Abstract
A severely damaged low carbon steel boiler tube was the object of this investigation. Detailed microstructural characterization was performed by optical microscopy, whereas scanning electron microscopy (SEM) was applied only in a few cases. Results show that a variety of microstructures was formed in the material of the damaged boiler tube during its exploitation. The failure of the tube is the result of very inhomogeneous overheating. The side of the boiler tube toward fire (F) was exposed to high overheating temperature, which in some locations was well above the A3 transformation temperature. The side toward boiler (BL) was subjected to lower temperatures, i.e. in the region mostly between A1 and A3 temperatures. Variations in temperatures and cooling rates, which resulted in microstructural inhomogeneity, are the main cause for the formation and multiplication of stresses leading to the rupture of the tube.
Highlights
The boiler tubes, designed to be exploited for a long period, operate in complex conditions involving high temperature, pressure and corrosive environment
Detailed microstructural characterization was performed by optical microscopy, whereas scanning electron microscopy (SEM) was applied only in a few cases
Variations in temperatures and cooling rates, which resulted in microstructural inhomogeneity, are the main cause for the formation and multiplication of stresses leading to the rupture of the tube
Summary
The boiler tubes, designed to be exploited for a long period, operate in complex conditions involving high temperature, pressure and corrosive environment. The failure of the tubes occurs when the effective strength, i.e., the load-bearing capacity of the tubes, falls below a critical level determined by component geometry and strength of the material [1]. Avoidance of such failure is of great importance since the failure results in a shortage of electric power and loss of industrial production. Hydrogen damage of boiler tubes is caused by a corrosive reaction between steam or cooling water and steel: The precipitation of molecular hydrogen (or methane), according to relation: 4H+ + Fe3C = 3Fe + CH4 occurs along grain boundaries, causing decarburization [4]. Tubes that have undergone this type of damage show a characteristic „window” fracture, in which the portion of the tube was detached [5]
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