Bainitic Rail Research Articles

Synchrotron X-ray radiation study of retained austenite stability and the effect on fatigue behavior of bainitic railway steels

The effect of retained austenite stability on the fatigue performance was studied in bainitic rail steel subjected to different heat treatments, including normalizing at 900 °C and normalizing at 900 °C combined with tempering at 320 °C. The normalized steel had yield strength of 851 MPa, tensile strength of 1131 MPa, uniform elongation 7.0 %, and total elongation 17.5 %. Tempering the steel at 320 °C resulted in similar strength and plasticity compared to the normalized condition. However, the fatigue performance of steel was improved significantly after tempering. The ultimate fatigue stress (endurance limit) after 1 × 107 cycles was 671 MPa, which was 38 MPa greater than the normalized steel. Microstructural analysis revealed similarities between normalized and tempered matrices at crystallographic interfaces and grain boundaries. Synchrotron X-ray radiation indicated that the initial retained austenite content was ∼15.5 % in both normalized and tempered conditions. However, at tensile stress of 800 MPa, the amount of retained austenite transformed in the normalized steel was ∼4 %, which was twice that of the tempered steel. The enhancement of retained austenite stability was an important aspect in improving the fatigue performance. The analysis of interphase spacing showed that the strain in the normalized matrix synchronized with the retained austenite, and the stress was concentrated in the retained austenite, which promoted transformation of retained austenite. The matrix of tempered steel was strained first, which prevented stress concentration in the retained austenite and rendered the retained austenite stable.

Effects of hydrogen on the formation of nanocrystalline at rolling contact surface of bainitic rail steel

A newly discovered peculiar microstructure at the rolling contact surface of bainitic rail steel has been investigated via positron annihilation technique and X-ray diffraction. The results show that the nanocrystalline layer is produced through low-temperature dynamic recrystallization. After hydrogen charged, low temperature dynamic recrystallization has been inhibited by hydrogen, and nanocrystalline layer at the surface decreases or disappears.

Influence of stratified tribologically transformed structure layers on contact fatigue crack initiation of bainitic rail steel

This study investigated the effect of stratified tribologically transformation structure (TTS) layers on contact fatigue crack initiation of bainitic rail steel serviced on a heavy-haul railway. Results demonstrate that the heterogeneous stratified TTS layer consists of alternating white etching layer and brown etching layer (WEL and BEL, respectively). The formation of stratified TTS is accompanied with the initiation of contact fatigue cracks, including vertical cracks, intra-layer cracks and inter-layer cracks. Unlike the current findings in conventional pearlitic rail steel, the intra-layer cracks are mostly initiated within BEL on bainitic rail steel. With help of transmission electron microcopy, nano-indentation techniques and in-situ micropillar compression tests, the initiation mechanism of contact fatigue cracks associated with stratified TTS on the bainitic rail are discussed in detail. The formation of brittle twined martensite and coarse oxidized particles is responsible for the intra-layer cracks initiated within BEL, while the inter-layer cracks nucleated from the interface are attributed to the difference in mechanical properties between different layers.

Study on microstructure and mechanical properties of continuous drive friction-welded joints of bainitic rail steel

Study on microstructure and mechanical properties of continuous drive friction-welded joints of bainitic rail steel

Design of cooling route for carbide-free bainitic rail steels and resultant microstructures and properties

In the present work, two new low-alloy, carbide-free bainitic rail steels (steel A: 0.29C + 2.49MnCr & steel B: 0.19C + 3.18MnCr wt.%) were designed to match the controlled cooling production process that combines forced air-cooling and nature air-cooling. The effect of alloy design and continuous cooling rate on the bainite production processing window and resultant microstructure was investigated, followed by the establishment of controlled cooling routes. The microstructure, transformation kinetics, and crystallographic characteristics of the specimens under the process were studied, and cuboid specimens were employed to validate the processing feasibility and investigate the correlation between related microstructures and mechanical properties. The results showed that both steels exhibited ultrahigh strength levels, with a tensile strength approaching 1400 MPa and elongation exceeding 16 %. However, the impact toughness of Steel A deteriorated due to the formation of granular bainite. Due to the homogeneous lath bainite morphology and refined bainitic ferrite substructure, Steel B exhibits excellent strength and toughness, essentially meeting the Chinese technical specifications for the procurement of 43 kg/m ∼75 kg/m rail.

