Kinetics of carbide precipitation during creep and its effect on creep rate of 10Cr-30Mn austenitic steels

The kinetics of carbide precipitation during creep and its effect on creep rate in a primacy or transient creep region were investigated for 10Cr-30Mn austenitic steels containing 0.003 to 0.55 mass% carbon. After solution-annealing, the specimens were subjected to creep tests at 873 K for up to 30 Ms (8300 h). In the high carbon steels containing above 0.2 mass% carbon, extensive precipitation of fine M23C6 occurred during creep and the creep rate decreased significantly at long times longer than 3-5 ks (1 h). The decrease in creep rate caused by the M23C6 precipitation was analyzed based on the Johnson-Mehl equation for diffusion-controlled growth of precipitates from supersaturated solid solution. The Johnson-Mehl equation with the time exponent n = 2/3 provided a reasonable approximation for the significant decrease in creep rate at long times. This resulted from a strain-enhanced precipitation of M23C6 on dislocation lines produced by creep deformation. In the high carbon steels, the stress exponent m for the minimum creep rate ((epsilon) over dot(min) = A sigma(n)) changed from very high values above 20 at relatively high stresses to a low value of about 5 at low stresses. The very high values of m are apparent values resulting from the M23C6 precipitation which decreases the minimum creep rate significantly with decreasing stress and increasing test duration.