Comparative Plasma Nitrocarburizing of AISI 316L and AISI 304 Steels Using a Solid Carbon Active Screen: Differences in the Developing Microstructures

Abstract

This paper features a direct comparison of the response of AISI 316L and AISI 304 austenitic stainless steels to active screen plasma nitrocarburizing applied for a wide range of treatment temperatures from 380 °C to 480 °C (653 K to 753 K). Thereby, an active screen made of solid carbon was used without applying bias plasma to the treated steel samples. Since it is believed that the result of treatment strongly depends on the concentration of in-situ generated hydrogen cyanide, the conditions of the treatments were adapted to keep this concentration constant. Under such treatment conditions, it was shown that AISI 316L has a lower tendency to form CrN than AISI 304 at comparable treatment temperatures, where the threshold temperature was found below 480 °C (753 K) for the former and below 440 °C (713 K) for the latter. In addition, the decomposition of expanded austenite generated on the treated AISI 316L at high temperatures leads to CrN and (mainly) Cr-depleted austenite phase, which is possibly accompanied by minor amounts of $$\alpha$$ α -ferrite. Instead, for the treated AISI 304, decomposition leads to CrN and transformed $$\alpha$$ α -ferrite phase. The modified layer thicknesses achieved after a given treatment duration and the kinetics of the layer growth showed a characteristic temperature dependence for each steel, which is discussed in accordance with the results of microstructure analysis.

Comparison of Nitriding Behavior for Austenitic Stainless Steel 316Ti and Super Austenitic Stainless Steel 904L

“…Nevertheless, several open questions remain for further discussion and investigation: why does the nucleation of CrN precipitates start at a much lower temperature in 904L-or at earlier times-during nitriding than in 316Ti? The alloy composition cannot be the only reason as the onset of CrN formation after nitrocarburizing is shifted by about 10 K towards higher temperatures for the higher-alloyed 316L compared to the alloy 304 [29].…”

“…The sputtering during nitriding is only a minor corrective term which does not significantly affect the diffusivity of the used short treatments [28]. Conversely, it is difficult or impossible to establish the formation of CrN, which should lead to a different "average diffusivity", as observed in the 5h treatments [29].…”