Fracture Toughness of Simulated Heat Affected Zones in NUCu-140 Steel

NUCu-140 is a copper-precipitation strengthened steel that is a candidate material for many new structural applications, including naval ship hulls.  Research has recently been completed to correlate the microstructure and evolution of copper precipitates in the heat affected zone (HAZ) to the changes in hardness and tensile properties.  This research builds on that previous work to include fracture toughness, impact toughness, and fractography of the base metal and four major regions of the HAZ.  Microhardness and grain size measurements were also performed.  The sub-critical, inter-critical, and fully recrystallized HAZ regions exhibited a reduction in hardness and increases in fracture and impact toughness relative to the base metal.  The base metal and these HAZ regions all exhibited a fracture mode of microvoid coalescence.  The coarse grain HAZ exhibited a slight increase in hardness and a mixed fracture mode of cleavage/microvoid coalescence, but the toughness was comparable to that of the base metal. Furthermore, there was an overall increase in impact toughness in the longitudinal-transverse (LT) orientation as compared to that of the transverse-longitudinal (TL) orientation.  The grain size in the HAZ did not change significantly from the base metal, except in the coarse grain region where a six fold increase was observed.  The increased toughness of the sub-critical, inter-critical, and fully recrystallized regions was attributed to softening associated with changes in the copper precipitates during welding.  The sub-critical HAZ exhibited partial dissolution of copper precipitates during the welding thermal cycle, whereas the inter-critical and fully recrystallized heat affected zones experienced complete dissolution of the copper precipitates followed by varying amounts of re-precipitation upon cooling.  Full precipitate dissolution occurred in the coarse grain HAZ.  Differences in impact toughness observed between the LT and TL orientations were due to anisotropy from the rolling procedure, and the anisotropy persisted throughout the HAZ samples.