Creep rupture properties of some high temperature reactor circuit materials in helium

Creep rupture tests in air and in helium containing nominally 500:500:50:50 ppM of H2:CO:H2O:CH4 have been carried out on three ferritic and three austenitic steels for durations of up to 10,000 h at temperatures ranging from 400 to 8000C. The helium environment was found to reduce the creep and rupture strengths of the austenitic steels at 650 to 7500C by 5 to 15 percent whereas a smaller environmental effect was observed on a 1 percent Cr steel at 5000C and a 21/4 percent Cr steel at 5500C. No effect of environment was detected on mild steel at 4000C. The creep and rupture behavior is shown to be consistent with the metallographic findings which indicated that specimens tested in helium often contained deeper surface cracks and in all cases showed differences in the oxidation behavior