The paper summarizes previously derived constitutive parameters for temperatures of 575, 590, 600, 620 and 640 8C. Values of the multi-axial stress rupture parameter, #, are reviewed and recorded. This constitutive parameter set is used to determine the thickness of the Type IV material zone (0.7 mm). Values of Type IV multi-axial stress rupture parameter # are determined for a wide range of butt-welded pipe and cross-welded uni-axial specimens, and an interpolation equation is derived in terms of temperatures and stress level. Finally, continuum damage mechanics (CDM) analyses are performed for pipes and cross-welded testpieces, which include a coarse-grained heataffected zone (CG-HAZ). It is shown that the constitutive parameter set, which corresponds to a minimum creep rate ratio of 1/2.5, with respect to the parent material, gives accurate predictions of lifetimes and damage distributions.

The paper reports the use of three-dimensional creep continuum damage mechanics techniques to study the creep failure of a medium-bore low-alloy ferritic-steel cylinder-cylinder branched pressure vessel welded connection, tested at a constant pressure of 4 MPa, at a uniform temperature of 590 • C. The development of computational techniques is reported to analyse this problem with a four-material model of the welded connection which includes: parent, type IV, heat-affected zone (HAZ) and weld materials. The results of analyses are presented for two sets of creep damage constitutive equations. For both equation sets, lifetimes are conservatively, yet accurately predicted; however, the results of metallographic examinations of a tested vessel are not accurately predicted. To overcome this deficiency further analyses of the vessel are recommended which include: a coarse-grained HAZ (CGHAZ), adjacent to the weld material; and, more-refined finite element modelling.

The paper concerns the development of constitutive equations for 316 stainless steel at 550 8C; it, firstly, considers time independent plastic straining at high temperature; and, secondly, describes how mechanisms-based creep constitutive equations have been formulated. It is shown how a power-law hardening model may be used to describe the effects of prior plastic pre-strain, by providing excellent comparisons with the results of experiments. The paper then develops constitutive equations for time dependent creep based on the theory of continuum damage mechanics. The corresponding state variable rate equations have been selected to include a description of strain hardening in primary creep, a major contributor to strain in this material, and also softening due to creep constrained cavity growth. Despite the paucity of data at lower stresses, corresponding to lifetimes in the region of 10 5 h, a model has been formulated which gives good predictions of experimental results over a wide stress range. The paper discusses how the time independent and time dependent models may be combined in a finite element analysis code to predict inelastic straining due to initial loading and to stress redistribution encountered in reheat cracking of welded pressure vessels. q

Constitutive equations are reviewed and presented for low alloy ferritic steels which undergo creep deformation and damage at high temperatures; and, a thermodynamic framework is provided for the deformation rate potentials used in the equations. Finite element continuum damage mechanics studies have been carried out using these constitutive equations on butt-welded low alloy ferritic steel pipes subjected to combined internal pressure and axial loads at 590 and 620 8C. Two dominant modes of failure have been identified: firstly, fusion boundary failure at high stresses; and, secondly, Type IV failure at low stresses. The stress level at which the switch in failure mechanism takes place has been found to be associated with the relative creep resistance and lifetimes, over a wide range of uniaxial stresses, for parent, heat affected zone, Type IV and weld materials. The equi-biaxial stress loading condition (mean diameter stress equal to the axial stress) has been confirmed to be the worst loading condition. For this condition, simple design formulae are proposed for both 590 and 620 8C.

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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.

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