Development of Lean Maraging Steels for Ultra high Strength Applications

Lean maraging steels were designed for several application sectors by providing very high strength and ductility, with the addition of relatively cheaper elements like manganese. In this work, the microstructural and mechanical properties of four niobium-containing (0.035 wt. %) and vanadium-containing (0.02 wt. %) Fe-7Mn-2Ni-1Ti-1Mo-0.03C (in wt. %) with different content of aluminium (1~2 wt. %) aged at different temperatures between 420 °C and 570 °C were investigated. As-quenched Fe-7Mn alloys exhibited a good combination of high strength (~700 MPa of 0.2 proof strength, ~ 850 MPa of UTS) and ductility (~ 10 % tensile elongation). The as-quenched microstructure consisted of lath martensite and a small amount (~ 0.3 vol. %) of micronsized (Ti, Mo, Nb/V)C carbides. The aging process significantly strengthened/hardened the Fe-7Mn alloys which is due to the formation of nano-sized Nix(Ti, Mn, Al) precipitates. Nix(Ti, Mn, Al) precipitates exhibit a very high number density (52.9×1014/m2 in the peak-aged state of Alloy 2, aged at 500 °C for 24 h) and fine size (average diameter was 17.4 ± 4.2 nm after aged at 500 °C for 168 h). The Vickers hardness increased with aging time in the under-aged stage which was due to the precipitate growth and the alloy was strengthened by Orowan bypassing mechanism. The hardness decreased with aging time after the peak hardness as the precipitate coarsened. There were two types of the crystal structure observed for Nix(Ti, Mn, Al) precipitates: The L21-Ni2(Ti, Mn, Al) phase (lattice parameter, a = 0.5863 ~ 0.5895 nm, which is co-planar with martensite matrix, with only 1.72 % of lattice misfit. And the L12-Ni3(Ti, Mn, Al) structure ( a = 0.3598 ~ 0.3613 nm). A short time-aging resulted in a yield strength above 1 GPa but led to embrittlement of Fe-7Mn alloys, which was believed to be due to the segregation of Mn to the grain boundaries. Both carbides and nano-precipitates formed along grain boundaries were likely to reduce the cohesion across the boundary plane, as well as resulted in stress-strain incompatibilities. However, the prolonged aging resulted in the formation of reverted austenite (RA) in the over-aged stage, which led to the recovery of ductility when aged at 570 °C as the austenite reversion removed the Mn solute from the grain boundaries. Reverted austenite exhibited lath-like shape with the length between 50 and 2000 nm. Both the size and volume fraction of RA increased with the increasing aging time and aging temperature. RA was formed with the diffusion-controlled mechanism, and it was observed exhibiting a Kurdjumov-Sachs (K-S) orientation relationship with the neighbouring aged martensite grains with an enrichment of Mn and Ni. Higher content of Al addition resulted in ~ 25 vol. % of δ-ferrite in the as-quenched microstructure, which was stable during aging. 2 wt. % of Al also resulted in higher volume fraction of nano-precipitates and increased the dissolution temperature of precipitates, however, it delayed the peak-aging time and austenite
reversion. Nb-containing alloys exhibited relatively finer size of prior austenite grains and (Ti, Mo, V)C carbides, larger size and higher number density of Ni(Ti, Mn, Al) precipitates, but slightly lower austenite fraction, compared to V-containing alloys. Based on the results, it is suggested that Alloy 3 aged at 570 °C for 2~6 h gives the optimized mechanical properties.