Finite element modeling of the cold pilgering process
The current project is aiming to develop validated finite element models (FEM) for the cold pilgering process in order to support design of a robust process. Cold pilgering is the major forming method used to produce high precision seamless tubes. These tubes are used in a wide range of industries where high precision is needed, e.g.: nuclear, aerospace, automotive, oil and gas. This method enables cost efficient production of tubes with tight geometric tolerances and high surface quality. Most metals are suitable for this method but the workability limits the maximum possible cross-sectional area reduction. Typical area reductions for stainless steels are up to 80 percent. In this method, the outer diameter and wall thickness of a tube preform is progressively reduced during several strokes to reach desired tube dimensions. Two roll dies are used to shape the outer surface while the mandrel determines the inner diameter, see Figure 1. The dimensional accuracy of the final tube is strongly dependent on the tool geometry, motion and deformation. Moreover, the large plastic deformation and friction generate a significant amount of heat leading to temperatures up to 300°C despite use of a coolant.
Figure 1: Illustration of the cold pilgering process.
In our first study, temperature and strain rate distributions are computed, using a 3D thermo-mechanical FEM, and the influence of temperature and strain rate on the rolling force is investigated, see Figure 2.
Figure 2: FE model of the cold pilgered tube at the end of process.
In the following study, the rigid tools and the tube in the previous model are supported with elastic springs to investigate the influence of vertical displacements of the roll stand and axial displacements of the mandrel and tube. In our recent study, the previous model is further developed using elastic roll dies to investigate the influence of roll die deformation on the material flow, contact region, roll separating force and tube dimensions. Full scales experiments are performed in the production mill to validate the model.
Figure 3: Left: The FE model with elastic rolls. Right: a section of the contact region.