Microstructure-property relationships of a novel metal-cored wire for welding of ultra-high strength steels
Research output: Thesis › Doctoral Thesis
Steel constructions for cranes or vehicles are frequently welded by gas metal arc welding. In energy efficient designs with a low component weight, thermomechanically processed ultra-high strength steels are a profound choice of material. Welding consumables for this type of steels are currently available up to a yield strength of min. 960 MPa. In the frame of this work the goal was to develop a new welding consumable with a minimum yield strength of 1100 MPa, while keeping the toughness at -20 °C above 47 J. All-weld metal samples with different alloying contents were produced for characterization of the mechanical properties and the microstructure. The objective of this thesis was especially to establish methods for an in-depth characterization of the microstructure and to correlate microstructural characteristics with the mechanical properties. A combination of light optical microscopy and electron backscatter diffraction (EBSD) was used to study the martensitic microstructure, and additionally atom probe tomography was employed for precipitate characterization. The all-weld metal is a multipass welding structure containing several sources of inhomogeneity, and the issue of sample preparation for locally restricted methods is addressed accordingly. With a combination of these methods, the effect of several alloying elements on the microstructure and the mechanical properties of the all-weld metal was studied. Particularly high effort was expended for analyzing the effects of microalloying elements on the weld metal. The investigations showed, that all microalloying elements acted negatively on the impact toughness. Titanium and vanadium formed (Ti,V)(C,N) clusters in the weld metal and strengthened it significantly. Because vanadium exhibited a moderate toughness loss compared to the strength increase, this microalloying element was recommended for strengthening the weld metal by V(C,N) cluster formation. This strengthening concept was combined with a toughening concept. The toughness of the material was improved by reducing the contents of carbon, silicon and manganese, which resulted in a refined martensitic microstructure with a higher amount of high angle grain boundaries, thus impeding crack propagation. Overall, the effects of changes in the alloying contents on the microstructure of the all-weld metal could be identified. The target values for yield strength and impact toughness were met. In the future, the service properties of the developed filler wire in welded joints will have to be evaluated.
|Translated title of the contribution||Mikrostruktur-Eigenschaftsbeziehungen eines neuartigen Metallpulverfülldrahts für das Schweißen von ultra-hochfesten Stählen|
|Publication status||Published - 2018|