Creep Lifetime Reduction in Heat Affected Zone in Boron-Added 9Cr Heat Resistant Steel

Boron-added 9Cr-3W-3Cr-VNb ferritic/martensitic heat resistant (MARBN12) steel is the candidate material for components used at intermediate temperatures, i.e., 923 K or less, in advanced ultra-supercritical (A-USC) power generation systems because they can suppress Type IV fracture under creep conditions. For evaluation of the creep strength of the heat affected zone (HAZ), simulated HAZ samples with peak temperatures of about 1173 K, 1223 K, 1273 K, and 1323 K with a heating rate of 100 K/s and a cooling rate of 40 K/s were crept at 923 K. Compared with the conventional Gr. 92 steel, the B-added steel showed about 10 times longer creep lifetime. Furthermore, minimum creep lifetime was observed around the AC3 point of about 1223 K. Electron back-scattered diffraction analyses revealed that clear fine-grained HAZ was not formed and that martensite remained in the simulated HAZ samples of the MARBN12 steel. Microstructural change occurred only around the prior austenite grain boundary (PAGB), i.e., fine grains were formed there. It generated grain boundary sliding in the vicinity around PAGB, leading to shorter creep lifetime than the base metal. Results show that the creep lifetime around HAZ in the MARBN12 steel was affected by the microstructure near PAGB. Also, that in the Gr. 92 steel was influenced by the grain size, i.e., Type IV fracture.