Walzen hartgedrehter Oberflächen

Until the mid-1990s, hardened steel could only be machined using grinding processes. The development of high-hardness cutting materials, such as CBN or ceramic, has meant that even steels with a hardness of over 45 HRC can be machined with a geometrically defined cutting edge. Since then, hard turning or hard milling has become widespread and is often used as an economical alternative to conventional grinding. In conjunction with high-precision machines, it is now possible to finish-turn components, such as roller bearings, directly on a lathe.

Today, however, we also know that progressive tool wear can damage the edge zone of the components. This can result in lower surface quality, residual tensile stresses or so-called white layers. These are extremely hard thin layers which, in combination with the underlying localised tempering zone, can lead to the surface flaking off.

Mechanical surface treatment can also help to adapt the surface and edge zone properties after hard turning. If a hardened material with more than 45 HRC is machined during rolling, this is referred to as hard rolling. Hydrostatic deep rolling tools are used for hard rolling. With this type of tool, a ball is pressurised hydrostatically via a hydraulic medium. The diameter of the hard material ball can vary; diameters between dk = 3 mm and 13 mm are common.

The ball is held directly above the equator so that it cannot fall out. The hydraulic pressure is then applied to the entire half surface of the ball. Rolling pressures of more than pw = 200 bar are usually used for hard rolling. Why this is the case will be described again later.

An important aim of hard rolling is to improve the surface quality, for example in the case of progressive tool wear. As part of my dissertation "Rolling hard-turned surfaces", I tried to answer this question back in 2003 [Roet03]. I used surfaces with new (wear mark width VB = 0µm), used (VB = 100µm) and worn (VB = 200µm) cutting edges and subsequently rolled the surfaces smooth.

It was found that the roughness of the surface deteriorates with increasing tool wear. If the surface is subsequently rolled smooth with different rolling pressures, the roughness also increases after deep rolling. It is interesting to note, however, that the relative improvement of the surface is independent of the pre-machining and depends only on the rolling pressure. As the pressure increases, the improvement increases, as shown in Figure 1.

This effect can also be seen for different degrees of hardness of the material (Fig. 2). However, the value of the respective improvement is smaller the greater the hardness of the component. This is understandable, as the material offers greater resistance to the rolling ball with increasing hardness.

Two other important parameters are the ball diameter and the rolling feed fw. If the ball diameter is varied, the surface becomes smoother the larger the ball diameter becomes. If the feed rate is increased, the improvement in surface quality initially remains constant and then decreases. This inflection point, at which the feed rate has an influence, depends on the ball size. With a diameter of dk = 3mm, the buckling point is at approx. 0.1mm feed; with a ball with a diameter of 13mm, however, it is only at 0.35mm feed. This is related to the so-called degree of overlap. It was later also shown that with different combinations of pressure, ball diameter and feed rate, a comparable surface quality is achieved if the degree of coverage is the same [Mais19].

With regard to the edge zone properties, hard rolling is also a process with major advantages. The negative edge zone properties introduced by hard turning, such as residual tensile stresses on the surface or reduced hardness below the white layers, can be reversed by hard rolling. Due to the high hardness, the contact point between the ball and the surface can be described in simplified terms using Hertzian pressure. With soft steels, however, the effect of plastic stretching is more relevant.

It can be deduced from the Hertzian contact that the maximum stresses during machining are below the surface. For this reason, there is also a very typical residual stress depth curve for deep rolling, as shown in Figure 3. The value of the maximum residual compressive stress increases with increasing rolling pressure, and the penetration depth of the residual compressive stresses increases with a larger ball diameter.