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.
Relevant microstructural characteristics ensuring a good mechanical strengthening up to 600 • C of a tempered martensitic steel containing 5% Cr (AISI H11) were investigated using transmission electron microscopy, energy-dispersive X-ray analysis, X-ray diffraction and extraction of carbides. Softening induced by tempering and cyclic loading is related to a strong reduction of the dislocation density estimated by X-ray peak profile analysis (modified Williamson Hall and modified Warren Averbach analysis). Moreover, the coalescence of chromium and vanadium carbides is involved in the yield strength decrease above 600 • C and during cyclic loading.
The present article focuses on the influence of machining on the fatigue life of a titanium alloy: Ti6Al4V. An experimental design was adopted in order to highlight the effects of machining parameters on surface integrity while generating very different surfaces with a view to subsequent fatigue testing (four point bending tests). Firstly, the impact of machining parameters on surface integrity was demonstrated. Then, the influence of surface integrity on fatigue lifetime was observed: no influence of the geometric and metallurgical parameters was observed. However, the mechanical parameter (e.g., residual stress) seemed to have a preponderant influence. To conclude, a machining plan of procedure was proposed to significantly improve the fatigue lifetime as compared with a reference industrial plan of procedure.
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.
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