Characterization of HAZ of API X70 Microalloyed Steel Welded by Cold-Wire Tandem Submerged Arc Welding

Significance Statement

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.

Characterization of HAZ of API X70 Microalloyed Steel Welded by Cold-Wire Tandem Submerged Arc Welding. Advances in Engineering

About The Author

Dr. Mohsen Mohammadijoo

Mohsen received his PhD in Materials Engineering from the University of Alberta, Canada in 2017. His expertise lies in physical and mechanical metallurgy of welding, fracture mechanics, welding engineering, additive manufacturing, metallurgical processes design and optimization, materials characterization and synthesis of nanostructured materials.

In his latest research achievement, he designed and developed a novel welding technique for HSLA steel pipe manufacturing through a research work conducted at the University of Alberta in collaboration with R&D Evraz Inc. North America for which he received two best paper awards. He is a member of American Welding Society and the program chair of Canadian Welding Association-Edmonton Chapter.

About The Author

Dr. Laurie Collins

Laurie Collins is currently the Director, Research & Product Development, at Evraz North America.  He is a graduate of Queen’s University in Kingston, Ontario and completed his PhD at MIT in Cambridge Massachusetts.  He joined Evraz in 1995 after spending 14 years at Materials Technology Laboratories of CANMET in Ottawa.

In addition to his current position, Laurie has held the positions of Superintendent, Rolling and Finishing in the Regina Steel Mill and Works Manager for Regina Tubular Operations.  He has been a member of ASM since 1981 and was the ASM Canada Council Lecturer in 1998.

About The Author

Professor Hani Henein

After completing the MEng at McGill University (1975) and a PhD at UBC (1981) in Canada, Hani took up a faculty appointment at Carnegie-Mellon University, Pittsburgh, PA.  In 1989, he moved to the University of Alberta actively teaching and doing research on pipeline steels, metal-matrix composites and rapid solidification.  He partners with industry in research and has extensive international collaborations.  Amongst the distinctions, he has received five best paper awards, the prestigious Killam Research Fellowship, and the Metals Chemistry Award.

He has been inducted Fellow of ASM International, CIM and the Canadian Academy of Engineers.  In service, Hani plays a leadership role in the profession as the 1998 MetSoc President, the 2014 President of the Minerals, Metals and Materials Society (TMS) and presently the President-Elect Designate of the American Institute of Mining, Metallurgical and Petroleum Engineers (AIME).

About The Author

Professor Douglas G. Ivey

Douglas Ivey is a professor in the Department of Chemical and Materials Engineering at the University of Alberta (Canada). He received his Ph.D. in Engineering Materials from the University of Windsor (Canada) in 1985.

His research focuses on applying high resolution microstructural characterization techniques to understanding the relationships between materials structure, properties and processing. Materials studied include microalloyed steels, semiconductor metallizations, hydrogen storage materials, materials for electrochemical energy storage (e.g., solid oxide fuel cells, supercapacitors and rechargeable batteries) and minerals in oil sands.

Reference

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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