Aluminium was added to a 0.2% C-2.5% Cr-1.4% Mo-11% Ni steel to modify the precipitation sequence during tempering treatment. The main goal was to obtain fine co-precipitation of an intermetallic phase and M 2 C carbides (where M is a combination of Cr, Mo and small amounts of Fe). Small angle neutron scattering, synchrotron X-ray diffraction, transmission electron microscopy and atom probe tomography were performed to characterize the nanometric precipitation. The tempering response of samples austenitized at 900 °C revealed a strong interaction between the two types of precipitation, leading to a significant modification of both the precipitation sequence of carbides and the arrangement of carbide nucleation sites compared with these sites in a single precipitation steel. Indeed, a microstructural investigation clearly showed that iron carbide precipitation was either delayed or did not occur during the tempering process, depending of the alloying elements added. Moreover, double precipitation directly influenced the mechanical resistance, as well as the toughness, leading to an ultrahigh-strength, high toughness steel.

A technique for the microstructural study of steels, based on the use of matrix dissolution to collect the very low number density precipitates formed in martensitic steels, has been considerably improved. This technique was applied to two different grades of alloy, characterized by high nickel and cobalt contents and varying chromium, molybdenum and vanadium contents. The technique was implemented at temperatures ranging between 900°C and 1000°C, in order to accurately determine experimental data including the crystallographic structure and chemical composition of the carbides, the carbide solvus temperatures, and variations in the chemical composition of the matrix. These experimental investigations reveal that the solubility of molybdenum in FCC carbides can be very high. These results have been compared with the behavior predicted by computational thermodynamics, and used to evaluate and improve the thermodynamic Matcalc steel database. This upgraded database has been validated on three other steels with different chemical compositions, characterized by the same Fe-Cr-Mo-V-C system.

In order to improve the knowledge of the precipitation mechanism in martensitic steels containing carbon, XRD synchrotron experiments were performed. Firstly, the influence of Ni, Co and Al were studied and it was found that the precipitation of iron carbides occurs in same way as in Fe-C steel. However, with the addition of molybdenum and chromium in same steels, XRD synchrotron investigations clearly showed alloyed carbides directly precipitate, thereby preventing the iron carbides formation.