Forming characteristics of austenitic stainless steel sheet alloys under warm hydroforming conditions

Stainless steel sheet alloys have been increasingly used in heating, ventilating, and air conditioning; appliance; sanitary and medical devices; as well as several structural and transportation applications, due to their high strength-to-weight ratio, corrosion resistance, biomedical compatibility, and esthetic appearance. Among various stainless steel alloys, austenitic stainless steels are the most commonly used type. Due to the forming limitations into complex shapes at room temperature conditions and stress-corrosion cracking issues, forming at elevated temperature has been considered as an enabling technique. Formability of stainless steels is affected by strain rate and temperature due to martensitic transformation of meta-stable austenite microstructure. In this study, deformation characteristics of three common austenitic stainless steels (AISI 201, 301, and 304) were investigated using closed-die hydroforming under different process conditions. Specifically, effects of pressure, temperature, and die/part geometry on the material forming characteristics quantified using cavity filling ratio and thinning distribution were investigated. Results suggested that, in terms of cavity filling, pressure and material grade are significant factors while temperature is not in the case of axisymmetric part. For the non-axisymmetric case, all parameters were found to be significant. In addition, finite element (FE) modeling was performed to simulate several forming cases that were experimentally conducted in this study. The FE model was simulated based on material flow curves obtained from previous studies under similar conditions (strain rate and temperature) by the authors. FEA results were shown to be in good agreement with experimental findings, particularly for cavity filling and part profile predictions. Hence, the FE model and the material models can be used for further predictions of complex and different parts with confidence.