Strengthening Mechanisms in a Medium-Carbon Steel Subjected to Thermo-Mechanical Processing

Strengthening mechanisms were examined in a Fe-0.43C-1.60Si-0.01Mn-1.1Cr-0.95Mo-0.08V-0.05Nb steel exhibiting a yield strength (YS) of 1310 MPa after tempering and 1550 MPa after tempforming. The dislocation strengthening gave a major contribution to the overall YS of the steel in the tempered condition, whereas dispersion strengthening was a major contributor to the YS of the steel in the tempformed condition. High values of dislocation strengthening after tempering were attributed to dislocations composing the lath boundaries. A high density of free dislocations provided nearly the same dislocation strengthening after tempforming. Warm rolling after tempering led to alignment of intercrystallite boundaries along the rolling direction that decreased the interparticle spacing between M23C6 carbides located at these boundaries and thus increased the magnitude of dispersion strengthening. The boundary strengthening contributed to overall YS significantly due to small lath thickness after tempering and nanoscale spacing between lamellar boundaries after tempforming.