It should be remembered that welding over weld metal is actually a very common occurrence. Multi-pass welds after all are manufactured by welding over weld metal! There are also many accepted procedures in which welds overlap. Pipeline circumferential welds cross the longitudinal seam welds, and pressure vessel nozzles cannot always be added in regions free from fabrication butt welds. Split tee connections will also require welds impinging on the original pipeline fabrication welds.
Even though welds on welds are common, there are nevertheless some potential concerns. Of particular importance are any differences between the welding procedures of the two overlapping welds. If the heat inputs are different, such as when a multi-pass girth weld overlaps a single-pass seam weld, the cooling rate of the weld metal in the two cases will be different. Thus the weld metal, which was designed to give satisfactory properties (in this case) with a slow cooling rate, may not perform so well in the HAZ of the faster cooling girth weld. Another factor to consider is compositional variation between the two welds. For example, alloying which may be required to obtain satisfactory properties in a high heat input weld, may result in poor microstructure and/or excessive hardness when incorporated into a low heat input weld by dilution.
Another issue to be careful of is hydrogen control. A high strength weld metal may not be able to tolerate hydrogen introduced by welding over it with a high hydrogen consumable.
In addition to metallurgical factors, it is worth bearing in mind that an existing weld is more likely than parent material to contain defects, such as fabrication flaws or cracking from service. Defects which are welded over, and not melted out, can suffer locally intensified strain age embrittlement by static or dynamic strain ageing at the region of concentrated strain at the flaw tip, leaving a planar defect with its tip in a region of low toughness (Dawes M G). It is therefore important to inspect the region to be repaired in advance (as well as afterwards), using appropriate non-destructive testing methods (see Further Information below).This is particularly important for repairs; however, in many cases new construction welds are also routinely inspected prior to over-welding.
FAQ: Standards commonly used in the UK joining industry: Quality and NDT
Dawes, M G; Francis-Scrutton, N: 'Locally intensified strain ageing of C and C:Mn steels and weld metals'. OMAE-95 V.3, ASME (1995) pp.471-477.