Elsevier

Tribology International

Volume 110, June 2017, Pages 66-76
Tribology International

A study of abrasive wear on high speed steel surface in hot rolling by Discrete Element Method

https://doi.org/10.1016/j.triboint.2017.01.034Get rights and content

Highlights

  • This work has overcome the challenge in determining the bond properties to reflect accurately mechanical properties of oxide material at 650 °C.

  • The effects of carbide shape, orientation, distribution and tip size on the abrasive wear of HSS have been examined.

  • Two realistic, ‘rod’ and ‘chicken-feet’ shape MC carbides have been presented and successfully embedded into oxide scale.

  • The work has taken advantage of DEM that can naturally produce material removal in the prediction of wear.

Abstract

In hot rolling, the asperities of oxidised strip and wear debris slide over High Speed Steel (HSS) work roll. Hence, abrasive wear occurs, and wear particles will be removed from the work roll surface. This work introduces a Discrete Element Method (DEM) model which has been applied successfully to study abrasive roll wear of HSS work roll with MC carbides embedded within the oxide layers at 650 °C. From this research, it has been found that the carbide orientation, distribution in the HSS roll, different scratching tip size and scratching depth affect the wear significantly.

Introduction

In hot strip rolling, the cost of roll wear was estimated to be as much as 10% of the total cost of steel production [1]; the longer roll life will extend the rolling campaign, increase productivity and reduce costs. Numerous empirical wear models have been used in the steel rolling industry on a trial and error basis but they are limited specifically to a particular plant. The ability to predict roll wear will improve significantly the product quality.

High speed steel rolls are preferred in hot strip mills due to their superior mechanical properties such as strong wear resistance, high hardness, and high temperature properties. In a hot rolling process, the roll surface is initially heated up to approximately 650 °C while in contact with the hot strip (850–1000 °C) for a short period, and subsequently cooled by water to around 70 °C in the same cycle. The thermal cycles on the rolls cause a superficial oxide scale, which plays an important role on the contact friction and wear. Hot rolls are subjected to high loading and shear force when the contact with rolled materials occurs, which results in fatigue and wear of the rolls. The extremely arduous working condition causes a deterioration of work rolls and their performance. Previous works on the oxide layer on HSS provide limited information on the effect of the mechanical properties on its wear resistance behavior. Most of the reported studies concentrated on the morphologies and microstructures of oxide layers but not their mechanical properties and wear behavior. Krzyzanowski et al. [2] developed an FEM model for oxide scale on steel strip, the morphology of which consists of three sub-layers, and the proportion of each layer is determined at different temperatures, oxidation times and each having a different composition and different mechanical properties. Other things that come into play are voids, roughness at the interfaces. However, the authors did not consider the oxide layer on the work roll in their model but instead focusing only on the strip surface.

HSS roll material is a complex multi-component alloy, with a carbon content ranging from 1.5 to 2.0 wt% and a significant amount of alloy elements such as V, W, Cr and Mo (4–10%, 6%, 3–8% and 6% respectively) [3]. The typical microstructure of HSS consists of primary carbides (10–20%) distributed in a matrix of tempered martensite and fine secondary carbides. The types of carbide in HSS are usually MC, M2C, M7C3 and M6C. As these carbides are much harder than the matrix, they contribute significantly to the mechanical strength, load bearing capacity and wear resistance. The mechanical properties of HSS rolls are affected by various microstructural factors such as the spheroidal precipitated carbides inside the steel matrix, the morphology and distribution of carbides. Rodenburg and Rainforth [4] found that different carbide sizes can lead to a difference up to 40% in the wear rate. The morphology, amount and distribution of carbides are dominant factors contributing to HSS wear resistance [5]. Luan et al. [6] concluded that with the smaller eutectic carbides and the more uniform distribution in the matrix, the wear resistance of HSS roll will be further improved. Qiang et al. [7], [8] found that different types of carbides in the material show large differences in their resistance to oxidation.

The wear of work rolls can be attributed to a combination of different wear mechanisms, such as abrasion, mechanical/thermal fatigue, adhesion, and high temperature oxidation. The most dominant roll wear mechanism is the abrasive wear, in which micro ploughing, and crack initiation occur simultaneously, depending on the morphology and hardness of abrasive particles. The worn oxide particles from the strip surface and carbides fall off from the roll can be captured in the roll strip interface and scratch the roll/strip surfaces. Recently, an iterative wear prediction procedure has been introduced, in which the contact pressure is calculated using nonlinear FEM analysis, and the geometry of the contact interface is progressively changed according to the Archard wear model [9]. However, the wear coefficients in Archard equation ignored the physically contact mechanics and varies according to different contact environment [10]. Pramanik et al. [11] developed an FEM model to investigate the behavior of metal matrix composite with a consideration of reinforcement and particles removal during orthogonal cutting. However, this work was carried out in 2D model and the reinforcements were uniformly distributed which is not realistic. Krzyzanowski and Rainforth [12] also proposed a combined discrete/finite element approach to model the oxide scale on the strip surface. They concluded that this combined method can account for the generation of abrasive particles and delamination of the oxide layer, and has the potential to simulate multi-scale and multi-phase phenomena that can lead to a more accurate modelling. However, they found the difficulty of this method is in determining accurately the bonding strength of the particles. Hakim et al. [13] employed the Finite Element Method (FEM) to investigate the wear behavior of oxide layers on HSS roll. The author combined FE simulation and nanoindentation experiments to develop a three dimensional (3D) FEM model to characterize the mechanical properties of the oxide layer formed on the HSS surface. The model has described well the mechanical properties of the oxide layer and the carbides. However, the FE mesh is still simplified and the detached elements (wear) occur only within a pre-defined thin layer on HSS top due to the difficulties of FEM in handling large mesh distortion and material removal. It did not consider the crack propagation during the scratching event. Moreover, the authors considered only one spherical carbide which is over-simplified and unrealistic.

