Effect of Processing Techniques on Microstructure and Mechanical Properties of Carbide-free Bainitic Rail Steels (original) (raw)
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Metals, 2018
An advanced bainite rail with high strength-toughness combination was produced in a steel mill and the effects of tempering on the microstructure and properties of the bainite rail steel were investigated by optical microscopy, transmission electron microscopy, electron back-scattering diffraction and X-ray diffraction. Results indicate that the tensile strength, elongation and impact toughness were about 1470 MPa, 14.5% and 83 J/cm 2 , respectively, after tempering at 400 • C for 200 min. Therefore, a high-strength bainite rail steel with good toughness was developed. In addition, the amount of retained austenite (RA) decreased due to bainite transformation after low-temperature tempering (300 • C) and RA almost disappeared after high-temperature tempering (500 • C). Moreover, as the tempering temperature increased, the tensile strength of the rail head first decreased due to the decreased dislocation density and carbon content in bainite ferrite and the coarseness of bainite ferrite, and then increased because of carbide precipitation at high-temperature tempering. Furthermore, RA played a significant role in the toughness of bainite rail. The elongation and toughness of the rail obviously decreased after tempering at 500 • C for 200 min because of the disappearance of RA and appearance of carbides.
Carbide-free bainitic steels for rail wheel applications
IOP Conference Series: Materials Science and Engineering, 2019
In South Africa, forged wheels for rail cars are imported but cast wheels are manufactured locally, although recent developments indicate that forged wheels will in future be manufactured locally. The forged wheels are generally manufactured from AAR Class C steel, which is a high carbon, pearlitic steel used for rail application. Railway wheels are a costly component of the railway wagon as they experience wear and damage during application. Improvements in the mechanical properties are thus desired. Against this background, a project is reported on the development of durable rail steel alloys for railway wheel applications. Carbide-free bainite is a novel microstructure comprising bainitic ferrite and retained austenite/martensite but without coarse carbides in the interlath positions. The absence of carbides is achieved through the addition of a high silicon (~2wt%) content to the steel. This carbide-free bainite can achieve high tensile strength (>1000MPa) and toughness (40J, 20°C) as well as good wear resistance. These alloys have found application in areas where high strength, toughness and wear resistance are required, such as in rail steels, and have been deemed the "next generation" of rail steels. Carbide-free experimental alloys were produced in the laboratory and tested for mechanical properties such as hardness, tensile strength and impact toughness. The properties of the laboratory alloys were compared to those of standard Class C alloys used in South Africa to determine their potential use as railway wheel alloys.
Influence of Microstructure on Mechanical Properties of Bainitic Steels in Railway Applications
Metals
Wheel–rail contact creates high stresses in both rails and wheels, which can lead to different damage, such as plastic deformation, wear and rolling contact fatigue (RCF). It is important to use high-quality steels that are resistant to these damages. Mechanical properties and failure of steels are determined by various microstructural features, such as grain size, phase fraction, as well as spatial distribution and morphology of these phases in the microstructure. To quantify the mechanical behavior of bainitic rail steels, uniaxial tensile experiments and hardness measurements were performed. In order to characterize the influence of microstructure on the mechanical behavior, various microscopy techniques, such as light optical microscopy (LOM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD), were used. Three bainitic grades industrially known as B360, B1400 plus and Cr-Bainitic together with commonly used R350HT pearlitic grade were studied. Influe...
An attempt was made to optimize the mechanical properties by tailoring the process parameters for two newly developed high-strength carbide-free bainitic steels with the nominal compositions of 0.47 pct C, 1.22 pct Si, 1.07 pct Mn, 0.7 pct Cr (S1), and 0.30 pct C, 1.76 pct Si, 1.57 pct Mn, and 0.144 pct Cr (S2) (wt pct), respectively. Heat treatment was carried out via two different routes: (1) isothermal transformation and (2) quenching followed by isothermal tempering. The results for the two different processes were compared. The bainitic steels developed by isothermal heat treatment were found to show better mechanical properties than those of the quenched and subsequently tempered ones. The effect of the fraction of the phases, influence of the transformation temperatures, the holding time, and the stability of retained austenite on the mechanical properties of these two steels was critically analyzed with the help of X-ray diffraction, optical metallography, scanning electron microscopy, and atomic force microscopy. Finally, a remarkable combination of yield strength of the level of 1557 MPa with a total elongation of 15.5 pct was obtained.
