Abstract
This paper presents the microstructural changes and mechanical properties of carbide-free bainitic steel subjected to various heat treatment processes and compares these results with similarly treated ferritic–pearlitic steel. A key feature of the investigated steel, which is common among others described in the literature, is that the Si content in the developed steel was >1 wt.% to avoid carbide precipitation in the retained austenite during the bainitic transformation. The phase identification before and after various heat treatment conditions was carried out based on microstructural observations and x-ray diffraction. Hardness measurements and tensile tests were conducted to determine the mechanical properties of the investigated materials. In addition, following the tensile tests, the fracture surfaces of both types of steels were analyzed. Changing the bainitic transformation temperature generated distinct volume fractions of retained austenite and different values of mechanical strength properties. The mechanical properties of the examined steels were strongly influenced by the volume fractions and morphological features of the microstructural constituents. It is worth noting that the bainitic steel was characterized by a high ultimate tensile strength (1250 MPa) combined with a total elongation of 18% after austenitizing and continuous cooling. The chemical composition of the bainitic steel was designed to obtain the optimal microstructure and mechanical properties after hot deformation followed by natural cooling in still air. Extensive tests using isothermal transformation to bainite were conducted to understand the relationships between transformation temperature and the resulting microstructures, mechanical properties, and fracture characteristics. The isothermal transformation tests indicated that the optimal relationship between the sample strength and total elongation was obtained after bainitic treatment at 400 °C. However, it should be noted that the mechanical properties and total elongation of the bainitic steel after continuous cooling differed little from the condition after isothermal transformation at 400 °C.
Highlights
It is crucial to produce harder materials for railways to compensate for the increased wear and enable faster train speeds
This work evaluated the compilation of bainitic steel in order to develop a new generation of rail steels
The microstructure of the bainitic steel in its continuous cooled state consisted of fine bainite, a thin film of retained austenite (RA), and the blocky austenite
Summary
It is crucial to produce harder materials for railways to compensate for the increased wear and enable faster train speeds. Steel is well-suited for various types of thermal processing, which can drastically change its mechanical properties One example of such a treatment is austempering, which is based on isothermal holding of the steel above the onset temperature for martensitic transformation (Ms). This heat treatment leads to a considerable improvement in the mechanical properties of the material, without reducing its plasticity. The chemical composition of steels was developed to obtain a desired bainitic structure after continuous natural cooling in still air immediately after the manufacturing process (without the necessity of heat treatment processes) For this reason, the analysis of the microstructure and mechanical properties of the examined steels was carried out. The tests clarified the correlation between the type and morphology of the austenite, as well as the cooling parameters and the temperature of the bainitic transformation of the analyzed steels
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