Kinetics of Carbide Precipitation during Creep and Its Effect on Creep Rate of 10Cr–30Mn Austenitic Steels

Abstract

The kinetics of carbide precipitation during creep and its effect on creep rate in a primary 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 (\dotεmin=Aσm) 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.