347-H stainless steel is widely used in superheater or reheater tubes of power boilers, where typical operating temperatures are in the range of 550 °C - 650 °C. During operation, microstructures are subject to changes because of the diffusion of alloy elements, giving place to phenomena like phase precipitation and phase growth. Carbide and sigma phase are the main precipitating phases at this temperature range. Grain refinement has not been reported in superheater or reheater tube steels. Evidence of grain refinement was detected during a previous study on the causes of a high corrosion rate of a 347H superheater tube which was operating in a 350 MW oil-fired boiler. The grain size reduction was detected in the hotter zones of the tube. Grain size measurements in specimens of three tubes were carried out confirming the presence of a grain reduction process in the hotter zones of 347H superheater tubes. TEM analysis revealed a polygonal structure of dislocations decorated by precipitation of fine M23C6 carbides. It was concluded that a recrystallization process took place induced by high temperature on a deformed structure generated during the tube manufacture process.

Subject
Grain size, Carburization, Materials, Carbides, Dislocation, Corrosion rate, Tubes, Transmission electron microscopy, Failure analysis, Steel, Boilers, Stainless steel, Grain
Keywords:
grain refinement, recrystallization, 347H stainless steel, superheater, microstructure, failure analysis
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2002
Association for Materials Protection and Performance (AMPP)
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