EFFECT OF BCC CHROMIUM PHASE ON HIGH-TEMPERATURE CREEP RESISTANCE IN AN FE-30CR-50NI-2MO ALLOY

A new corrosion resistant Fe-50%Ni-30%Cr-2%Mo austenitic alloy developed for super heaters has a high creep rupture strength at temperatures as high as 1073K. The creep rupture strength of this alloy is equivalent to that of 17-14CuMo, one of the strongest commercial alloys. Unlike ordinary commercial austenitic heat resisting steels strengthened by carbide dispersion, which tend to decrease in the creep rupture strength at higher testing temperatures and under lower creep stresses, this alloy retains its strength in a temperature range between 823 and 1073 K for more than 10 000 h. This excellent long term creep rupture strength was found to be due to a high density and a uniform dispersion of intragranular precipitates of bcc chromium phase. The dispersion strengthening due to the bcc chromium was umabiguously demonstrated by a creep test of a preaged specimen with a controlled dispersion of the precipitates. Coarsening of the intragranular bcc chromium precipitates was slower than that of the carbides, which lead to the smaller decrease in the creep rupture strength at higher testing temperatures and under lower creep stresses.