Microstructural Evolution and Mechanical Properties of Fusion Welds in an Iron-Copper-Based Multicomponent Steel

Investigation of Strength Recovery in Welds of NUCu-140 Steel Through Multipass Welding and Isothermal Post-Weld Heat Treatments

“…The initial peak temperatures were chosen based on dilatometry results presented by Farren et al (Figure 8 of Ref. [9]) to represent each region of the HAZ, where the Ac1 (start temperature of ferrite to austenite transformation during heating) and Ac3 (finish temperature of ferrite to austenite transformation during heating) temperatures for NUCu-140 steel were found to be 979 K and 1097 K (706°C and 824°C), respectively. [9] Farren et al have shown that there is no change in the precipitate characteristics or softening in the subcritical HAZ region.…”

“…Site-specific samples for atom probe tomography (APT) were prepared using dual-beam focused-ion beam (FIB) microscope (FEI Helios Nanolab, Hillsboro, OR, USA) by following a standard lift-out procedure, the details of which are described elsewhere. [9] APT experiments were done at a specimen base temperature of 60 K in ultrahigh vacuum (<10 À8 Pa), and the data obtained were analyzed using the program IVAS 3.6 (Cameca, Madison, WI). The mean radius, hRi, number density (N v ), and volume fraction (F) of Cu precipitates were determined using the envelope method, which is based on the maximum separation distance algorithm.…”

“…This surface energy manipulation has been shown to give good agreement between simulated and experimentally obtained precipitate characteristics for the BM and single weld pass thermal treatments. [9] The temporal evolution of the mean precipitate radius hRi, number density N V , and phase fraction u was first simulated for the BM thermal history as described above. These precipitate characteristics were then used as input for simulations for the initial weld HAZ thermal cycles followed by either another weld pass or isothermal PWHT.…”

“…[8] Research is starting to emerge on the weldability of this new class of Cu-precipitation-strengthened steels. A recent study by Farren et al [9] has provided insight into the welding behavior of NUCu-140. Local softening of the heat-affected zone (HAZ) was observed in NUCu-140 weldments.…”

“…However, there was no observation of coarsened Cu precipitates that are often associated with the overaged HAZ region that is typical of precipitation-strengthened alloys. [9] Precipitation-strengthened alloys typically exhibit local softening in the HAZ that is associated with precipitates that have either dissolved and/or coarsened during the weld thermal cycle. [10][11][12][13][14][15][16] The precipitates that have dissolved in the high-temperature region of the HAZ can be re-precipitated with a direct aging treatment after welding.…”