Influence of precipitate evolution on the impact fracture behavior in Nb-containing 16Cr–25Ni superaustenitic stainless steel weld metal

Abstract

In this work, the precipitate evolution and its influence on the impact fracture behavior of Nb-containing 16Cr–25Ni superaustenitic stainless steel weld metal were systematically studied via scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). Results show completely transgranular ductile fracture mode of as-welded condition, of which the M6C carbides and Nb(C, N) carbonitrides contributed to the crack initiation, but the interface between M6C carbide and matrix was more prone to crack under impact load. In contrast, due to the semi-continuous distributed M23C6 carbides on the grain boundary, an intergranular/transgranular mixed mode was observed in the post-weld heat treatment (PWHT) sample. Additionally, the crack sources in grain interior were not limited to primary precipitation phases, whereas secondary M2(C, N) carbonitrides and M23C6 carbides also acted as preferential sites for crack initiation. The massive M2(C, N)/matrix and M23C6/matrix interfaces led to easier crack propagation path simultaneously. Moreover, the deformation mechanism were governed by dislocation slip and twinning in the impact fracture of as-welded sample, but the mechanical twins disappeared in the fractured sample at PWHT condition, which was attributed to the local stress concentration in the matrix could not exceed the critical twinning stress. Therefore, the combination of secondary precipitates induced embrittlement and weakened plastic deformation capacity of matrix, leading to the impact toughness declined from 91.0 ± 3.6 J to 34.3 ± 4.6 J after PWHT.