Fracture Toughness Evaluation of Laser Beam-Welded Joints of 780 MPa-Strength Class Steel

Abstract

For fracture assessment of steel structures with laser beam-welded joints, it is significantly important to evaluate the fracture toughness of the weld metal. However, the fracture toughness of weld metal is often impossible to measure by standard Charpy impact testing in the brittle-to-ductile transition or upper-shelf temperature region, because of fracture path deviation (FPD) from the weld metal due to narrow weld bead and a high degree of overmatching in strength. In this work, evaluated fracture toughness of laser beam-welded joints of 780 MPa-strength class (HT780) steel is examined by Charpy impact testing and CTOD fracture toughness testing. The improved Charpy impact tests, using the specimen with three parallel welds or the side-groove specimen, are conducted in order to prevent FPD. The applicability and significance of the improved impact test methods to the laser beam-welded joints of high-strength steel are investigated by means of Weibull stress analysis. The Weibull stress analysis indicates that the 3-weld method could not necessarily prevent FPD in laser beam-welded joints of HT780 steel, since side beads are not as hard as main weld beads, due to reheating at the main weld. On the other hand, a side-groove specimen is effective for avoiding FPD, but intrinsic VE of the weld metal could not necessarily be obtained by using this specimen.