I finished my Master Thesis August 2004 under the supervision of Arne Olsen and Rolf Benjamin Johansen (DNV).
Master Thesis: A TEM study of the structural changes in “supermartensitic stainless steel” during deformation and welding (PDF).
Summary: The crystal structure and morphology of a martensitic steel ( 12.5 wt % Cr, 0.02 wt % C , 5.9 wt % Ni and 2.1 wt % Mo) are studied by selected area diffraction with TEM, XRD and using the sensitive triangles created by the Kikuchi patterns. The cell dimension is 2:878 Å , with an accuracy of 0.004 Å . The crystal structure is BCC or BCT with a very small difference between the and c- axis. The sensitive triangles created by the Kikuchi patterns can be used to measure the tetragonality of the structure when the steel has a carbon content above 0.1 wt %. The steel examined in this thesis has a carbon content of 0.2 wt % therefore the method could not be used to measure the tetragonality of the crystal structure of this steel. Both the base metal and the weld metal contain retained austenite. During deformation of the steel, about 20% of the austenite was probably transformed to martensite. The dislocation density was measured on (S)TEM images using EELS to calculate the thickness of the sample. The thickness was measured with a 1 nm accuracy. The dislocation density dropped drastically when deforming the 3 deformation cycles. When deforming in opposite direction than the previous loading, the yield point decreases 26%, creating the Bauschinger effect. The effect was probably due to the increase in dislocation density, the amount of low angled boundaries and the retained austenite.
Publications, presentations and posters from my Master work:
- Annett Thøgersen, Rolf Benjamin Johansen, Przemyslav T. Zagierski, Anette E. Gunnæs og Arne Olsen. Extended abstract. Structural changes in Supermartensittic, Stainless Steel under deformation and welding, Proceedings SCANDEM 2003.