America's Cup 4340 yacht welding (foreshadowed)

Foreshadowed at the end of the layer 2 cut — Tom is about to launch into the case where a client wanted to use 4340 (a high-hardenability, >180 ksi steel requiring 600°F preheat and post-weld heat treatment) for racing yacht components. Will be developed in the next portion of the lecture. ## Figures referenced

With the weld metal you get martensite, and you try to keep your hardness in your weld metal below Rockwell C 30 — the petroleum guys like to keep it below Rockwell C 22, but they can't use the higher strength. It turns out the bridge guys, when you're building a bridge, can use steels up to 100 ksi, but we don't use steels any stronger than that, because you'd have to do post-weld heat treatment. You can't post-weld heat treat a bridge, not economically anyway. If you go above 180 ksi — so this is up to 120, this is like 120 to 180, if you're greater than 180, it's hard martensite. Now we're talking preheats of 600 degrees Fahrenheit and post-weld heat treatment required, very expensive. This is the problem when they came to me and said, oh, we want to use 4340 in our America's Cup yachts —