Effect of Molybdenum on Ductility Dip Cracking in Nickel-Base Weld Filler Metal 52XL
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Date
2018-05
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The Ohio State University
Abstract
The nuclear industry has an increasing demand for thick cross-section walls from nickel-based alloys for structural and repair applications. Due to the high restraint and residual stresses developing in these welds, ductility dip cracking (DDC) is a major problem, especially when using high chromium nickel-based filler metals. Previous research has shown that alloying elements, such as niobium (Nb) and tantalum (Ta) decrease the susceptibility to DDC by the formation of eutectic carbides along the solidification grain boundaries. These carbides pin the grain boundaries in the weld metal, which results in tortuous grain boundaries causing a mechanical locking effect, so that DDC formation by grain boundary sliding is mitigated.
The objective of this undergraduate research is to determine the effect of molybdenum (Mo) on DDC in nickel based Ta-bearing FM 52XL. Mo is an alloying element in nickel-based alloys to increase high temperature strength. The influence of Mo on DDC is not well understood, and contradictory results have been reported. Microstructural characterization was done on weldability samples with different Mo content to evaluate the effect on eutectic formation, grain size, tortuosity, grain boundary precipitates and hardness. Image analysis on light optical micrographs (LOM) and scanning electron micrographs (SEM) was used to determine the amount of eutectic in the weld metal and the weld metal grain size as a function of the Mo content. Results with the LOM showed no conclusive correlation, but the SEM images showed an increased amount of eutectic with Mo additions. Grain size measurements have shown that the Mo bearing weld metal exhibits a finer grain structure. Tortuosity measurements show that the sample without Mo has the highest curvature but the lowest DDC resistance, this could indicate that there could be other factors that increase DDC resistance in FM 52XL. M23C6 precipitates were found to be continuous along the grain boundaries, and were noticed to be coarser in the variant without Mo. Hardness measurements determined that both Mo and Ta have a solid-solution strengthening effect in FM 52XL.
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Keywords
Molybdenum, Ductility dip cracking, High chromium nickel base alloy