To improve generation efficiency, it is needed to increase temperature and pressure of steam condition. High temperature creep resistance material is required for this steam generation condition, therefore the extensive researchs have been performed to develop the high strength-heat resistance steel.
9~12% Cr steel is attractive for high strength-heat resistance steel since it has good properties, such as low cost, higher creep strength, high temperature fatigue strength, oxidation resistance and corrosion resistance.
In this study, three different 10% Cr-based alloys which contain W and B were developed. The effects of W and B on high temperature creep strength and microstructure were discussed by investigation of room temperature mechanical properties, microstructures and high temperature creep properties.
The conclusions of this study are as follows.
The K3 alloy (0.14C-10.5Cr-0.5Mo-1.7W-0.2V-0.05Nb-0.076B) showed best creep rupture strength at 650℃ creep rupture test. The creep rupture strength is increased by the addition of Mo and W element (K3, K7) comparing with Mo addition alloy(K1), and the creep rupture strength increased with increasing W content. K3 alloy showed higher creep rupture strength than reference alloy such as TMK1 and E911.
K1 alloy has the lower impact energy than K3 alloy after tempered at 700℃ although the lower hardness. That is due to the δ-ferrite phase in K1 alloy results in lower impact energy and creep rupture strength.
It is investigated that the K3 alloy has more amount of fine precipitates and the recovery rate of substructure and recrystallization rate is lower than K1 alloy. It is analysed that the above results is attributed to the W and B addition. However, it is possible that the long-term creep ductility in K3 alloy is decreased due to the Fe$_2$W laves phase during creep in K3 alloy.
The creep exponents(n) were measured as 8.5, 18.7 and 9.7 in K1, K3 and K7 alloy, respectively.