Abstract
Rotary swaging is an incremental cold forming process that changes beneath the geometry also the microstructure and mechanical properties of workpiece. Especially a new process design with Eccentric Flat Shaped Dies (EFSD) influences both the kind and amount of stress and plastic strain and consequently the material structure and hence the material and workpiece properties. Eccentric rotary swaging typically provides a helical material flow. According to the process parameters the microstructure features a typical eddy pattern with a spiral shaped grain orientation. The forming process can be carried out in one or more process steps. In a multi-stage process, it is possible to change the feed direction and, hence, the material flow helix direction. This approach can be used as a possibility to improve the homogeneity of the workpiece and material properties. In addition, for this aims an intermediate heat treatment in multi-stage forming operations could be realised. Following the goal of optimising the final properties, the question arises how these mechanical and thermal treatments affect the material microstructure and the forming properties of the workpiece and how they interact. Experiments were conducted with austenitic stainless steel rods of grade AISI304. The effects of the varied feed direction, feed velocity and heat treatment between the forming operations are discussed.
Highlights
Metastable austenitic steels are characterized by an elevated phase hardening due to strain-induced transformation of austenite into martensite especially by multi stage processing
An improvement of material characteristics and a better formability of parts are typical for Ultra Fine Grained (UFG) materials that are characterized by good strength in combination with sufficient ductility [1, 2]
The application of the eccentric flat swaging dies generated a significant reduction of the number and the depth of cracks on the surface of the specimens in comparison with eccentric process without Eccentric Flat Shaped Dies (EFSD) [7]
Summary
Metastable austenitic steels are characterized by an elevated phase hardening due to strain-induced transformation of austenite into martensite especially by multi stage processing. I.e. decrease of the strain and increase of the ultimate tensile strength more than twice compared to the initial values, could be achieved by cold forming processes like rotary swaging [3]. In this process the workpiece is fed axially into the swaging head with a feed velocity vf. The variation of the process parameter during rotary swaging leads to changing of the mechanical properties and of the swaged parts [4] It can be used at preparation of semi-finished products with both a calibrated diameter as well as predefines material properties of the parts. To modify the microstructure of swaged rods with small diameter about
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