Microstructural evolutions and impact toughness in simulated welding heat affected zones for a high-strength carbide-free bainitic rail steel

Systematic experimental investigations were conducted to study the microstructures and impact toughness of each heat affected zone (HAZ) formed during rail flash-butt welding. A high-strength carbide-free bainitic rail steel was subjected to different thermal simulation cycles to separately reproduce each HAZ subzone by tailoring the peak temperature (PT) with respect to 700, 850, 920, 1000 and 1350 °C, and hence to generate the corresponding microstructures by using Gleeble-3500 simulator. Results show that the HAZ subzones exhibit complicated microstructures depending on the PTs, and with increasing PT the dominant bainitic microstructure type evolves from polygonal bainitic ferrite (700 °C) to a mixture of fine bainitic ferrite and granular bainite (850–1000 °C), and finally to coarse bainitic ferrite and granular bainite (1350 °C). Impact tests demonstrate that the impact toughness initially increases significantly as the PT reaches 920 °C (i.e., fine-grained HAZ), beyond which the impact toughness starts to decrease. The fine-grained HAZ displays optimal impact toughness in HAZs, yet which is lower than the base metal. Moreover, the morphology and distribution of martensite-austenite (M-A) constituents is strongly dependent on the welding PT, and the high fraction blocky and coarse slender M-A constituents is considered to be detrimental for the impact toughness.

Heterogenous structure and formation mechanism of white and brown etching layers in bainitic rail steel

The failures of conventional pearlitic rail steels are strongly affected by the formation of hard and brittle white and brown etching layers (WEL and BEL, respectively) on the rail raceway during service. Bainitic rail steels with an excellent strength–toughness balance are promising candidates for next generation rail steels. It is generally difficult to generate hard and brittle WEL/BEL in bainitic rails because of the low carbon content. This study reports, for the first time, the formation of a unique multilayer heterostructured WEL/BEL in a field-tested bainitic rail. The heterogenous structures of WEL/BEL were characterized using transmission electron microcopy, focused ion beam, energy-dispersive X-ray spectroscopy, electron energy loss spectroscopy, electron probe microanalysis, atom probe tomography, and nano-indentation techniques. Results demonstrate that WEL is mainly composed of fine-grained martensite or ferrite and retained austenite, whereas BEL is a composite structure containing nanocrystalline martensite or ferrite, retained austenite, cementite, and oxide or O-rich particles. The oxide or O-rich particles are primarily formed within BEL through local oxidation, resulting in secondary partitioning of the alloying elements between the oxidation product and its surrounding matrix. BEL is hard but brittle due to the formation of brittle twinned martensite, cementite, and oxide particles, whereas the adjacent WEL is relatively soft and ductile. Based on these findings, the heterostructured WEL/BEL formation mechanism is preliminarily discussed in terms of inhomogeneous grain refinement, localized oxidation, chemical alterations (including both chemical segregation in the initial microstructure and secondary partitioning of the alloying elements during local oxidation), and phase transformation.

Wear and rolling contact fatigue competition mechanism of different types of rail steels under various slip ratios

This study aims to explore the wear and rolling contact fatigue (RCF) competition mechanism of the hypoeutectoid, eutectoid rail, and bainitic rail steels under different slip ratios. The results showed a clear competition relationship between wear and RCF of rail steel. With the increase in slip ratio, the wear rate of hypoeutectoid and eutectoid rail steels was increased and then remained stable, and the wear rate of bainitic rail steels was increased. Bainitic rail steels suffered catastrophic wear when the slip ratio exceeded 2%. With the slip ratio increased, the RCF crack propagation rate of the hypoeutectoid rail and eutectoid steels rail increased rapidly at first and then increased slowly. Except for the slip ratio of 5%, the RCF crack propagation rate of the bainitic rail increased with the increase in slip ratio. With the increase in slip ratio, the degree of wear rate increase and crack propagation rate was not consistent, which led to the competition between wear and RCF. The competitive relationship between wear and RCF was reflected in the variation of rail wear form. As the slip ratio increased, the main wear form of the hypoeutectoid and eutectoid rail steels was transformed from adhesive wear to adhesive wear and fatigue wear, whereas for bainitic rail steel, it was changed from adhesive wear to fatigue wear, and finally to adhesive wear. At high slip ratios, the retained austenite of bainite rail steel was seriously broken, so cracks in the bainitic rail steel could easily propagate along the plastic deformation layer.

