The effects of tungsten addition on the microstructural stability of 9Cr-Mo Steels

The effects of tungsten addition on the microstructure and high-temperature tensile strength of 9Cr-Mo steels have been investigated by using three di fferent steels: M10 (9Cr-1Mo), W18 (9Cr-0.5Mo-1.8W), and W27 (9Cr-0.1Mo-2.7 W) steels. The tungsten-added 9Cr steels have revealed better high-temperat ure tensile strength. Microchemical analysis for (Cr,Fe)(2) (C,N) revealed that the tungsten addition increased the Cr/Fe ratio, which resulted in the lattice expansion of (Cr,Fe)(2) (C,N), and then the enhanced pinning effec t on the glide of dislocation. In addition, in M10 steel, the M23C6 carbide s quickly grew and agglomerated, while the tungsten-added 9Cr steels reveal ed a fine and uniform distribution of M23C6 carbides. Dislocation recovery during tempering treatments was delayed in tungsten-added 9Cr steels, which was correlated with the stabilized precipitates and the decreased self-dif fusivity of iron. It is, thus, believed that the excellent high-temperature tensile strength of tungsten-added 9Cr steels is attributed to the stabili zed M2X carbo-nitrides and M23C6 carbides and the decreased self-diffusivit y of iron. (C) 2001 Elsevier Science B.V. All rights reserved.