Fracture Toughness of NUCu-140 Simulated Heat Affected Zones

NUCu-140 is a copper-precipitation strengthened steel that is a candidate material for many new structural applications, including naval ship hulls. It has a simple chemistry and is manufactured using basic techniques. Before this new alloy can be fully utilized, its mechanical behavior following welding must be determined. Previous research has been conducted by Farren et al.1 to correlate the microstructure and evolution of copper precipitates in the heat affected zone (HAZ) to the mechanical behavior seen in those locations. This research builds on that previous work by extending the study to include fracture toughness and fractography of the base metal along with the four major regions of the HAZ. Microhardness and grain size measurements were also performed. Results show that a reduction in hardness, increase in fracture toughness, and failure mode of microvoid coalescence is seen from the base metal up to and including the fully recrystallized HAZ, followed by an increase in hardness, decrease in fracture toughness, and regions of cleavage failure in the coarse grained HAZ (CGHAZ). These observations are primarily a result of the evolution of the copper precipitates in those regions, with a contribution from the overall microstructure. Grain size does not change significantly, except in the CGHAZ where a 6 fold increase was seen.