Study of MA Effect on Yield Strength and Ductility of X80 Linepipe Steels Weld
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MARTENSITE-AUSTENITE (MA) islands usually consist of martensite and retained austenite which form in the Inter-Critical Heat Affected Zone (ICHAZ) and Inter Critical Coarse Grain HAZ (ICCGHAZ); regions which are heated to the inter-critical transformation temperatures between Ac1 and Ac3. The effect of MA on low temperature toughness has been often observed for years as an embrittling microconstituent.[1–4] It has been shown that differences in MA fraction, location and morphology can lead to significant variation in fracture or impact toughness.[4] However, few studies have considered the effect of MA on quasi-static strength and ductility.[3,5,6] This is of particular interest for strain based pipeline design applications, and particularly during pipeline construction where there is a potential for strain localization in the weld HAZ when the welded pipe is lowered below the ground during construction. To better understand how to control the fraction and morphology of MA, many studies have considered the
NAZMUL HUDA and ADRIAN P. GERLICH are with the Centre for Advanced Materials Joining (CAMJ), Department of Mechanical and Mechatronic Engineering, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada. Contact e-mail: [email protected] ROBERT LAZOR is with the TransCanada Pipelines Ltd, Calgary, AB T2P 5H1, Canada. Manuscript submitted January 18, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
influence of base metal microstructure and welding thermal cycle. Several researchers studied the effect of MA on tensile behavior. Yong et al. have shown by TEM that formation of MA in the grain boundary can change the direction of crack propagation by 90 deg in an ultra-fine acicular ferrite microstructure. They concluded that a combination of MA and low-angle grain boundaries could have similar effect on crack deflection as high-angle grain boundaries in terms of crack propagation during tensile tests.[7] Chen et al. concluded that yield strength depends on both dislocation movement barrier and crack deviation in MA islands.[8] Moreover, Pedrosa et al. reported a decrease in yield strength for X80 linepipe materials which was attributed to MA dissolution in the aging process.[9] Lanzillotto et al. reported an increase in all strength parameters contributed by the presence of MA in dual phase steel.[10] An increase in tensile strength attributed to MA has been reported by other researchers.[9,11] Han et al. reported that there could be an increase in ultimate tensile strength, with low transition temperatures with higher MA area fraction.[12] In contrast, Lambert et al. have used acoustic emission technique to show that MA is a crack initiation point during tensile tests.[3,13] Moreover, Davis and King. also pinpointed MA as the crack initiation point in high strength low alloy steel.[4] These reports all contradict Matsuda et al. who reported that MA does not have significant effect on strength.[14] In addition to the effect that MA has on tensile strength, researchers have also studied the f
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