Damage evolution during creep of steels

Using high-precision constant-stress machines, normal creep curves are recorded over tensile stress ranges giving creep lives up to about 1000 h or more for 0.5Cr-0.5Mo-0.25V ferritic steel, 1Cr-1Mo-0.25V bainitic steel, 316L austenitic steel and the oxide-dispersion-strengthened 14% Cr ferritic steel, Incoloy MA957. Although the curve shapes are material and test condition sensitive, a minimum rate rather than a secondary or steady-state value always occurs when the decaying primary rate is offset by the tertiary acceleration. The observed behaviour patterns are analysed with special reference to the measured values of the creep damage tolerance parameter and microstructural evidence on the various damage processes governing tertiary creep and fracture