The newly developed grades of austenitic heat resistant steels (Super 304H, HR3C, TP 347 HFG) exhibit superior corrosion resistance in steam as well as creep strength thanks to their fine-grained microstructure, especially in case of Super 304H and TP 347 HFG. They are worldwide used in USC power plant boilers and substitute steels AISI 316, AISI 321 and/or AISI 347 formerly used in superheater and reheater tubes. On the other hand, the structural stability of these new steels is under examination, especially form the viewpoint of -phase precipitation which is known for its embrittlement effect.The precipitation of -phase was already found in the mentioned steels after long-term annealing at working temperature in the base material of tubes as well as in the tube bends. However, it seems that the presence of applied stress (and/or deformation) significantly promotes the precipitation of this phase. Therefore, much more and coarser particles of -phase were detected in the weldments after creep exposure.The paper presents the results of stress rupture tests of welded tubes made of steels Super 304H and HR3C, the assessment and prediction of long-term creep life and describes also the structural changes that appeared in these steels, especially precipitation of -phase, when significant difference in the amount of -phase between the both steels was revealed.Keywords: Super 304H, HR3C, welded joint, creep strength, sigma phase
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