Found some pictures here of the failed San Bruno pipe, along with some experts’ (?) comments. I can’t draw even superficial conclusions as the pictures aren’t detailed enough and I lack the expertise anyway. However the zig-zag fracture with long straight edges and a couple of near-90º corners looks distinctly odd, and not at all what one would expect from ductile steel with sound welds. So suggestions about cracked welds seem plausible.
Any other thoughts?