| Summary: | Advances in steel production processes in the last two decades have enabled the production of novel materials with improved strength, weldability and usability. Many industries are implementing these novel materials into production, primarily to benefit from the higher strength-to-weight ratio. Improved material properties are especially important for industries engaged in advanced structural engineering and applications such as construction plants, piping systems of nuclear power plants, and products in the automotive and aeronautical sectors. Another trend in modern manufacturing is increased use of dissimilar metals welding, and welding of dissimilar steel grades is becoming common in the economically critical energy sector. Dissimilar welding of high-strength steels is especially advantageous for regions with extreme weather conditions, such as sub-Saharan Africa and Arctic regions. When dissimilar joints for high strength steel (HSS) are welded, gas metal arc welding (GMAW) is extensively used because of its adaptability and controllability (control of input and output) and well-established production quality. The most important issue in dissimilar welding of HSS is control of the thermal cycle, as these steels have rather narrow process parameter windows and are prone to softening in the heat-affected zone (HAZ). This thesis addresses the issue of improving the weld quality of dissimilar material welds through improved understanding of the relationship between the welding parameters and resulting microstructure determining the mechanical properties of the joint. The materials used in this thesis belong to the classes of high strength steels, ultra-highstrength steels (UHSS) with a static strength of 690 to 960 MPa, steels manufactured by a thermo-mechanically controlled process (TMCP) and quenched and tempered (QT) steels. A specific objective is to define favourable heat input values that can improve the quality of dissimilar joints of S690QT-TMCP and S700MC-S960QC. The main difficulty in welding of ...
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