Carbide-Free Bainitic Weld Metal: A New Concept in Welding of Armor Steels
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TRODUCTION
CARBIDE-FREE bainite is a relatively recent development in steel microstructures. In contrast to conventional bainite, it is made up of extremely thin plates of bainitic ferrite (down to 20 nm in thickness) dispersed in a matrix of stable carbon-enriched austenite. The plates grow in clusters, giving rise to a characteristic sheaf microstructure. Within each sheaf, the plates are parallel and share a common crystallographic orientation.[1] This unique microstructure develops when cementite precipitation is retarded during bainitic transformation, typically by judiciously alloying the steel with silicon.[2,3] While the carbide-free version of bainite is known for a long time, it is only recently that it could be engineered to realize its full potential, mainly due to Bhadeshia and co-workers at Cambridge University.[4–9] Carbide-free bainitic (CFB) steels have a simple, yet tricky, chemistry, with a minimum of ~1.5 wt pct silicon. One important question in designing CFB steels is how to avoid large blocks of retained austenite in the final microstructure. These blocky austenite regions between the sheaves of bainite seriously embrittle the steel when they undergo stress-induced transformation to martensite.[1,7,10–12] To minimize these regions, it is necessary to maximize the volume fraction of bainite. Additions of cobalt, aluminum and nickel are helpful in this regard, which shift the T0 curve (the locus of points N. KRISHNA MURTHY, Research Scholar, G.D. JANAKI RAM, Associate Professor, and B.S. MURTY, Professor, are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600 036, India. Contact e-mail: [email protected] G.M. REDDY, Scientist G, is with the Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad 500058, India. T.J.P. RAO, Senior General Manager, is with the D&H Se´cheron Electrodes Pvt. Ltd., Kila Maidan, Indore 452006, India. Manuscript submitted February 6, 2014. Article published online July 10, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
where austenite and ferrite of the same composition have identical free energies) to higher carbon concentrations.[4,5,8,13,14] On the other hand, manganese and, to a lesser extent, chromium shift the T0 curve to lower carbon concentrations and, therefore, reduce the volume fraction of bainite.[4,10] Nevertheless, they are commonly used in CFB steels for ensuring sufficient hardenability. For maximizing the volume fraction of bainite, it is also necessary to reduce the transformation temperature. This helps reduce the scale of the microstructure (thickness of ferrite plates) as well, which is beneficial for strength. However, at low temperatures, the transformation time can be unrealistically long. Thus, a compromise between transformation time and properties would have to be reached.[13] For reducing the transformation temperature, it is necessary to bring down the bainite start (BS) and martensite start (MS) temperatures of the steel. This can be easily done by increasing the carbon
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