Effects of Heat-Affected Zone Microstructure on Fracture Toughness of Two X70 Pipe Girth Welds
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SIGNIFICANT advances in the thermomechanical controlled process (TMCP) technology of steel pipe production have been made in recent decades. As a result, pipe’s strength levels have increased along with toughness by controlling steel chemistry and microstructure. Weld metal properties are also well controlled by developing suitable filler metals. However, heat-affected zone (HAZ) is exposed to high temperatures and variable thermal cycles produced by welding, and it can undergo (unanticipated) microstructural variation from the fusion boundary to the unaffected base metal, albeit invariant steel chemistry. Especially, on-site girth welding of pipelines is more difficult to acquire a high quality of weldment as it is difficult to achieve controlled conditions. Considering the transformation of microstructure in the HAZ, heterogeneous material properties across weldments (through weld metal, DONG-YEOB PARK, JEAN-PHILIPPE GRAVEL, and MUHAMMAD ARAFIN are with CanmetMATERIALS, Natural Resources Canada, 3303 - 33 Street N.W., Calgary, Alberta T2L 2A7, Canada. Contact e-mail: [email protected] BABAK SHALCHI AMIRKHIZ, RENATA ZAVADIL, and PEI LIU are with CanmetMATERIALS, Natural Resources Canada, 183 Longwood Road South, Hamilton, Ontario L8P0A5, Canada. YIYU WANG and LEIJUN LI are with the Department of Chemical and Materials Engineering, University of Alberta, 9107 - 116 Street, Edmonton, Alberta T6G 2V4, Canada. ANQIANG HE is with the University of Alberta nanoFAB, 9107 - 116 Street, Edmonton, Alberta T6G 2V4, Canada. Manuscript submitted September 19, 2016. Article published online April 14, 2017 3248—VOLUME 48A, JULY 2017
HAZ, and base metal) give rise to concerns in structural integrity, specifically from a fracture toughness perspective.[1,2] Due to the heterogeneous material property, a welded joint generally relies on the level of relative strength of base and weld metals.[3] This is the reason that an overmatched strength of weld metal relative to base meal is intentionally used in structural components where defects and flaws in the weld metal and HAZ are of concern under loads, as the overmatched weld could transfer plastic flow to the base metal.[4] On the other hand, an undermatched weld is used for high-strength steels where hydrogen-induced cold cracking is concerned.[5] In addition to the strength mismatch of base and weld metals, another mismatch can arise by softening in the HAZ region when exposed to welding thermal cycles, which is frequently observed in highstrength low-alloy steels.[6,7] A considerable loss in hardness and strength in the HAZ negatively affects the mechanical properties of the structure, regardless of the HAZ width.[8] The HAZ of high-strength low-alloy steels is generally composed of coarse-grained HAZ (CGHAZ) adjacent to the fusion line, followed by fine-grained/intercritical HAZ merging with the tempered base metal. The highest toughness is generally observed in the fine-grained/ intercritical region and the lowest toughness in the coarse-grained region.[9–11] The deterioration
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