Investigation of bond formation behaviour in composite ring rolling
Introduction
As summarized by Allwood (Allwood et al., 2005), ring rolling is an incremental bulk metal forming process used to manufacture seamless rings with a wide variety of sizes and shapes as well as processible materials which allows application of rolled rings in automotive, bearing, energy or aerospace industries. As described in the “Metals Handbook” (American Society for Metals, 1988), a ring-shaped pre-form’s diameter increases as a result of reducing its cross-section in two roll gaps, cf. Fig. 1. The pre-form’s wall thickness is reduced by moving a non-driven mandrel towards the driven main roll in the radial roll gap, while its height is reduced by two driven truncated cone shaped rolls arranged vertically, of which the upper roll moves downwards. Two guide rolls ensure circularity as well as alignment of the ring in the machine’s axis by inducing lateral forces aiming inwards on both sides of the ring. Fig. 1 shows a schematic overview of the tools’ arrangement in the machine.
Usually, rings produced via ring rolling consist of a single material, which has to withstand all loads acting on it during use, even if these loads only act locally. Especially for conflicting material properties, e.g. for roller bearings which need high hardness and wear resistance on the raceways as well as high toughness as a whole component, this leads to high costs either for special materials or to the need of post-treatment of the rings. A different approach, using composites comprising at least two different materials on the inside and outside of rings, is investigated here. The process of roll bonding is an established technology for joining flat products by generating material bonds, as stated by Manesh and Taheri (Manesh and Taheri, 2005) in an investigation of generating three-layer sandwich strips by cold roll bonding. In this study an analytical model based on the upper bound theorem is used to investigate material flow and to predict rolling force and power with medium to high accuracy depending on which types of material combination from copper, aluminium and steel alloys are used. Although Jing et al. (Jing et al., 2014) show that roll bonding of steel strips is a current research topic, this principle has only been studied marginally for application in ring rolling.
Kluge et al. (Kluge et al., 1995) published first studies in the 1990s, in which composite ring rolling was investigated by successfully bonding two concentrically arranged rings, i.e. a construction steel outer ring and a duplex steel inner ring. To achieve a material bond between both rings, a higher ring growth tendency for the inner ring was shown to be necessary. Additional limitations consisted of welding of the rings’ front edges to prevent oxidation as well as utilisation of rings with a high ratio of height to wall thickness. While both materials show very similar flow curves, no detailed conclusions about feasible material combinations, process strategies or process limits were given.
Currently, Küsters et al. (Kuesters et al., 2017) investigate cold composite ring rolling. In this case, a softer material is used for the inner ring than for the outer ring, so as to generate a force fit between both rings during the ring rolling process. As a result of a higher plasticisation and a lower elastic spring back on the inside, an interference fit is generated in the radial roll gap.
The present authors recently published (Seitz et al., 2016) an investigation focused on process layout for combining a stainless steel outer ring and a tempering steel inner ring with an approx. 50% lower flow stress. First experiments showed an outgrowing of the inner ring, which is in accordance with simplified numerical simulations which assume friction between the outer and inner ring, cf. Fig. 2, upper part. This is induced by a too high ring growth tendency of the inner ring, which results from the flow stress difference to the outer ring as well as a small contact length of the mandrel with the inner ring. Increasing the mandrel diameter to increase the contact length was shown to sufficiently inhibit its growth tendency to allow a stable rolling process.
Furthermore, a symmetrical joint line in addition to a stable rolling process could only be seen for rings with sufficiently high ratio of height to wall thickness, cf. Fig. 2, lower part. The rolling table was shown to induce asymmetrical material flow across the rings’ heights, which is suppressed by friction with the rolling tools for large but not for small rings.
Using these results, the present authors (Guenther et al., 2017) further examined the composite ring rolling and shifted the focus to preform preparation, which was shown to be crucial to achieving a material bond in roll bonding processes. A strong interference fit was used in order to compensate a higher thermal expansion coefficient of the outer ring, which would lead to a gap between both rings when heating. As the rings were heated in a natural gas furnace, the joining surface of both rings was exposed to oxygen and therefore showed strong oxidation after rolling. This oxidation could not be prevented by the interference fit, even though contact pressure between the rings still remained after heating. Because oxidation has a strong detrimental effect on material bond formation, no material bond was achieved as a result.
Considering this knowledge of previous investigations, the aim is now to produce a composite ring made from two steels. To achieve this, first a model to describe the material bond evolution between the steels is introduced and parametrised. This model is then combined with finite element (FE) ring rolling simulations to observe whether not only material bond formation but also breaking of bonds occurs during the process and if so, at which locations. Subsequently, an approach for preparing and producing composite rings with an adjusted process strategy which favours bond formation is developed.
Section snippets
Material bonds established by roll bonding
As stated by Groche et al. (Groche et al., 2014), material bonding between two materials can be achieved by a wide variety of forming processes, such as composite extrusion, explosion forming, composite forging and roll bonding. Out of all joining-by-forming processes, roll bonding of steel is the most similar to the process investigated here, as almost all analytical considerations about conventional ring rolling take similarities to flat rolling as their point of origin. This holds true for
Experimental investigation of the composite ring rolling process
In this study, the stainless steel X5CrNiMo17-12-2 was used for the outer ring, while the tempering steel 13CrMo4-5 was used as the inner ring’s material. This material combination could e.g. be used for heat exchanger pipes in energy plants, as both materials are permissible for high temperature use while the stainless and more expensive steel is more resistant to sulphur gas corrosion.
Numerical investigation
To gain further insights into the bond formation characteristics during the composite ring rolling process, finite element simulations of the process with the incorporated empirical bond strength model introduced in this study are employed.
Conclusion and outlook
Composite ring rolling offers the opportunity of combining materials with widely different properties and could therefore be used to produce rings tailored to specific applications.
In this paper, composite ring rolling of a stainless steel outer ring and a considerably softer tempering steel inner ring was investigated. When aiming to understand the material bond generation between both rings during the process, FE simulations considering the bond evolution are necessary. Therefore, a contact
Acknowledgement
This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) [project-ID 284309777].
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