Several T23 and T91 steel superheater tubes in a subcritical pressure 700 MW unit boiler had failed for short-term overheating after serving for 105 h.The mechanical properties and microstructure of the failed tubes were studied.The results show that there was an obvious change of microstructure for the failed T23 steel

whose microstructure was made of ferrite and no-orientation carbide.Its tensile strength and hardness increased weakly

while its proof strength of non-proportional elongation(Rp)decreased dramatically.The results for T91 went the opposite way

with a slight change of microstructure and a significant rise of tensile strength and hardness.It indicates that the factor determining the microstructure of failed superheater tubes made by ferrite high temperature-resistance steel is the cooling rate of furtance

but not the cooling rate of ejected fluid.