Tandem submerged arc welding (TSAW) is widely used in the fabrication of high-strength low alloy steel pipelines, pressure vessels, wind turbine towers, and structures. TSAW has several advantages over other welding procedures including high deposition rates, high quality welds, deep penetration, and the capacity to weld thick plates due to its high heat input. However, fracture toughness of the weld heat affected zone, specifically the coarse-grained heat affected zone, is often weakened due to the high heat input and thermal cycles that the steel experiences in the course of welding.
The drop in toughness of the coarse-grained heat affected zone can be traced to the formation of large prior austenite grains as well as martensite-austenite constituents. These are characterized as localized brittle zones that result from the high peak temperatures and fast cooling rates in the coarse-grained heat affected zone. Previous studies have indicated that the formation of a network of enlarged martensite-austenite constituents along the prior austenite grain boundaries leads to cleavage crack initiation in the heat affected zone.
A cold-wire tandem submerged arc welding (CWTSAW) process was developed through research work carried out by Dr. Mohsen Mohammadijoo under the guidance of Professor Douglas Ivey and Professor Hani Henein from the University of Alberta in Canada, in collaboration with Dr. Laurie Collins and his group at Evraz Inc. North America, in a bid to enhance the microstructure as well as the mechanical properties of the heat affected zone of an X70 microalloyed steel (pipeline steel with a yield strength of 70 ksi or 480 MPa) weld while retaining suitable weld geometry. The inclusion of the cold wire in the tandem weld pool increased the deposition rate, leading to better welding productivity for the entire process without increasing the heat input relative to conventional tandem submerged arc welding. Their research work is published in Metallurgical and Materials Transactions.
The research team evaluated the influence of cold-wire addition to tandem submerged arc welding through characterization of the macrostructure, microstructure alterations and mechanical properties in the heat affected zone of a microalloyed steel by varying the feed rate of the cold wire in the CWTSAW process. The authors then performed Charpy V-notch impact testing as well as microhardness testing in a bid to investigate and correlate the property changes with microstructure alterations in the heat affected zone of the prepared weld samples.
The incorporation of a cold wire in tandem submerged arc welding moderated the heat input by consuming some of the excess energy of the trail electrode. This reduced the amount of heat introduced to the weldment. Therefore, better quality welds were realized at lower heat inputs per mass of deposited material and with a considerable drop in the arcing time, which led to the formation of a smaller and shorter weld pool.
Cold wire inclusion led to a reduction in the prior austenite grain size in the coarse grained heat affected zone. In their study, the research team through microstructure analysis also observed that the fraction of martensite-austenite constituents was reduced and their size, shape and distribution were affected when a cold wire was included in the TSAW process. A cold wire addition rate of 25.4 cm/min provided the optimal change in microstructure with a corresponding enhancement of the fracture toughness of the heat affected zone because of a decrease in the welding heat input and the prior austenite grain size.
Mohsen Mohammadijoo, Stephen Kenny, Laurie Collins, Hani Henein, and Douglas G. Ivey. Characterization of HAZ of API X70 Microalloyed Steel Welded by Cold-Wire Tandem Submerged Arc Welding. Metallurgical and Materials Transactions 2248—VOLUME 48A, MAY 2017.
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