Journal
METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE
Volume 52, Issue 9, Pages 3788-3798Publisher
SPRINGER
DOI: 10.1007/s11661-021-06339-w
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This study used atom probe tomography to measure elemental segregation of the bondline in ERW linepipe steel, identifying a new mechanism for carbon depletion. Post-weld heat treatment homogenized alloying elements across the bondline, while segregation of elements in the heat-affected zone followed NPLE kinetics for bainite transformation.
The bondline of electric-resistance-welded (ERW) linepipe steel, often etched white (i.e., ferrite) in optical microscopy, is generally believed to be carbon depleted. The mechanism for the carbon depletion, however, is not fully understood by researchers. To this end, atom probe tomography (APT) was used to measure elemental segregation of the as-welded and post-weld heat-treated bondline regions of X70 linepipe welds. The thin vertical features at the bondline in the as-welded condition were identified as carbon-rich martensite-austenite (M-A) constituents, and the majority ferrite phase in the bondline was identified as carbon-depleted ferrite. Following the post-weld normalization, all alloying elements, except Nb and Mo, are homogenized across the bondline and heat-affected zone. The carbon depletion in the ERW bondline was accurately measured. A new mechanism for carbon depletion has been proposed using Scheil calculations of elemental partitioning during weld formation. Segregation of elements in the heat-affected zone was shown to follow the negligible partitioning local equilibrium (NPLE) kinetics for bainite transformation.
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