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Machine Hammer Peening (MHP)

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.


In addition to the well-known technologies of roller burnishing and deep rolling or shot peening, machine hammer peening (MHP) is a comparatively new process. Here, a hammer is blown on a component's surface at high frequency. It is therefore an incremental surface forming process.


Unlike during roller burnishing or deep rolling, the tool is not in continuous contact with the surface. As with shot peening, the kinetic energy of the tool is used to reshape the material using an impulse. However, the impact energy of a single blow during MHP is significantly higher than during shot peening, which is why the edge zone is influenced even more deeply by this technology than with any other process.

 

The MHP process is determined by different process parameters. These include, among others, the size and shape of the hammer head. Here, hemispheres with radii between 4 and 25 mm are usually used. The impact pattern on the surface is also determined by the path distance and the relationship between impact frequency and feed rate. The incremental forming process leads to a regularly structured surface that is similar to the surface shape after shot peening, but differs in the regular distance between impact points. The last important parameter for MHP is the impact energy. It determines the degree of deformation and thus the intensity of the edge zone influence.

* stroke h, path distance s, feed v, diameter of hammer head d, hammering frequency f

These parameters describe every MHP process regardless of tool design. Depending on the manufacturer, different tool systems are offered. The oscillation of the hammer head is always achieved in different ways, for example electromagnetically or by a pneumatic system. In contrast to ECOROLL's tool approach, all other tool systems require an additional form of energy in the machine. With ECOpeen, ECOROLL relies on a self-sufficient system that can be clamped directly into the milling spindle and is driven by its rotation.

 

First applications for MHP were post-treatments of welding seams and the smoothing of dies in tool and mold construction. When processing welds, mobile systems are often used directly on the construction site. Although these systems are very practical, consistent quality of the process is not guaranteed, since a manually guided tool does not produce even results, which usually leads to reworking.

Overall, MHP can significantly reduce the surface roughness of a component. Due to high impact energy, it is possible to easily achieve roughness values ​​of Rz < 1 µm. The targeted structuring of surfaces, for example for lubrication pockets, has also been investigated. The biggest advantage, however, lies in the significantly greater residual compressive stresses. Due to the impact impulse, the effective depth of the residual compressive stresses is even greater than with rolling. Various measurements have shown that MHP can introduce residual stresses to a depth of 4 to 4.5 mm which is particularly crucial for increasing the service life of large components.