Evaluation of stress relaxation cracking susceptibility in Alloy 617 for advanced USC boilers

Abstract

In this study, the stress relaxation cracking susceptibility of Alloy 617, which is a candidate material for advanced ultra-supercritical (A-USC) power plant pipe work, was evaluated. In order to evaluate the effect of aging and grain size on stress relaxation cracking susceptibility, materials aged at 700 °C and coarse grain materials heat treated at 1200 °C for 1 h, 1225 °C for 2 h, 1250 °C for 3 h were tested in addition to as-solution-annealed materials. A three-point bending test with notch placed at the center of each specimen was used for the examination. The test results showed that only the aged material suffered stress relaxation cracking. The time-to-crack tends to be shortened when initial strain in the test is increased. This suggests that higher residual stress in the weldment would increase the risk of stress relaxation cracking. It was proposed that the larger magnitude of creep strain occurs via stress relaxation during the three-point bending test due to higher yield strength caused by γ' phase strengthening, and low ductility due to grain boundary carbides in aged materials promoted stress relaxation cracking. The critical conditions to cause crack could be estimated from the stress relaxation cracking boundary in the map of the relationship between the initial strain and the creep strain during the three-point bending test.