published transition temperature for precipitation hardening – a complete loss of diffraction intensity is found for the whole measured 2θ-angular range of 35° – 54° after 30 min. For higher temperatures, a broad peak corresponding to a martensitic phase appears, indication a rather small grain size. No diffraction peaks corresponding to a nitrogen containing phase are detected. This very strong variation of the results with the nitriding temperature shows that special attention is necessary when using a commercial nitriding process to maintain consistent results.