Evolution of microstructure and acceleration of creep rate in tempered martensitic 9Cr-W steels

The effect of microstructural evolution on creep rates in the acceleration creep region has been investigated for tempered martensitic 9Cr-W steels. Creep tests were carried out at 823, 873 and 923 K for up to 15 000 h. The creep rupture strength increased with increasing W concentration or by the addition of V and Ta which formed fine MC carbides. In the accelerated creep region, the logarithm of creep rate increased linearly with strain but not linearly with time. The acceleration of creep rate by strain correlates with the microstructural stability and increases with increasing W concentration or by the addition of V and Ta.