Deformation and martensitic phase transformation in stainless steels

Sammanfattning: The use of high-energy synchrotron x-rays which has enabled three- dimensional structural characterization from the nano- to the macroscopic scale, and now to the meso-scale, such as individual grains and dislocation structures, is a major scientific advance. This is the first technique with sufficient spatial resolution and penetration power to probe the local structure embedded deep within the material. This, together with good time resolution makes it suitable for investigations of e.g. phase transformations kinetics, stress-strain behaviour, and texture evolution in stainless steels. The micromechanical response of a metastable austenitic stainless steel and a duplex stainless steel during loading has been investigated by a series of high-energy x-ray diffraction experiments at the Advanced Photon Source (APS) in Argonne, IL, USA. Different measurement scales of the steels are tested, ranging from the behaviour of individual grains up to the macroscopic material behaviour. The experimental data is used as input to material models to validate and improve existing models for strain-induced martensite and mechanical properties. The x-ray investigations have revealed that autocatalytic Ü-martensite transformation is triggered by strains induced by the transformation itself in 301 and this was evidenced as bursts of Ü-martensite transformation during tensile loading. To the author's knowledge this behaviour has not been previously reported, and is of significant importance for the mechanical properties of the metastable stainless steels, since it provides strong local hardening and increases the time to neck formation. The å-martensite formation was investigated for 45 individual austenite bulk grains in 301 and the resolved shear stress was determined. Out of the 45 austenite grains probed one was observed to form å-martensite. The grain that formed å-martensite had the highest Schmid factor for the active slip system during fcc to hcp transformation. The behaviour of 301 during tensile loading at different strain rates was also investigated and it was concluded that even moderate strain rates produce adiabatic heating sufficient to suppress the martensite formation. The strain-induced martensitic transformation and the stress-strain behaviour was predicted by an extended Olson-Cohen model, finite element simulations for the temperature evolution and a radial return algorithm for the stress-strain behaviour. The measured and modelled results were in fair agreement. In addition, the phase specific stresses were measured during the experiments and these were in good agreement with the predicted results from the finite element model. Thus, it was concluded that the employed iso- work principle was a good assumption for the stress distribution between the phases. One way of tailoring the metastable austenitic stainless steels' microstructure with different phase fractions and deformation structures is by the reverse transformation from martensite to austenite. This was investigated for the cold rolled 301 steel, and the reverse transformation was observed to occur via two different mechanisms, one diffusion controlled and the other a diffusionless transformation. The onset of the diffusion reversion was about 450°C and the shear reversion became active at higher temperatures. The microstructure of shear reversed austenite consists of highly faulted austenite with an inherited lath like structure. The stress response of 15 individual austenite and ferrite grains deeply embedded in the bulk of a duplex stainless steel was measured during tensile loading. These results showed large intergranular stresses acting between grains due to grain interaction. The large intergranular stresses will have a significant effect on the two-phase behaviour during loading.