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Basic principles of deep rolling
Boundary layer strengthening processes are based on three physical effects: Burnishing, residual stresses and cold strengthening. It is only in the case of deep rolling that all three effects combine whereby this process can bring about extraordinary high increases in dynamic strength. Deep rolling can be applied to all metallic materials. The areas of application currently extend from steels and cast materials to lightweight alloys such as aluminium, titanium and magnesium.
When roller burnishing or deep rolling, a rolling element, which can be a ball as well as a roller, is pressed against the surface of the work piece. The compression in the contact area exceeds the yield point and a plastic forming takes place in the boundary layer. According to Hertz a comparison stress depth profile ensues which has a maximum below the surface and approaches zero deep within the work piece. Through this only part of the boundary layer is plastically formed, whilst in deeper levels only elastic forming takes place. This leads to residual compressive stresses which as for the Hertz comparative stress always pass through a minimum below the surface.
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Forming happens cold i.e. the boundary layer is consolidated cold, below the recrystallisation temperature of the material. Plastic forming induces disruptions in the lattice structure. The increased dislocation density increases the strength of the boundary zone and can thereby prevent cracking or retard the rate of crack growth. In respect of its kinematics a deep rolling process is similar to turning or milling and can be carried out by plunging (for small radii) feeding or with a linear feed movement. To avoid sharp gradients in the boundary layer the build up of rolling force or pressure is prolongated. By means of this careful increase a notch effect is prevented. Because of the simple kinematics the process can be used with conventional machine tools. For large series production of crankshafts and piston rods however special machines are used for deep rolling and roller burnishing.
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Individual factors
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Burnishing
The burnishing of the surface is based on plastic forming of the surface finish peaks. During forming the surface peaks in particular bear the weight of the rolling element. By this means the peaks are pressed downward and the material flows sideways to fill the valleys, which leads to raising of the valley height (Fig. 3).
The assumption that compaction of the boundary layer takes place is only true for porous materials. The changes to diameter from rolling are in line with the forming only in micron range and can be taken into consideration at the pre-machining stage. Current ECOROLL tooling technology allows hardened work pieces with up to 65 HRC (840 HV) to be rolled. The hydrostatic tool principle which entails a hard-material ball being rolled across the surface with high pressure, enables the peak-to-valley height to be reduced to approx. one quarter of that of the pre-machining stage. For unhardened, ductile materials a reduction of peak-to-valley height by factor 20 is possible.
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Residual stresses
Many processes for increasing component strength or the service life under dynamic loads are based on inducing residual compressive stresses in the boundary layer. These include heat treatments such as hardening and nitriding but also shot peening and deep rolling. Chip removal processes such as upcut milling or a well-matched grinding process can also induce significant residual compressive stresses in a component.
Residual stresses through deep rolling are based on the inhomogeneous deformation of the boundary layer contiguous to the surface. the rolling element in the form of a roller or ball creates stresses in the boundary layer which at some levels cause plastic and at others only elastic deformation.
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According to Hertz, when similar maximum comparative stress is concerned, a larger rolling element results in a greater depth of penetration of the comparative stresses. The residual stresses correlate with these comparative stresses so that the shape of the rolling element has a great influence on the path of the residual stresses. With the larger rolling element therefore a greater depth of penetration is always caused by rolling compared to shot peening where significantly smaller peening material is used. The depth path of residual stresses and hardness has a significant effect on the service life of a component.
In order to achieve optimum results the path of the load induced stresses must also be observed. Adverse conditions in the boundary layer can result in the crack exit is deflected from the area of the surface to below the surface. This is not desirable since there is no external sign to indicate a failure.
Residual stress measurement using mechanical and X-ray techniques is customary today. ECOROLL will be pleased to conduct residual stress measurements on your behalf when testing deep rolling applications.
Cold work hardening
As a result of plastic forming of the boundary layer at temperatures below the recrystallisation temperature cold work hardening takes place when deep rolling. When plastic forming takes place layers of atoms slide past each other. In the microstructure the sliding is impeded on the grain boundaries. At these points the space lattice is distorted and results in prevention of sliding. Cold work hardening leads to a greater number of distortions of the space lattice. These prevent further sliding of the material when further loading takes place. That means that a cold work formed zone will be hardened and brittle. Further load will then result in fracture rather than sliding. Picture 15 illustrates this interrelationship. The embrittlement which for the forming of sheet metal for example can become critical does not represent a problem for deep rolling since the cold hardening only occurs in the boundary layer. However, raising the elastic limit and tensile strength in the boundary layer contributes significantly to increasing service life.
Cold work hardening can be measured with an X-ray diffractometer using the half width or by measurement of hardness. When measuring hardness however the cold work hardness is overlapped by the residual stresses.
In order to achieve optimum results with deep rolling it is necessary to establish the optimum machining parameters with trials. ECOROLL will be pleased to carry out optimization of deep rolling processes on behalf of customers. This can include trial machining of components or samples, analysis of residual stresses and hardening or examination of dynamic strength.
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Summary
Deep rolling is the only mechanical rim zone strain hardening process which results in smoothing the surface, the induction of compressive residual stresses and cold work hardening. This results in an extraordinary increase in fatigue strength. The scope of deep rolling extends over almost the entire range of metallic materials.
Through the use of deep rolling results in significant increases in the life of a component by the interaction of the reduction in surface roughness with strain hardening.
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