Introduction            

Climate change is progressing faster and faster. OEMs are setting ambitious goals for a carbon-neutral future, putting many of their suppliers under pressure. In industrial companies, there are many different levers to pull to achieve a carbon-neutral future. One possibility is to consider the CO2 footprint directly in the design stage to reduce the impact of the machine or individual component. Measures such as reducing power losses, lightweight construction using new materials, or the use of high-strength materials are just a few examples. For this, however, the greatest influencing variables in the design must be understood. In various research projects, for example the "Antriebsstrang 2025" project funded by the German Government (BMWK), it has been shown that one of the biggest influencing factors on the CO2 footprint of a component is the proportion of material required [1], i.e., the amount of raw material. Subsequent manufacturing steps, such as machining, forming or heat treatment, have a significantly smaller impact. This is also where the challenge lies in the lightweight construction measures mentioned above. The use of new or even higher-strength materials usually requires compromises and also higher energy consumption for production or disposal.

A simpler way to improve the climate balance is to exploit the so-called surface and subsurface properties. These properties can significantly influence the performance of a component. For example, by producing a specially ground or honed surface, the proportion of friction can be significantly reduced when the component is used. Furthermore, the service life of dynamically loaded components can be reduced by targeted exploitation of residual compressive stresses in the subsurface area. This is known from shot peening of turbine blades or gears. The targeted use of residual compressive stresses can increase the service life by more than 1.2 - 5 times, which means that the components can be designed smaller and lightweight construction effects can be exploited.

If the design specifies the use of residual compressive stresses, the question naturally arises as to how these residual compressive stresses can be introduced in a targeted and process-reliable manner. Shot peening would be an option, but always requires an additional machine system. The processes of deep rolling or machine hammer peening, on the other hand, can be used directly on the machine tool and do not require any additional plant technology.

During deep rolling, or deep cold rolling, a rolling element is pressed onto the surface of a component with a defined force. The contact between the component and the rolling body creates high stresses and the material is plastically deformed locally on the surface. This deformation smoothens the surface and causes changes in the microstructure in the near-surface area [2], resulting in a significant increase in service life.

The aim of this research is to evaluate the effect of deep rolling on the reduction of CO2 emissions. Therefore, different analyses, experiments and literature reviews are considered.

Evaluating the CO2 Footprint of Deep Cold Rolled Parts

Extending the lifespan of machine parts can be advantageous in several ways. According to a McKinsey report, aftermarket services, such as the provision of parts, repair, maintenance, and digital services for the equipment sold by original equipment manufacturers (OEMs), provide stable revenue and often higher margins than the sales of new equipment. The report also suggests that OEMs should take a more detailed approach to the design of their products to ensure that they are durable and long-lasting. This can be achieved by using high-quality materials, optimizing the design, and conducting rigorous testing.

The paper discusses a simple and modular framework to evaluate the CO2 emissions of a deep rolled part. The author suggests that the framework should start by defining the part service life and loading situation. The main challenge is to identify the optimum rolling parameters, which cannot be predicted simply to achieve a specific service life for a specific material without trying. Experiments, literature, or expert knowledge can be used to identify the optimum parameters.

The next step is to identify a lifetime increasement factor, which can be done by estimations, literature review, or actual service life tests. This step is followed by a redesign or lightweight design with the goal of reducing the amount of material used for the part. Finally, the actual CO2 footprint is evaluated using a Life Cycle Assessment (LCA).

Experimental Procedure

To test this framework, two kinds of experiments are used. On the one hand, lifetime test with rotating bending specimens is conducted. The specimens have a working diameter of d = 13 mm and are made of AISI 4140. Two different specimens are machined. One specimen is produced by turning and the second type is produced by turning with additional deep rolling using an ECOROLL tool type EG5-1-40M. Both specimen types are tested in a rotating bending tester from SincoTec. 

On the other hand, literature data is used to evaluate the CO2 footprint for deep rolled roller bearings. In this case, the data is given in [4] and [5]. In the cited research, the roller bearings are hard turned and deep rolled in comparison to a conventional manufacturing chain of grinding and honing.

Measurement of CO2 Footprint for Cold Deep Rolling

The rotating bending test shows an increase of 10% in service life by using deep rolling. Assuming a lightweight design with a test diameter of dred = 12.59  mm, the same lifetime is achieved as for the just turned specimen. This means about 8.6% less raw material is used for the specimen. 

The LCA was conducted within a project by the Mittelstand-Digitalzentrum Hannover. The energy and material input for the process chain “raw material”, “turning” and “deep rolling” was considered. For the deep rolling, an energy measurement was conducted. All information is considered within the tool “Umberto” to use well-known CO2 databases. Umberto is a commercial software and database for evaluating the CO2 footprint. 

The model shows very clearly that the most important factor to consider is the amount of raw material. The data shows that just by reducing the material amount by 8.6%, the equivalent carbon emissions are reduced by 96g CO2-eq. per part. To conduct the additional rolling process, about 15g CO2-eq. is emitted. In total, there is a reduction of 7.3%. 

Potential of CO2 Reduction for different Applications

For roller bearings, the effect is even greater. By using a bearing type NU205 instead of NU206, the material can be reduced by 33%. The basis for this conclusion is 2.5 x L10(small/rolled) = L10(large/not rolled). Assuming the same loads, the dynamic load rating C can be calculated. For NU206, this calculation leads to the value of NU205. With the additional rolling process, only 18g CO2-eq. are emitted. This leads to a CO2 reduction of about 31.6%. Assuming just 50,000 bearings per year manufactured, this results in 21 to. CO2-eq. reduced (Fig. 3). 

Conclusion and Outlook

The presented results show a very clear effect of the cold deep rolling process on the resulting carbon emissions of a dynamically loaded part. If all life cycle parameters of a part are taken int account, the use phase is the most important factor in reducing carbon emissions. However, the definition of the amount of CO2 emissions is given in the design phase. This is why a consequent use of subsurface properties is necessary. Therefore, designers must know about mechanical surface treatments such as cold deep rolling, roller burnishing or machine hammer peening.  

References

1. Denkena, B., Wichmann, M., Kettelmann, S., Matthies, J., Reuter, L.: Ecological Planning of Manufacturing Process Chains. Sustainability, 2022, Vol. 14(5)

2. https://www.surfacematters.tech

3. Maiß, O., Röttger, K., Meyer, K.: Increase the Resource Efficiency by Evaluation of the Effects of Deep Rolling within the Design and Manufacturing Phase. Future Automotive Prodction Conference 2022, Springer Vieweg, S. 86-96

4. Neubauer, T.: Betriebs- und Lebensdauerverhalten hartgedrehter und festgewalzter Zylinderrollenlager. Dr.-Ing. Diss., Gottfried Wilhelm Leibniz Universität Hannover, 2016

5. Maiß, O.: Lebensdauererhöhung von Wälzlagern durch mechanische Bearbeitung. Dr.-Ing. Dissertation, Gottfried Wilhelm Leibniz Universität Hannover, 2019

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