Auswirkungen des Festwalzens auf die CO2-Bilanz eines Bauteils

One of the key tasks for engineers in the coming years will be to develop climate-neutral products. However, the question of climate neutrality is not easy to answer when it comes to components. A life cycle assessment (LCA) is primarily concerned with determining the_{CO2 emissions} for a predefined area of consideration. Or to put it more simply, you have to clearly define which_{CO2 emissions} are included in the assessment and which are not.

Typically, this is referred to as Scope 1, 2 and 3, which is still relatively simple in terms of a company's climate neutrality. Do you only consider incoming energy? How do you take the company's vehicle fleet into account? Or are the emissions caused by employees travelling to and from work included?

If you want to balance a component, you need to differentiate more precisely and, above all, consider whether the manufacturing and/or utilisation phase should be considered. In a complete LCA, the entire life cycle is considered, i.e. from the manufacturing phase through to delivery, use and finally disposal/recycling of the product.

In order to reduce the emissions of a system in this complete LCA, consistent lightweight construction is an option. This primarily affects the utilisation phase. For example, more and more plastics are being used in vehicle construction to make cars lighter and thus reduce emissions when driving. However, this trend can have a negative impact on the manufacturing and disposal phase, as this results in significantly higher energy requirements.

One possibility for lightweight construction with metallic components is the optimal utilisation of surface and edge zone properties. It has been scientifically proven for many years that the properties of the material and also the tribological properties of the component can be specifically adapted to the requirements of the application through the manufacturing process. Even today, there are still a large number of research projects that are looking at how the dynamic strength of components can be optimised through the targeted introduction of residual compressive stresses.

Through mechanical surface treatment, we can utilise precisely this and manufacture components in a targeted manner to increase their service life, which in turn has a positive effect on their_{carbon footprint}.

But how does the increased service life actually affect the_{CO2 balance} of the component?

There are two traditional approaches to answering this question. A distinction must be made as to whether it is a component that is designed for the entire service life of the overall system (e.g. vehicle) or whether it needs to be replaced at regular intervals?

Let's go through these two scenarios using the example of a roller bearing. As part of a research project by the German Research Foundation (DFG), the extent to which the service life of a cylindrical roller bearing of type NU206 can be increased by changing the process chain from grinding and honing to hard turning and deep rolling was investigated [1].

The results have shown that deep rolling very specifically introduces residual compressive stresses where the highest stresses occur during bearing operation. The residual compressive stresses counteract the load stresses there and reduce the equivalent stress according to von Mises. Compared to a conventional bearing, this means that the hard-turned and hard-rolled bearings in the project had a 150% longer L10 service life, meaning that the bearings can be used 2.5 times longer.

Based on this increased service life, both scenarios can now be positively influenced. Let's start with the component that has to be replaced regularly during operation; in other words, in this machine, this bearing is replaced regularly, for example once a year. With a machine service life of 20 years, this means that this bearing has to be replaced 20 times over the course of the machine's entire service life, meaning that 20 bearings have to be manufactured.

The manufacturing emissions of 20 bearings must therefore be taken into account for the_{CO2 balance}. This means that steel has to be provided, energy has to be used for the manufacturing machines and a large amount of thermal energy has to be used for heat treatment. In addition, the bearings have to be transported and brought to the machine. We know from research projects, such as the "Drive Train 2025" project funded by the Federal Ministry for Economic Affairs and Climate (BMWK), that a change in production is unlikely to have a particularly large impact on the_{carbon footprint} of a bearing. In other words, whether the bearing is ground and honed or hard turned and rolled is not decisive for the_{carbon footprint}. It is more important to save material.

And this is precisely where the lever for improving the_{CO2 balance} through rolling lies. If the same bearing can now be used for 2.5 times as long, then fewer bearings need to be produced. In our example of a total service life of 20 years from above, only 8 bearings need to be produced, which saves around 60% of_{CO2 emissions}.

In the second scenario, we assume that a component is designed for the entire service life of the machine. If the same bearing is used here, but it is hard rolled, then the_{CO2 balance} worsens, as we have to use a small amount of additional energy for rolling. However, the bearing is disposed of after 20 years, even if it is still completely intact. In order to positively influence the_{CO2 balance} in this case too, the approach here is to achieve the same service life with a smaller bearing.