Submarine first-dive stress relief

Submarines cannot be furnace-stress-relieved after outfitting (rubber, plastics, O-rings inside). The Navy uses hydrostatic compression on the first deep dive to mechanically relieve weld residual stresses — and requires shipyard top management to ride that first dive. Tom's "favorite quality-control story."

So what does the US Navy do? You can't take a submarine with all the components and stick it in a big furnace. That's what they do with a pressure vessel for something like a nuclear reactor — and that pressure vessel for a nuclear reactor is twice the diameter of this room and three times as tall, and they have furnaces that size. They'll stick it in there at 1200°F and stress relieve it. If they don't have a furnace that size, they will build a special furnace around it in the field, in thirty-foot-diameter vessels. But you can't do that once you have all the equipment and the O-rings, the rubber and plastic and everything else inside the submarine. So how do they stress relieve it? The first deep dive — you've got hydrostatic compression and all those welds get stress relieved mechanically on the first deep dive. That's one of my favorite stories about quality control — the requirement is that all the top management of the shipyard has to go on the first deep dive. They make sure that's a quality submarine. It's one of the best things I know. The threat of extinction really focuses the mind.

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The hull welds of a new submarine are plastically deformed and stress-relieved on the first deep dive, on which the shipyard managers themselves are aboard — a quality-control incentive structure Tom highlights as exemplary.

When does the submarine see plasticity in its welds? The very first deep dive. And who gets to go on that dive? All the management of the shipyard. One of the best quality control techniques I've ever heard is, you send the managers on the first deep dive. They have to go — not all of them, but most of them, and I don't know if they draw straws to see who takes the chance, but they have real incentive to make sure, in the submarine shipyard, that they do good quality control, at least on the hull welds. Because they get to go on the first deep dive, and on that first deep dive you mechanically stress relieve the welds. So they do stress relieve them.

Used as the teaching counterpoint to thermal post weld heat treatment. A nuclear submarine pressure hull is too large for furnace stress relief and contains heat-sensitive internals; instead, mechanical stress relief on the first deep dive accomplishes the same end. Tom adds the cultural point that shipyard top management goes on the first deep dive — quality control via skin-in-the-game.

One pressure vessel that is so large that we can't do a thermal stress relief is a nuclear submarine. We're talking three, four hundred feet long, thirty feet in diameter. And even if you could have a furnace that big, it's already got a few other things inside that don't want to go up to 1100, 1200 degrees. You're going to destroy the insides of the ship. So you can't really put it in a furnace. With submarine steel, you'd have to do local stress relief, and you have to worry about what you're doing to everything else. The problem with submarine steel is you've already heat treated it to get maximum strength, and you want that strength. For pressure vessels — not saying we don't use quench-and-tempered steels for pressure vessels, we do — but for a nuclear submarine you're basically using all that strength for the strength of the vessel, to be able to dive.