Effect of overheating temperature on the microstructure and creep behavior of HP40Nb alloy

HP40Nb alloy has been widely used as a high temperature material in petrochemical plants. However, overheating or local temperature excursion occurs occasionally in service and leads to serious damage on the material. Effect of temperature on the microstructure and creep performance of the HP40Nb alloy is investigated in the present work. Several specimens are cut from serviced components of the alloy and heat-treated at different temperatures from 900 °C to 1250 °C for its possible working conditions, in which the temperature of 1200 or 1250 °C is used to simulate the overheating condition of HP40Nb tubes. The microstructure of specimens is examined by scanning electron microscope (SEM) and transmission electron microscope (TEM). The creep behavior is evaluated through using impression creep tests with a flat-ended cylindrical indenter. The content of inter- and intra-dendritic carbides in the specimens is represented by the surface fraction of each phase, which has been estimated by image processing method. The results show that the total of the surface fraction of inter- and intra-dendritic carbides in the HP40Nb alloy does not significantly change at the temperature lower than 1100 °C. However, the surface fraction of inter-dendritic carbides reaches the maximum at 1100 °C. A maximal steady state impression rate is also observed at 1100 °C. The results suggest that the content of inter-dendritic carbides is the main influencing factor on the creep performance of HP40Nb alloys comparing with that of the intra-dendritic carbides.

► Effect of overheating temperature on the microstructure of HP40Nb is investigated.
► We measure the creep properties of HP40Nb (ε) through using impression creep testing.
► Effect of overheating temperature on ε is investigated.
► The content of inter-dendritic carbides is the main influencing factor on ε.