Age hardening and creep ductility in Cr–Ni–Ti,Fe alloys

In alloys such as 22Cr–33Ni–Fe (wt-%), titanium can improve creep strength, but may also give excessive hardening and low ductility in some regimes. Experimental casts having various Ti, Mn, Mo, and Si contents were solution treated at 1250°C and age hardening at 550–650°C was studied. Two commercial 800 type alloys were included for comparison. Slow strain rate tensile tests were carried out at 550°C to relate flow stress to creep ductility and age hardening. Age hardening was related to Ni₃Ti (γ′) particle size and equilibrium volume fraction, and to titanium solute depletion. The critical effect of free titanium ([Ti] – 3·4[N]) on the factors given above was apparent. Manganese suppressed an early stage of hardening at 600°C, apparently by hindering the clustering on cooling that nucleated a fine γ′ precipitate. Aluminium enhanced the effect of titanium, but this could not be attributed solely to substitution in γ′. Intermediate treatments at 850–900°C improved the inherent creep ductility at 550°C by precipitating coarse (Cr, Fe)₂₃C₆ on grain boundaries. These treatments also preprecipitated some of the titanium as TiC with some reduction in the subsequent creep flow stresses.

MST/877