It later had 75% deep circumferential cracks, which were welded back together

This photo dates from the 1960s and it shows a boiler after it has failed a hyrdostatic pressure test. As you can see, the failure is what one might call "catastrophic". After the test, the boiler was cut in half and the half without the hole was welded to another half. The boiler was then used in the nuclear power station, Sizewell A.