Creep behavior and stability of MX precipitates at high temperature in 9Cr–0.5Mo–1.8W–VNb steel

The growth behavior of MX carbonitrides during aging and creep was investigated for 9Cr–0.5Mo–1.8W–VNb steel (ASME-P92). The stress exponent of minimum creep rate decreases with increasing testing temperature. The value of stress exponent is 5.7 at 1023 K over a wide range of stress examined, while the value is 12.7 at 923 K. The low stress exponent at 1023 K may be due to the growth of MX carbonitrides during creep. The MX carbonitrides grow to a size of 60 nm during aging at 1023 K for 400 h. The size is much larger than the critical value over which the coherent strain between the MX and matrix decreases. The growth rate of the MX carbonitrides is larger in gauge portion than in head portion of crept specimens, implying a stress or strain effect.