Welded steel crane component failure

A two-inch-thick steel crane component with un-stress-relieved welds failed in brittle fracture under bending, dropping a seventy-ton water bag onto a worker (back injury). Boiler-and-pressure-vessel code requires post-weld heat treatment above 1.5 inches; this code was not followed.

Did I tell you how the US Navy stress relieves the submarines? I don't think I told the class — I think this was a conversation right afterwards. Steel is usually thermally stress relieved. You build a big pressure vessel out of steel and you weld it, and the boiler and pressure vessel code says that if the steel is more than one-and-a-half inches thick, it must be post-weld heat treated, because the welding process introduces residual stresses. At one-and-a-half inches thick, the residual stresses will be equal to the yield strength of the steel, and that steel could now fracture in a brittle manner when ordinarily it would have been ductile. I have a crane component that fractured and a seventy-ton water bag fell on top of a man. He hurt his back pretty badly, because they didn't stress relieve the welds, which were about two inches thick on steel that was about two inches thick. No post-weld stress relief, residual stresses, and when this thing got bent — started getting a bending load that it wasn't supposed to get but it got it — instead of just bending over like a paper clip, it snapped. Not quite like a piece of glass, but it snapped, and the thing came down while he was underneath it. He shouldn't have been underneath it, but nonetheless he was, and he got hurt. So stress relief is important.