Sea Wolf welding wire high-side chemistry / hydrogen cracking

Two-billion-dollar Navy problem traced to welding wire drawn from an enriched (high-side-chemistry) region of a twenty-ton ingot. Yield strength came in at 140 ksi instead of 100 ksi; hydrogen sensitivity scaled accordingly. All within spec but at the wrong end of the range. Used to land the teaching point that microsegregation in the original casting propagates through downstream processing.

Why can that be a problem? The whole Sea Wolf problem, two billion dollars it cost the Navy, was because the welding wire was too rich. It had too much of certain alloying elements, and it probably came from an enriched area of the casting. It was all drawn into wire, but they didn't draw the whole twenty-ton ingot into wire all at once. They cut it up in pieces, and so this part of the ingot was rich, this part was poor in alloying element. The lot they used turned out to be what they called high-side chemistry. If you had gone to that same ingot, you would find low-side chemistry elsewhere. The problem with the high-side chemistry: it had a yield strength of about 140 ksi, not 100 ksi, because it was enriched in alloy. At 140 ksi, your sensitivity to hydrogen is much worse. You can't tolerate anywhere near as much hydrogen. It was all within the Navy spec range, but it happened to be all the way at one end of the high-side chemistry. They always give you a range because there are these inhomogeneities in the casting. They got bit by segregation all the way back in the original ingot. Some of these things actually are important, and they do follow through the whole system and can sometimes create real problems for you.