Tsung-Yuan Kuo

Effects of filler metal composition on joining properties of alloy 690 weldments

Abstract This work investigates the influence of filler metal composition on the corrosion resistance and mechanical properties of alloy 690 weldments. Alloy 690 (61 wt.% Ni, 30 wt.% Cr) was used as the base metal. Inconel I-52 (61 wt.% Ni, 29 wt.% Cr) and I-82 (73 wt.% Ni, 20 wt.% Cr) rods were used as filler metals. Manual gas tungsten arc welding was performed using four weld passes in three layers for a single-V groove butt weld. The subgrain structure near the centerline of the fusion zone middle layer was cellular and columnar dendritic in the I-52 weld, but was dominantly equiaxed dendritic in the I-82 weld. Both had white particles dispersed in the fusion zone. The I-82 weld had more white particles and a denser subgrain structure. Compositional analysis showed the I-52 welds interdendritic region had higher Al, Si, Ti, N content than the dendritic core. The I-82 welds interdendritic region had higher Al, Si, and Nb content than the dendritic core. The Ni and Cr content of interdendritic white particles of the I-52 and I-82 welds decreased to 38, 23 wt.% and 11, 9 wt.%, respectively, much lower than the base and filler metals. This situation causes high corrosion at the white particle sites during Modified Huey testing. Thus, the I-52 weld had better corrosive resistance than the I-82 weld. However, I-82s superior weld joint strength is attributed to its finer fusion zones subgrain structure and giving it higher tensile strength and elongation.

Evaluation of effects of niobium and manganese addition on nickel base weldments

Abstract The present work investigates the influence of different concentrations of Nb (from 0.1 to 3.35 wt-%) or Mn (from 0.78 to 3.32 wt-%) on the microstructure and mechanical properties of an Inconel 690 weldment. Welding electrodes are produced by coating Inconel filler metal 52 with a flux containing various percentages of Nb or Mn. Weldments with a bevel edge are butt welded via a manual shielded metal arc welding process, using identical parameters and procedures. The microstructure and mechanical properties of the resulting weldments are then analysed. The experimental results indicate that the subgrain structures of the welds are primarily dendritic. Under tensile testing, it is found that each specimen ruptures in the fusion zone and that the fracture surfaces exhibit entirely ductile features. It is noted that as the content of Nb increases, the welds tend to show a finer subgrain structure, i.e. having smaller dendritic spacing. Consequently, the tensile strength and microhardness of the fusion zone increase slightly and the tensile rupture mode changes from slant to flat fracture. It is determined that the interdendritic precipitates are mainly Nb rich eutectic type and Nb rich type constituents. The presence of these precipitates increases with higher concentrations of Nb in the flux and results in a significant decrease in the ductility of the weldment. Regarding the relative influence of the Mn additions, there appears to be no significant change in the subgrain structure as the percentage of Mn increases. However, the ductility tends to increase and it is also found that the tensile strength and microhardness of the fusion zone also increase slightly. Accordingly, the tensile rupture mode exhibits a slight tendency to change from flat to slant fracture. Although the interdendritic precipitates identified in the Mn series are similar to those in the Nb series, it should be noted that the precipitates appear in lower numbers and are smaller than their Nb counterparts.