Effect of Processing Techniques on Microstructure and Mechanical Properties of Carbide-free Bainitic Rail Steels

et al. 2021

Metall Mater Trans A

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.

Section: Resultsmentioning

confidence: 99%

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

Adamczyk‐Cieślak

Koralnik

et al. 2021

Metall Mater Trans A

“…[25] On the other hand, filmy austenite is a feature that positively influences the fracture process and hinders crack propagation. [24,[26][27][28][29][30][31] It was also found that packets are responsible for significant changes in the crack path, while the close-packet boundaries inside the packets and M/A constituents influence the local changes of the crack path. [19] Generally, the packet boundaries are characterized by high-angle misorientation boundaries (HABs) near the 48 and 55 deg, while block boundaries are characterized by the lower misorientation angle.…”

Section: Introductionmentioning

confidence: 99%

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

Królicka

Lesiuk

et al. 2022

Metall Mater Trans A

The low-carbon bainitic steel after a continuous cooling process was subjected to fracture toughness investigations using the J-integral approach. The research was focused on the determination of microstructural factors influencing the fracture processes considering the crystallographic units, as well as dimensions and morphology of phases. It was found that the fracture surface is characterized by complex fracture mechanisms (quasi-cleavage, transcrystalline cleavage–ductile, and ductile mode). It was found that the main features influencing the cracking processes are bainitic ferrite packets and prior austenite grain boundaries. The changes in the crack path were also related to the changes in the misorientation angles, and it was found that changes in the crack path direction occur mainly for the bainitic ferrite packets (HABs). Also, the fracture process zone induced by the crack tip was identified. At a distance of about 4 to 5 µm from the fracture, the retained blocky austenite transformed into martensite was observed. Due to the high carbon content in the retained austenite, the transformed martensite was brittle and was the site of microcracks nucleation. Another origin of microcracks nucleation were M/A constituents occurred in the initial microstructure. In the crack tip area, the reduced dislocation density in the bainitic ferrite, which was caused by the formation of sub-grains, was also determined. Finally, the prospective improvement of the fracture toughness of bainitic steels was determined.

“…In just 2 years, significant research reports have been published in terms of low-carbon bainitic steels intended for applications in railway infrastructure. The latest investigations cover a wide range of issues including microstructure optimization considering alloying additives [1] and processing techniques [2]; manufacturing process [3]; heat treatments methods [4]; wear resistance [5]; rolling contact fatigue [6]; lowcycle fatigue performance [7]; fatigue crack growth rate [8]; and analysis of welding processes [9]. The mechanical properties of bainitic steels, compared with those of conventional ones, are promising and * E-mail: aleksandra.krolicka@pwr.edu.pl indicate the possibility of enhancing the durability of railway tracks, especially under heavy loads.…”

Section: Introductionmentioning

confidence: 99%

Decomposition mechanisms of continuously cooled bainitic rail in the critical heat-affected zone of a flash-butt welded joints

Królicka

Żak

Materials Science-Poland

et al. 2021

The joining process of bainitic rails is significant in terms of their industrialization in high-speed and heavy-loaded railways. This paper demonstrates the microstructure changes in the critical zone of the welded joint, which is responsible for the greatest deterioration in mechanical properties. Extensive progress in the decomposition of the retained austenite and bainitic ferrite occurs in the low-temperature heat-affected zone (LTHAZ) of the flash-butt welded joint of low-carbon bainitic rail. The decomposition products of the retained austenite were mainly a mixture of cementite and ferrite. The cementite was mainly precipitated at the boundary of the bainitic ferrite laths, which indicates lower thermal stability of the filmy austenite. Moreover, it was found that a part of the refined blocky retained austenite was decomposed into the ferrite and nanometric cementite, while another remained in the structure. The decomposition mechanisms are rather heterogeneous with varying degrees of decomposition. A relatively high proportion of dislocations and stress fields prove the occurrence of residual stresses formed during the welding process.