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. A high strength weld metal may not be able to tolerate hydrogen introduced by welding over it with a high hydrogen consumable. 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.Īnother issue to be careful of is hydrogen control. Another factor to consider is compositional variation between the two welds. 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. 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. Of particular importance are any differences between the welding procedures of the two overlapping welds. Split tee connections will also require welds impinging on the original pipeline fabrication welds.Įven though welds on welds are common, there are nevertheless some potential concerns. Pipeline circumferential welds cross the longitudinal seam welds, and pressure vessel nozzles cannot always be added in regions free from fabrication butt welds. Multi-pass welds after all are manufactured by welding over weld metal! There are also many accepted procedures in which welds overlap. It should be remembered that welding over weld metal is actually a very common occurrence. National Structural Integrity Research Centre.Structural Integrity Research Foundation.
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