Microstructural effects on central crack formation in hot cross-wedge-rolled high-strength steel parts

J Mater Sci (2020) 55:9608–9622 METALS & ORROSION Metals &Ccorrosion Microstructural effects on central crack formation in hot cross-wedge-rolled high-strength steel parts Xianyan Zhou1 1 2 , Zhutao Shao1, Famin Tian2, Christopher Hopper1, and Jun Jiang1,* Department of Mechanical Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK Dyson School of Design Engineering, Imperial College London, Imperial College Road, London SW7 1AL, UK Received: 10 March 2020 ABSTRACT Accepted: 11 April 2020 Central cracking in cross-wedge-rolled workpieces results in high wastage and economic loss. Recent cross-wedge rolling tests on two batches of steel showed that one batch formed central cracks, while the other was crack-free. The batches were both nominally of the same chemical composition and thermomechanical treatment history. In addition, both batches had passed all the standard quality assessments set for conventional forging processes. It was suspected that the different cracking behaviours were due to differences in microstructure between the two as-received steel billets, and the material in cross-wedge rolling (CWR) was more sensitive to the initial microstructure compared with other forging processes due to its specific loading condition including ostensibly compression and large plastic strain. Nevertheless, no previous study of this important problem could be identified. The aim of this study is, therefore, to identify the key microstructural features determining the central crack formation behaviour in CWR. The hot workability of the as-received billets was studied under uniaxial tensile conditions using a Gleeble 3800 test machine. Scanning electron microscope with energy-dispersive X-ray spectroscopy and electron backscatter diffraction was applied to characterise, quantitatively analyse, and compare the chemical composition, phase, grain, and inclusions in these two billets, both at room temperature and also at the CWR temperature (1080 °C). Non-metallic inclusions (oxides, sulphides, and silicates) in the billets were determined to be the main cause of the reported central cracking problem. The ductility of the steels at both room and elevated temperatures deteriorated markedly in the presence of the large volumes of inclusions. Grain boundary embrittlement occurred at the CWR temperature due to the aggregation of inclusions along the grain boundaries. It is suggested that a standard on specifying the inclusion quantity and size in CWR billets be established to produce crack-free products. Published online: 22 April 2020 Ó The Author(s) 2020 Address correspondence to E-mail: https://doi.org/10.1007/s10853-020-04677-5 J Mater Sci (2020) 55:9608–9622 Introduction Cross-wedge rolling (CWR) is widely used to manufacture axially symmetric products, such as the camshafts, gear shafts, or preforms for forging [1]. The formation of central cracks (i.e. the cavities formed in the centre of the workpiece), also known as the Mannesmann effect, was acknowledged as the most common defect limiting the development of CWR [2]. To drive further development of CWR into areas such as the more safety-critical aerospace industry, it is of great importance to understand the fracture mechanisms of central crack formation and determine a proper fracture criterion or damage model to produce crack-free CWR products. The research in this area is globally active and ongoing. Pater et al. [3] compared nine fracture criteria to find the one most suitable for the prediction of central crack formation. Yang et al. [4] studied central crack formation on a microstructural scale and revealed the ductile fracture mechanism of steels at high temperature. Zhou et al. [5] considered the combined effects of the shear and normal stress and proposed a novel fracture criterion, which was validated quantitively. However, there is not an agreement on the fracture mechanisms of central crack formation due to the complex mechanical and microstructural behaviours during CWR. It is well known that the workability of a material is usually determined by two factors: the process-related parameters (including die geometries, thermal history, strain rate, etc.) and the material-related parameters (such as phase composition, grain size, and chemical composition). Intensive studies have been conducted to investigate the effects of stress states on central crack formation. Dong et al. [6] used finite element methods to analyse the stress distribution during CWR and determine the cause of central cracks. Li et al. [7] systematically investigated central crack formation under various die geometries and proposed a non-dimensional fracture criterion for producing central crack-free products. By adopting the Cockcroft–Latham damage model, Pater concluded that using a large forming angle and small spreading angle during CWR could effectively avoid central cracking [8]. After careful analysis of the shear and tensile deformation at the central region of the workpiece during CWR, Yang et al. [4] suggested that the forming angle had the greatest effect on central crack formation. Although many studies have been 9609 conducted to analyse the effects of process parameters (e.g. die geometries), however, limited attention has been devoted to understanding the effects of material-related parameters (grain size, phase, and chemical composition) on the central crack formation problem. Thus, in order to control central crack formation accurately and efficiently, it is necessary to understand the microstructure evolution during CWR and determine the critical microstructural feature for central crack formation. Cross-wedge rolling companies previously found that when using the same working process-related parameters (i.e. die geometry, thermal history, and strain rate), central cracks were frequently observed in some batches of the CWR-formed steel parts, but not in other batches, even though the batches had the same nominal chemical composition [9]. The initial steel feedstock billets met all the typical technical specifications required, including strength, ductility, chemical composition, and porosity. It is, therefore, of great interest to investigate which material-related parameters may influence central crack formation, and the mechanisms involved. Cross-wedge rolling companies suffer heavy losses due to this problematic phenomenon. In discovering the material-related fracture mechanisms and proposing possible solutions to mitigate the risk of cracking, it is thought that the findings from this study could have a large beneficial impact on the CWR industry. Some studies have been conducted to understand the microstructure distribution and evolution during CWR. For example, Wang et al. [10] studied AISI 5140 steel both numerically and experimentally, considering phase transformation, grain recrystallisation, and grain growth. Huo et al. [11] established a unified constitutive model coupling micro (...truncated)