Strength mis-match effects on fracture behavior of structural steel welds

For the welding of many structural metallic materials, laser beam (LB) offers many advantages over traditional welding processes. The advantages include a low heat input, narrow bead width and heat affected zone (HAZ), low distortion, welding in an exact and reproducible manner at high speed. However, the mechanical testing methods of laser welded similar and dissimilar joints and the response of conventional materials to this new process have not yet been fully established. Laser welded joints are not yet commercially used in Turkey. The use of such joints are in the experimental phase. The aims of this project are to gain more information on the heterogeneity of the welded region by means of experimental procedures, following the fracture mechanics methods, to help establish the mechanical and metallurgical properties of laser welded joints from the fracture mechanics aspects and also help to set-up a standard test procedure method for laser welded joints. It is strongly believed by many researchers that laser welding which offers possibilities for automotization and robotization for welding to be carried out in areas of difficult access in severe operating conditions, among its many other advantages will be one of the most widely accepted welding techniques worldwide in near future. This study was undertaken to investigate t'ıe microstructural evaluation occurring during welding and to determine the mechanical properties and fracture behavior of laser welded similar and dissimilar ferritic and austenitic steel joints. A complete information on the tensile and fracture toughness properties of the fusion zone is essential for pre-qualification and understanding of the joint performance in service as well as for conducting the defect assessment procedure for such very narrow weld joints. An experimental investigation on the mechanical properties of laser weld joints were carried out to establish a testing procedure using flat microtensile specimens for determination of the tensile properties of the weld metal and heatIll affected zone (HAZ) of the laser beam welds. These results are compared to those obtained with standard transverse tensile and round tensile tests. To accomplish this, three similar and two dissimilar butt-joints were produced by CO2 laser, using 6 mm thick steel plates. The mechanical properties of laser welded specimens have been further examined by extensive microhardnçss measurements and microstructural evaluation has been carried out on the joints. The CTOD (Crack Tip Opening Displacement) and CMOD (Crack Mouth Opening Displacement) tests were performed at Room Temperature and -40 ° C, to determine the fracture toughness of similar and dissimilar LB welds and the variation of the fracture behavior with temperature by using three point bend specimens. The effect of mechanical heterogeneity (mis-matching) at the vicinity of the crack tip on the CTOD fracture toughness values was also discussed. This study is part of a joint project between Dokuz Eylül University and GKSS Research Center, Germany on the research on LB welds carried out at GKSS Research Center. It is financially supported by TÜBİTAK, Turkey and International Bureau Jülich, Germany.