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

Abstract

NUCu-140 is a ferritic copper precipitation-strengthened steel that is a candidate material for use in many naval and structural applications. Previous work has shown that the heat-affected zone (HAZ) and fusion zone (FZ) of NUCu-140 exhibit softening that is due to dissolution of the copper-rich precipitates. This study aims to recover the FZ and HAZ strength by re-precipitation of the copper-rich precipitates through either multiple weld passes or an isothermal post-weld heat treatment (PWHT). The potential use of multiple thermal cycles was investigated with HAZ simulations using a Gleeble thermo-mechanical simulator. The HAZ simulations represented two weld thermal cycles with different combinations of peak temperatures during the initial and secondary weld passes. To investigate the potential for a PWHT for strength recovery, gas tungsten arc weld samples were isothermally heated for various times and temperatures. Microhardness measurements revealed no strength recovery in the multipass HAZ samples. The time-dependent precipitate characteristics were modeled under the HAZ thermal cycle conditions, and the results showed that the lack of strength recovery could be attributed to insufficient time for re-precipitation during the secondary weld pass. Conversely, full strength recovery in the HAZ was observed in the isothermally heat treated samples. Atom probe tomography analysis correlated this strength recovery to re-precipitation of the copper-rich precipitates during the isothermal PWHT.

Effect of forced cooling after welding on CGHAZ mechanical properties of a martensitic steel

“…A peak temperature of 1350 °C was chosen to promote rapid austenite grain growth resulting in a CGHAZ with a low impact and fracture toughness. [18,19] The specimens were heated to 1350 ∘ C at 400 ∘ C/s where they were held for one second before rapid cooling at 50 or 15 ∘ C/s. These cooling rates correspond to t8/5 times of 6 and 20s.…”

Effect of forced cooling on the tensile properties and impact toughness of the coarse-grained heat-affected zone of a high-strength structural steel

“…A peak temperature of 1350 °C was chosen to promote rapid austenite grain growth resulting in a CGHAZ with a low impact and fracture toughness. [16,17] The test specimens used in the Gleeble tests were cut parallel to the longitudinal rolling direction of rolled plates with dimensions 10x5x55 mm for the Charpy V impact testing and 120xØ6 mm for the tensile testing.…”

Effects of welding and post-weld heat treatments on nanoscale precipitation and mechanical properties of an ultra-high strength steel hardened by NiAl and Cu nanoparticles

“…In particular, the precipitation of coherent nanoparticles, such as body-centered cubic (bcc) Cu-rich nanoclusters [2][3][4][5][9][10][11][12] and B2-ordered NiAl intermetallic nanoparticles [13][14][15][16][17], is very appealing, because they can precipitate on a sufficiently fine scale (less than 5 nm in diameter), providing an extremely high strengthening response. Currently, research is starting to emerge on the weldability of this class of high-strength steels, mostly of Cu-rich nanoclusters strengthened steels [18][19][20][21][22][23]. Previous welding studies showed that the hardness of heat-affected zone (HAZ) and fusion zone (FZ) of Cu-strengthened steels was undermatched to the base metal (BM), which was considered to be due to the partial dissolution of preexisting Cu-rich nanoclusters within the HAZ and full dissolution of nanoclusters within the FZ, as characterized by atom probe tomography (APT) [18][19][20][21].…”

“…Previous welding studies showed that the hardness of heat-affected zone (HAZ) and fusion zone (FZ) of Cu-strengthened steels was undermatched to the base metal (BM), which was considered to be due to the partial dissolution of preexisting Cu-rich nanoclusters within the HAZ and full dissolution of nanoclusters within the FZ, as characterized by atom probe tomography (APT) [18][19][20][21]. It has been found that the reprecipitation of nanoparticles can be achieved by designing an appropriate post-weld heat treatment (PWHT) or multi-pass welding procedure, which provides a promising way to recover the mechanical properties of welds [22,23].…”