scope:

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.