The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Effect of Strain-Induced α'-Martensite Transformation on Mechanical Properties of Metastable Austenitic Stainless Steels
Juho Talonen
Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Mechanical Engineering for public examination and debate in Auditorium K216 at Helsinki University of Technology (Espoo, Finland) on the 1st of June, 2007, at 12 noon.
Dissertation in PDF format (ISBN 978-951-22-8780-2) [14614 KB]
Dissertation is also available in print (ISBN 978-951-22-8779-6)
Abstract
Metastable austenitic stainless steels undergo a strain-induced martensitic transformation, where the metastable austenite phase is transformed to the thermodynamically more stable α'-martensite phase due to plastic deformation. The strain-induced martensitic transformation enhances the work hardening of the metastable austenitic stainless steels, and affects their ductility. This thesis concentrated on the effects of the strain-induced martensitic transformation on the mechanical properties of the metastable austenitic stainless steels, focussing on the interaction between the strain-induced martensitic transformation and the work hardening. The effects of chemical composition, temperature and strain rate on the strain-induced martensitic transformation were studied.
The experiments were carried out on the steel grades EN 1.4318 (AISI 301LN) and EN 1.4301 (AISI 304). Mechanical testing was performed by means of uniaxial tensile tests at temperatures ranging between −40 and +80°C and at strain rates ranging between 3×10−4 and 200 s−1. The α'-martensite volume fractions were measured with a Ferritescope. X-ray diffraction was used for phase identification, dislocation density measurements and to measure the stacking fault energies of the test materials. Microstructure investigations were carried out by means of the scanning electron microscopy, transmission electron microscopy and optical metallography. Load distribution between the phases was studied by in-situ X-ray diffraction stress measurements.
The effects of applied stress and the stacking fault energy on the formation of the shear bands, acting as the nucleation sites for the α'-martensite, were demonstrated by using the model developed by Byun (2003). An excellent correlation between the theoretical predictions and the scanning electron microscopy findings was found. The suppression of the strain-induced α'-martensite transformation with increasing strain rate and temperature was attributed to the temperature dependence of the stacking fault energy. A direct relationship between the work-hardening rate and the rate of the α'-martensite transformation was found. The α'-martensite transformation was concluded to govern the uniform elongation by affecting the work-hardening rate. In the optimum condition the transformation effectively shifts the intersection of the stress-strain and work-hardening curves to higher strains. The higher was the transformation rate, the higher was the work-hardening rate. The dislocation density of the austenite phase was found to increase with increasing plastic strain and stress. Instead, the dislocation density of the α'-martensite was substantially higher and remained relatively constant. The work hardening sequence of the metastable steels was divided in four stages. During the first stage, the work-hardening rate decreased rapidly due to the dynamic softening effect caused by the strain-induced α'-martensite transformation. During the stage II, the work-hardening rate started to increase due to the dispersion hardening caused by the strain-induced α'-martensite. The dispersion hardening effect was analysed by means of quantitative optical metallography and the theory developed by Ashby (1971). At the onset of the stage III, the α'-martensite forms a percolating cluster extending through the whole body. This manifested itself by an abrupt change in the relations between the flow stress, α'-martensite volume fraction and dislocation density of the austenite. During the stage III the work-hardening rate continued to increase. The stage IV was related to the high α'-martensite volume fractions, where the α'-martensite became the matrix phase, and the work-hardening rate started to decrease.
Keywords: austenitic stainless steel, work hardening, strain-induced martensite, stacking fault energy
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© 2007 Helsinki University of Technology
Last update 2011-05-26