STT and DTT Diagrams of Austenitic Cr–Mn–Ni As‐Cast Steels and Crucial Thermodynamic Aspects of γ → α′ Transformation

The mechanical behavior and microstructure evolution during deformation of novel austenitic Cr–Mn–Ni as‐cast steels with varied Ni content were investigated at various temperatures using static tensile tests, optical microscopy, and the magnetic scale for the detection of ferromagnetic phase fraction. To summarize all knowledge about the deformation‐induced processes, the STT and DTT diagrams were developed for Cr–Mn–Ni steels. The diagrams illustrate the different deformation mechanisms depending on temperature and tension load, and quantify the elongation of the deformation mechanisms. The deformation‐induced ε‐ and α' martensite formation and twinning – the TRIP and TWIP effects – occur in the Cr–Mn–Ni steels depending on the chemical composition and temperature. The differences of deformation‐induced processes depend on thermodynamics and are confirmed by thermodynamic calculations. The nucleation threshold of γ → α′ transformation was determined for the investigated Cr–Mn–Ni steels.     

Cyclic Deformation Behaviour of Three Austenitic Cast CrMnNi TRIP/TWIP Steels with Various Ni Content

This study presents the cyclic deformation behaviour of three high‐alloyed austenitic cast steels which are characterized by different chemical compositions leading to different austenite stabilities and stacking fault energies. Thus, depending on the chemical composition different deformation mechanisms arise which have a significant influence on the cyclic deformation behaviour and life time relations. The materials were characterized under total‐strain control. The fatigue life relations of Basquin and Manson‐Coffin are applied successfully for all steel variants. The cyclic stress‐strain response is described using the Ramberg‐Osgood relationship. It is shown that the parameters n' and K' depend strongly on the accumulated plastic strain λ_p. The mechanical properties are discussed together with microstructural investigations of deformation structures and martensitic transformations as well as twinning, respectively.