Strain induced martensite in incremental forming - Formation, effect and control

Stainless steels are increasingly today applied in industrial use. The metastable structure of austenitic stainless steels enables strain induced martensite formation during plastic deformation. Thus, in order to effectively apply these steels in incremental sheet forming (ISF), it is essential to know their α'-martensite transformation tendency in the process. For the four different austenitic stainless steels in the present study, the transformation was found to be very sensitive to the applied process parameters. The martensite formation was more profound with the unstable grades, however, with external heating the martensite formation could be diminished. By optimizing the ISF process, the amount of transformed martensite can be controlled and products with exceptional property combinations can be produced. The novelty of the present paper is to, first, provide information on the influence of strain induced martensite on the incremental forming process and product properties. In addition, based on the observations, propose means to control the transformation. Furthermore, the paper establishes that ISF favours a moderate rate of martensite transformation for extreme formability.