Microstructural characterisation of Tristelle 5183 (Fe-21%Cr-10%Ni-7.5%Nb-5%Si-2%C in wt%) alloy powder produced by gas atomisation

Nitrogen gas atomised powders of the hardfacing alloy Tristelle 5183 (Fe-21%Cr-10%Ni-7%Nb-5%Si-2%C in wt%) were sieved into different particle size ranges and their microstructures have been investigated. Powder particles larger than approximately 53 μm are composed of dendritic fcc γ-Fe as the principal phase with smaller quantities of: α-Fe, an interdendritic silicide phase isostructural to Fe5Ni3Si2, and Nb(C,N). Particles <53 μm have increasing quantities of either dendritic α-Fe or cellular silicide phase with decreasing amounts of γ-Fe as the particle size decreases, along with ~5% Nb(C,N). Coarse (> 10 μm) sized Nb(C,N) particles, that are seen in all powder size fractions, pre-existed in the melt prior to atomisation, whereas micron-sized Nb(C,N) particles that are found within α-Fe, γ-Fe or silicide are the primary solidification phase. Nanoscale Nb(C,N) also formed interdendritically in the last stages of solidification. Compared with a mould cast sample, a significant difference is the suppression of M7C3 formation in all powder size ranges. The increasing quantities of α-Fe and silicide in smaller sized powder particles is consistent with increased undercooling prior to nucleation permitting metastable phase formation.