The evolution behavior of second phases during long-term creep rupture process for modified 9Cr-1.5Mo-1Co steel welded joint

Abstract In this paper, evolution of Laves phase in weld metal (WM) during long-term creep exposure was investigated for modified 9Cr-1.5Mo-1Co steel welded joint (WJ) by microstructure characterization and theoretical calculation based on a series of creep tests at 620 °C. It was found that some rod-like Mo-rich Laves phase precipitated adjacent to M 23 C 6 carbides on grain boundary and other isolated polygonal Laves phase particles formed in grains after long-term creep exposure. Number and size of Laves phase increased simultaneously at early stage of precipitation. The volume fraction nearly approached to equilibrium fraction while coarsening was still ongoing after 2500 h. Johnson-Mehl-Avrami and Ostwald ripening model indicated that precipitation of Laves phase was controlled by grain boundary diffusion with a decreasing rate. EDS results showed Laves phase in WM had the highest Mo content in the whole WJ. The pinning effects on grain boundaries by Laves phase became weaker during creep exposure due to its coarsening. More high angle boundaries and sub-structure in WM from EBSD results provided more potential sites for Laves phase nucleating. With Laves phase particles growth on grain boundaries in WM, solid solution elements like Mo in matrix were severely depleted, resulting in loss of solute strengthening effect and precipitation strengthening effect, thus leading to the rupture in WM.