Structural stability of austenitic CrMn(Mo)N steels for high-strength, corrosion-resistant retaining rings

The improved temperature resistance of insulating materials enables the service temperature of retaining rings on electric generator shafts to be raised from 90 to 130 degreesC. Therefore the structural stability of two high nitrogen austenitic steels in respect to cold expansion and ageing was investigated by XRD, STEM and internal friction. Cold working caused the formation of about 5 to 10 % epsilon-martensite, which was promoted by the low stacking fault energy of about 30 mJ/m(2). Ageing between 90 and 170 degreesC was accompanied by the precipitation of Fe3N which dissolved during subsequent aging at 400 degreesC because of the stronger bond of nitrogen to dislocations. Their pinning prevented long-term structural changes of a ring which was shrink fitted at 400 degreesC and more than 60.000 h in service.