Basic Principles of Deep Rolling

Deep rolling – like roller burnishing, mechanical hammer peening or shot peening – belongs to the group of processes for mechanical surface treatment. These processes are always used when surface and edge zone properties have to be adjusted to pre-defined target values in order to improve a component's quality or properties. As with the other processes in this group, deep rolling involves a local plastic deformation of the surface through mechanical stress.

Fig.: Cold forming

As with roller burnishing, a rolling element is pressed onto the surface of a component with a pre-defined force during deep rolling. The entire component is positively influenced by the rolling element moving over the entire surface. The local contact pressures between the rolling element and the surface lead to a plastical deformation of the surface's roughness peaks. This deformation is usually still noticeable to a depth of 0.2-1 mm below the surface, affecting the rim zone. The local stresses within the surface zone during machining lead to residual compressive stresses being introduced, increased hardness and strain hardening of the surface.

Fig.: Effects of deep rolling

It is precisely this deliberately influenced edge zone that distinguishes deep rolling from roller burnishing. Roller burnishing processes are intended to achieve the smoothest surface possible. The hardening of the edge zone is a positive side effect. With deep rolling, it is exactly the other way around. Here, the smoothing of the roughness is a side effect. The goal is to achieve a defined condition of the edge zone. For this reason, the requirements for deep rolling are higher than for roller burnishing. The process parameters must be clearly defined in advance, and the respective rolling parameters must be monitored precisely. Usually, differences of the edge zone's conditions can not be checked without destructive testing of the component, so that process control has to be much more precise. New smart tools, such as ECOsense tools from ECOROLL AG, are therefore able to monitor the critical variable "rolling force" during the process and to detect errors instantly.

Fig.: Impact of deep rolling on service life [Source: Denkena et al., 2016, Bearing World]

The positive effects of deep rolling are particularly evident in dynamically stressed components. There is countless evidence in literature that fatigue strength for deep-rolled components increases significantly compared to non-rolled parts. Improvements of 2 to 5 times the previous service life are possible through this simple and highly productive process. Deep rolling is therefore a highly reasonable process, especially in times of a necessary increase in resource efficiency, as it is very efficient and at the same time comparatively cost-effective. This means that components can be modified with this process in such a way that they can withstand greater loads over a longer period of time. The use of cheaper materials or less material (i.e. lightweight construction) is therefore possible. The benefit of such effects can contribute to reducing the carbon footprint of components in many different industries.

Like roller burnishing, deep rolling can be carried out on conventional lathes and milling machines. Depending on component geometry to be processed, special tools are designed to carry out the required machining. In comparison to other hardening processes, such as shot peening or mechanical hammer peening, deep rolling has advantages in many areas. For example, the depth of penetration of the residual stresses is significantly higher in deep rolling than in shot peening. The reason for this is simple and lies in the larger dimensions of rolling elements compared to blasting material. This creates a larger contact area on the component, which in turn leads to a greater penetration depth of the stresses.