Influence of microstructural aspects on impact toughness of multi-pass submerged arc welded HSLA steel joints

Highlights: • Acceptable welding parameters were determined for welding HSLA steel. • The HAZ toughness with high heat input is lowered by coarse brittle microstructures. • Acicular ferrite is responsible for good WM toughness regardless of heat inputs. • The optimal range of inclusion size is determined about from 0.45 to 1 μm. • Martensite–austenite constituents with large size are proved to be potential initiation sites for cracking. Multi-pass submerged arc welding was performed on the HSLA steels using multi-microalloyed electrodes in the present work, and three different heat input processes were employed to investigate the microstructure evolution and corresponding mechanical properties of weldments. The emphasis was placed on studying the influence of microstructure aspects on impact toughness of weld metal and HAZ with different heat inputs to reveal fracture micromechanism and to optimize the welding system. According to the experimental results, the acceptable welding parameters were ascertained for the studied steel to obtain a good balance of high strength and toughness welded joint. The optimal range of size distribution of non-metallic inclusion formed in the weld metal was determined and the essential causes of deteriorated HAZ toughness were revealed.