Development of New High-Strength Carbide-Free Bainitic Steels
Metallurgical and Materials Transactions A, 2011
An attempt was made to optimize the mechanical properties by tailoring the process parameters for two newly developed high-strength carbide-free bainitic steels with the nominal compositions of 0.47 pct C, 1.22 pct Si, 1.07 pct Mn, 0.7 pct Cr (S1), and 0.30 pct C, 1.76 pct Si, 1.57 pct Mn, and 0.144 pct Cr (S2) (wt pct), respectively. Heat treatment was carried out via two different routes: (1) isothermal transformation and (2) quenching followed by isothermal tempering. The results for the two different processes were compared. The bainitic steels developed by isothermal heat treatment were found to show better mechanical properties than those of the quenched and subsequently tempered ones. The effect of the fraction of the phases, influence of the transformation temperatures, the holding time, and the stability of retained austenite on the mechanical properties of these two steels was critically analyzed with the help of X-ray diffraction, optical metallography, scanning electron microscopy, and atomic force microscopy. Finally, a remarkable combination of yield strength of the level of 1557 MPa with a total elongation of 15.5 pct was obtained.
Influence of the Austempering Time on the Mechanical Properties of Carbide-Free Bainitic Cast Steels
International Journal of Metalcasting, 2020
Three medium-carbon, high-silicon cast steels with different alloy contents were austempered at 330°C for different holding times in order to obtain carbide-free bainitic microstructures. Aiming at evaluating the influence of holding time and microstructural features on strength and ductility, tensile properties were measured for each steel at selected austempering times. The results obtained indicate that it is possible to adjust holding time in order to obtain the best strength/ductility combination at determined austempering temperature. Moreover, it has been shown that the mechanical stability of retained austenite is the key factor in controlling tensile performance. Short austempering times result in low carbon enrichment of the austenite (low stability) and promote higher ultimate tensile strength and lower ductility. For longer austempering times, steels present a slight decrease in ultimate tensile strength but a marked increase in ductility. This work shows that it is possible to obtain cast steels with ultimate tensile strength of 1682 MPa, yield strength of 1493 MPa and total elongation of 12.5% by means of bainitic reaction. This strength/ductility combination and others reported in this study are remarkable for cast steels.
ISIJ International, 2008
In order to improve the ductility of carbide free bainitic microstructures, consisting of a bainitic ferrite matrix and a mixture of austenite and martensite, the TRIP effect i.e. the strain induced transformation of retained austenite to martensite, should be controlled. In this sense, the effect of the chemical composition on the mechanical stability of the retained austenite and the morphology, size, and distribution of this phase has been studied to determine the role that plays on the ductility behaviour of advanced bainitic steels. Results suggest that apart of the retained austenite, the morphology of the bainitic matrix is an important factor controlling ductility. Bainitic microstructures formed by coiling with coarse and blocky bainite morphology have shown higher uniform deformation values than those obtained by air cooling with the typical thin bainite platelets.
The Role of Retained Austenite on Tensile Properties of Steels with Bainitic Microstructures
MATERIALS TRANSACTIONS, 2005
In high-carbon, silicon-rich steels it is possible to obtain a very fine bainitic microstructure by transformation at low temperatures (200-300 C). This microstructure consists of slender ferrite plates, with thicknesses of several tens of nm, in a matrix of retained austenite. Whereas strength is mainly provided by to the fine scale of the ferrite plates (stronger phase), ductility is mostly controlled by the retained austenite (softer phase). Further improvement in ductility is achieved by strain induced transformation of austenite to martensite, the so called TRIP effect. In order to take full advantage of this effect, the mechanical stability of the austenite, i.e., its capability to transform to martensite under strain, must not be too low nor excessively high.
Materials Science Forum, 2012
The influence of pre-straining and bake-hardening on the mechanical properties of thermomechanically processed 0.2C-1.5Si-1.5Mn-0.2Mo-0.004Nb (wt%) steel was analysed using tensile test, transmission electron microscopy (TEM) and atom probe tomography (APT). This steel after processing had high strength (~1200MPa) and good ductility (~20%) due to the formation of fully bainitic microstructure with nanolayers of bainitic ferrite and retained austenite. The bake hardening (BH) of pre-strained (PS) samples increased the yield strength of steel up to 690MPa and showed the bake-hardening response of 220MPa due to the operation of several strengthening mechanisms such as transformation induced plasticity during pre-straining and pinning the dislocations by carbon during bake-hardening treatment. The carbon content of the bainitic ferrite and retained austenite before and after bake-hardening treatment, the solute distribution between these phases and the local composition of fine Fe-C clu...
Materials Science and Engineering: A, 2019
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