The microstructure and fracture behavior of the dissimilar alloy 690-SUS 304L joint with various Nb addition

Abstract

This study investigates the microstructure and fracture behavior of dissimilar weldments of alloy 690 and SUS 304L for various additions of niobium (0.1, 1.03, 2.49, and 3.35 wt pct) in the flux. With identical parameters and procedures, weldments were butt welded by the shielding metal arc welding (SMAW) process using three layers, with each layer being deposited in a single pass. The results indicate that the microstructure of the fusion zone was primarily dendritic and that the contents of Ni, Cr, and Fe within this zone remain relatively constant and resemble alloy 690. With Nb addition, it is noted that the microstructure changes from a cellular to columnar dendrite and equiaxed dendrite. Meanwhile, the dendrite arm spacing reduces and the secondary arms grow longer. Moreover, the composition of the interdendritic phase, whose precipitate volume percentage increases from 5 to 25 pct, changes from Al-Ti-O to Nb rich. The spread of the interdendritic phase is less in the root bead than in the cap bead due to the greater influence of base metal dilution in this region. Mechanical tests indicate that Nb addition increases the average hardness of the weldment and reduces its elongation prior to rupture. However, the tensile strength is essentially unchanged by Nb addition. It is found that the average hardness of the root bead is generally lower than the cap bead, and that the tensile specimens all rupture in the fusion zone, with the fracture surfaces exhibiting ductile features. It is noted that the cap bead tends to rupture interdendritically with increasing Nb addition. Finally, fractography shows that the dimples in the root become larger and shallower with Nb addition and are rich with an interdendritic phase.

Characterization of Microstructure and Mechanical Properties of Inconel 625 and AISI 304 Dissimilar Weldments

“…The maximum hardness at the weld interface and in the weld zone could also be attributed due to the formation of equiaxed dendrite due to the higher amounts of Nb. This is also well matching with the results of Lee et al 14) such that the Nb addition enhanced the hardness of the fusion zone. It is also well proven from the tensile studies that the fracture occurred at the parent metal of AISI 304 giving an indication that the weld region was found to be stronger than the parent metals.…”

“…This is exactly matching with the results of this work in such a way that on employing PCGTA welding, the hot cracking tendency could be totally avoided due to the lower segregation of Mo at the weld region. In the similar manner, the chromium carbide precipitation effect at the weld interface of Inconel 625 has not been observed on employing the PCGTA welding and using appropriate filler metal which is also in agreement with other researchers 14,15) It is evident from the SEM/EDAX analysis at the HAZ of AISI 304 that there is a region of the base metal that was heated to below the liquidus temperature but above the solidus temperature, so it was only partially melted. This zone is known as the partially melted zone.…”

“…As reported by Lee et al,14) the increased content of Nb results in a denser spacing of the dendrite arms normally result in the weld hardness. The authors further reported that the non-uniform distribution of Nb throughout the fusion zone would be due to the compositional difference between the alloy 690 and SUS 304L using different filler wires and may also be influenced by the variation in constitutional super-cooling on employing SMAW.…”

Dissimilar welding of nickel-based Alloy 690 to SUS 304L with Ti addition

“…Previously, Lee et al [25] stated that a higher Nb addition results in larger inter-dendritic phases, which prompt the weldment to rupture within the inter-dendritic region. The results of the present study suggest that an appropriate amount of Ti addition may cause a finer microstructure and smaller inter-dendritic phases, which increases the elongation of the weldment rather than lowering its strength under the conditions of high heat input.…”

Precipitated Phases and Corrosion Behavior in the Dissimilar Alloy 690-SUS 304L Joints Formed by EBW and GTAW

“…It has been reported that the addition of suitable quantities of niobium and titanium to the filler metal refines the grain size, and hence improves the mechanical strength of the weldment. [12][13][14][15] However, due to the Nb-rich and Ti-rich eutectic-like structure of the fusion zone and the precipitation of chromium carbide at the grain boundaries, the welding process and method mentioned above are not able to prevent hot cracking completely or improve corrosion resistance effectively. In addition, many of the components used in nuclear power plants are fabricated from thick plates, the number of welding passes required to accomplish the joint is often very high.…”