Magnetism variance analysis of Super304H austenitic stainless steel after service

Abstract Magnetism variance of Super304H austenitic stainless steel superheater tubes after 36891h and 41000h service in the ultra-supercritical boilers was investigated by using optical microscope, scanning electron microscope, X-ray diffractometer, PPMS system (vibrating sample magnetometer, VSM), EBSD and electrochemical workstation. The results show that grain size of the 36891h serviced tube is extremely large with grade 2, and magnetization was detected in the tube, which was confirmed with a small amount of ferromagnetic phase distributing along some grain boundaries by EBSD observation, while the tube of 41000h service has a small grain size of grade 7–8 and no ferromagnetic phase or magnetization is present. The morphologies after DL-EPR test show that the intergranular corrosion susceptibility of the 41000h tube is higher, but the depth and width of the Cr-depleted zones at grain boundaries are significantly smaller than the 36981h tube. The precipitates in Super304H after service did not cause significant changes in its magnetic properties. Coarse grains, which led to reduction of the diffusion channels, made Cr difficult to diffuse from the matrix to Cr-depleted zones. As a result, the Cr content in the Cr-depleted zones maintained a lower level and resulted in the increase of Ms of the Cr-depleted zones, which induced martensitic transformation of Super304H tubes during the shut-down to room temperature of the boiler.