Microstructures and Creep Strength of Welded Joints for W Strengthened High Cr Ferritic Steel
Masaaki TABUCHI, Takashi WATANABE, Kiyoshi KUBO, Masakazu MATSUI,@Junichi KINUGAWA and Fujio ABE
Abstract:On an increasing demand to reduce CO2 emissions and save energy, high strength ferritic heat resisting steels of 9-12%Cr have been developed with a view to elevating metal temperatures of thermal power plants. Type IV creep cracking in heat affected zone (HAZ) of welded joints is a serious problem for these high strengthened steels. The present paper investigates the microstructures and creep properties of welded joints for W strengthened 11Cr-0.4Mo-2W-CuVNb steel. Most of the welded joint specimens were ruptured at HAZ and creep lives decreased than that of base metals. The comparison of creep properties of simulated HAZ specimens showed that fine grains produced by heating around Ac3 temperature was responsible for such degradation of creep strength. The growth of precipitates on grain boundaries and recovery of dislocation structures during creep was faster for fine-grained HAZ. The electron beam (EB) welded joints indicated two times longer creep life than gas tungsten arc (GTA) welded joints, however the brittle Type IV fracture was occurred even in the EB welded joints for long-term creep test. The FEM creep analysis by using the creep data of simulated HAZ specimens could explain the experimental results for creep properties of welded joints. Key Words:11Cr-0.4Mo-2W steel, creep properties, GTA welded joint, EB welded joint, simulated HAZ, Type IV crack, FEM analysis