Characterization of HAZ of API X70 Microalloyed Steel Welded by Cold-Wire Tandem Submerged Arc Welding
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TANDEM submerged arc welding (TSAW), i.e., submerged arc welding with two to five electrodes,[1–3] has been extensively utilized to fabricate high-strength low-alloy steel pipelines, pressure vessels, structures, and wind turbine towers.[4,5] TSAW offers several advantages over other welding processes, such as high deposition rate, deep penetration, high-quality welds and the ability to weld thick plates due to its high-heat input.[6,7] The fracture toughness of the heat-affected zone (HAZ), particularly the coarse-grained heataffected zone (CGHAZ), of microalloyed steel welds tends to weaken due to the high-heat input and thermal cycles that the steel experiences during welding. The reduction in toughness in the CGHAZ is attributed to the formation of large prior austenite grains (PAGs) and MOHSEN MOHAMMADIJOO, HANI HENEIN, and DOUGLAS G. IVEY are with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, T6G 1H9, Canada. Contact e-mail: [email protected] STEPHEN KENNY is with the Department of Chemical and Materials Engineering, University of Alberta, and also with the R&D Division, Evraz Inc. NA, P.O. Box 1670, Regina, SK, S4P 3C7, Canada. LAURIE COLLINS is with the R&D Division, Evraz Inc. NA. Manuscript submitted August 8, 2016. Article published online March 10, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A
martensite-austenite (M-A) constituents, which are characterized as localized brittle zones (LBZ), as a result of the high peak temperature and relatively fast cooling rate in the CGHAZ.[8–12] Davis et al.[13,14] and Reichert et al.[15] found that the formation of a network of enlarged M-A constituents along the PAG boundaries resulted in cleavage crack initiation in the HAZ. Moeinifar et al.[16] suggested that a reduced fraction of M-A constituents in the CGHAZ was beneficial to the impact toughness. However, the fraction and size of M-A constituents are essentially dependent on the PAG size. Yu et al.[10] and Li et al.[11] showed that the fraction and shape of M-A constituents were increased by coarsening the PAG size in the CGHAZ. They found that a coarse PAG size, associated with coarse M-A constituents, is the dominant factor in promoting brittle fracture in the CGHAZ. Gharibshahiyan et al.[17] reported that the formation of coarser PAGs in the CGHAZ has a detrimental effect on the toughness of the HAZ. Yang and Bhadeshia[18] and Garcia-Junceda et al.[19] showed that the martensite start temperature (Ms) increases with an increase in the PAG size, which results in a higher volume fraction of martensite. In the present work, cold-wire TSAW (CWTSAW) is developed to improve the microstructure and the mechanical properties of the HAZ of an X70 microalloyed steel weld, while retaining appropriate weld VOLUME 48A, MAY 2017—2247
geometry. The additional cold wire fed into the tandem weld pool essentially increases the deposition rate, resulting in better welding productivity for the welding process without increasing the heat input compared with the TSAW process.[20–22
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