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

Abstract

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 M 23 C 6 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.

A comparison of three weld consumables for P92 under creep

“…Such precipitate free zones have been observed before in 9Cr weld metal [4] and it was shown in that work that the PFZ developed during creep testing of the weld metal due to carbon diffusion down activity gradients associated with local variations in the weld metal composition. These variations were quantified, with the main variation being in chromium level, where the PFZs were observed to have chromium levels typically 1.5 wt% less than those of the surrounding material, causing the carbon to diffuse down the activity gradient to the high chromium material.…”

“…However, creep failures within this type of weld metal have also been reported, typically with cracking occurring along the coarse columnar grain boundaries [4]. Therefore, selection of a consumable that provides a strong weld metal can be significant in increasing the service life of a welded pipe.…”

Solidification phenomena in creep resistant 9Cr weld metals and their implications on mechanical properties

“…Zhang [8], Zhang et al [9] and Allen et al [10] reported creep damage occurring within P91 and P92 weld metals. Among other things, so-called white bands, which were formed between the individual welding layers, have been identified as the cause of damage.…”

“…The total damage was caused by the combination of the individual crack phenomena. Zhang et al [9] also described the "white bands" with regard to creep strength as a detrimental factor, since they cause Type I cracks in the columnar weld metal areas to grow together and accelerate the damage, in particular under a load transverse to the weld seam.…”

“…According to [8,9], the "white bands" can be described as follows. Within these zonestypically 100 to 200-μm widea reduced number of stabilizing precipitates is found.…”