Investigation of Strength Recovery in Welds of NUCu-140 Steel Through Multipass Welding and Isothermal Post-Weld Heat Tr
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INTRODUCTION
COPPER precipitation-strengthened materials such as high-strength low-alloy (HSLA) 80 and 100 have been used extensively in naval and structural applications due to their excellent combination of strength and toughness. The ever increasing need to minimize cost has made it desirable to develop a less expensive HSLA variant that can achieve high-strength levels (‡825 MPa (120 ksi)), while maintaining suitable toughness.[1] Recent research at Northwestern University[1–6] has led to the development of NUCu-140 as a candidate material that achieves strength levels in excess of 825 MPa while retaining toughness levels that are similar to HSLA-80 and HSLA-100 alloys.[7] Similar to HSLA-100 steels, NUCu-140 also has a low-C content but does not contain Cr and Mo. This results in its lower JASON T. BONO, Research Assistant, and JOHN N. DUPONT, R.D. Stout Distinguished Professor, are with the Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015. Contact e-mail: [email protected] DIVYA JAIN, Graduate Student, is with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208. SUNG-IL BAIK, Postdoctoral Research Associate, and DAVID N. SEIDMAN, Walter P. Murphy Professor, are with the Department of Materials Science and Engineering, Northwestern University, and also with the Northwestern University Center for Atom-Probe Tomography (NUCAPT), Northwestern University, Evanston, IL 60208. Manuscript submitted March 23, 2015. Article published online August 21, 2015 5158—VOLUME 46A, NOVEMBER 2015
hardenability and a ferritic microstructure that is not as susceptible to hydrogen-induced cracking during welding as its martensitic counterparts. The absence of Cr also eliminates harmful Cr6+ vapors during welding, providing safety to the welder.[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. The fusion zone (FZ) and high-temperature HAZ region exhibited hardness equal to that of the base metal (BM). Microstructural characterization by atom probe tomography determined that the softening of the HAZ was due to dissolution of the Cu-rich precipitates during the welding thermal cycle. The lack of softening in the FZ and high-temperature HAZ region was attributed to the formation of the acicular ferritic microstructure that was observed in these regions. 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.[10–16] The precipitates that have dissolved in the high-temperature region of the HAZ can be re-precipitated
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