Robert Skorupski

Influence of Surface Morphology on the Fatigue Behavior of Metastable Austenitic Steel

Using a low temperature turning process with carbon dioxide cooling in the cutting zone a variation of the morphology at the specimen surfaces of the metastable austenitic steel AISI 347 was realized. In LCF and HCF fatigue tests at ambient temperature and 300 °C the influence of the surface morphology on the cyclic deformation behavior and fatigue life was investigated by the measurement of stress-strain hysteresis. An additional magnetic measurement allows the characterization of the phase transformation from paramagnetic austenite in ferromagnetic α´-martensite during the turning processes and during cyclic loading. The surface morphology was studied in detail by SEM and x-ray investigations.

Influence of Morphology of Deformation Induced α´-Martensite on Stress-Strain Response in a Two Phase Austenitic-Martensitic-Steel

In this research work specimens of the metastable austenitic steels AISI 304 and AISI 347 with one phase (fully austenitic) and two phase (austenitic-α ́-martensitic) microstructure were monotonically loaded at ambient temperature. Using stress-strain and temperature measurements the deformation behavior was characterized in detail. To study the influence of morphology of deformation induced α ́-martensite on the stress-strain response a phase field model for α ́-martensite transformations was developed. With this approach it was possible to model the two phase austenite-α ́-martensite microstructure and investigate the deformation behavior on the micro level. With optical microscopy, magnetic and x-ray measurements the microstructure characterization of fully austenitic and austenitic-α ́-martensitic steels was realized.

Influence of Mechanical Loading, Temperature and Chemical Composition on the Deformation Induced Martensite Formation in Metastable Austenitic Steels

The deformation induced α´-martensite formation in the metastable austenitic steels AISI 304, AISI 321 and AISI 348 was investigated in tensile and low cycle fatigue tests at ambient and low temperature. By means of stress strain and magnetic measurements, the mechanical behavior and phase transformation were characterized. The susceptibility of deformation induced α´-martensite formation depends on the chemical composition, the temperature and the degree of cumulated plastic strain. On the basis of comprehensive experimental data a mathematical model was developed to describe and predict the α´-martensite formation under cyclic loading in the temperature range -60 °C to 25 °C. The influence of test temperature and austenite stability of the model parameters was studied.