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Description / Abstract:
INTRODUCTION
Post-weld heat treatment (PWHT) is applied to steel assemblies
in order to reduce the likelihood of brittle fracture; this is
achieved through the combined effect of a reduction in residual
stresses and the tempering of hard, brittle microstructures.
For fabrications where the need for a welded repair has been
identified, but for which a post-weld heat treatment would be
impractical or expensive, it is possible to affect a repair by
using a procedure which is intended to give rise to significant
refinement and tempering of the heat-affected zone (HAZ)
microstructure. Furthermore, for materials operating within the
creep regime, and particularly for a repair with near-matching weld
metal, whilst the imposition of a PWHT would substantially reduce
the level of residual stress, it is likely to shorten creep life by
inducing precipitate growth and substructure recovery in the
HAZ.
Several slightly different approaches have been developed for
repair without PWHT, from conventional buttering to a half-bead
technique, and later a two-layer approach. A schematic diagram of a
weldment, together with the relevant regions of the Fe-C phase
diagram is shown in Figure 1 [1]. The principle of the various
controlled deposition repair strategies is that weld beads in the
first layer are overlapped such that at least part of the
coarse-grained region produced by the previous bead is replaced
with fine-grained HAZ. A second layer, with appropriate
penetration, can achieve further refinement, and further layers may
be added to achieve tempering.