Dissimilar welds are widely used in tubular systems of power plant boilers to compensate for different operating conditions. Redistribution of interstitial elements across the fusion line is regarded to be the most dangerous degradation mechanism in heterogeneous welds. Carbon and nitrogen move along the gradient of their chemical potential, in our case from P23 steel towards P91 steel. The paper deals with a TEM characterization of precipitation processes in fusion zones of two variants of heterogeneous P23/P91 welds after creep tests at 550 °C with times to rupture up to ca 80 000 hours. Both carbon extraction replicas and thin foils were studied. In carburized areas of welds heavy precipitation of carbides was observed, most of them corresponded to M23C6 carbides. The Cr/Fe ratio in these carbides continuously and systematically changed from the onset of carburized areas towards WM91 or P91 base material. This proved that carburization started in fusion zones, characterized by a continuous gradient of chemical composition. Furthermore, a low fraction of M6X particles occurred in carburized areas. In adjacent partly decarburized areas of WM23 or P23 steel M23C6 and M7C3 particles dissolved during creep at 550°C. Furthermore, SAED investigations revealed precipitation of Laves phase of Fe2W type in partly decarburized areas of both variants of P23/P91 welds. Precipitation of Laves phase has not been reported in P23 steel yet. Thermodynamic calculations proved that a reduction of carbon content in partly decarburized areas of WM23 or P23 steel made it possible to stabilize this minor phase.Keywords: TEM, heterogeneous welds, fusion zone, minor phases
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