High-temperature creep mechanism and microstructure evolution of Haynes230 alloy for ultrahigh-temperature gas-cooled reactors

High-temperature gas-cooled reactor (HTGR) is one of the Generation IV nuclear energy systems, and its core outlet temperature is up to 800-950°C. The operating temperature of the intermediate heat exchanger (IHX), therefore, increases significantly. Haynes230 alloy is a very competitive candidate for IHX tubing, and it is important to evaluate the creep properties of Haynes230 alloy to maintain the long-term performance in high temperature and pressure. Creep tests from 800 to 950°C, data processing for creep behavior and microstructural analysis were performed. The Norton model of steady state creep stages at 800°C, 850°C, 900°C and 950°C was established to obtain the steady state creep rate and stress equation, and the creep exponent n was obtained, and the creep mechanism was inferred to be the dislocation slipping and climbing. The Monkan-Grant relation for Haynes 230 alloy was obtained by fitting the derivatives of the steady state creep rate and the time to creep rupture. Through the creep damage calculation, the creep damage tolerance and creep rupture time of Haynes230 alloy were plotted. It shows that the fracture was mainly caused by the growth of internal crack cavities which reduced the effective area and induced the fracture. SEM, EDS, EBSD and EPMA were used to analyze the microstructure and fracture mechanism of the alloy, and the results show that the precipitated phase of Haynes 230 alloy in solid solution condition was W-rich M₆C in the vicinity of grain boundaries, and granular M₂₃C₆ and M₆C re-precipitated at grain boundaries during creep. Creep cracks originated at coarse intergranular carbides, followed by expanding along a direction perpendicular to the creep stress or strip carbides. Dislocations slipped from the intracrystalline to the grain boundary, accumulated, induced a local stress concentration and microcracks.