Study on microstructure-toughness relationship in heat affected zone of EQ70 steel by laser-arc hybrid welding

Highlights: • Toughness reduction occurred when the notches traversed CGHAZ and ICCGHAZ. • Grain boundaries above 45° can notably block micro-crack propagation. • Block width was found to be the effective grain size controlling toughness. • The increase of block width together with the necklace grain boundary M-A constituents both contribute to the toughness reduction. The microstructures and toughness of the laser-arc hybrid welded joint of EQ70 steel were investigated by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron back-scattered diffraction (EBSD), micro-shear test and Charpy impact test. The micro-shear toughness reduction and impact toughness reduction both occurred when the micro-zones traversed the coarse grained HAZ (CGHAZ) and inter-critically reheated coarse grained HAZ (ICCGHAZ). The CGHAZ were composed of lath martensite (LM), granular bainite (GB) and polygonal ferrite (PF). Necklace M-A constituents were distributed at the ICCGHAZ grain boundaries. The All-Euler maps showed that high angle boundaries (HABs) above 45°can notably block the micro-cracks propagation. The block width was the effective grain size controlling the toughness. The increase of block width in CGHAZ and ICCGHAZ, together with the necklace grain boundary M-A constituents, both contribute to the toughness reduction.