Abstract
Hardness distribution, micrographs, and chemical analysis of the nitrided surface layer under various conditions were investigated for a 2.25Cr-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.
2) Nitride 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 ductility.
4) Combination of high creep rupture strength and good ductility were achieved by nitriding for relatively short period.