Effect of Segregation and Contact Stress on Fatigue Wear Behavior of Bainitic Rail Steel

Effect of Segregation and Contact Stress on Fatigue Wear Behavior of Bainitic Rail Steel

The impacts of retained austenite on the microstructure and property changes of carbide-free bainite during sliding wear

With the rapid development of Chinese railway industry, there is an increased demand for better toughness, wear resistance and fatigue resistance of wheel/rail materials, and the development of carbide-free bainitic wheel/rail materials with stable and outstanding comprehensive properties is of great significance to the acceleration of railway development. This paper conducts sliding wear experiment on the currently developed FCB bainitic wheel steel (abbreviated as FCB in the following content) and AB1 bainitic rail steel (abbreviated as AB1 in the following content), and investigates the impacts of retained austenite (RA) on the microstructure and property changes of carbide-free bainite. During the process of sliding wear, the wear morphology of FCB is chiefly featured by abrasive wear; As wear rotations increase, adhesion wear and fatigue wear gradually appear. Compared with AB1, FCB with similar chemical composition has more RA, which causes it to undergo plastic deformation more easily during sliding wear. The deformation gives rise to higher strain in the surface layer and makes the grains caused by it much smaller, leading to white etching layers (WELs) with higher thickness and hardness more easily. Meanwhile, more RA means that more martensite phase with high thickness will be generated during the wear process, which increases the hardness of FCB. These two factors work together to make wear resistance of FCB better.

Corrosion Behavior of Multiphase Bainitic Rail Steels

Pearlitic steel experiences excessive corrosion in a hot and humid atmosphere. The multiphase bainitic/martensitic structure was developed for a better combination of strength and ductility, especially rolling contact fatigue, but little attention to corrosion has been investigated. Corrosion behaviors of multiphase steels obtained from bainitic-austempering (BAT) and bainitic-quenching and -partitioning (BQ&P) processes were investigated via immersion and electrochemical tests in 3.5 wt.% NaCl solution. The corroded surface and rust after immersion and electrochemical tests were analyzed via electron microscopy, Fourier transform infrared spectra, and x-ray diffraction. The multiphase bainite + martensite/retained austenite island showed higher corrosion resistance than that of the pearlitic one. The acicular bainite obtained from the BQ&P process showed slightly higher corrosion resistance than the granular bainite + martensite structure obtained from the BAT process.

Strain amplitude-dependent cyclic softening behavior of carbide-free bainitic rail steel: Experiments and modeling

Experimental investigations of Carbide-free bainite (CFB) rail steel under different strain amplitudes are conducted to indicate the cyclic deformation feature and hysteresis loop evolution. A modified yield function with an exponential evolution coefficient is proposed to consider the strength differential effect, and a strain range-dependent coefficient is introduced to consider the transient Bauschinger effect. Moreover, a Logistic dose–response function and a cyclic softening function are introduced into the isotropic and kinematic hardening rules. Finally, the comparison between the simulated and experimental results demonstrates that the proposed model can reasonably describe the strain amplitude-dependent cyclic softening of CFB rail steel.

The effect of microstructure evolution on the ratchetting-fatigue interaction of carbide-free bainite rail steels under different heat-treatment conditions

Low-cycle fatigue experiments emphasizing the ratchetting are performed on the carbide-free bainitic (CFB) rail steel under different heat-treatment conditions. The effects of microstructure and retained austenite (RA) on the ratchetting-fatigue interaction are investigated through macroscopic and microscopic characterizations. The results indicate that the CFB rail steel exhibits a non-saturated and strain amplitude-dependent cyclic softening feature and shows different ratchetting behaviors and failure modes at various stress levels. Ratchetting promotes the development of stable dislocation substructure and the transformation of blocky RA. The fatigue resistance of 1380 grade CFB steel under the online controlled cooling conditions is better than that of 1280 grade CFB steel under the air-cooling condition.

Effect of heating on the microstructure and mechanical properties of hot rolled high strength bainitic steel rail

ABSTRACT In this paper, the microstructural evolution and mechanical properties of the hot-rolled U25CrNi high-strength bainitic steel rail under different tempering conditions were investigated. The results indicate that both the hot-rolled and tempered microstructures of the tested steel consist of bainitic, martensite and retained austenite. When the tempering temperature rises to 400°C, relatively large volume fraction of residual austenite as well as a smaller amount of carbide precipitation in the tested steel have been detected. When the tempering time is extended to 360 min at the same temperature, the precipitation of carbide particles in the steel sample increases. When the tempering temperature is 500°C, the bainitic ferrite in the tested steel is obviously coarsened, accompanied by the precipitation of a large amount of carbide. In a word, the optimum tempering process for this hot-rolled U25CrNi high-strength bainitic rail is 400°C × 200 min.

