Einfluss von Legierungselementen auf die Eigenschaften von niedriglegiertem, warmfestem CrMoV-Schweißgut

The aim of the present master thesis, which was conducted in cooperation with the company voestalpine Böhler Welding, was to examine the influence of the alloying elements Cr, Mo, V and C on the mechanical properties of a creep resistant bainitic 2,25Cr-1Mo-0,25V-weld metal and to define the respective correlations with the microstructure. The evaluation was carried out via a fully factorial 2^4-Design of Experiment (DoE) with two discrete levels for each element. The microstructure of the resulting 16 different welds, produced via stick electrode welding, was examined using light optical microscopy, a scanning electron microscope and a scanning transmission electron microscope and by doing electron backscatter diffraction and high energy X-ray diffraction utilizing synchrotron radiation. The findings of this thesis revealed a more acicular bainitic microstructure and a higher toughness with increasing Cr-content, as well as a higher strength but no influence on the creep behavior with increasing Mo-content. With increasing C-content, the number of carbides and carbonitrides (fine and coarse) rises, which is beneficial for the strength and the creep behavior, whereas the coarse/fine distribution of these carbides and carbonitrides is mainly influenced by the V-content. A higher amount of V leads to less coarse carbides and more fine MX-carbonitrides (M=metal, X=carbon or nitrogen), which are most beneficial for the creep behavior due to their stabilizing effect on the microstructure and dislocation substructure at high temperatures. According to these structure-property relationships, a high V-, medium C- and low Cr- and Mo-content is recommended for producing a weld metal with high creep resistance and balanced strength and toughness. Furthermore, this master thesis showed, that a DoE is a useful statistical tool for evaluating the influence of alloying elements on the mechanical properties and microstructure of a material.