Influence of the Austempering Time on the Mechanical Properties of Carbide-Free Bainitic Cast Steels
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opyright Ó 2020 American Foundry Society https://doi.org/10.1007/s40962-020-00512-4
Abstract Three medium-carbon, high-silicon cast steels with different alloy contents were austempered at 330 °C for different holding times in order to obtain carbide-free bainitic microstructures. Aiming at evaluating the influence of holding time and microstructural features on strength and ductility, tensile properties were measured for each steel at selected austempering times. The results obtained indicate that it is possible to adjust holding time in order to obtain the best strength/ductility combination at determined austempering temperature. Moreover, it has been shown that the mechanical stability of retained austenite is the key factor in controlling tensile performance. Short austempering times result in low carbon enrichment of the austenite (low stability) and promote higher ultimate
tensile strength and lower ductility. For longer austempering times, steels present a slight decrease in ultimate tensile strength but a marked increase in ductility. This work shows that it is possible to obtain cast steels with ultimate tensile strength of 1682 MPa, yield strength of 1493 MPa and total elongation of 12.5% by means of bainitic reaction. This strength/ductility combination and others reported in this study are remarkable for cast steels.
Introduction
steels, i.e., steels homogenized and forged or rolled before austempering.1–3 These investigations proved that CFB steels can reach remarkable mechanical properties, with UTS values ranging from 700 to 2200 MPa, and elongation up to 35%. A recent study showed an extraordinary combination of a tensile strength of 2068 MPa with a total elongation of 21.3%.5
Over the last decades, carbide-free bainitic (CFB) steels have gained interest since they are potentially able to reach remarkable mechanical properties at low manufacturing cost.1 This has encouraged the study of its use in structural components for automotive vehicles, among other applications.2–4 CFB is commonly obtained by means of an isothermal heat treatment (austempering). Its microstructure consists of bainitic ferrite plates, carbon-enriched retained austenite and, in some cases, small amounts of martensite.2 The high silicon content of these steels ([ 1.5 wt.%) greatly retards the carbide formation during the bainitic transformation.3 As a result, the carbon partitioned from the bainitic ferrite after its growth remains in solution in the surrounding austenite, decreasing its martensite start (Ms) temperature. If the carbon enrichment is sufficiently high, austenite becomes thermally stable at room temperature.1 Most of the studies focusing on the processing and properties of CFB steels have been carried out on wrought
International Journal of Metalcasting
Keywords: cast steels, high silicon, austempering, carbide-free bainite, mechanical properties, austempering time, retained austenite
Meanwhile, the development of CFB microstructures from cast steels received very little attention. Cast steels present some
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