Bainite Formation in Medium-Carbon Low-Silicon Spring Steels Accounting for Chemical Segregation
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IN the past decades, the continuous market demand for improving vehicle performance and the rising awareness about CO2 emission and energy consumption created the need to develop new, more energy efficient production processes that include commonly used heat treatments. To reduce the overall weight of vehicles, the automotive industry needs to focus on each individual component. This research is related to the definition of alternative heat treatments to produce automotive springs. Low-alloyed, medium- or high-carbon steels are the most commonly used materials for automotive spring production. Recently, research on spring steels started to concentrate on springs with bainitic microstructures, instead of the commonly used tempered martensitic microstructures. Bainitic microstructure offers in principle significant advantages over the most commonly used tempered martensite. First of all, it offers a fine microstructure with high strength and acceptable ductility. The fully bainitic microstructure has a lower crack growth rate than tempered martensite.
C. GOULAS, Ph.D. Candidate, is with the Materials innovation institute (M2i), Electronicaweg 25 2628 XD Delft, The Netherlands, and also with Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands. Contact e-mail: [email protected] M.G. MECOZZI, Senior Postdoctoral Researcher, and J. SIETSMA, Full Professor, are with the Department of Materials Science and Engineering, Delft University of Technology. Manuscript submitted May 18, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
Tomita and Okabayashi showed that lower bainite exhibits increased fracture ductility and notch toughness, because micro-crack propagation is effectively inhibited by the fine bainitic ferrite plate morphology.[1] The bainitic microstructure, containing fine and evenly dispersed hard cementite precipitates, can be expected to effectively retard the fatigue crack growth by crack interlocking.[2] Additionally, during processing, the tempering treatment can be avoided, which is an indispensable and costly step of the tempered martensite production. In this way, temper embrittlement is avoided as well and a lower energy amount is needed for the heat treatment. Although bainite and its advantages have been known for decades, the incomplete understanding of the influence of chemical composition and process parameters on the resulting bainite fraction and morphology limits the industrial development of bainitic steels. For almost a century, a large body of research has been done on the bainite reaction and two conflicting ideas about mechanism of formation of bainite have been developed. When formed at low temperatures, bainite looks like martensite and it is sometimes even impossible to distinguish them microscopically. This experimental finding suggests that the mechanism of formation of bainite is similar to that of martensite and therefore is diffusionless in nature. On the other hand, an increasing number of experimental and theoretical stu
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