Creep rupture behaviour of circumferentially welded mod. 9Cr-1Mo steel pipe subject to internal pressure and axial load

The multi-axial creep strength of circumferential welds in power piping, including failure mode and failure life against a wide range of stress ratios and stress levels, was newly examined in this paper. The creep rupture behaviour of modified 9Cr-1Mo steel (9Cr-1Mo-VNb steel; ASME P91) pipe with a circumferential weld subject to combined internal pressure and axial load was experimentally investigated at 650 degrees C. The test results, with several kinds of stress ratios of macroscopic axial and hoop stress can be summarised as follows. Along with the increased stress ratio (axial/hoop), both failure location and failure mode changed from a base metal failure caused by hoop stress to an FGHAZ (fine-grained heat affected zone) failure due to axial stress. The stress ratio where the failure mode changed was '0.8'. Strength reduction in FGHAZ failure by axial stress should be considered in the structural design of circumferential welds. A series of FEM creep analysis was carried out to discuss the relation of the failure mode to both the local stress distribution and the damage. The failure mode variation along with the increase in the stress ratio is also discussed based on simplified ductile creep failure analysis.