Creep strain, damage and life prediction for welded 0.5CMV steel

Creep damage as cavitation and cracking has historically occurred somewhat earlier in the inspections of steam lines made of 0.5CMV steel, when compared to other low alloy steels. The difference has been attributed to modest creep ductility of 0.5CMV steel, and has resulted in attention paid to inspection and maintenance. As the steel is still being used in vintage plants, it is of interest for users while also serving as a model material that can show some essential features of long term creep in much shorter laboratory testing. Particularly challenging service can combine welded thick-wall components with pre-existing fabrication defects, resulting in an unfavorable combination of high stresses, multi-axial loading, and locally weak material. Examples are shown of the observed damage evolution in welded 0.5CMV steam line material in front of a crack-like defect. Long-term multi-axial loading does not necessarily result in type IV failure of welds, but can also induce creep damage and cracking close to the fusion line. This particularly applies to welds with undermatching weld metal such as those made with consumables corresponding to 2.25Cr-1Mo steel. With the LICON approach, it has been shown that this IIIa type of damage can be reproduced in about 5 000 h of multi-axial creep testing, while in plant such damage may require more than 100 000 h of service. The microstructural features of the observed damage and a comparison of uniaxial and CT test data suggest that the LICON approach of life prediction could be applied also for welded components where the location of maximum damage deviates from the type IV position.