The influence of Laves phase precipitation on a fracture process and creep rupture strength of W alloyed 9%Cr ferritic steel was investigated. Additionally, the change in strengthening factors during creep was examined by Vickers hardness and nano-indentation tests in order to clarify the material degradation mechanism. The main results obtained are as follows.
(1) Laves phase is the preferred site for cavity to initiate. This cavity initiation at Laves phase and subsequent small crack formation cause the fracture of long-termed creep specimen.
(2) The creep rupture strength at 600°C decreases with pre-aging at 650°C. Laves phase is closely associated with the decrease in the rupture strength, because the rupture time decrease as the amount of Laves phase increases.
(3) Nano-indentation testing technique revealed that the matrix softening during thermal aging was caused by the decrease in the amount of W and Mo in solid solution due to Laves phase precipitation. This decrease in solidsolution strengthening due to Laves phase precipitation causes the above-mentioned decrease in the rupture strength.
(4) The annihilation of dislocation is the predominant factor of the matrix softening in the transient creep region, while the matrix hardness decreases as the amount of W and Mo in solid solution decreases with Laves phase precipitation after the transient region.
