Characterization and formation mechanism of periodic solidification defects in deep-penetration laser welding of NiCrMoV steel with heavy section

With the thickness increasing of steel, application of laser beam welding on heavy section has been arousing broadly interests. Deep-penetration laser welding was performed on thick plate of NiCrMoV steel, and the characteristics of welding defects were analyzed in terms of their formation and distribution. The periodic defects were found to occur frequently at the central longitudinal section of the bead-on-plate welds that have a certain distance from fusion line. The formation and distribution of these defects were deemed as unique for deep-penetration laser welding on heavy plate due to mismatch of welding parameters with weld profile. Defect susceptibility was raised firstly and reduced subsequently as the heat input decreased, and a large amount of periodic solidification defects should occur when penetration-to-width ratio (aspect ratio) is higher than 4.0. By means of detail characterization on defect by optical microscopy (OM), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS), the periodic defects were proved to be hot cracking defects induced by contact and rapid shrinkage of transverse columnar crystals in mid part of the welds before feeding in sufficient liquid. The formation of periodic solidification defects in welds was greatly related to weld profile and cooling rate limited by the heat dissipation conditions. The optimized parameters to avoid periodic solidification defects were determined for deep-penetration laser welding of this steel.