The macroscopic analysis

physico-chemical analysis

electron microscopic analysis

and energy spectroscopic analysis have been carried out for specimens of the superheater tube ruptured during the time of start-up after shutdown of one gas furnace used to supply heat in one power plant

results shows that the openings

microfissures

and oxides of grain boundary corrosion are main causes leading to brittle rupture of materials.At the same time

it has been pointed out that the rupture of lower elbow bend on the superheater is mainly due to micro-damage existing in the material proper and oxygen corrosion resulted from conservation after shutdown.The factor in two said aspects intensifies stress corrosion after putting the boiler into operation

and finally resulting in brittle rupture.