Phase changes of Fe−20Mn−13Cr−3Co/1.2Ti alloys during solution nitriding

Recently, solution nitriding, a method of nitrogen permeation into the surfaec of stainless steel by heat treatment in a nitrogen gas atmosphere, has been studied as a new nitrogen-addition method. The phase changes of Fe-20Mn-13Cr-3Co/1.2Ti alloys have been studied during solution nitriding at 1150 DGC in a nitrogen gas atmosphere. Investigations have focused on the phase change, the variation of nitrogen content, the hardness change, and the corrosion resistance. Fe-20Mn-13Cr-3Co alloy displayed austenite single phase by solution nitriding at the surface layer, while the interior region, where the nitrogen did not permeate, exhibited a mixture of austenite, ferrite, and e-martensite. For the alloy of Fe-20Mn-13Cr-1.2Ti, titanium nitrides of square type at the outmost surface and of rod type at the next surface were detected in the austenite matrix, while ferrite and austenite appeared in the interior region. From observation of the microstructure of two austenites of Fe-20Mn-13Cr-1.2Ti alloy, it was found that few stacking faults existed in the austenite of the surface layer, likely due to a high SFE caused by high nitrogen content. In contrast, many stacking faults were observed in the nitrogen-free austenite of the interior region, possibly due to a low SFE. The nitrogen content of Fe-20Mn-13Cr-3Co alloy decreased gradually with increasing depth below the surface after showing a maximum of 1.30 % at the outmost surface, which was in line with the trend of the hardness decrease. For the Fe-20Mn-13Cr-1.2Ti alloy, Ti diffused from the interior to the surface when the nitrogen permeated from the surface into the interior owing to strong affinity between Ti and N, which resulted in a maximum Ti content at the outmost surface. Solution nitriding remarkably improved the corrosion resistance in Fe-20Mn-13Cr-3Co alloy, while the precipitation of titanium nitride deteriorated the corrosion resistance in Fe-20Mn-13Cr-1.2Ti alloy.