CREEP FAILURES IN STEAM LINES OPERATING BELOW LIMIT TEMPERATURES FOR CREEP DESIGN

A couple of unexpected and serious creep failures in live steam pipe components occurred in Sweden some years ago. The service temperature was 450°C which is 30°C below the limit temperature for creep design for the used steel grade in the cases of point, 13 CrMo 4 4 (1Cr0,5Mo). This steel is also the most commonly used material in Sweden for steam lines operating at 420 – 480°C. Consequently, a number of steam pipe systems at other plants with similar service data were inspected with respect to creep for the first time. Creep cracks and creep cavitations were found in several of them. Damages were foremost observed in welds in T-joints. Some of these T-joints were retired and retained for metallographic investigations. Among those that had operated at 450°C significant creep cavitations were observed in vicinity to cracks but in general the spreading of cavities was smaller than typically observed in the creep range. Furthermore, parametric stress and creep crack propagation analyses have been carried out to find out the conditions that may lead to creep failure at service temperatures below the limit temperature for creep design. High local stress is a key factor. For example, small initial defects together with component design with respect to yield strength only were shown to give sufficient stresses for relatively fast initiation and propagation of creep cracks at 450°C.