The effect of tungsten on creep properties of high chromium steels for steam turbine

The effect of tungsten addition on creep properties and microstructure was studied for high chromium ferritic steels with various tungsten contents. Tungsten was distributed among matrix, Laves phases and carbides during high temperature exposure. The tungsten content in the matrix was reduced down to about 0.5 mass% during creep and/or ageing for more than 10 000 h at 600 degrees C regardless of the initial tungsten content. Solid solution strengthening due to about 0.5 mass% tungsten was maintained during long-term creep. Laves phase precipitated at martensitic lath interfaces, grain boundaries and in lath during high temperature exposure. At martensitic lath interfaces and grain boundaries, continuous formation of Laves phases and M(23)C(6) carbides was recognised. This morphology played a role to suppress the collapse of lath shape, and contributed to maintain the long-term rupture strength. On the other hand, fine Laves phases precipitated in lath contributed to increase the short-term rupture strength. In order to obtain higher creep rupture strength throughout the long-term, it is important not only to obtain the appropriate amount of Laves phase but also to maintain the high amount of solid solution elements in the matrix. This purpose was realised by doping rhenium or increasing the molybdenum equivalent to over 1.5.