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

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 micro-segregations 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. •Carbon transport occurs between 350 °C and 450 °C in 18MND5/Alloy 52 welds.•Micro-segregations in steel and temperature affect carbon peak value in the weld.•Hardness in the weld increases with carbon content.•Hardness increase is owed to nano-precipitation of M23C6 chromium-rich carbides.•Thermo-kinetic modelling supports the observed trends.