Characterization of complex carbide–silicide precipitates in a Ni–Cr–Mo–Fe–Si alloy modified by welding

•Ni-based alloy with Cr, Mo, Si, Fe and C was welded, examined with SEM, EBSD, and TEM.•Original Ni2(Mo,Cr)4(Si,C) carbides changed from equiaxed to lamellar shape in HAZ.•Composition and crystal structure remained almost unchanged in HAZ.•Original carbides changed to lamellar Ni3(Mo,Cr)3(Si,C) in some cases in weld metal.•Precipitates were mostly incoherent, but semi-coherent in some cases in weld metal.

Nickel based alloys of the type Hastelloy-N™ are ideal candidate materials for molten salt reactors, as well as for applications such as pressure vessels, due to their excellent resistance to creep, oxidation and corrosion. In this work, the authors have attempted to understand the effects of welding on the morphology, chemistry and crystal structure of the precipitates in the heat affected zone (HAZ) and the weld zone of a Ni–Cr–Mo–Fe–Si alloy similar to Hastelloy-N™ in composition, by using characterization techniques such as scanning and transmission electron microscopy. Two plates of a Ni–Cr–Mo–Fe–Si alloy GH-3535 were welded together using a TiG welding process without filler material to achieve a joint with a curved molten zone with dendritic structure. It is evident that the primary precipitates have melted in the HAZ and re-solidified in a eutectic-like morphology, with a chemistry and crystal structure only slightly different from the pre-existing precipitates, while the surrounding matrix grains remained unmelted, except for the zones immediately adjacent to the precipitates. In the molten zone, the primary precipitates were fully melted and dissolved in the matrix, and there was enrichment of Mo and Si in the dendrite boundaries after solidification, and re-precipitation of the complex carbides/silicides at some grain boundaries and triple points. The nature of the precipitates in the molten zone varied according to the local chemical composition.

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