Deciding about the toughness of ERW fusion lines based on MTR values
Is it always trustable to decide about the toughness of ERW fusion lines based on MTR?
Initiation/Propagation of cracks through ERW fusion lines is a likely failure mechanism especially in casing strings like P110 tubing.
Center-line segregation and fusion line embrittlement due to metallurgical effects as well as hydrogen embrittlement and a low tempering temperature (for QT pipes) may result in very brittle fusion lines.
Having said that, some codes like Canadian CSA Z245.1 allow the CVN notch to be WITHIN 3 mm from ERW fusion line (not exactly at the line), so MTR CVN values for the fusion line may not show the embrittlement problem of the weld. Moreover, there is no minimum tempering temperature requirement (for P110 for instance) in API 5CT while tempering at low temperatures may result in severe ERW embrittlement.
Here are some good reads: