As design temperatures increase and steel strength increases, the limits on yield strength of the nickel-alloy weld consumables make it more difficult to achieve overmatched weld areas, increasing the difficulty of a strain-based design assessment. Testing of Alloy 625 girth welds in X65 pipe clad with Alloy 825 found that the welds were approximately matching in yield strength to the steel in circumferential direction tests. Cross-weld tests at 20°C and 150°C found localized yielding up to 15% strain in the weld cap area followed by distribution of strain into the steel and failure in the steel.
The use of clad pipes for offshore oil and gas production has been worldwide, with multiple methods of pipe laying considered (Hval et al. 2014) (Jones et al. 2011). Assessments of the allowable flaw sizes in these girth welds generally indicate that there is value in increasing the strength of the girth welded area toward overmatching of both the yield and ultimate strength of the girth weld compared to the base metal, particularly in cases with cyclic plastic strain (Yang et al. 2009) (Tronskar et al. 2015) (Tkaczyk et al. 2015) (Carlucci et al. 2014).
There has been a desire to continue to use a single electrode for the entire girth weld to advance strength, but also provide internal corrosion resistance. Some patented methods are available for increasing the strength of girth welds, while retaining other desirable properties (Ayer et al. 2013) (Ayer et al. 2014). These methods have not widely entered welding practice, as users have primarily worked with Alloy 625 and Alloy 688 consumables.
Under these circumstances one way of improving the assessment of the allowable flaw size for strain-based design conditions would be to include a better understanding of the yielding behavior and strain aging capability of both the steel and the weld metal into models describing their behavior during reeling, installation, and service.
This phase of the project looks at the tensile behavior of example weld areas both at room temperature and at elevated temperatures.
