Effect of Co on Creep Deformation and Precipitation Behavior of High-Cr Ferritic Steels

The creep behaviour of alpha iron with Co was investigated at temperature of 600°C under stress of 30 to 40 MPa. The minimum creep rate for model alloyed with Co was two orders of magnitude lower than that of alpha iron. The changes of strengthening factor in alpha iron with Co are consistent with the content of Co. The creep behaviour of martensitic 10% Cr steels with Co was also investigated at temperature of 600°C under stress of 250 to 350 MPa. In contrast to alpha iron with Co, the changes of strengthening factor in martensitic steels with Co show no monotonic, but the curve has a maximum, at around 3% Co, as a content of Co, associated with the solid solution strengthening of Mo and W. An excess of Co content like 5% Co acccelerates the Laves phase precipitation reaction in a way similar to increasing the Mo and W content, suggesting that Co increases the supersaturation of Mo and W. This leads to the appearance of the curve with a maximum.