Carbide-forming elements (W, Mo, Nb, V), as well as elements that influence only the tempering kinetics (Co, Ni), were added to a 5% Cr tempered martensitic steel in order to modify its precipitation. The main goal was to shift the secondary hardening peak towards higher tempering temperatures. Small angle neutron scattering and X-ray diffraction experiments, as well as transmission electron microscopy, were performed to characterize the precipitation of nanometric carbides. A significant modification of the volume fraction and/or chemistry of the very fine secondary precipitation was observed only for Mo, V and Ni additions. Moreover, the mechanical properties showed that the volume fraction of small precipitates (VC, Fe 3 Mo 3 C) directly influences the mechanical resistance at high temperature but has a detrimental effect on Charpy impact energy.

In comparison with the conventional AISI H11 tool steel, which contains approximately 1 wt.% silicon, the modified steel AISI H11 (∼0.35 wt.% silicon) exhibits improved tensile and fatigue properties at 550 • C -the estimated tool surface temperature during the highpressure injection of aluminium alloys. The effect of silicon on the stability of secondary carbides was studied using transmission electron microscopy and small-angle neutron scattering. Silicon has a considerable influence on the precipitation of secondary carbides. A higher volume fraction and density of small particles were observed in the low-silicon-grade steel, both after heat treatment and after fatigue testing. The final discussion focuses on the influence of silicon in the precipitation sequence. It is concluded that silicon has a detrimental effect as it shifts the secondary hardening peak towards lower tempering temperatures.