Aging-Induced Microstructural and Property Changes in Direct Laser Energy Deposited NiCrFeSiBC Hardfacing Alloy Bushes

Abstract Wear resistant NiCrFeSiBC hardfacing alloy bushes for fast breeder reactor applications are fabricated through direct laser energy deposition (DLED) using CO 2 laser with optimized parameters for laser power (2.5 kW), scan speed (4.2 mm/s), powder feed rate (4 g/min) and 60% of track to track overlap. DLED bushes have significantly different microstructure than weld deposited ones. As-deposited bushes have predominantly uniform fine dendritic solidification structure of γ-Ni + Ni 3 B, γ-Ni + Ni 3 B + Cr 3 C 2 and Ni-B-Si eutectics with low volume fractions of Cr-rich borides and carbides. Microstructural heterogeneity is observed only in layer overlap regions characterized by coarsening of carbides that resulted in lower hardness (625 ± 11 HV0.1) compared to layer interiors (740 ± 19 HV0.1). Effect of aging at 550 °C on the microstructure and properties of the bushes is investigated using experimental and thermo-kinetic simulation techniques. Microstructure of the bushes remains stable without significant coarsening of the γ-Ni + Ni 3 B eutectic structures even after heat treatment for 4000 h. However, precipitation and coarsening behavior of Cr 7 C 3 and Cr 23 C 6 carbides are affected by aging which is reflected in the variation in hardness. Hardness of the bushes increases up to 100 h of aging due to the precipitation of fine Cr-rich carbides, and with increase in the duration of aging, carbides coarsen thereby reducing the hardness. Based on the study, it is concluded that direct laser energy deposited bushes may have better stability with respect to microstructure and properties at service temperatures.