Development of New High-Strength Carbide-Free Bainitic Steels
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TRODUCTION
CARBIDE-FREE bainitic steels are emerging as a very promising class of steels, because they exhibit a remarkable combination of high strength with good ductility and can be produced at low costs.[1] The mechanical properties of carbide-free bainitic steels were found to supersede those of quenched and tempered alloys of the same hardness because of their interesting microstructure consisting of thin plates of bainite in a matrix of retained austenite. This microstructure can be generated by quenching to a certain temperature followed by a simple isothermal holding treatment route. The strength of these steels is due to the small length scale of the microstructure (relative fineness of bainite lathes) and high dislocation densities, whereas ductility was attributed to the film of retained austenite present in between the bainitic lathes.[2–4] The detrimental effect of carbides is overcome by suppressing their precipitation with the addition of a sufficient amount of silicon,[5,6] and the thin film of austenite acts as a cushion for any crack propagation.[7,8] A carbon-enriched austenite results in increased stability and thereby helps in minimizing the fraction of retained austenite transforming to martensite.[9] A series of bainitic steels with strengths more than 1200 MPa along with an elongation of 13 pct can be used as high performance alloys possessing good toughness and tribological properties.[10] The design criterion for the development of very S. SHARMA, Research Scholar, S. SANGAL, Professor, and K. MONDAL, Assistant Professor, are with the Department of Materials Science and Engineering, Indian Institute of Technology, Kanpur 208016, India. Contact e-mail: [email protected] Manuscript submitted December 13, 2010. Article published online August 9, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
strong bainitic steels lies in the engineering of metastable temperature (To) where the free energies of austenite and ferrite are the same. Coarse blocks of retained austenite can be avoided by shifting the To curve to higher carbon concentrations in austenite.[10–12] One of the best combinations of strength and toughness was achieved p inffiffiffiffia bainitic steel exhibiting 1600 MPa and 130 MPa m, respectively, by meeting this criterion.[7,8] Chakraborty et al.[13] recently showed improved mechanical properties of 52100 steel (1.1C, 1.46Cr, 0.27Si, 0.33Mn, 0.14V, 0.04Ni, 0.02P, balance Fe, all in wt pct) with a dualphase structure made of bainite and martensite over quenched and tempered structure by adopting the austempering route. Some prior cold deformation was given to refine the thickness and size of the bainitic sheaves. A maximum strength of 2250 MPa with an impact strength of 72 J was achieved. Brown and Baxter[14] found the isothermal transformation characteristics of a high carbon and high silicon steel, which exhibited a tensile yield stress of 1673 MPa with an elongation of 8 pct. They designated them as super bainitic steels, which are virtually free of any carbides, and the sheaf thickness is very fin
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