For the assessment of welds under high-temperature conditions in the creep or creep-fatigue regimes, the damage location and its temporal evolution is of high importance. The failure behaviour of similar welds of ferritic-martensitic steels in the creep regime is well known. The fracture locations shifts from base metal to the outer heat affected zone (Type IV cracking) with increasing time and decreasing stress. For creep fatigue loads the behaviour of welds is still subject of research but it seems that the HAZ limits the life time as well. This local failure behaviour is not reflected in design guidelines using weld reduction factors, like the weld strength factor (WSF, see ECCC guidelines) or in typical assessment approaches. The evaluation of local strains and stresses in the heat affected zone is unavoidable. For the improvement of design and inspection guidelines, a more detailed consideration of weld behaviour is of interest.

In this paper, some current developments in the assessment of welds under creep, fatigue and creep-fatigue loading conditions are given. The influence of multiaxial stress states on weld behaviour under creep will be discussed. For this, an assessment approach for creep failure including the prediction of rupture time and location will be presented. The assessment is based on numerical analyses considering the different behaviour of base and weld material as well as heat affected zone represented by 3 different ranges. A numerical damage model is included in the constitutive equations describing the behaviour of the above mentioned materials.  The approach will be validated with the simulation of uniaxial crossweld tests, creep crack tests and component tests. For the component tests pipes with internal pressure and optional additional axial load were used.

Whereas the creep behaviour of the HAZ compared to base metal is quite well known, there is only little knowledge of their cyclic behaviour. Using a set of fatigue tests on HAZ, base metal specimens and crossweld specimens, the influence of fatigue and creep-fatigue on the lifetime and failure location of a weld will be discussed. For the numerical simulations a viscoplastic material law of chaboche type is used and an evaluation of the local strains in the HAZ allows an attempt to explain the observed failure locations. A current ongoing validation test on a circumferential weld in a pipe will be presented and an outlook will be given.


Damage Assessment of Similar Martensitic Welds Under Creep, Fatigue and Creep-Fatigue Loading

Division / Sponsors: 
Pressure Vessels & Piping (PVP)


Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2020/volume/83815

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