Hardness distribution, micrograph, and chemical analysis of the nitrided surface layer under various conditions were investigated for a modified 9Cr-1Mo steel. Moreover, relationships between nitriding conditions and creep rupture properties were examined.
The results obtained are as follows.
1) The hardness of the nitrided surface layer increased and nitrogen diffused inside deeper with increasing nitriding time. However, increasing nitriding time more than 24 hours had liltle effect on maximum hardness and depth of hardened layer.
2) Nitrides on the surface spalled or decomposed to diffuse inside as solved nitrogen during heat treatment after nitriding.
3) Nitriding increased markedly creep rupture strength but nitriding for longer time decreased rupture ductilities.
4) Rupture ductilities were increased by decreasing nitriding time, polishing the nitrided layer, and nickel plating on the specimen.
