Research Specifications

Nanostructured bainitic steels are known as valuable engineering materials which can be obtained through a simple austempering heat treatment process as large chunks. The well-known MUCG83™ thermodynamic model has been vastly used in order to design the proper chemical composition for obtaining these unique alloys based on thermodynamic theories alone. This article aims to study first how experimental results match with those predicted by thermodynamic theories determining the times needed for both reconstructive and shear transformations being started. Secondly it aims to investigate the austemperability of designed steel which isothermally is going to be kept in salt bath furnaces at different heat treatment temperatures to obtain nano bainite. For this aim, austemperability was evaluated based on the critical diameter of the material. At first step, ideal diameter was measured according to the chemical composition and using the Jominy end quench test method. Consequently the quench severity factors of the salt bath furnaces (H factors) were determined experimentally and were used for determining the critical diameters at each transformation temperature. It has been shown that practical results were in a good agreement with those predicted by thermodynamic theories for estimating the incubation times needed for starting the bainitic transformation even if it was less accurate for reconstructive transformations. Therefore thermodynamic model has been shown to be a good approach for designing the steel in order to attain nanostructured bainite. This in turn could omit the try and error methods and enhances the efficiency for steels production. Moreover, it has been demonstrated that large chunks of steel with enough thick cross sections could be austempered successfully without any diffusional transformation products taking place within the microstructures. Decreasing the transformation temperature increased the H factor of the identical salt bath so thicker steel c