Evolution of Microsegregation-Induced Precipitations and Bending Fracture Mechanisms of 9% Ni Steel Weldments Filled with Nickel-Based Alloys

Abstract

A combined experimental/modeling approach had been performed to investigate the evolution of microsegregation-induced precipitates and its effects on the crack initiation and propagation behavior of 9% Ni steel weldments filled with different kinds of nickel-based alloys. The study found that Nb was more inclined to segregate at the inter-dendritic region than Mo during the cooling of weld metal (WM), which facilitated the precipitation of the Laves phase for Ni-Cr-Mo-Nb weld metal (Ni-WM2), while the segregation of Mo resulted in the precipitation of the P phase for Ni-Cr-Mo weld metal (Ni-WM1). Despite the approximate widths over a range of 1 μm~3 μm for various precipitates, the Laves phase was almost twice as long as the P phase. Besides, the elongated Laves phase had sharp edges and a larger area fraction than the P phase. This caused Ni-WM2 with Laves phase to exhibit quite more deteriorative bending ductility than Ni-WM1 with the P phase. Then, the crack initiation and propagation behaviors were investigated based on the microstructure-based simulation and experimental observation. Further, the great disparity in fracture mechanisms of Ni-WM1 and Ni-WM2 induced by the bending test was revealed.

Keywords: microsegregation, Precipitation, Crack initiation and propagation, Bending ductility