Creep Life Assessment Method for Super304H Boiler Tubes Based on HTS-SQUID Gradiometer

This paper presents a novel method for the creep life assessment of Super304H (KA-SUS304J1HTB, ASME code: 2328) boiler tubes based on a discrete-type eddy-current-testing (ECT) HTS-SQUID nondestructive evaluation (NDE) system. Six specimens were subjected to internal pressure creep tests at a stress level of 110 MPa and a temperature of 700 °C. The specimens were inspected at various creep test times using our NDE system, which included an ECT probe and a HTS-SQUID gradiometer. By studying the physical characteristics and microstructures of the specimens at different test times, we found that the electrical conductivity and permeability of the specimens varied with an increasing test time. The detected signals may have been generated from creep voids and/or surface microcracks. Based on the NDE inspection results, creep life assessment curves that can be used for assessing the remaining creep life of the Super304H boiler tubes were plotted. The results confirmed the possibility of assessing the remaining creep life using our HTS-SQUID-based system in a nondestructive manner.