The resulting tensile residual stresses on the surface of cold full-forward extruded parts are unfavorable for the fatigue life of these parts. The final stress state is determined by the combination of two process stages: forming and ejection. This is due to the fact that the workpiece undergoes a second plastic deformation after forming during the ejection from the die. So far, literature is focusing mainly on the identification of the parameters affecting the residual stresses during the first stage. In the present paper, the attention is drawn to the ejection phase during cold extrusion of workpieces made out of the austenitic stainless steel AISI 316 L. First of all, a system consisting of an active die is presented. This technology allows the control of the applied pre-stress on the die during the process. It is experimentally and numerically demonstrated that a significant shift of the residual stress state in the near-surface region can be achieved. Even compressive axial and tangential residual stresses can be induced in this area. Also the limits of this system are numerically investigated. It is observed that a different deformation mechanism occurs above a certain pre-stress level. Finally, an analytical model is created and observations are presented relatively to the mechanisms that influence the plastic deformation during ejection.

The austenitic stainless steels 1.4307 and 1.4404 significantly benefit from cold forming, due to their high work hardening capability. Great potential to improve the component's fatigue properties is expected by optimizing the forming process chain such that specific residual stresses are induced in critical component areas. In this work, an analysis of the formation of residual stresses during rotary swaging is carried out. Through this incremental forming process, high strain hardening and a complex material flow history are induced in the workpieces. Therefore, measuring strategies for the residual stress measurement of cold de-formed austenitic steels by X-Ray diffraction, using the sin2Ψ-method, were developed. Here, especially the 1.4307 is a challenging material due to cold forming induced martensite formation. Despite phase changes, both cold formed materials exhibit anisotropic microstructures as well as coarse grained areas. Moreover, particular notched geometries are produced on the workpieces by rotary swaging. The measuring techniques are further developed for these complex geometries and the residual stresses are investigated.

The austenitic stainless steels 1.4307 and 1.4404 significantly benefit from cold forming, due to their high work hardening capability. Great potential to improve the component's fatigue properties is expected by optimizing the forming process chain such that specific residual stresses are induced in critical component areas. In this work, an analysis of the formation of residual stresses during rotary swaging is carried out. Through this incremental forming process, high strain hardening and a complex material flow history are induced in the workpieces. Therefore, measuring strategies for the residual stress measurement of cold deformed austenitic steels by X-Ray diffraction, using the sin 2 Ψ-method, were developed. Here, especially the 1.4307 is a challenging material due to cold forming induced martensite formation. Despite phase changes, both cold formed materials exhibit anisotropic microstructures as well as coarse grained areas. Moreover, particular notched geometries are produced on the workpieces by rotary swaging. The measuring techniques are further developed for these complex geometries and the residual stresses are investigated.

Cold extruded components are characterized by residual stresses, which originate from the experienced manufacturing process. For industrial applications, reproducibility and homogeneity of the final components are key aspects for an optimized quality control. Although striving to obtain identical deformation and surface conditions, fluctuation in the manufacturing parameters and contact shear conditions during the forming process may lead to variations of the spatial residual stress distribution in the final product. This could lead to a dependency of the residual stress measurement results on the relative axial and circumferential position on the sample. An attempt to examine this problem is made by the employment of design of experiments (DoE) methods. A statistical analysis of the residual stress results generated through X-Ray diffraction is performed. Additionally, the ability of cold extrusion processes to generate uniform stress states is analyzed on specimens of austenitic stainless steel 1.4404 and possible correlations with the pre-deformed condition are statistically examined. Moreover, the influence of the coating, consisting of oxalate and a MoS2 based lubricant, on the X-Ray diffraction measurements of the surface is investigated.

ZusammenfassungAusgehend von vorherigen Ergebnissen zur Prozessoptimierung fließgepresster Proben aus dem austenitischen Werkstoff 1.4404 mittels einer aktiven Prozessführung zur Verbesserung des Eigenspannungszustands, wurde das gleiche Verfahren auf den metastabilen austenitischen Werkstoff 1.4307 angewandt. Es wurden neue Erkenntnisse erwartet, da dieses Material bereits bei geringen Umformgraden zur Bildung von verformungsinduziertem Martensit neigt. Zum Vergleich der erwarteten Optimierung des Eigenspannungszustands wurden konventionell fließgepresste Proben hegestellt und spannungsarmgeglüht.Es wurde gezeigt, dass die Reduktion der Zugeigenspannungen durch die aktive Prozessführung beim 1.4307 insbesondere in der martensitischen Phase auftritt, während diese beim 1.4404 im Austenit erfolgt. Durch einen zusätzlichen Prozessschritt der Wärmebehandlung konventionell fließgepresster Proben werden ebenfalls Zugeigenspannungen abgebaut. Allerdings zeigt die Kaltumformung mit aktiver Prozessführung ein höheres Potenzial zur Verbesserung des Eigenspannungszustands.