Beneficial effect of re on the long-term creep strength of high cr ferritic heat resistant steels

Abstract

There has been a great demand for superior heat resistant steels in order to raise the thermal efficiency of fossil-fuel power plants and to reduce C0 2 emission to the global environment. To this end, by using the d-electrons concept, 9-12% Cr ferritic steels were designed for use of a steam turbine rotor operated in the USC power plants at the steam temperature of 620 to 650°C. The crucial issue for the design is to suppress the deterioration of the long-term creep strength by alloying. First, the Re addition was found to give a beneficial effect on the creep strength of a 10%Cr-4%W steel. Then, the creep tests were performed with the six Re-free and 3.5% W ferritic steels to get an optimum Cr content in the range of 8.5 to 11.5%. As the result, it was found that an excess amount of Cr yielded a detrimental effect on the creep properties, and the 9% Cr steel was the best in view of the very long-term creep strength tested in the condition of 650°C, 98 MPa. Subsequently, a series of creep tests was conducted with the steels by fixing at 9% Cr but by varying the W content from 2 to 4% and the Re content from 0 to 0.5%. From the prolonged creep tests for more than 40,000 h, it was shown that the 9Cr-4 W-0.5Re steel had the longest creep rupture life among all the high Cr ferritic steels so far developed in the world.