Thermally Stable Nanocrystalline Steel
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INTRODUCTION
NANOCRYSTALLINE steels, commonly referred to as superbainite, have been the subject of a large number of studies since their development by Caballero et al.[1] due to their combination of strength and toughness, achieved in large volumes with neither rapid cooling nor severe deformation.[1–9] The structure consists mostly of alternating thin plates of bainitic ferrite, ab , and retained austenite, cr , with a small fraction of retained austenite blocks forming the residue of the sample. The austenite films and bainite plates are typically below 50 nm in width, providing a potent strengthening mechanism without compromising toughness. The retained austenite is able to accommodate a large amount of plastic work by either one of or both dislocation glide and the formation of stress-induced martensite. Nanocrystalline steels represent a formidable combination of mechanical properties; their transformation ultimately relies on the addition of a large quantity of carbon. Carbon serves to depress both the martensite-start temperature, Ms , and the bainite-start temperature, Bs , but the former more than the latter.[2] There is then a sufficiently wide temperature range in bainite that may form with ever finer platelets as the transformation temperature is lowered. The large carbon content is further enhanced in the retained austenite due to partitioning after the bainitic
CHRISTOPHER NEIL HULME-SMITH, SHGH WOEI OOI, and HARSHAD K.D.H. BHADESHIA are with the Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge, CB3 0FS, UK. Contact e-mail: [email protected] Manuscript submitted January 9, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
transformation. At temperatures where the atomic mobility of carbon atoms is sufficient, there will then be a tendency for the austenite to decompose into a mixture of ferrite and cementite. Many studies have observed a carbon supersaturation with respect to cementite in both austenite[5,6,10–15] and ferrite.[12,13,15–19] There is therefore a large driving force for the formation of cementite in both phases. Rapid decomposition of austenite into carbides and ferrite has been observed in nanocrystalline steels upon heating.[20,21] The resulting loss of austenite compromises both the strength and toughness of the steel and hence it is unsuitable for service at elevated temperatures. The aim of the current work was to design new nanocrystalline steel alloys that are able to tolerate exposure to high temperatures while retaining an acceptable level of strength and toughness.
II.
ALLOY DESIGN
Two approaches were considered to develop novel alloys: an extension of previous work[22] to introduce as many atoms that are insoluble in cementite as possible and a new concept to minimize the carbon content while still obtaining the desired microstructure. Thermodynamic modeling was conducted using the calculation software MTDATA version 4.73 from the National Physical Laboratory, Teddington, U. K.[23] with various thermodynamic databases
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