Studies of Bainitic Steel for Rail Applications Based on Carbide-Free, Low-Alloy Steel

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.

The Role of Microstructure Morphology on Fracture Mechanisms of Continuously Cooled Bainitic Steel Designed for Rails Application

“…It was determined that in the crack area the dislocation density of bainitic ferrite decreased compared to the base material (Table V). A similar result was also found in the research on cyclic deformation, [12] therefore also in the case of the area crack tip/matrix interaction, the change in dislocation density may be caused by the formation of sub-grains. Also, it is another observation supporting this hypothesis along with the size of the crystallites.…”

“…It was found that the dislocation density decreased at the crack tip, which was supported by the formation of sub-grains. [12] Due to the technological properties and the prospects for industrialization, [70] bainitic steels subjected to continuous cooling are vastly promising. The variety of bainite morphology and different proportions of phases after continuous cooling directly influence the mechanisms of their fracture.…”

“…The probability of sub-grain and cell structures formation was also proposed in investigations. [12] On the other hand, the presence of blocky retained austenite was excluded in the fracture process zone [Figure 17(c)].…”

A Comparative Study of Deterministic and Stochastic Models of Microstructure Evolution during Multi-Step Hot Deformation of Steels

“…As far as steels are concerned, recent papers on complex phase [6], multiphase [7], ferritic/martensitic [8], pipeline [9], and bainitic steels [10] should be mentioned. Modern bainitic steels are used for manufacturing rails [11] and rods [12]; the latter mainly has further application as stock for cold heading processes [13].…”

Supervised pearlitic–ferritic steel microstructure segmentation by U-Net convolutional neural network

“…Depending on the cooling rate of steels, the ferrite (F), pearlite (P), bainite (B), or martensite (M) microstructures could be formed due to the displacive and reconstructive transformations of austenite (A) crystal structure, which are accompanied with cementite precipitation at different diffusion rates [1]. It is well known that the volume fraction, dimension, and morphology of these microstructure phases are greatly responsible for the mechanical properties of steels [2]. In this case, the correct classification of these microstructure features is crucial during the metallographic investigation to understand their role in material performance.…”