Elemental Inter-diffusions for the Transformation of δ/M₂₃C₆/γ Phases During Heat Treatment Processes in Heavy Gage 316H-Type Austenitic Stainless Steel

The δ-ferrite and M₂₃C₆ carbide are easy to form and usually have detrimental effects on service performance in heavy gage 316H-type austenitic stainless steel. In order to clarify their transformation behavior in the austenite matrix, elemental inter-diffusions between δ/M₂₃C₆/γ phases were investigated by the in situ observation of heating and cooling processes and the concentration analysis of C, Cr, Mo and Ni elements. The M₂₃C₆-type carbides were found to precipitate preferentially along the interface between austenite and retained δ-ferrite with cooling rate of less than 100 °C/s, where the Mo, Cr and C elements were measured to be enriched and Ni element was depleted. The decomposition of M₂₃C₆ during heating at above 950 °C, on the other hand, could enhance the enrichments of Mo and Cr on the δ-ferrite side, and thus increase the difficulty of δ-ferrite dissolution. The nano-indentation tests indicated that nano-hardness values were increased sequentially for austenite, δ-ferrite and M₂₃C₆ phases with the increase of Mo, Cr and C concentrations among them. Moreover, the relative concentration difference of Mo and Cr between two phases could easily cause electrochemical corrosion, leading to preferentially corrosion in the phase with lower Mo and Cr concentration.