Characterisation of creep-weak zones (white bands) in grade 91 weld metal

A microstructural study of creep failure in grade 91 weld metal has revealed two primary modes of creep failure. In addition to creep fractures along columnar grain boundaries (typical of weld metal creep failure), creep fractures were also found along creep-weak 'white bands' which had formed at the inter-bead boundaries. The white-band regions consisted of material where the M23C6 carbides had dissolved during creep testing; the loss of carbides had allowed recrystallisation of the martensitic structure to ferrite and consequently this material was much softer than the bulk weld metal. The element mapping over the weld metal by laser-induced breakdown spectroscopy demonstrated that there was significant inhomogeneity in the distribution of certain elements, most significantly, chromium. This inhomogeneity resulted in strong activity gradients in carbon (even though the carbon concentration was homogeneous following welding) resulting in carbon loss from the alloy-depleted regions, the associated dissolution of carbides and the recrystallisation that accompanied this, and thus the poor mechanical properties which resulted in creep failure.