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

摘要

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 0 B,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 M_(23)C_(6) carbides and a small number ofηphases,which was controlled by the outward diffusion of C and Ti combined with the inward diffusion of Ni and Cr from theγmatrix.In addition,M_(23)C_(6) carbides in boron-free alloy were in continuous chain and needle-likeηphases were precipitated near them,while M_(23)C_(6) carbides in boron-containing alloys remained in granular distribution and noηphases precipitation around them.Adding B could delay the agglomeration and coarsening of M_(23)C_(6) 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 bo ron-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 M_(23)C_(6) carbide distribution due to the addition of B.