Microstructure and Hardness Profiles of Hybrid Laser-Arc Welded Joint for UltrahighStrength Steel.

In this investigation, hybrid laser-arc welding was performed on 6-mm thick ultrahigh-strength quenched and tempered steel  plates. Laser-arc welds were characterized for hardness and metallurgical properties in three defined cooling cycles. Destructive  tests of the joints revealed that the joints were characterized by tensile strength similar to that of the base material. Microstructural  characterization was made by optical microscopy, scanning electron microscopy and X-ray diffraction. It was revealed that the  weld beads consist of lath martensitic structure with small percentage of retained austenite. Quantitative analysis of mixed mi crostructures of the heat affected zone (HAZ) revealed 0%, 38% and 62% of the upper bainite for the cooling cycles- cycles A,  B and C respectively. In addition, the hardness profile of the cross-section of the weld was characterized by Vicker’s microhard ness test. The hardness fluctuation values within the HAZ for the cycles A, B and C are 450-510, 370-425 and 320-375 HV  respectively, thus increasing as the upper bainite percentage increases with each cycle.