More recently, Phan et al. [14] have overcome those difficulties and successfully proposed a 3D DEM model to simulate the abrasive wear phenomenon of oxide layer formed on HSS work roll at room temperature. The model can describe the removal of material which is the advantage of DEM model over the FEM-based one. In this work, the 3D CAD geometries can be easily imported from the external CAD software such as SolidWorks which can model complex structures. In addition to that, the carbides (clusters) can easily be created and distributed in the domain with the desired orientation and position. Moreover, the proposed model has the capability of creating different bonding between particles in one system which is a promising way to study the complex composite materials containing different material properties.

The model has shown the capability to predict the abrasive wear of work rolls which accounts for the carbides, and illustrated that the spherical shape of the carbides has more wear resistance than the cluster shape. Moreover, it has also confirmed that the presence of carbides increases the wear resistance of HSS roll [14].

In the current work, the oxide material formed on the HSS roll surface at 650 °C, which manifests a brittle behavior, has been successfully modelled by 3D DEM at micro scale. This work has overcome the challenge in determining the bond properties to reflect accurately the mechanical properties of oxide material at 650 °C which has not been reported elsewhere. Qiang et al. [7], [8] found that the V-rich MC carbide is the most prevalent carbide-type in HSS roll with the volume fraction occupying approximately 9% compared to the total volume fraction of 13% of all the carbides in HSS material, which include MC, M2C, M6C and M7C3. The V-rich MC carbides itself has various morphologies such as rod-like, chicken-feet, branch, chrysanthemum or coral-like carbides [6], [15]. However, the rod-like and chicken-feet-shaped MC carbides are two popular MC shapes. Thus due to the complexity of the 3D structure in creating other shapes and the expensive computational time in modelling, the current work focuses only on these two shapes of MC carbides. The rod- and chicken feet MC carbides and their orientation have been presented and successfully distributed into the oxide scale. Moreover, the hard iron oxide debris particles trapped on the strip surface have been modelled to investigate their size influence on the HSS wear rate. In addition to that, the penetration depth of these iron oxide debris particles has also been considered to investigate their influence on the roll wear. The current work has taken advantage of DEM that can naturally produce material removal to predict HSS roll wear at 650 °C.

Section snippets

Mechanical properties of oxide scale and V-rich MC carbides at elevated temperature (650 °C)

HSS is a composite material that contains many types of carbides, and a steel matrix. HSS has inherently a high wear resistance and mechanical resistance at the working temperature [16]. According to Badisch and Mitterer [17], the large primary carbides (1–10 µm in diameter) are mainly MC and M6C-types which are harder than the matrix. Due to the high hardness, the carbides can significantly improve the wear resistance of HSS roll when they are dispersed in the martensitic matrix. Krzyzanowski

Bond configuration determination in DEM model for oxide at 650 °C

In Finite Element Method, the measurements performed on laboratory specimens can be used to obtain input properties such as modulus and strength. In this study, the Hertz-Mindlin bonding contact model is used in the 3D DEM model. But in DEM, the bond properties (such as normal/shear stiffness, critical normal/shear stresses) are not known. It is assumed that the oxide scale at 650 °C behaves in a brittle manner [12], [18], [19], [20], and to accurately determine the bond configuration, a DEM

Conclusions

A 3D Discrete Element Model has been proposed to simulate the abrasive wear phenomenon of oxide layer formed on HSS work roll. The bond properties of the oxide scale at elevated temperature are determined by implementing 3D tensile test and 3D nano-indentation test. The model is capable to predict the abrasive wear of work roll at an elevated temperature (650 °C) with two different shapes of MC carbides embedded within the surface layer of the oxide scale. The following conclusions can be made:

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Acknowledgements

The authors acknowledge financial support from the Australian Research Council Discovery Project DP130103973 and Bao steel project BAJC 12045, and the scholarship support from the University of Wollongong for the first author. The authors are also grateful for the support with the EDEM software given by Professor Peter Wypych and Dr David Hastie from the Bulk Materials Handling Centre.

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