Whether safety critical welds in stainless steel would be fit-for-service in the chilly realm of -160◦C was the great imponderable confronting one of TWI's industrial members recently.
The company, a key player in the oil and gas field, asked TWI's structural integrity and materials teams to demonstrate using fracture mechanics that submerged arc welded longitudinal seam welds in 304L fittings would perform up to scratch at such low operating temperatures.
How did TWI tackle the work? First, Charpy V-notch tests were conducted on the longitudinal weld of a 72in elbow, supplied by the client. It had a nominal wall thickness of 20mm, and this indicated that the specified minimum lateral expansion of 0.38mm might not be achieved at -175◦C in the weld metal of fittings made according to the same weld procedure specification.
Furthermore, Feritscope measurements showed ferrite numbers (FN) greater than eight. This exceeded the client's specified limit, understood to be imposed for Charpy notch toughness considerations.
Ferrite numbers greater than eight were also measured in 56 and 18 inch elbows, but Charpy tests achieved 0.38mm lateral expansion at -196◦C.
To establish whether fittings operating down to -160◦C are fit-for-service, it was agreed with the client that:
- A fracture mechanics based assessment procedure would be used to demonstrate fitness-for-service.
- The longitudinal weld in the 72in elbow was to be used to determine fracture toughness and provide necessary input for the fracture mechanics analyses of all fittings.
Tests were conducted at -160, -168 and -175◦C. Ferrite number determination and metallography were both carried out and the Engineering Critical Assessment performed. This uses data from the mechanical tests together with stress data provided by the client to enable allowable flaw size to be estimated. The results of these tests and analyses were then discussed in the context of assessing fitness-for-service of the fittings for LNG service.
Given the nature of flaws that might be present and the radiographic inspection carried out by the fabricator, TWI considered that the presence of flaws, exceeding the limiting size calculated by fracture mechanics, was highly unlikely. Consequently, it was considered that the fittings were fit for intended service.
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