Operating life and reliability of steam boiler and piping systems are considerably determined by long-term creep rupture strength (CRS) of welded joints of boiler tubes and pipes. Consequently, the appreciable research effort has been focused to reveal the most important factors affecting the creep properties of weldments. The presented work deals with the evaluation of creep rupture strength of welded joints in steam boiler piping system made from low-alloy and chromium modified steels. Cross-weld creep rupture tests and, at the same time, comparative creep tests of parent material are considered as the optimum approach how to evaluate the decreasing of creep resistance of weldment towards the base metal and how to quantify this effect e.g. by using strength reduction factor (SRF) and lifetime reduction factor (LRF) of welded joints. The necessary condition for such an approach is that the mechanism of creep damage (type IV cracking) in laboratory tests must be the same that acts in in-plant welded joints of the mentioned steels. The obtained results confirmed that the precipitation strengthened steels (mainly Cr-Mo-V type) show more pronounced decreasing of creep rupture strength of the critical locality of welded joint – intercritical part of heat affected zone when compared to Cr-Mo steels and that they are, therefore, much more prone to the premature failure.Keywords: Creep, Cr-Mo-V steel, steel P91, strength reduction factor, type IV damage
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