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Abstract
The forward rod extrusion of ferritic stainless steel X6Cr17 (DIN 1.4016) is here investigated with the objectives to experimentally identify and numerically verify the effect of the lubrication system and die opening angle on residual stresses. Three lubricants – MoS2, soap and polymer – are considered whose tribological properties are characterized via double cup extrusion tests. The effect of material flow is also studied by forming in conical dies featuring three different opening angles. The extrusion experiments revealed a decrease in the near-surface tensile stresses with decreasing friction for all the considered opening angles. An opening angle of 2α = 90° led to the highest tensile residual stresses. Both an increase to 2α = 120° and a decrease to 60° resulted in reduced tensile stresses and even a shift to compressive stresses.Furthermore, a previously developed numerical model of forward rod extrusion is optimized and validated against the experimentally measured residual stresses. The effect of the spatial and temporal discretizations of the model on the predicted residual stresses is investigated. Based on the experimental and numerically verified results, the recommendation to reduce friction in forward rod extrusion is derived as a means to obtain a less detrimental to the lifecycle of extruded parts residual stress state.
Highlights
For the production of rotationally symmetric steel components—such as screws, bolts or axles—in large quantities, forward rod extrusion is used
The experimentally determined residual stress state of the reference parts is presented in Fig. 9, together with the corresponding results from the numerical model
The decreasing trend in residual stresses was verified by the numerical model
Summary
For the production of rotationally symmetric steel components—such as screws, bolts or axles—in large quantities, forward rod extrusion is used. The high material strength compared to hot forming and the extensive work hardening during the component production lead to highly resilient components [13]. An inhomogeneous material flow occurring during forming leads to residual stresses remaining in the component after removal from the tool. Compressive residual stresses lead to an extension of the fatigue strength, while tensile residual stresses promote crack initiation and growth [29]. In order to remove any tensile residual stresses that may be present, formed components are often subjected to an energy-intensive heat treatment in which they are stress-relieved by annealing. A targeted adjustment of the residual stresses that are generated in the component during extrusion offers great potential for improving the component properties while shortening the process chain and improving the energy footprint by eliminating the need for heat treatment
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