Microstructural and mechanical investigation of the near fusion boundary region in thermally aged 18MND5 / alloy 52 narrow-gap dissimilar metal weld

Abstract

Detailed characterization of the near fusion-boundary region of 18MND5 / Alloy 52 dissimilar metal weld joints was performed to investigate the effect of thermal aging on the microstructure and mechanical properties and to quantify the diffusion of carbon into the weld metal. It was shown that the microstructural features of dissimilar metal welds, such as the size of the partially mixed zone in the weld and the adjacency to microsegregations in the low-alloy steel have an effect on the peak carbon concentration in the enriched region of the weld. Aging conditions (time and temperature) influence the position of this concentration peak in the weld. Those trends were supported by thermodynamic calculations.

Multi-technique study of precipitation in the near-fusion boundary region of an aged narrow-gap dissimilar metal weld

“…Wang et al [15] characterized the crystal structures of Cr23C6 carbides and alloy 52M weld metal and found that both precipitate and matrix have FCC crystal structure, and that the lattice constant of Cr23C6 is 3 times the size of that of alloy 52M, in agreement with prior investigations on the crystal structure of Cr23C6 [16]. The presence of chromium-rich carbide precipitates isostructural to Cr23C6 was qualitatively demonstrated in the present DMW [2].…”

“…Accordingly, many authors have reported an increase in the micro-hardness in weld metal next to the fusion boundary [5][6][7][8][9][10], which is related to this carbon migration. The magnitude of carbon transport during aging in service conditions was investigated in the present weld and was shown to be strongly correlated to changes in the local mechanical properties [2].…”

“…This migration of carbon leads to a number of microstructural changes that can be summarized as follow. On the LAS side, carbides dissolve and a carbon-depleted zone forms in the region adjacent to the fusion boundary, over a few hundreds of microns [1,2]. In the nickel-based weld metal, the low diffusivity of carbon and its precipitation in Cr-rich carbides [3,4] results in its accumulation close to the fusion boundary over only a few tens of microns.…”

“…Due to the pronounced effect of carbide precipitation on mechanical properties, it is desirable to understand the kinetics of the different phenomena involved in the precipitation sequence in nuclear power plant DMWs. The various factors that govern the long range carbon transfer during thermal aging have already been examined [2]. The aim of this work is now to investigate the effect of thermal aging on the formation and growth of Cr-rich precipitates in the fusion boundary region between 18MND5 low alloy steel and Inconel 52 Ni-based weld metal.…”

“…The aim of this work is now to investigate the effect of thermal aging on the formation and growth of Cr-rich precipitates in the fusion boundary region between 18MND5 low alloy steel and Inconel 52 Ni-based weld metal. However, given the characteristic size of the precipitates with respect to the DMW itself [2], it is challenging to obtain quantitative and representative measurements that are critical to kinetic studies. In order to maximize the relevance and the reliability of the measurements, the present study relies on small angle X-ray scattering (SAXS) in correlation with atom probe tomography (APT) to combine the large-probe, averaged information of the former with the near-atomic resolution of the latter.…”