Mechanical properties after thermomechanical processing of cryogenic high-strength materials for magnet application

Abstract

Due to the constant increase of requirements for structural materials used in superconducting magnets, e.g. for plasma confinement in fusion reactors, the present work deals with the possibilities of increasing the mechanical properties of austenitic stainless steel at cryogenic temperatures. Scope is to systematically develop a new industrial-feasible thermomechanical processing technology in high-nitrogen 316LN austenitic stainless steel to tailor mechanical properties for cryogenic application. Based on available processing maps and numerical simulations a medium-sized upsetting experiment is proposed to enhance the mechanical properties of conventionally processed high-nitrogen 316LN austenitic steel. The numerical simulation software DEFORM HT/3D using the finite element method, is used to predict the distribution of strain and temperature in the hot/cold processed material. All processing parameters are chosen with the industrial manufacturability in mind. The main object of interest is the microstructural, deformation behavior and mechanical properties of 316LN at cryogenic temperatures. Light microscopy and scanning electron microscopy with electron backscatter diffraction is used for microstructural characterization and the evaluation of the damage mechanism from the sample’s fractures. Deformation behavior is studied by tensile and fracture tests in a temperature interval between room temperature and 4.2 Kelvin.