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| Microstructure and Mechanical Properties of a Low Carbon 7.7%Ni Steel Subjected to Intercritical Quenching |
| ZHU Xu-xiang,LIU Dong-sheng |
| Institute of Research of Iron and Steel, Shasteel, Zhangjiagang 215625, Jiangsu, China |
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Abstract To save the cost of constructing liquefied natural gas (LNG) storage tanks, a lean steel composition (low C CrNi steel containing 7.7% mass fraction of Ni) was proposed as a substitute of the conventional 9% Ni steel. The mechanical properties of the subject steel subjected to austenitizing followed by water quenching+lamellarizing+tempering (QLT) were tested. Microstructure evolution in the tested steel during the QLT processing was studied. The microstructures consist of mainly ferrite (F)+lath-like martensite (LM), martensite/austenite (M/A) constituents and reversed austenite (γ′). With the increase of lamellarizing temperature (tL), the fraction of LM gradually increases while that of F decreases, resulting in refined microstructure and increased fraction of large-angle grain boundaries (HAGB). The fraction of γ′ maintains is at approximately 7% when tL is between 650-680℃. It decreases with further increase of tL. The cryogenic toughness is mainly determined by γ′ content and the fraction of HAGB. Excellent mechanical properties can be achieved in the current steel processed by the QLT heat treatment showing yield strength (Rp0.2) greater than 530MPa, tensile strength (Rm) greater than 670MPa, Charpy impact toughness (KV2) greater than 150J even at -196℃. The mechanical properties of the intercritically quenched steel are equivalent to those of 9%Ni steel.
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Received: 20 March 2013
Published: 25 November 2013
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2013, Vol. 48 
