Addressing the issue of precipitates in maraging steels - Unambiguous answer

Despite several decades' long study, the identity of the precipitating phases responsible for the strengthening of maraging steels is still not clear. In the current work, this issue was extensively investigated using experimental and theoretical approaches. First, in-depth characterization of the precipitates in C250 steel and the precipitation order were performed through a combination of various Transmission Electron Microscopy (TEM) methods. In parallel, thermodynamic calculations were used for the prediction of the phase content at equilibrium. Then, in order to isolate the effects of the different precipitates, model alloys were cast and aged. It was shown that the phases responsible for the strengthening during the initial stages of aging are Ni3Mo and Ni3Ti. In the over-aged (close to equilibrium) condition, the steel consists of martensite, reverted austenite, Ni3Ti, and Fe-Mo phases. This conclusion was found to be in perfect agreement with thermodynamic calculations. The formation of Ni3Mo at early stages of aging, despite its calculated lowest driving force for formation, was attributed to a low barrier for formation.