Cast 17-4PH stainless steel exhibited pitting and fine surface cracks after gas nitriding. Parts also experienced bending at mid-radius. Parts were activated for gas nitriding by adding PVC pipe to the nitriding load.
Cast 17-4 PHSS will have about 2- 10% delta ferrite depending on the cooling rate of the cast section. Some amount of Cu segregation at the interdendritic regions could also be expected. The non-homogeneity in the microstructure/composition of the as-cast microstructure could affect the gas nitriding results.
There is no mention of the heat treatment condition of the material prior to nitriding and the nitriding conditions. Activation of SS surface by chloride is required for effective nitriding. The air-formed oxide layer need to be destabilized for effective diffusion of nitrogen. This chloride activation could also lead to pitting if moisture was present in the nitriding atmosphere and Cr level was low at local regions. Higher nitriding temperature (> 500 deg.C) and higher activity of nitrogen would increase the nitriding kinetics and may result in surface cracking. Solution treatment before nitriding and nitriding at ~400 C may help mitigate this problem.
Disclaimer: I am not an expert on nitriding or gas nitriding of any material. My view is purely based on general physical metallurgy principles.
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