Aging behaviour of 18Mn–18Cr high nitrogen austenitic steel for end rings

Abstract

The end windings of electrical generators are held in place by end rings which are heated to over 300°C during the fitting process. A new non-magnetic 18Mn–18Cr–0·6N steel has recently been developed for this application and the possibility of sensitization during service has been investigated. Precipitation of grain boundary nitrides is slow and from observations between 400 and 800°C the extrapolated time at 300°C for 10% of the boundaries of the as-received material to contain precipitates is 10⁵ h. Cellular precipitation occurs at high temperatures but is only detected at 600°C after 32 h. A re-solution treatment for 1 h at 1080°C reduces the subsequent rate of precipitation, whereas prior cold work accelerates precipitation. The precipitates are Cr rich but contain Mn and Fe. Below 700°C only the M₂₃X₆ structure forms, but above 700°C (CrMnFe)₂N is also observed. Both structures coexist for up to 68 h at 800°C. At temperatures above 550°C the rate of precipitation appears to be governed by Cr bulk diffusion, but at low temperatures the activation energy is lower, probably as a result of dislocation pipe diffusion.