This work is concerned with determining the effects of Niobium on mechanical properties and precipitation behavior of 20% Ni maraging steel.
Heats of 20% Ni maraging steel with and without Nb (0.5 w/o) were prepared by vacuum melting. The ingots were subjected to hot forging and rolling, followed by solution treatment and aging.
Hardness, tensile properties, and impact energy of the Nb-free and the Nbcontaining alloy were measured under the solution treated and maraged conditions. Transmission electron Microscopy was employed to identify morphology and composition of precipitates.
Under the solution treated condition, the yield strength of the Nb-containing alloy (138Ksi) was about 4% higher than that of the Nb-free alloy (133Ksi). The 4% increase was attributed to solid solution hardening by adding 0.5% Nb in the matrix (Fe-Ni-Ti-Al). Upon maraging heat treatment, the yield strength of both the Nb-free and the Nb-containing alloy increased by an amount of 78% over the solution treated condition.
The large increase in strength resulted from precipitation hardening. The yield strength of the maraged Nb-free alloy was 237 Ksi, compared with 246 Ksi of the maraged Nb-containing alloy. The increase of 4% (9 Ksi) in the yield strength was obtained by adding 0.5 w/o Nb. However, the Charpy impact energy tested at room temperature decreased by the amount of 25% by the addition of 0.5% Nb.
This result shows that the addition of 0.5% Nb in the 20% Ni maraging steel is not desirable in the view of having optimum combination of strength and fracture toughness.
Transmission electron microscopic study revealed that precipitate phases formed in the Nb-free alloy were FCC $Ni_3$ (Ti.Al) ｒ' and HCP $Ni_3$ Ti $\eta$ phase, while is the Nb-containing, rod shape of orthorhombic $Ni_3Nb$ existed in addition to the ｒ' and $\eta$ phases.