Microstructure and mechanical characterization of high strength low alloy steel welded joint by hybrid laser arc welding

Abstract

Hybrid laser arc welding was developed to joint EQ70 steels. Welds with no visible defects were obtained. Investigations on microstructure and mechanical properties showed that: the welded metal (WM) consisted of acicular ferrite (AF), granular bainite (GB) and M-A constituents. The arc zone and laser zone possessed semblable HAZ microstructures. Coarse grained heat affected zone (CGHAZ) and fine grained HAZ (FGHAZ) were both composed of lath martensite (LM), GB and polygonal ferrite (PF). The intercritically HAZ (ICHAZ) consisted of martensite and M-A constituents. The microstructure of the subcritically HAZ (SCHAZ) was tempered sorbite. Partly cracked blocky M-A constituents were found at the grain boundary of intercritically reheated CGHAZ (ICCGHAZ). Some M-A constituents was slender and distributed in the grain of ICCGHAZ. The arc zone and laser zone had similar hardness and micro-shear strength distributions. The highest hardness and micro-shear strength of the backing and double-pass welded joints appeared near CGHAZs, reheated WM and ICCGHAZ. Slightly soften showed up at the SCHAZs. SCHAZ, CGHAZ and ICCGHAZ showed a reduction of micro-shear toughness. -40°C sub-size Charpy impact testing showed that FL specimens possessed the lowest impact values. The coarse CGHAZs and cracked blocky M-A constituents of ICCGHAZ may be the reason caused the toughness deterioration.