Microstructure and creep strength property of the precipitation strengthened 15Cr ferritic steels with addition of W and Co

The effects of W and Co on the creep deformation behaviour and microstructure of fully annealed and precipitation strengthened 15Cr ferritic steels with a ferrite matrix at 923 and 973K have been investigated in detail in order to discuss the strengthening effects of W and Co in a long-term and high temperature service conditions. The effect of Co addition on the creep rupture strength of the steel is higher than that of W content except for the low stress condition of 50 MPa at 973K, where the effect of an increase in W content is higher than that of Co, and the creep rupture life of the steel with an increased W content is longer than that of the steel with Co. Increase in W content stimulates the precipitation of Laves phase, mu phase and chi phase. The addition of Co promotes the precipitation of fine carbide particles identified as M23C6 within the grains. So it has been concluded that precipitation strengthening of carbide promoted by Co addition is effective in improvement of short-term creep strength in higher stress region, and that of intermetallic compounds due to W is more effective in lower stress and a long-term condition.