High-Temperature Cycle Deformation and Fracture Behavior of Advanced Austenitic Heat-Resistant Steel Sanicro25 Alloy for A-USC Power Plants

Cyclic deformation mechanism and fracture characteristics for Sanicro25 alloy were investigated at temperatures and mean stresses ranging from 973 K to 1023 K and 130 to 260 MPa with stress amplitudes between 70 and 130 MPa. The results demonstrated that the increase of mean stress and stress amplitude increased the cyclic strain rate and reduced the fracture life. The minimum strain rate and mean stress were found to follow the power-law relationship and the similar values of apparent stress exponent (n(a) = 6.5 to 10.4 at sigma(a) = 70 MPa and n(a) = 6.0 to 9.2 at sigma(a) = 130 MPa) showed the parallel trend at different stress amplitudes. The average values of apparent activation energy were 593.6 and 689.5 kJ/mol and were found to increase with increasing stress amplitude. The threshold stresses were estimated by a linear extrapolation method and decreased with increasing temperature and stress amplitude. By invoking the concept of threshold stresses, the normalization of cyclic strain rate can be achieved with a true stress exponent of 5. Moreover, the value of the true activation energy Q = 270kJ/mol was equal to the lattice self-diffusion of gamma-Fe, implying that the cyclic strain process was controlled by the lattice self-diffusion. As for the correlation of ruptured life, a d mage accumulated strain (epsilon(d)) was proposed to make a modified Monkman-Grant equation ((epsilon) over dot(min)(alpha') . tr/epsilon(d) = C-MMG) and all min data can be normalized into a narrow band under different loading conditions. (C) The Minerals, Metals & Materials Society and ASM International 2022

Automated summary

The cyclic deformation mechanism and fracture characteristics of Sanicro25 alloy were investigated. The results showed that increasing the mean stress and stress amplitude led to higher cyclic strain rates and shorter fracture life. The relationship between the minimum strain rate and mean stress followed a power-law equation, with similar apparent stress exponent values at different stress amplitudes. The apparent activation energy increased with stress amplitude. Threshold stresses decreased with increasing temperature and stress amplitude. The cyclic strain process was found to be controlled by lattice self-diffusion, and the rupture life could be normalized using a modified Monkman-Grant equation.