Effect of Cr Content on Softening during Aging in High Cr Ferritic Heat Resistant Steel

In order to clarify the effect of Cr content on creep strength in high Cr ferritic steels for turbine rotors, a series of experiments is carried out using the steels in the tampered state and after simple aging at 650 degrees C. In the tempered state, the Vickers hardness of 10.5 Cr steel is higher than that of 9 Cr steel, but it decreases remarkably in 10.5 Cr steel and becomes almost the same value in both steels at after aging for 6700 h. In the tempered state, 10.5 Cr steel shows large number density and small average particle diameter of the fine Laves phase compared with 9 Cr steel, and this condition continues even after aging for 1000 h. However, M23C6 carbide and the Laves phase grow with increasing aging time and their average particle sizes in 10.5 Cr steel become greater than those in 9 Cr steel after aging for over several thousand hours. In 10.5 Cr steel, the amounts of carbomtrides such as NbX, Cr2X, and VX decrease after 3000 to 6000 h due to precipitation of the Z phase, whereas in 9 Cr steel, carbomtrides are still present beyond 6000 h due to retardation of the Z phase precipitation.