Creep damage characteristics and evolution of HR3C austenitic steel during long term creep

Abstract

HR3C austenitic heat resistant steel has been gradually applied as tubes to manufacture components of superheaters and reheaters in ultra-supercritical (USC) power plants. Creep tests were conducted at 650 °C under the applied stresses in the range of 150 MPa–250 MPa. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD) were employed to investigate the creep damage characteristics and evolution. The results show that intergranular microcracks and dense creep cavities along grain boundaries are the major features of creep damage. Different states of creep damage characteristic distribution are related to the applied stress and creep duration at high temperature. The wedge-cracking is prone to take place at the triple junctions of grain boundaries, and creep cavities are nucleated preferentially at M23C6 precipitates along grain boundaries. The nucleation and growth of creep cavities and wedge-cracks have a crystallographic orientation relationship with the neighboring austenite grains. The adjacent grains have a large difference of Schmid factor, and one of them is close to <1 1 1>//the stress axis orientation, which has greater resistance to deformation. The Orientation Deviation (KAM) obtained from EBSD analysis implies that the creep damage is obviously affected by the stress level. Microcracks under high stress are mainly attributed to the local deformation, and dense creep cavities under low stress are dominated by time-dependent diffusion. Under the intermediate stress level, local deformation and time-dependent diffusion act together, leading to the most serious damage.