Not the bolts, the wrong NUTS. The bolts were the original: the mechanic wanted to change them. They asked for replacements and were shown a box of parts from w

Not the bolts, the wrong NUTS. The bolts were the original: the mechanic wanted to change them. They asked for replacements and were shown a box of parts from which they selected the nuts and re-used the bolts. From reading the OP it appears the failure was the replacement nuts, not the re-used bolts. This wasn't a failure of the boiler vessel itself, although the article says that the stop valve was defined as "a boiler-boundary". BTW 600 psi and 850 F is a lot of heat and energy. UPDATE a post below talks about 1200 psi 975 degree superheated steam.