W含有高Crフェライト系耐熱鋼のクリープ強度とサブグレイン組織安定性の相関

Abstract

This paper aims at discussing the role of W in creep strength of high chromium ferritic steels with a tempered martensitic lath structure. Creep tests of a 9Cr-1.8W-MoVNb steel (NF616) were performed at 923 and 973 K at stresses ranging from 80 MPa to 120 MPa. Microstructural degradation (recovery of subgrain structure and agglomeration of M23C6 carbides and Laves phases) in the course of creep was studied by TEM and SEM observations. The experimental results were compared with those of a Mod.9Cr-1Mo steel without W and an 11Cr-2.6W-MoNNb steel (TAF650). The most noticeable change in microstructure during the creep of the high Cr ferritic steels is the recovery of subgrain structure. Subgrain width increases and dislocation density within subgrains decreases with the progress of creep deformation. The growth rate of subgrain width in the W containing steels is slower than that of the steel without W, and the growth rate decreases with increasing W concentration. The fine and stable subgrain structure of the W containing steels suppresses the accumulation of creep strain, resulting in their longer rupture lives. Precipitation of Laves phase and M23C6 carbide on sub-boundaries retards the recovery of subgrain structure. The W addition stabilizes the M23C6 carbides, and the number of the carbides increases with increasing W concentration. These facts are the origin of the stable subgrain structure of the W containing steels. The number of M23C6 carbides in a unit area is 5 to 10 times larger than that of Laves phases, suggesting that the stability of M23C6 carbides is more important in the strengthening of the ferritic steels.