It's almost 4 am, Unit 2 is down with another tube leak, and I just got back from the 14th floor, looking at which tubes need to be cut and repaired, and which can be muscled back into line.
Boiler tubes are tough. They have to endure and perform in horrendous conditions. These particular tubes have the hottest steam in them, and are in one of the hottest places in the boiler. The steam they carry is around 1100 deg F, and so I think that on the outer skin, it must be at least 50 degrees hotter, and maybe more. Recently we had a huge clinker in this boiler, and it acted like a funnel to direct the gas along the sides of the boiler. It also insulated a lot of tube area, causing other areas to have to accept more heat. This tube failed in one of the hottest areas. It is a stainless steel tube, and they are really good in heat. But if the temperature is up around 1200-1400 deg F, and it might well be on the tube surface, sigma phase degradation starts to form. It is actually a type of intergranular corrosion. In steel with less than 18% chrome the grains move around and you get little voids and the steel is said to have 'creep'.
Stainless steels don't creep in the same way, but chromium carbide does form in between the grain bounderies causing a weakened area. In time the carbides get bigger, and the strength is degraded, and then finally it fails and a bunch of guys spend several days and nights fixing it.