Evaluation of microstructure and toughness of SAW steel weld metals with optimum Al/O ratio

A study was made to understand the possible effect of the Al/O ratio on the Charpy V-notch impact properties of submerged arc weld metals within the optimum range of Al/O ratio previously reported to be 0.45–0.75. To vary the Al/O ratio of weld metals in such a narrow range, two base plates different in aluminum content were selected, and a bead-in-groove weld was made on each plate using the same welding condition. From the chemical compositions obtained, the Al/O ratio of each plate was determined to be 0.47 and 0.75. Charpy impact tests showed that the ductile–brittle transition temperature (DBTT) of a low Al/O ratio was about 25 °C lower than that of a high Al/O ratio. The improvement in toughness is concluded to be attributable to the microstructural refinement that can be quantified by two different parameters: AF content increased from 70 to 79% and grain size decreased from 8 to 6 μm. These results also indicated that even a slight variation in aluminum content of base plates could cause a marked variation in weld microstructure and toughness. In addition, the inclusions in the weld of low Al/O ratio were confirmed to be covered by the TiO layer, but no manganese-depleted zone (MDZ) was observed.