Microstructure evolution and stress rupture properties of K4750 alloys with various B contents during long-term aging

Abstract

The relationship among B content, microstructure evolution and stress rupture properties of K4750 alloy during long-term aging were investigated. After aging at 800 ℃ for 1000 h, the decomposition degree of MC carbides of K4750 alloys with 0B, 0.007 wt. % B and 0.010 wt. % B were basically identical, which indicated that B has no inhibition on MC carbide decomposition during long-term aging. The MC carbide decomposition was accompanied by the formation of M23C6 carbides and a small number of η phases, which was controlled by the outward diffusion of C and Ti combined with the inward diffusion of Ni and Cr from the γ matrix. In addition, M23C6 carbides in boron-free alloy were in continuous chain and needle-like η phases were precipitated near them, while M23C6 carbides in boron-containing alloys remained in granular distribution and no η phases precipitation around them. Adding B could delay the agglomeration and coarsening of M23C6 carbides during long-term aging, which was because the segregation of B at grain boundary retarded the diffusion of alloy elements, thus weakened the local fluctuation of chemical composition near grain boundary. The stress rupture samples of K4750 alloys with various B contents after aging at 800 ℃ for 1000 h were tested at 750 ℃/380 MPa. The results indicated that the stress rupture properties of boron-containing alloys were significantly better than that of boron-free alloy, which could be attributed to the increase of grain boundary cohesion strength and the optimization of M23C6 carbide distribution due to the addition of B.