Continuum damage mechanics predictions of creep damage initiation and growth in ferritic steel weldments in a medium bore branched pipe under constant pressure at 590 °C using a five-material weld model

Abstract

The paper reports three-dimensional creep continuum damage mechanics (CDM) analyses of creep failure in a medium bore Cr–Mo–V low alloy ferritic steel welded branched-pressure vessel that has been tested under a constant pressure of 4 MPa, at a uniform temperature of 590 °C. The use of the CDM computer software Damage XXX to analyse the initiation and growth of creep damage and subsequent failure in the branch weld is reported for a five-material model that includes: parent, Type IV, refined heat affected zone (R-HAZ), coarse grained heat affected zone (CG-HAZ) and weld materials. The results of the analyses are presented for two cases: the first without the CG-HAZ; and, the second with the CG-HAZ included. For both cases, lifetimes are conservatively, yet accurately predicted. It is shown that it is necessary to use a Type IV thickness of 0.7 mm to accurately predict the failure location and mode. The results of metallographic examinations of a tested vessel and the predicted damage fields are in close accord. Failure is predicted to take place, by steam leakage, from the interior of the vessel, through the Type IV zone adjacent to the main pipe, connecting through the R-HAZ to the CG-HAZ, where leakage takes place at the weld toe in the crotch plane.