Characterisation of microstructures in nickel based transition joints

The microstructural changes occurring at the low alloy steel fusion line in nickel based transition welds are believed to be critical to the developme nt of creep cavities and cracks. The progressive changes in Type I carbides observed following laboratory ageing at 625 degrees C and from interrupted creep specimens tested at 590-625 degrees C have been evaluated by optical and electron microscopy. In general, these carbides appear to be M23C6 typ e and grow in an approximately elliptical shape with the long dimension in the plane of the interface. The behaviour noted is in good general agreemen t with a cubic growth law and predictions made from laboratory results are in agreement with previous observations from plant welds. The accuracy of t hese predictions is such that measurements of carbides in service can be us ed to estimate an equivalent operating temperature. (C) 2000 Kluwer Academi c Publishers.