Abstract
This paper describes the development and characterisation of bainitic steel for rail applications based on carbide-free, low-alloy steel. The results show that after rolling and subsequently cooling, the designed carbide-free bainitic steel exhibits better mechanical performance than standard pearlitic steel. This is because of its fine, carbide-free bainitic microstructure, which consists of bainitic ferrite and retained austenite laths. Microstructural and mechanical property analysis was carried out using scanning and transmission electron microscopy, X-ray diffraction, hardness measurements, tensile and low-cycle fatigue tests. The obtained results demonstrate that during low cyclic deformation, a partial transformation of the retained austenite into deformed martensite α′ takes place, and strain-induced martensitic transformation occurs. The initial strengthening of the material during low-cycle fatigue was caused by the transformation of austenite into martensite and the increase in the dislocation density of the steel. In addition, an optimal amount of retained austenite in the form of thin layers and islands (dimensions not exceeding 1 µm) made it possible to obtain a high yield while maintaining the high plasticity of the steel. These microstructural features also contributed to the high crack resistance of the tested carbide-free bainitic steel.
Highlights
SOME of the most popular and widely used types of rail steel are pearlitic carbon–manganese steels
The mechanical properties of the examined steels were strongly influenced by the stability of the retained austenite, which depended on multiple factors, including the chemical composition, morphology, size, distribution, stress state, temperature, isothermal bainite transformation parameters, and strength of the surrounding phases.[37,38,39,40]
C and Mn are austenite stabilisers that reduced the temperature at which the transformation of austenite to martensite began (Ms)
Summary
SOME of the most popular and widely used types of rail steel are pearlitic carbon–manganese steels. These materials (e.g. R260 grade or carbon–manganese steels used for the production of heat-treated materials) develop moderate strength after rolling, followed by continuous cooling without heat treatment.[1] harder steels (e.g. R350HT) are used in certain rail locations, such as turnouts or bends with radii up to 800 m, because these sections of rail typically wear out more intensively during service. For turnouts and rail crossings that are most subject to wear and damage as a result of cracking, Hadfield steel is used to produce the most wear-resistant and durable structures.[2] When designing new steels for railway solutions, attention should be paid to the rate at which contact—fatigue phenomena occur.
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