Microstructural investigation on phase transformation during flow forming of the metastable austenite AISI 304Donnerstag (17.09.2020) 15:00 - 15:20 Uhr Metallographie
Flow forming is a metal spinning process that has gained recently relevance in industries like automotive, due to the manufacturing flexibility, resource optimization and the possibility to produce near-net-shape components . The use of austenitic stainless steel in combination with advanced manufacturing techniques makes the production of high-performance products possible. During forming processes not only the geometrical characteristics of the material but also its microstructure is transformed, modifying the mechanical properties. New investigation trends try to develop control systems during forming processes to obtain components with defined geometries and at the same time with specific mechanical properties. The formulation of material models that correlate parameters measured during the manufacturing process with the properties, plays a crucial role in the development of such control systems.
This investigation focuses on a characterization of the microstructure evolution of the metastable austenitic steel AISI 304L, specifically regarding the phase transformation from austenite into alpha’-martensite during forming processes. Therefore, specimens were produced by flow forming of tubes at room temperature, using different forming steps and tool feeds, to get thickness reductions between 0.25 and 1 mm. A combination of non-destructive micromagnetic measurements, hardness tests and microscopic investigations, including electron diffraction microanalyses, have been carried out to describe the material behavior and the microstructure evolution. The electron backscatter diffraction technique shows qualitative and quantitative results of the phase transformation, that can be correlated with micromagnetic parameters like magnetic Barkhausen noise and mechanical properties like hardness. The correlations between measurements, properties, and process parameters allow the formulation of a material-process model, that can be used in control systems. This intelligent-controlled flow forming process enables manufacturers to adjust geometries and properties, to produce high-added-value components.