Cu precipitation kinetics during martensite tempering in a medium C steel

The Cu precipitation kinetics during martensite tempering of an Fe-0.44C-0.60Mn-0.21Si-0.11Cr-1.53Cu (wt.%) steel was quantitatively investigated by separating the Cu precipitation from the cementite precipitation through electrical resistivity, small-angle neutron scattering (SANS), dilatometry, and thermodynamic calculations. The cementite precipitation was already finished during continuous heating to 450 °C, and then Cu precipitation occurred above 450 °C. The Cu precipitation kinetics was accelerated with increasing tempering temperature. The fcc ε-Cu particles were precipitated mainly at cementite interfaces, while bcc Cu-rich particles were formed in the tempered martensite matrix, and transformed to 9R, 3R, and fcc ε-Cu during further tempering, resulting in higher hardness of a medium C steel. The activation energy for isothermal Cu precipitation (64.9 ± 13.3 kJ/mol) during martensite tempering of the present medium C steel was even lower than that of a low C steel due to the greater cementite fraction.