Relationship between Microstructure and Properties of 1380 MPa Grade Bainitic Rail Steel Treated by Online Bainite-Based Quenching and Partitioning Concept

According to the concept of the bainite-based quenching and partitioning (BQ&P) process, we designed the online heat treatment routes of bainitic rail steel for heavy haul railway. The new heat treatment process reduced the fraction and size of the blocky martensite/austenite (M/A) islands formed during the conventional air-cooling process. The M/A islands are coarse and undesirable for mechanical properties. A new kind of 1380 MPa grade bainitic rail steel with more uniform microstructure and better mechanical properties was produced by the online BQ&P process. We characterized the multiphase microstructures containing bainite, martensite, and retained austenite of 1380 MPa grade bainitic rail steels via optical microscope, scanning electron microscopy, transmission electron microscopy, and X-ray diffractometer. We investigated in-depth the relationship between the microstructure, retained austenite stability, and mechanical properties, particularly the resistance to wear and rolling contact fatigue, of the new 1380 MPa grade bainitic rail steels. Meanwhile, the conventional air-cooling bainitic rail steel was studied as a comparison.

Effect of non-uniform microstructure on rolling contact fatigue performance of bainitic rail steel

Rolling contact fatigue (RCF) is the main failure mode of wheel-rail system. In this work, the influence of non-uniform microstructure on the rolling contact fatigue performance in bainitic rail steel was investigated. According to the results, an increase in degree of segregation enabled material to improve the wear resistance through the improvement of the local hardness of the rail, but seriously reduced the rolling contact fatigue performance of the rail. The crack propagated along the segregation band, thereby leading to the failure of the specimen. The main cause of fatigue crack initiation was the non-uniform strain distribution under cyclic loading due to different hardness values between the matrix and the segregation region. A large number of dislocations produced by plastic deformation on the surface of the specimen segmented grains into nanocrystals. Meanwhile, it can be seen from the in-situ heating transmission electron microscope (TEM) that the friction heat generated during RCF test was enough to support the occurrence of dynamic recovery.

Head check resistance of B320 bainitic rail steel grade

This work focuses on the degradation mechanisms of a B320 carbide-free bainitic steel grade rail and of its weld after laboratory tests. The microstructure of the tested rails is investigated, focusing mainly on retained austenite. The Head Check resistance of B320 grade base material is high, notably due to the TRansformation Induced Plasticity (TRIP) effect, inducing a reduction in crack propagation speed. Inside the Heat Affected Zone, a few Head Check defects have been detected and are attributed to a lower retained austenite content coming with a higher Martensite-Austenite Compound content prior to rail testing. In this zone, the transformation process from austenite to martensite following TRIP effect is less pronounced. A novel degradation mechanism is proposed considering the phases contribution in the deformation process of B320 carbide free steel grade.

Formation Mechanism of Abnormal Martensite in the Welded Joint of the Bainitic Rail

Formation Mechanism of Abnormal Martensite in the Welded Joint of the Bainitic Rail

Tekerlek Ray Etkileşimi, Perlitik ve Beynitik Çeliklerin Yorulma Hasar Direnci Üzerine Bir Sonlu Elemanlar Analizi

Bu çalışmada ülkemizde ve dünyada demiryolu hatlarında işletme şartlarında çoğunlukla kullanımda olan perlitik mikroyapıdaki çelikler ve perlitik çeliklere alternatif olarak sunulan, gelişmiş mekanik özelliklere sahip olan beynitik mikroyapıdaki çelikler ele alınmıştır. Geniş bir literatür özetiyle konvansiyonel ray sınıfları ve gelişmiş ray sınıfları, kimyasal kompozisyonları ve temel mekanik özellikleri dikkate alınarak değerlendirilmiştir. Ayrıca dünyada farklı bölge ve ülkelerde geliştirilen beynitik ray çelikleri incelenmiştir. Daha sonra tekerlek ray etkileşimi, sürtünme şartları ve temas yüzeyleri dikkate alınarak, geliştirilen sonlu elemanlar modelinde rayın yorulma hasar direnci analiz edilmiştir. Sonuç olarak analiz edilen beynitik mikroyapıdaki çeliğin yorulma hasar direncinin perlitik mikroyapıya göre üstün olduğu ve 386,69 MPa yük altında on milyon çevrime dayanabildiği anlaşılmıştır.

Study on fatigue wear competition mechanism and microstructure evolution on the surface of a bainitic steel rail

The wear and fatigue competition mechanism of a bainitic rail steel was studied through rolling contact fatigue test. Results showed that the damage process could be divided into initial (before 2.0 × 105 cycles), middle (around 2.0 × 105 cycles) and late (after 2.0 × 105 cycles) stages, with the dominant damage mechanisms of wear, wear and fatigue, and fatigue in each respectively. With increasing rolling cycles, the retained austenite transformation played an important role in achieving dynamic equilibrium between wear and fatigue. Moreover, the untransformed retained austenite could release crack tip stress and effectively control the excessive propagation of fatigue cracks.