Microstructure and mechanical properties of a 2000 MPa grade co-free maraging steel
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A great amount of research has been carried out over the years on the aging microstructure, mechanical properties, and strengthening mechanisms of 18Ni maraging steels.[1–20] Dense, fine, and complex microstructures form in maraging steels during aging treatment, with precipitation having complicated diffraction patterns. These, coupled with compositional variations of different steels, have resulted in differing opinions among the research literature. For example, regarding the nucleation process of aging reaction, classical nucleation theory[1–4] as well as spinodal decomposition[5,6] were proposed. Different precipitation phases have also been identified, including -Ni3Mo,[1,3,6–8] -Ni3Ti,[1–4,6,8–11] Laves-Fe2Mo,[1–3,6,9] -FeMo,[1] -Fe7Mo6,[1,4] FeTi,[1,7] Fe2Ti,[1] ,[11,12,13] and dispersion austenite,[10] either singly or simultaneously occurring. Moving on to precipitation strengthening mechanisms, there are a dislocation looping mechanism[2,14] and a shearing mechanism.[3,15] Many authors observed reverted austenite of different morphology under overaging or special treatment, its orientation relationship with martensite varying in the range of Nishiyama–Wassermann (N–W)[3,16,17] and Kudjumov–Sachs (K–S)[3,6,17,18] types. In terms of mechanical properties, Rack and Kalish[19] and Zhu et al.[20] found embrittlement in 18Ni (350 ksi grade) maraging steels after low-temperature aging at 673 to 723 K. Overaging decreases strength, ductility, and YI HE, formerly Postgraduate Student, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China, is Research Associate, Laboratory of Materials Science, Delft University of Technology, 2628 AL Delft, The Netherlands. KE YANG, Professor, is with the Institute of Metals Research, Chinese Academy of Sciences. WEI SHA, Professor, is with the Metals Research Group, School of Civil Engineering, Architecture and Planning, The Queen’s University of Belfast, Belfast, BT7 1NN, United Kingdom. Contact e-mail: [email protected] Manuscript submitted December 22, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
toughness,[14,19] but there were also reports of improved strength and toughness due to reverted austenite.[20,21] To date, research on Co-free maraging steels has concentrated on T-250 and other grades with 1800 MPa strength level.[2,14,16,22–25] Discrepancy among the results exists similar to that for 18Ni Co-containing maraging steels. For example, Vanderwalker believed that in T-250, austenite nucleated first in martensite, followed by nucleation of Ni3Ti from the austenite site,[22] contrary to Vasudevan et al.’s work showing Ni3Ti to be the only precipitation phase.[2] Sha et al. studied T-300 (18 pct Ni-2.4 pct Mo-2.2 pct Ti-0.2 pct Al-0.2 pct Si-0.1 pct C, at pct ) and found that Ni3Ti formed first followed by the precipitation of a spheroidal Fe7Mo6 phase after longer aging times, mostly at different sites from Ni3Ti.[23] No austenite was observed after 360 hours at 783 K. Sinha et al. studied the mechanical properties of T-250
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