Modeling of precipitation hardening during the aging and overaging of 18Ni-Co-Mo-Ti maraging 300 steel

The precipitation hardening behavior of an 18Ni-Co-Mo-Ti maraging 300 solution treated and aged in the 440-650 °C range was investigated. The aging at 440, 480, 510 and 560 °C was modeled by an equation ΔH = (Kt)n. The activation energy for precipitation was determined (132 kJ/mol) and compared to published values for 18Ni-Co-Mo-Ti maraging 250 and 350 steels. The model for overaging based on the Orowan mechanism was tested in samples treated at 560, 600 and 650 °C but was found to be valid only for 560 °C. The softening of samples aged at 600 and 650 °C is rather due to austenite precipitation than to Ni3(Mo, Ti) and Fe2Mo coarsening. Equations H = H1tp were fitted for the overaging at 560, 600 and 650 °C.