Abstract
Colossal N supersaturation of ferritic as well as austenitic stainless steels during low temperature gaseous nitridation treatments has lately gained much technological significance. However, available thermodynamic models to calculate the N paraequilibrium solubility limits have failed to explain the levels of colossal N supersaturation observed in several cases of nitrided ferritic/austenitic stainless steels. In this work, we show that consideration of N dissolution induced spinodal decomposition is essential in calculating the N paraequilibrium solubility limit for both ferritic and austenitic stainless steels. This modification in the thermodynamic model has led to the successful explanation of the thermodynamic cause for the colossal N supersaturation in ferritic and austenitic stainless steels. Available experimental observations in literature support the occurrence of spinodal decomposition.
Highlights
Surface modification of stainless steels for the purpose of simultaneously achieving improved corrosion resistance and surface mechanical properties is of great technological significance[1,2,3,4]
Because of the importance of understanding and optimizing the colossal levels of N dissolution, researchers have made attempts to calculate the maximum N paraequilibrium solubility which can be realized for applied chemical potential of N12 in nitriding atmosphere for different ferritic and austenitic stainless steels while suppressing Cr-nitrides development[7,11,13,14]
Ferritic/austenitic solid solution becomes equal to that constantly maintained in the nitriding atmosphere. Such calculations for the case of controlled gaseous nitriding of stainless steels, which allow precise control of chemical potential of N in gas phase, are expected to correctly predict the measured nitrogen contents. Such thermodynamic model calculations appear theoretically correct, until now such calculations have failed to explain the measured levels of N supersaturations realized in stainless steels; the calculated N paraequilibrium solubility limits for AISI 316 austenitic stainless steel was much greater than the observed N contents, whereas the calculated paraequilibrium N content for delta-ferrite of 17-7 PH stainless steel is much smaller than the observed content[11,15]
Summary
Ferritic/austenitic solid solution (in the work piece) becomes equal to that constantly maintained in the nitriding atmosphere. Since phase separations has been shown (from Figs 1 and 2) to be feasible at relatively very low N-contents than those observed, the paraequilibrium that is closer to reality is that between the spinodally decomposed (phase separated) solid solutions and the nitriding atmosphere rather than between the un-decomposed, homogeneous solid solution and nitriding atmosphere This kind of chemical equilibration involving two distinct solid solution compositions pertaining to the same phase (austenite or ferrite, respectively) with the gas atmosphere was achieved by imposing the condition of global minimization of Gibbs free energy of the solid solution mixture while imposing the condition that the chemical potential of N in all solid solution phases is equal to that in the gas phase. As will be shown in the discussion, the close correlation between the calculated results and experimental observations in literature suggests that such extrapolations carried out in this work are quite reasonable
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