Kinetics of low temperature precipitation in a ULC-bake hardening steel

The kinetics and the mechanism of the precipitation stage of the strain aging process have been studied in a very dilute Fe-C system, i.e. a bake hardenable ULC steel, through measurement of changes in the yield stress. The results showed that within the range of temperatures studied, the stages of the precipitation and the kinetics could be measured accurately using strength measurements. It was possible to detect easily the transition between low temperature carbide to epsilon -carbide precipitation from strength measurements and also from an analysis of the strain hardening behavior of the aged specimens. Results showed that the epsilon -carbide forms only at tempratures > 70 deg C and that the transition from low temperature carbide to epsilon -carbide is a continuous phenomenon. The time exponent n (0.42-0.48) and the activation enthalpy value (74plus/minus0.9 kJ/mol) for the formation of epsilon -carbide suggest that the process is essentially controlled by the diffusion of carbon from the ferrite matrix. The activation energy for the formation of epsilon -carbide was found lower than the activation energy of the low temperature carbide/cluster formation process (85.6plus/minus0.4 kJ/mol).