Optimization of Cr content in high Cr heat resistant steels on the basis of long term creep strength

摘要

There have been many investigations to improve creep strengths of 9-12% Cr ferritic steels, aiming to increase the efficiency of power generation and to reduce CO{sub}2 emission to the global environment. We have tried to develop a heat resistant ferritic steel for a steam turbine rotor which would be tolerable even in the ultra super critical condition (e.g, steam temperatures of 620 to 650℃). In the beginning of alloy design, the Cr content was increased to strengthen the average chemical bond between the atoms in steel, which resulted in high creep strength in the test for a short term under the condition of large applied stresses. However; it was found from the creep test for a longer term under the condition of 650℃, 157 MPa that a 10% Cr steel was much superior in creep strength to an 11.5% Cr steel, contrary to the results obtained from the creep test for a short term. Thus a series of creep tests was performed with six ferritic steels varying in Cr content in the range of 8.5 to 11.5%. As a result, it was found that an excess amount of Cr had a detrimental effect on the long-term creep strength, because it acted in some ways to promote the recovery and the softening of the martensitic microstructure in the steels. By this effect, a 9% Cr steel exhibited the highest creep strength among the six steels under the very long-term creep condition of 650℃, 98 MPa. Thus the developing of advanced ferritic steels necessitates the optimization of the Cr content.