Elsevier

Materials Letters

Volume 305, 15 December 2021, 130772
Materials Letters

Simulation, microstructure and austenite reconstruction of a medium carbon micro-alloyed steel subjected to an austenitising bending process

https://doi.org/10.1016/j.matlet.2021.130772Get rights and content

Highlights

  • An austenitising bending process was conducted on the thermomechanical simulator.

  • Work hardening was enhanced on thickness edge positions due to dislocation.

  • Finer prior austenite grains at edge position led to the improvement of hardness.

  • The numerical simulation was verified in good agreement with experimental results.

Abstract

The austenitising bending process was conducted on a medium-carbon micro-alloyed steel by Gleeble 3500 thermomechanical simulator to comparatively investigate the microstructural and mechanical evolution through thickness of the bent plate. A combination of bainite ferrite, lath martensite and acicular ferrite with a presence of partial dynamic recrystallisation was the main microstructural features. The increased work hardening in the compressive and tension zones after austenitising bending operation can be ascribed to synergistic effects of dislocation pile-up/cells, and finer prior austenite grains. Also, the reliability of this study was verified from the consistency between the results of simulation and experiment, which would provide a valuable reference for the practical manufacturing technology of railway spring clips.

Introduction

High-speed railway transport has achieved rapid development over the past few decades. To ensure the safety of railway transportation, the requirements for the quality of railway infrastructure are constantly improved [1]. As a vital part of the railway fastening system, the spring clip connects rail steels and sleepers for fixing the railway in a stable position [2]. Nowadays, to meet a high standard of practical application, it is necessary to optimise the forming process, which directly affects the microstructure and mechanical property of the spring clip.

Medium carbon Si-Cr steel as a new generation of spring steels has been widely concerned owing to its superior mechanical property, including excellent strength, satisfactory elongation, and well-pleasing toughness [3]. Since the microstructure after the hot-working process is closely related to the final performance of products, a satisfactory forming design may be established to reflect the correlation between microstructure and forming conditions [4]. The bending investigation is relatively easy to be achieved at room temperature if the lab condition permits, while nearly no studies have been proposed at the high-temperature bending process due to experimental limitations. A deep understanding of the hot bending mechanism can be well adopted in industrial production. To forward this objective, the current study presented the FE simulation combined with physical experiments to illustrate the potentiality of the heating strategy on the loading force and stress distribution throughout the plate thickness.

Section snippets

Experimental methodology

Cuboid specimens with a dimension of 80 × 10 × 2 mm extracted from 55SiCr hot-forged rods (0.55C, 1.4Si, 0.65Mn, 0.65Cr, 0.11Ni, 0.12Cu, 0.012S, 0.018P, Balanced Fe, in wt%) were assembled with a custom-designed die to conduct the austenitising bending experiment at 900 °C in a vacuum environment by a Gleeble 3500 thermomechanical simulator. To obtain a homogenised temperature distribution during testing, the heating rate and holding duration were set as 10 °C/s and 90 s, respectively.

Results and discussion

Fig. 1(a) displays the assembly relationship between specimen and die on Gleeble 3500 as well as the bending process. To better simulate the entire process, bending modelling was established based on the actual test conditions with related mechanical properties according to our previous study [3], [5]. In this work, the accuracy of the FE model can be verified from the good agreement of experimental and simulation results (Fig. 1(b)). Clearly, the loading force increased at the early stage of

Conclusion

During the austenitising bending process of the medium-carbon micro-alloyed steel, the work hardening degree was comparatively aggravated at compressive and tensile zones due to intensive dislocation. Austenite reconstruction is applied to illustrate that finer prior austenite grain and martensite packet size observed at the tensile zone would improve the mechanical property from another aspect. The high consistency between simulation and experimental results further verifies the reliability of

CRediT authorship contribution statement

Yao Lu: Writing – original draft, Investigation, Methodology. Haibo Xie: Writing - review & editing. Jun Wang: Data curation, Methodology. Fanghui Jia: Software. Zhou Li: Conceptualization. Fei Lin: Formal analysis. Di Pan: Visualization. Jingtao Han: Conceptualization, Resources. Zhengyi Jiang: Supervision, Writing - review & editing, Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was financially supported by the Australian Research Council (ARC) ITTC-Rail centre.

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    To obtain the micro-hardness transition of the plates bent at different temperatures, Fig. 4(a) displays the hardness measurement scheme of five testing columns in the bent regions. Through the colour legend shown in Fig. 4(a), it is intuitively observed that hardness decreases with increasing temperatures, while the colour distribution in the hardness maps appears to be asymmetrical for each plate due to the development of microstructure and dislocation accumulation [31]. Moreover, since the hardness gradient presented subtle deviation (less than 2%) of each bent plate, it may be speculated that the microstructural variations throughout the thickness were not significant (see Fig. 4(b)).

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