Microstructure Evolution of Heat-Affected Zone in Submerged Arc Welding and Laser Hybrid Welding of 690 MPa High Strength Steel and its Relationship with Ductile-Brittle Transition Temperature

The comparative study of submerged arc welding (SAW) and laser hybrid welding (LHW) was carried out for a 690 MPa high strength steel with thickness of 20 mm. Microstructure and ductile-brittle transition temperature (DBTT) evolution in welded zone were elucidated from the aspect of crystallographic structure, particularly, digitization and visualization of 24 variants. The impact toughness of each micro zone in LHW joint is better than that of SAW, in which the DBTT of equivalent fusion line and heat-affected zone (HAZ) can reach - 70 and - 80 & DEG;C, while that of SAW is only - 50 & DEG;C. LHW technology induces narrowing of the HAZ and refining of the microstructure obtained in weld metal and HAZ. Meanwhile, the austenite grain size and transformation driving force in the coarse grained heat-affected zone (CGHAZ) are reduced and increased, respectively. It makes variant selection mechanism occurring in CGHAZ of LHW dominate by close-packed plane grouping, which promotes lath bainite formation with high density of high angle grain boundary, especially block boundary dominated by V1/V2 pair. While for SAW, the lower transformation driving force inferred from the large amount of retained austenite in CGHAZ induces Bain grouping of variants, and thus triggers the brittle crack propagating straightly in granular bainite, resulting in lower impact toughness and higher DBTT.