An Evaluation Method for Laser Weld Metal Toughness by Side-Notched Charpy Test

Abstract

Ultra-fine grained (UFG) steel has been developed on a laboratory scale, which has both high strength and superior toughness. When welded, however, grain growth takes place at the heat-affected zone (HAZ), and consequently HAZ softening occurs. To avoid the detrimental effect of HAZ softening, laser welding is one of the promising methods. The hardness distribution of laser welded joints is so steep in a narrow region that fracture path deviation (FPD) is often observed in the standard Charpy V-notch (STD-Cv) test in and above the transition temperature region. Side-notch Charpy (SN-Cv) test is widely used to prevent FPD. The SN-Cv test gives a higher transition temperature and lower absorbed energy as compared with the STD-Cv test. Therefore, it is necessary to evaluate quantitatively the difference between the STD-Cv and the SN-Cv test. In this paper, STD-Cv and SN-Cv tests were carried out on laser weld metal for UFG steel and on base plates of conventional steels from mild steel to 800 MPa high strength steel. The fracture surface of Cv test was categorized into three types, that is, fibrous fracture at the notch tip and at the edge of the specimen, brittle fracture, and slant fracture at both sides. The area of each fracture surface was measured for all specimens tested and each unit fracture energy was determined by using the regression analysis method. Then, the difference in the formation of each fracture surface between STD-Cv and SN-Cv tests was discussed, and finally a method to estimate STD-Cv energy from the SN-Cv test results was proposed.