Nanoscale co-precipitation and mechanical properties of a high-strength low-carbon steel

Nanoscale co-precipitation in a novel high-strength low-carbon steel is studied in detail after isothermal aging. Atom-probe tomography is utilized to quantify the co-precipitation of co-located Cu precipitates and M2C (M is any combination of Cr, Mo, Fe, or Ti) carbide strengthening precipitates. Coarsening of Cu precipitates is offset by the nucleation and growth of M2C carbide precipitate, resulting in the maintenance of a yield strength of 1047 ± 7 MPa (152 ± 1 ksi) for as long as 320 h of aging time at 450 °C. Impact energies of 153 J (113 ± 6 ft-lb) and 144 J (106 ± 2 ft-lb) are measured at −30 °C and −60 °C, respectively. The co-location of Cu and M2C carbide precipitates results in non-stationary-state coarsening of the Cu precipitates. Synchrotron-source X-ray diffraction studies reveal that the measured 33% increase in impact toughness after aging for 80 h at 450 °C is due to dissolution of cementite, Fe3C, which is the source of carbon for the nucleation and growth of M2C carbide precipitates. Less than 1 vol.% austenite is observed for aging treatments at temperatures less than 600 °C, suggesting that transformation-induced plasticity does not play a significant role in the toughness of specimens aged at temperatures less than 600 °C. Aging treatments at temperatures greater than 600 °C produce more austenite, in the range 2–7%, but at the expense of yield strength.