Friction analysis during deformation of steels under hot-working conditions

https://doi.org/10.1016/j.triboint.2021.106928Get rights and content

Highlights

  • A multiscale analysis has been developed to characterize the interfacial friction phenomena in hot forging conditions.

  • The role of the oxide scale on stick-slip phenomena is highlighted under hot forging contact conditions.

  • A lubricant film breakdown mechanism occurs due to the accumulated oxides.

Abstract

An original multiscale friction analysis is developed to characterize the interfacial friction phenomena during the hot forging contact conditions in the manufacture of wheels for the railway industry. Hot upsetting sliding tests and roll-on-disc tests are performed to reproduce tribological conditions at meso and micro-scale.

Crushed and embedded oxides are found to be present on the sample's surfaces. Stick-slip phenomena occur in the roll-on-disc tests due to accumulation of the crushed oxides. This transform the interfacial contact conditions from a lubricant/oxide configuration to an oxide/oxide condition. Microanalysis measurements on the specimen's wear tracks clearly show that the lubricant layer is broken down and transferred from the tool to the specimen surface by means of the accumulated oxide particles.

Introduction

In industrial hot-working processes employed in the manufacture of steel components, friction conditions at the tool/workpiece interface are severe due to the relatively high temperatures (850°C–1250 °C) and loads involved in these operations. Under these processing conditions, the tool/workpiece interface usually involves the presence of an oxide layer and a lubricant film. At these high temperatures, the oxide scale layer is mainly composed of an inner wüstite layer, besides the intermediate magnetite and outer hematite layers [1,2]. The quantity of each oxide depends on the oxidation temperature [3]. Hematite can have a hardness of about 1000 HV at room temperature (RT) and promotes abrasive wear, whereas magnetite has a hardness of 500 HV at RT. On the other hand, wüstite is well known to have a lower hardness of 300 HV at RT and the highest lubricity among these oxides [4].

Basically, due to the elevated temperatures of the workpiece, lubricants for hot forming can only be applied to the tools. Thus, an appropriate lubricant should reduce friction and wear and be environmentally friendly [5]. The mechanical properties of the oxide scale and lubricant employed are important factors, which determine the magnitude of the friction coefficient at the tool/workpiece interface during hot forming. Therefore, it is important to understand the oxide and lubricant behavior, as well as their interaction when the friction analysis of an industrial hot forming process is conducted.

As shown in different investigations, the deformation behavior of the oxide layer formed on the surface of the steel components depends on a number of parameters. These include deformation temperature, strain applied, as well as oxide scale characteristics (thickness and composition) and surface topography, among others. As an example, Utsunomiya et al. [6] studied the hot rolling process of a low carbon JIS SS400 steel grade and determined that the oxide scale showed limited plasticity during deformation below 850 °C. However, it was observed that the oxide scale ductility increased with the increase in temperature and the decrease in the scale thickness (<26 μm). Furthermore, it was determined that the oxide layer could have an important lubricating effect during hot rolling, by decreasing the friction coefficient from 0.2 to 0.12 in the oxidized conditions. A similar study was conducted by Cheng et al. [7] on a 430 ferritic stainless steel hot rolled in the temperature range of 1050°C–1090 °C. This investigation allowed the observation that thick oxide layers of iron oxides and Cr spinel formed on the alloy surface tended to fracture and pulverize during deformation, whereas thin layers tended to be plastically deformed at rolling reductions below 46%. Additional investigations on the hot rolling behavior of a micro-alloyed low carbon steel at 900 °C carried out by Yu et al. [1] corroborated the significant effect of thickness reduction of the oxide scale behaviour. These authors reported that oxide cracking and blistering could be observed even if the material undergoes low rolling thickness reductions of about 3%, whereas, at higher thickness reductions (>28%) the oxides were observed to be descaled.

The lubrication effect of the oxide layer formed on the steel surface has also been analyzed experimentally under compression testing conditions. In this sense, Matsumoto et al. [8] investigated the behavior of a chrome JIS SCr420 steel grade deformed in the temperature range of 800°C–1150 °C, by means of hot ring compression tests. According to these authors, the oxide layer can act as a lubricant during the hot forging processes, giving rise to a decrease in the friction coefficient from 0.6 to 0.3 when the oxide layer thickness increases from 6 μm to 300 μm. However, it was also observed that the oxide layer fractured and fragmented during the tests. Behrens et al. [9] also employed compression tests for investigating the flow stress behavior of different iron oxides at different temperatures and strain rates. For this purpose, cylindrical samples were sintered by employing iron oxides powder materials with different oxygen contents and deformed under hot compression conditions. The flow stress behavior was formulated on the basis of the Hansel-Spittel constitutive law. The numerical simulation of such tests showed that the forming force of a specimen covered by an oxide layer is expected to be less than that computed when the oxide layer is not present. The authors attributed these results to the smaller flow stress of the oxide scale, as compared with that of the steel substrate.

