Determination of mechanical and fracture properties of laser beam welded steel joints

Abstract

Laser beam (LB) welding is increasingly being used in welding of structural steels. The thermal cycles associated with laser beam welding are generally much faster than those involved in conventional arc welding processes, leading to a rather small weld zone that usually exhibits a high hardness for C-Mn structural steels due to the formation of martensite. It is rather difficult to determine the tensile properties of a laser weld joint area due to the small size of the fusion zone. Complete information on the tensile and fracture toughness properties of the fusion zone is essential for prequalification and a complete understanding of the joint performance in service, as well as for conducting the defect assessment procedure for such weld joints. Therefore, an experimental investigation on the mechanical properties of laser welded joints using flat microtensile specimens (0.5 mm thick, 2 mm wide was carried out to establish a testing procedure to determine the tensile properties of the weld metal and heat-affected zone (HAZ) of the laser beam welds. In the present work, two similar joints, namely, ferritic-ferritic and austenitic-austenitic and one dissimilar ferritic-austenitic joint were produced with a CO 2 laser using 6-mm-thick steel plates. In addition to the testing of flat microtensile specimens, the mechanical properties were examined by microhardness survey and conventional transverse and round tensile specimens. The results of the microtensile specimens were compared with standard round tensile specimens, and this clearly showed the suitability of the microtensile specimen technique for such joints. The crack tip opening displacement (CTOD) tests were also performed to determine the fracture toughness of the LB welds using three-point bend specimens. The effect of strength heterogeneity (mismatchingj across the weld joint and at the vicinity of the crack tip on the CTOD fracture toughness values was also discussed.