Correlation between creep strength and stability of subgrain structure in high chromium ferritic heat resistant steel with tungsten

This paper aims at discussing the role of W in creep strength of high chrom ium 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 stres ses ranging from 80 MPa to 120 MPa. Microstructural degradation (recovery o f subgrain structure and agglomeration of M23C6 carbides and Laves phases) in the course of creep was studied by TEM and SEM observations. The experim ental 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 microstru cture during the creep of the high Cr ferritic steels is the recovery of su bgrain structure. Subgrain width increases and dislocation density within s ubgrains decreases with the progress of creep deformation. The growth rate of subgrain width in the W containing steels is slower than that of the ste el without W, and the growth rate decreases with increasing W concentration . The fine and stable subgrain structure of the W containing steels suppres ses the accumulation of creep strain, resulting in their longer rupture liv es. Precipitation of Laves phase and M23C6 carbide on sub-boundaries retard s the recovery of subgrain structure. The W addition stabilizes the M23C6 c arbides, and the number of the carbides increases with increasing W concent ration. 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 M 23C6 carbides is more important in the strengthening of the ferritic steels .