Effect of carbon on microstructure and mechanical properties of HR3C type heat resistant steels

Abstract

The modified HR3C austenitic heat-resistant steels for applications of ultra-supercritical (USC) power plants were developed and investigated. As the C content breaks through the limitation of the HR3C composition range (>0.1 wt%), the evolution of carbides and mechanical properties after isothermal aging at 700 °C have not been understood. In this study, two modified HR3C with different C content were studied by tensile test and Charpy impact test at room temperature after long-term aging up to 10,000 h. The M23C6 carbide is rapidly precipitated at the interface (GBs, TBs or NbC/γ), and these carbides at grain boundaries (GBs) are gradually changed from a continuous distribution to a semi-continuous distribution, then finally agglomerate and coarsen near the GBs. Moreover, a new morphology type lamellar M23C6 carbide forms in the grain. The effect of lamellar carbides on mechanical properties at room temperature (RT) is not obvious because the strength of GB is rapidly weakened. Besides, the Larson-Miller parameter was obtained and the creep strength of the modified HR3C was extrapolated, by conducting creep rupture experiments on un-aged sample. The creep tests reveal that the rupture lives decrease with increasing C content. Lamellar carbides are precipitated and weaken the strength of grain during high temperature creep.