Eigenspannungsentstehung beim Festwalzen

Deep rolling is one of the mechanical surface finishing processes. Like the other processes in this category, the aim of deep rolling is to adjust the surface and edge zone properties in a targeted and reliable manner in order to give the component an additional function.

The great advantage of deep rolling is that, in addition to the edge zone properties, the surface quality is also significantly improved; virtually as a by-product. However, as with shot peening or surface hammering, the actual focus of the process lies in the introduction of so-called residual compressive stresses [1].

Residual stresses are stresses in a component that are introduced by the manufacturing process and differ from the state of the basic structure. Generally, residual tensile stresses have a negative effect on the service life of a dynamically loaded component. They act as crack amplifiers and add up with the tensile load stresses. On the other hand, there are residual compressive stresses. These counteract crack propagation and generally lead to a significant increase in service life.

During deep rolling, a rolled body is pressed onto the surface of the component. The shape of the rolling element is designed in such a way that the contact area is as small as possible. This means that a moderate rolling force can lead to a high surface pressure and the material can be significantly influenced.

Depending on the material, the effect of plastic stretching or Hertzian pressure occurs in the contact zone. In both cases, there is a significant increase in the local stress distribution in the material. The material clearly exceeds the yield point, leaving stresses in the workpiece (Fig. 1).

In the case of plastic stretching, the largest deformations are located directly on the surface and therefore the maximum residual compressive stresses are also present directly on the surface. This tends to occur with softer materials.

The three main parameters of the rolling process are the rolling pressure, the rolling feed/coverage ratio and the rolling ball size. The correlations can be explained very well with the effects of Hertzian contact. Analogue to Hertzian contact, the maximum residual compressive stress also increases when the rolling force (analogue to the rolling pressure) is increased. It is interesting to note that the position, i.e. the depth at which the maximum residual compressive stress occurs, changes only minimally as a result of the rolling force (Fig. 2).

In contrast, the ball diameter has an effect almost exclusively on the depth of the maximum residual compressive stress and the penetration depth. Here, for the same Hertzian pressure pmax, the depth of the maximum residual compressive stress increases significantly with increasing ball diameter (Fig. 3). However, the value of the maximum residual compressive stress remains at a similar level. The latest research is working here with the process signature approach, which is being investigated in the SFB TR136 at the University of Bremen.

The deep rolling process is particularly notable for its outstanding repeatability. As part of a study at the SMART Surfaces industry forum organised by the Institute of Production Engineering and Machine Tools (IFW), 15 identical rolling processes were carried out. An HG6 tool was used for the machining of 42CrMo4. The rolling pressure was kept constant at pw = 250 bar and a degree of coverage of u = 75% was set. The residual surface stresses were then measured using an X-ray diffractometer. The scatter of the measurement of all 15 samples was clearly within the measurement uncertainty range of the measuring device of +/- 25 MPa (Fig. 4). This means that, from a scientific point of view, all samples were absolutely identical and no difference could be detected.