Process-reliable deep rolling with hydrostatic rolling tools

Many components are designed to withstand dynamic loads. In order to achieve sufficient strength, either an appropriate material can be used or the component can be geometrically designed accordingly. In terms of resource efficiency, however, it would be better to use lower-quality materials or generally smaller and lighter components. For this purpose, the component can be processed by mechanical surface treatment after machining. Processes for this treatment include machine hammer peening, shot peening or deep rolling. In the following, we’d like to focus on the latter.

During deep rolling, a rolling element is pressed into the surface of a component with a defined force. This plastically deforms areas close to the surface and affects the so-called edge zone. This results in work hardening, increased hardness and residual compressive stresses. The major challenge with deep rolling, however, is that the process result cannot usually be tested non-destructively after the process. In order to determine whether the necessary compressive residual stresses have been applied, the finished workpiece must be destroyed. For this reason, the tools used must be particularly reliable and as resilient as possible to process fluctuations.

Hydrostatic rolling tools optimally meet these requirements. Alongside mechanical tools, this is the second group of tools used for roller burnishing and deep rolling. With mechanical tools, the rolling element, a roller, is mechanically supported and pressed into the surface with a defined force. In hydrostatic tools, also known as HG tools, a ball is used as the rolling element. This ball is pressurized on one complete half via a hydraulic medium. This generates a force that increases in proportion to the applied pressure.

Since the ball floats in the medium, the tools have very low-friction and low-wear, but can still generate very high pressures between ball and surface, resulting in high compressive residual stresses.

A distinction is made between tools with and without following system. The following system ensures that the ball is always pressed into the surface with a constant force over a stroke range of several millimeters. This means, for example, that non-round or uneven components can also be machined. In principle, the possible component geometry varies greatly and can range from a simple cylindrical to a free-form surface. The only exceptions are surfaces that end very close to edges, such as grooves or, of course, internal or external threads.

HG tools are used for a wide variety of applications. In particular, they are used when it is necessary to reliably increase the strength of a component by introducing compressive residual stresses or strain hardening, even with component hardnesses of more than 45 HRC, i.e. in so-called hard machining.

Due to the hydrostatic bearing, the ball rolls freely in the ball holder and is therefore significantly more durable compared to alternative processes such as diamond burnishing. Compared to shot peening, however, the process not only produces an increase in hardness, but also ensures that the surface is smoothed significantly.

In addition to the tool, which is available in various designs and with different ball sizes from 2-25 mm, the process also includes a high-pressure unit. Depending on material and required compressive residual stress, the rolling pressure can be up to 600 bar. A ball size of d_k = 6 mm and a pressure of p_w = 400 bar results in a rolling force of approx. F_w = 1,000 N. The combination of ball size and force induces a Hertzian pressure (contact pressure between two components) of p_max > 6 GPa. This is far above the strength of steels, but is necessary to generate sufficient compressive residual stresses.

To ensure reliable operation of the tool, the hydraulic system is part of the HG rolling tool technology unit. Although the medium used can be that of the CNC machine, i.e. cutting oils or cooling lubricants, suitable flow rates, filtering and cooling systems are required to ensure that a sufficient quantity is always available, even in 24/7 operation. The greatest danger with this technology is the risk of the medium running dry. In the worst-case scenario, this would result in the not-moving ball being pressed against the surface at full force, which would lead to the immediate failure of the ball insert.

As briefly mentioned above, a very high Hertzian contact pressure is applied during deep rolling with HG tools. Due to the depth effect, this is also responsible for the introduction of compressive residual stresses. The high pressures ensure that basically all previously introduced properties of the near surface area are compensated. In 2003, for example, Dr. Karsten Röttger, CEO of ECOROLL, demonstrated that different compressive residual stresses caused by tool wear during hard turning can be compensated for by using the same deep rolling process. As long as the depth effect of rolling is greater than the previously introduced surface integrity, deep rolling is the decisive process. This means that "errors" from previous processes can also be reliably compensated for.

All in all, the design of hydrostatic tools already ensures that the rolling force as the critical process variable is kept at a constant level. Geometric deviations or changes in the subsurface area resulting from previous machining can be reliably compensated for and a constant subsurface condition can be generated. By using coordinated hydraulic units, media management can also be designed safely, and the service life of the rolling balls can be significantly extended.