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 oC - 650 oC. 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.
Keywords: grain refinement, microstructure, failure analysis,
recrystallization, 347H stainless steel, superheater.