Here in, a boiler tube is heated in the range 800-1,000ºC, the tube blows up like a balloon and even­tually bursts. Overheating of pressurized boiler tubes might be due to lack of cooling of one or more tubes either at partial dry running of the boiler or because of blocking-up of the tube.

As opposed to a short but severe overheating, a long term overheating often causes less deformation limited to a minor area around the crack. A metallurgical examination of the material in the vicinity of the crack shows clear signs of creeping with development of voids in the grain boundaries and several parallel cracks. Although overheating damages can be divided into short-term and long term, damage categories between these extremes do occur. Often, a short term overheating will cause a crack to occur in a tube already weakened by long term overheating.

Undoubtedly, the most frequent cause of cracks in boilers. Corrosion fatigue arises due to simultaneous influence of varying mechanical stresses and a corroding medium; stress-induced corrosion also takes place during stress free periods, viz during stand-still. Varying mechanical stresses are mostly set up due to fluctuating loads which lead to differential in expansion rates of the individual boiler components. These stresses also arise during start-up from cold due to the fact that it takes some time for the individual boiler parts to reach the temperature balance.  For example, the boiler tubes pick up the tem­perature fluctuations quicker than the heavy gauge upper and lower drums. During periods of stand still with boiler water containing high concentration of  oxygen, will lead to a corrosion taking place in the actual crack, as a galvanic oxygen concentration element is formed between the crack and its surroundings.