An Atom-Probe Tomographic Study of Arc Welds in a Multi-Component High-Strength Low-Alloy Steel
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HIGH-STRENGTH low-alloy (HSLA) steels represent an important class of alloys that are used for a variety of structural applications. Modern HSLA steels, such as HSLA-100, contain a low carbon concentration of 690 MPa (100 ksi), while maintaining ductility and fracture toughness.[1] Recently, several variations of HSLA steels have been developed at Northwestern University for direct substitution of existing HY60, HY100, and HSLA-100 steels used currently in naval applications.[2–12] These alloys, designated as NUCu-X, where X refers to the alloy yield strength in ksi, include NUCu-70, NUCu-140, and NUCu-170 steels. NUCu-70 is fabricated via hot rolling and the composition is designed to co-precipitate NbC and Cu-rich precipitates during air cooling. In addition to Nb and Cu, Ni is added to reduce hot-shortness cracking during hot rolling caused by insoluble Cu,[13] ALLEN H. HUNTER, Ph.D. Candidate, and DAVID N. SEIDMAN, Walter P. Murphy Professor, are with Northwestern University, Evanston, IL. Contact e-mail: [email protected] JEFFREY D. FARREN, Ph.D. Candidate, and JOHN N. DUPONT, Professor, are with Lehigh University, Bethlehem, PA. Manuscript submitted February 23, 2012. Article published online November 10, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
Mn is added for solid-solution strengthening and to getter S in MnS inclusions, and Si is added for additional solid-solution strengthening.[5–12] NUCu-70 is classified under ASTM Standard A710 as alloy ‘‘Grade B’’[14] and is currently utilized in bridge construction.[8] To increase the yield strength of NUCu-70 above 70 ksi, additional Ni, Al, and Nb are added to the composition to increase both the volume fraction of precipitates and the precipitation strengthening potential. The alloy, designated as NUCu-140, is solution treated and aged for 2 hours to achieve the required yield strength.[2–4] The Cu precipitation strengthening of the NUCu-X series of alloys has an additional benefit of increasing the low-temperature toughness compared to Cu precipitate-free alloys (e.g., HSLA-65). The Cu precipitates increase the dislocation activation volume by approximately a factor of three and subsequently reduce the temperature dependence of the flow stress, leading to improved toughness at low temperatures.[15] A third NUCu composition has an increased yield strength of 170 ksi, achieved by increasing the Cu concentration to 2.09 wt pct.[4,16–18] NUCu-170 displays, however, a significantly reduced toughness,
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