Study on the surface microstructure evolution and wear property of bainitic rail steel under dry sliding wear

doi:10.1016/j.wear.2020.203217

Wear

Volumes 448–449, 15 May 2020, 203217

https://doi.org/10.1016/j.wear.2020.203217 Get rights and content

Highlights

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During sliding wear, WELs would form on the worn surface of AB1 bainitic steel, with the thickness of less than 20μm.
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The microstructure evolution law of AB1 bainitic steel during sliding wear was presented.
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The factors that affected the wear property of bainitic steel were discussed.

Abstract

As the industry of railway transportation is thriving, much more attention has been paid to the properties of wheel/rail steel materials. Although conventional wheel/rail steel materials have proved their competence so far, it is necessary to think ahead and develop new wheel/rail steel materials in order to meet new challenges raised by railway transportation in the future. Since bainitic steel is the first choice for the new wheel/rail steel material, it is especially important to fundamentally investigate its wear property. Through the sliding wear experiment on AB1 bainitic rail steel, the microstructural evolution of bainitic steel during the wear process and the factors that affected its wear property were probed into in this paper. Under sliding wear, it was discovered that ploughing wear, adhesion wear and flake-like peeling were the primary morphological characteristics of AB1 bainitic rail on the worn surface, and the flakes generated by peeling were relatively thin; WELs appeared on the worn surface of AB1 bainitic rail steel, with the thickness of less than 20μm. The main microstructure inside the WELs contained nanoscale bainitic ferrite grains and martensite grains. Under the impact of stress, the surface microstructure of AB1 bainitic rail steel underwent the following changes: the direction of bainitic ferrite laths gradually became perpendicular to that of the stress, and with the increase of the deformation strain and strain rate, bainitic ferrite laths were segmented into numerous short bar-shaped subgrains and even into nanoscale equiaxed grains with random orientations; residual austenite, no longer stable, underwent martensite transformation, and after the formation of martensite, under the influence of pressure stress, martensite lath grains were multidirectionally segmented into pieces that formed nanoscale martensite grains. Residual austenite inside the microstructure of AB1 bainitic rail steel, the thickness of bainitic ferrite laths and the WELs formed during the wear process would all affect the wear property of bainitic steel.

Introduction

In recent years, with the development of railway transportation, the operational speed of trains has been constantly improved, axle load, passenger flow and freight volume have been gradually increased, and various kinds of wheel/rail flaws and frequent occurrences of crack damage, which all lead to higher wheel/rail maintenance and replacement costs [1,2]. Therefore, to meet these service conditions of high expectations, the properties of wheel/rail materials will be accordingly improved, which should possess higher strength, wear property and fatigue resistance. Currently, pearlitic steel is the commonly used wheel/rail material. Reducing the spacing between pearlitic lamellae or adding alloying element can enhance rolling contact fatigue property and wear property of this steel. Different technologies have been adopted in the in-line hardening treatment of pearlitic steel, such as forced air cooling, cold water cooling and dipping in synthetic quenching medium [[3], [4], [5], [6]]. Nonetheless, in view of the present development stage of casting technology and materials chemistry, the research into how to enhance the properties of pearlitic steel has met the bottleneck and any effort to further improve its wear property seems quite impossible [[7], [8], [9]]. Thus, it is reasonable to develop new wheel/rail steel materials (for instance, bainitic steel).

Bainitic steel, which possesses better fracture toughness, wear property and fatigue resistance [[10], [11], [12], [13], [14], [15], [16], [17]], is the best choice to supplant pearlitic steel as the new wheel/rail material. Even though some literature also pointed out that the wear property of bainitic steel was worse than that of pearlitic steel [[18], [19], [20]], Clayton and others [7,21] claimed that this conclusion was made with no consideration of test conditions, chemical compositions and other mechanical properties in their research. Moreover, the type of the bainite microstructure inside bainitic steel, the mixed microstructure containing other detrimental phases like allotriomorphic ferrite mingled with bainite and the existing carbide all affected the fracture tenacity and wear property of bainitic steel material [12,22]. In order to fundamentally understand the wear property of bainitic steel, in this paper, sliding wear experiment was conducted on the newly developed bainitic steel. Through the observation of the microstructure changes of the bainitic steel in the wear process, the factors that influence its wear property had been probed into, which indeed would provide theoretical basis and guidance for the following applications of bainitic steel into wheel/rail materials.

Section snippets

Experimental materials and methods

A piece of AB1 bainitic rail steel was utilized in this experiment, with the chemical composition of 0.18-0.28 wt% C, 1.50-2.50 wt% Mn, 0.90-1.90 wt% Si, 0.30-1.00 wt% Cr, and 0.10-0.60 wt% Mo. The hardness of the surface under sliding wear approximated 400HV_0.1. Fig. 1 showcased the microstructure morphology of typical AB1 bainitic rail steel under OM. The original bainitic rail steel consisted of lath-like bainitic ferrite (BF), small amounts of residual austenite (RA) and blocky M/A. The

Wear mass losses

Fig. 3 showed the wear mass losses of AB1 test ring and CL65 test block during the process of sliding wear. In the initial stage of sliding wear, the wear mass loss of the test block was negative, meaning its total mass was slightly increased. This indicated adhesion wear occurred in the process of sliding wear: part of the material on the worn surface of the test ring to adhered to that of the test block. In addition, in this stage, the wear mass losses of the test ring were dramatically

The microstructural evolution of AB1 bainitic rail steel during dry sliding wear

After the observation of the cross-sectional microstructure of AB1 bainitic rail steel after sliding wear, what was found was that since the original microstructure contained lath-like BF, RA and blocky M/A, the microstructural evolution laws of lath-like BF and RA under plastic deformation could be respectively observed when the microstructure evolution law of AB1 bainitic steel was investigated.

Conclusion

In order to fundamentally investigate the wear property of bainitic steel, in this paper, sliding wear experiments were performed on AB1 bainitic rail steel. Through the observation of its microstructural evolution during the wear process, the factors that affected its wear property were probed into and the following significant conclusions were obtained:

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Under sliding wear, it was discovered that ploughing wear, adhesion wear and flake-like peeling were the primary morphological characteristics

Author statement

Dear Editor.

We hereby confirm that this manuscript is our original work and has not been published nor has it been submitted simultaneously elsewhere. We further confirm that all authors have checked the manuscript and have agreed to the submission.

Declaration of competing interest

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Study on the surface microstructure evolution and wear property of bainitic rail steel under dry sliding wear”.

Acknowledgements

This research was supported by National Key Basic Research Program of China (No. 2015CB654802)

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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.
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Study on the surface microstructure evolution and wear property of bainitic rail steel under dry sliding wear

Highlights

Abstract

Introduction

Section snippets

Experimental materials and methods

Wear mass losses

The microstructural evolution of AB1 bainitic rail steel during dry sliding wear

Conclusion

Author statement

Declaration of competing interest

Acknowledgements

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J. Mater. Process. Technol.

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Eng. Fail. Anal.

Mater. Sci. Eng., A

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Mater. Sci. Eng.

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Acta Mater.

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