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

Abstract

This study investigates the correlation between the microstructure and the corrosion resistance properties of the fusion zone of Alloy 690-SUS 304L stainless steel dissimilar weldments formed by electron beam welding (EBW). The effects of the EBW process are evaluated by comparing the microstructure and corrosion resistance properties of the EBW weldment with those of Alloy 690-SUS 304L weldment formed by gas tungsten arc welding (GTAW). The experimental results reveal that the interdendritic region of the fusion zone of the EBW weldment contains fine TiN precipitates and Cr-Ni rich phases. The TiN precipitates are originated from the Alloy 690 base metal, while the Cr-Ni rich phases, a new formation of precipitates, is precipitated in the region around TiN during solidification. Microscopic analysis of the samples following a modified Huey test indicates that the matrix around TiN precipitate and the Cr-Ni rich phase precipitate provide the preferred sites for corrosion pit initiation. Due to the rapid cooling in the EBW process, relatively fewer and smaller TiN precipitates and Cr-Ni rich phases are formed in the weldment. Consequently, only limited corrosive pitting is observed which indicates better interdendritic corrosion resistance properties in comparison to joints with GTAW process. Furthermore, rapid solidification in the fusion zone results not only the suppression of chromium carbide precipitation but also the chromium depletion at the grain boundaries. As a result, the intergranular corrosion resistance and interdendritic corrosion resistance of the EBW weldment are significantly higher than that of the GTAW weldment.

Numerical and experimental investigation into effect of temperature field on sensitization of Alloy 690 butt welds fabricated by gas tungsten arc welding and laser beam welding

“…Since Alloy 690 has lower carbon content than Alloy 600, the sensitization effect is particularly pronounced at temperatures in the range 540-760 • C. Therefore, in improving the corrosion resistance of welded Alloy 690 components, it is essential to develop a detailed understanding of the thermal welding cycles within the weldment in order to predict the susceptibility of the weldment to the sensitization phenomenon, although the time required in the sensitization temperature range to cause the sensitization in Alloy 690 has not yet been determined. While the published literature contains many experimental investigations into the microstructure and corrosion resistance properties of Alloy 690 (Kuo and Lee, 2002;Lee et al, 2007), relatively few studies have performed a systematic numerical investigation into the effects of the temperature field on the sensitization of Alloy 690 butt welds such as those used in many nuclear power plant components. Accordingly, the present study performs a series of ANSYS simulations based upon a moving heat source model and the physical property data maintained in the JMatPro database to compare and contrast the welding thermal cycles induced in butt welds fabricated using the GTAW method and the LBW method, respectively, and to investigate the effects of these thermal cycles on the sensitization tendencies of the two weldments.…”

Predicting Effect of Temperature Field on Sensitization of Alloy 690 Weldments

“…17,18) Therefore, in improving the corrosion resistance of welded Alloy 690 components, it is essential to develop a detailed understanding of the thermal welding cycles within the weldment in order to predict the susceptibility of the weldment to the sensitization phenomenon. While the published literature contains many experimental investigations into the micro-structure and corrosion resistance properties of Alloy 690, 4,[17][18][19][20] relatively studies 21,22) have performed numerical investigations of the effects of the temperature field on the sensitization of Alloy 690 butt welds. There is a clear need for more systematic and accurate investigation of these welds, which are used in many nuclear power plant components.…”