The oxide and lubricant interaction during deformation constitutes another important subject, which has also been analyzed in several previous studies [1,2,[9], [10], [11]]. Concerning this topic, Tran et al. [10] reported that the presence of an oxide layer formed onto a 316 stainless steel could provide undesirable tool sticking behavior during high temperature deformation and that it could be inhibited by means of a crystalline sodium borate lubricant. Also, Yu et al. [1] investigated the interaction between the oxide scale and nanoparticles of a graphite lubricant during the hot rolling of a micro-alloyed low carbon steel. A lubrication mechanism advanced by the authors proposed that the propagation of cracks along the magnetite grain boundaries could create a repository to collect graphite nanoparticles during lubrication, which could give rise to a decrease in the wear rate of the material. Kong et al. [11], on the other hand, studied the interaction of the oxide scale with a polyphosphate lubricant during the hot rolling of an interstitial-free (IF) steel. These authors reported that under unlubricated conditions, a significant number of cracks could be found in the oxide scale after deformation. However, under lubricated conditions, the oxide scale became compact and smooth. Similar results reported by Bao et al. [12] indicate that during hot rolling of an ASTM 1045 steel at 1000 °C the oxide layer becomes thinner and denser under lubricated contact conditions due to the addition of 0.3 wt% of nano-SiO2 particles.

The interaction between oxide scale and lubricants, which could occur during metal forming processes has also been recently investigated by means of wear tests. As an example, Wang et al. [2] studied the lubrication mechanisms of sodium metasilicate by means of ball-on-disc tests, aimed at reproducing the hot rolling contact conditions of a low C steel grade. For this purpose, the tests were conducted at temperatures ranging from 550 °C to 920 °C, at a sliding velocity of 0.094 m/s and a normal load of 10 N. A low carbon steel and a high speed tool steel were used as disc and ball materials, respectively. The authors reported that during the sliding tests, melting at the interface occurred mainly due to the interaction between the outer hematite layer of the oxide scale and the sodium metasilicate film. Accordingly, the involvement of Fe3+ into the silicate melt contributes to the friction reduction under high temperature and pressure conditions.

Most of the investigations described above have mainly addressed the problems concerning the deformation behavior of the oxides and the oxide-lubricant interaction under different hot forming processing conditions. However, the problems concerning the lubricant film breakdown mechanisms by its interaction with an oxide layer under such conditions have been addressed to a much lesser extent. The present work proposes a multiscale research strategy to analyze in detail the mechanisms occurring at the tool/billet interface during the hot forging of an ER7 steel grade. For this purpose, hot upsetting-sliding tests as well as roll-on-disc tests were carried out under conditions similar to those found during the industrial practice. The evolution of the oxide layer and of the lubricant deposit are studied thanks to the surface topography, the evolution of the friction conditions and to EDS analyses.

Section snippets

Multiscale research strategy

The current research is based on a multiscale analysis of the hot forging interfacial contact conditions that exist at the tool-billet interface. The analysis has been conducted in order to study in more detail the tribological behavior of an ER7 steel grade deformed under hot forging conditions. This takes into account the oxide layer deformation behavior and its interaction with a lubricant film. According to the multiscale strategy, the interfacial contact conditions are analyzed at macro

Macro-scale results: sliding velocity, contact pressure and temperature profile

The FEM simulations are also used to identify the essential contact parameters which characterize the forging process, such as sliding velocity, contact pressure, tool and billet temperature, as well as interface temperature. These thermomechanical simulations are conducted employing the code Forge NxT 3.0 (TRANSVALOR S.A.). A 2D axisymmetric model is used with a tetrahedral mesh for modeling the billet and dies. The elements for the dies are considered to be rigid. The initial mesh element

Conclusions

A multiscale methodology has been applied to analyze the friction phenomena during the hot forging of railway wheels. This leads to the following conclusions:

At the macro-scale, the interface conditions are obtained from an analysis of the industrial forging process and from FEM simulations. According to the operating parameters (contact pressure, contact temperature, sliding velocity) and the interface constituents (lubricant, oxide layer), a reduced-scale analysis has been designed to

Funding

No funding was received for this work.

Intellectual property

We confirm that we have given due consideration to the protection of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication, with respect to intellectual property. In so doing we confirm that we have followed the regulations of our institutions concerning intellectual property.

CRediT authorship contribution statement

Ivan Serebriakov: Writing - original draft. Eli Saul Puchi-Cabrera: Methodology, Formal analysis, Writing - review & editing. Laurent Dubar: Conceptualization, Project administration, Funding acquisition. Philippe Moreau: Writing - review & editing, Software, Visualization, Investigation. Damien Meresse: Conceptualization, Writing - review & editing. Jose Gregorio La Barbera-Sosa: Investigation, Data curation.

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.

Acknowledgments

The authors gratefully acknowledge the financial support of Valenciennes Metropole granted to I. Serebriakov and to Professor Puchi-Cabrera, as well as the infrastructure provided by the Science for Wheelset Innovative Technology laboratory (SWIT'lab), Université Valenciennes MetropolPolytechnique Hauts de France, Valenciennes, France. The special collaboration of S. Salengro and F. Demilly (MG Valdunes) is also gratefully acknowledged.

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