Simulation and experimental investigation of strain rate impact on martensitic transformation in 304L steel through dome
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The present study aims to investigate the effect of strain rate on volume fraction of martensite (VFM) in 304L austenitic stainless steel through dome test. Findings of this study show that martensitic transformation affects the deformation mode and formability of this material. Three strain rates are applied by controlling forming punch speed to conduct the stretching experiments and VFM is measured using magnetic saturation method. Results of the present study reveal that maximum VFM of 56% forms at the pole at low strain rates, whereas VFM of 47% is seen at high strain rates, and transforms in the region close to the flange. It is noted that blanks show good formability under low strain rate and poor formability at high strain rates. The relationship between VFM and strain rate, strain, percentage of shell reduction, equivalent of stress, and triaxiality are discussed. The experimental procedure is simulated and predicted findings show good agreement with the experimental results.
I. INTRODUCTION
There has, in recent years, been much interest in the development of austenitic stainless steel with face centered cubic (FCC) structure, in particular 304 stainless steel, for various applications such as pressure vessel, pipes for transferring corrosion liquids, high temperature application, because of their good mechanical properties, corrosion resistance, good process-ability, nonmagnetic properties, and favorable combination of strength and formability.1–5 Low yield and fracture strength at room temperature are the main disadvantages of stainless steel.5 Both strength and ductility can be improved by introducing martensitic transformation to the material, because such transformation enhances its plasticity.6 This can be brought about by subjecting the material to large strain. Martensitic transformation in steels with unstable or metastable austenite phase leads to increase strain hardening during the forming process. This phenomenon causes an increase in the ductility and strength of the material, which is known as the transformation induced plasticity (TRIP) effect.7,8 There have, in recent years, been many attempts to produce modern material such as unstable austenitic steel due to promise of enhancement of their mechanical properties through transformation.9 While it is recognized that phase transformations in many materials can critically influence material performance, such transformations must be carefully controlled to avoid complications during operation.10 Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.251
Martensitic transformation is controlled by nucleation mechanism. Some basic studies were reported about the mechanism and kinetic of martensite nucleation by G.B. Olson and M. Cohen, in 1972 and 1975. They found that new sites or embryos, which are included in strain induced-nucleation, are generated during plastic deformation. Strain induced-nucleation plays an important role in the martensitic transformation. ά-emb
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