For fabrications which need repair, but for which a post-weld heat treatment would be impractical or expensive, it may be possible to effect a repair by using a procedure which is intended to develop a heat-affected zone (HAZ) with a fine grain structure.
The approach is applicable to steel or an alloy which undergoes a phase transformation during a weld thermal cycle. (In the HAZ of a steel, the initial normally body-centred crystal structure, which is present at room temperature, changes to the high temperature form (austenite), and then back to a body-centred structure - or a variant of this - as it cools down again. The boundaries of the austenite grains can usually be revealed in the HAZ by using an appropriate etchant. Close to the fusion boundary, where growth of the prior-austenite grains occurred, is the coarse-grained HAZ and beyond it the fine-grained HAZ). Generally, the technique involves the deposition of two or more layers of near-matching manual metal arc (MMA) weld metal with small (2.4 or 3.2mm) diameter electrodes. Each bead overlaps the previous one substantially (typically by ~50%), so that the coarse-grained HAZ region produced by one bead is largely replaced by a fine-grained region produced by the subsequent bead. Further weld layers are deposited at a higher heat input (typically 1.5 to 2 times that of the first layer), using larger diameter electrodes (3.2 or 4mm),so that any coarse-grained HAZ remaining from the first layer is replaced by fine-grained HAZ produced by the second layer. A two-layer procedure, which is appropriate for C-Mn steels, is given in Ref. [1, 2]. In more hardenable steels, further layers may be added to effect tempering of the HAZ. A four-layer technique is described in Ref. .
As an alternative to near-matching weld metal, Ni-base electrodes may be used for a controlled deposition repair to a steel fabrication. Such electrodes operate at lower currents, and create a narrower HAZ than a ferritic electrode of the same diameter. The weld metal is of higher toughness than ferritic weld metal, and the higher solubility of hydrogen in such weld metal will avoid the risk of hydrogen-assisted cracking. However, the dissimilar metal combination will hinder considerably any non-destructive examination. Repairs effected with Ni-base consumables are more likely to be considered as temporary repairs. The advantages and disadvantages of using ferritic or nickel base electrodes for repair welding of Cr-Mo steels has been discussed in Ref.   .
Consideration must be given to any code requirements, and to obtaining the agreement of interested parties as appropriate. Also, where a repair introduces a high level of restraint, consideration should be given to the possible need to employ a higher preheat and minimum interpass temperature than would be used in conventional welding. Consideration should also be given to the residual stresses and their consequences during early service. Refinement of the grain size in the HAZ, and subsequent possible tempering, should reduce the maximum HAZ hardness. However, care is needed around the periphery of the repair to avoid leaving or creating any higher hardness as-deposited coarse-grained HAZ. Run-off strips may be added around the repair, and ground away upon completion of all welding, but all tacking must be carried out on the inside of the repair.
Please see TWI Knowledge Summary on controlled deposition repair welding.
- Alberry P J: 'Simple test reveals level of two-layer (weld microstructure) refinement', Welding and Metal Fabrication, 1981, 49(9), 543-547.
- Jones R L: 'Development of two-layer deposition techniques for the manual metal arc repair welding of thick C-Mn steel plate without post-weld heat treatment', The Welding Institute Research Report 335/1987, April 1987.
- Friedman L: 'The EWI/TWI controlled deposition repair welding procedure for 1.25%Cr-0.5%Mo and 2.25%Cr-1%Mo steels', Welding Research Council Bulletin 412 June 1996, 27-34.
- S J Brett, D J Abson and R L Jones: 'The repair welding of power plant without post-weld heat treatment', Proc. Int. Conf. on Integrity of high temperature welds, 3-4 November, 1998, Nottingham, UK, The Institution of Mechanical Engineers, 53-63.
- Lant et al., 'Review of weld repair procedures for low alloy steels designed to minimise the risk of future cracking', International Journal of Pressure Vessels and Piping, vol.78, no.11-12. Nov.-Dec.2001.
- S. Issler et al., 'Weld repair of ferritic welded materials for high temperature application', International Materials Reviews, vol.49, no.5, 2004.