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Abstract
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It is critical to obtain nanocrystalline materials in large chunks rather than only in thin film or rod shapes and nanostructured bainitic steels are good examples of bulk nano materials. This article aims to investigate the hardenability of nanobain steel to estimate the biggest cross section that can be successfully austempered to form nanocrystalline bainite. Isothermal heat treatments were conducted at different temperatures using salt bath furnaces, the quench severity factors of which were determined at bainitic transformation temperature ranges. It was found that, the proper chemical composition selection increased the hardenability and shifted the upper C-curve of the Time–Transformation–Temperature diagram to longer heat treatment times. Accordingly, large chunks of nanostructured bainite with almost 330, 340 and 390 mm diameters could be austempered to obtain nanobain successfully in salt bath furnaces of 300, 250 and 210 _C, respectively. The nanocrystalline bainite was composed of nanoscale bainitic subunits of 30–70 nm thicknesses depending on transformation temperature.
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