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Microstructure and mechanical properties of simulated weld heat affected zones in X80 linepipe steel Mandal, Madhumanti

Abstract

Low carbon micro-alloyed steels are used for linepipe applications as they provide a good combination of strength, toughness and weldability. An important part of the construction for long distance pipelines is the in-field joining of pipes. Owing to the complex thermal cycles during welding, the microstructures and mechanical properties of the base material are altered adjacent to the weld in the region known as the heat affected zone (HAZ), in particular, the regions, i) next to the fusion line (the coarse grain heat affected zone, CGHAZ) and ii) where the thermal fields from multi-pass welds overlap (for example: the intercritically reheated coarse grain heat affected zone, ICCGHAZ). Using a Gleeble thermomechanical simulator, bulk microstructures, that were representative of the thermal conditions found in the CGHAZ and ICCGHAZ regions, were produced in two high strength low alloy steels. It was found that the cooling rate after the first weld pass had a large effect on the microstructure produced relevant to the CGHAZ. The second thermal excursion involves intercritical annealing of the initial microstructures (relevant to the ICCGHAZ). This produced a nearly continuous necklace of martensite along the prior austenite grain boundaries. The effects of i) the different morphologies of bainite and ii) the intercritical austenite fraction (which transforms to martensite-austenite (M/A) constituents during cooling) on the tensile and Charpy impact properties were systematically studied. The fine bainite microstructures formed at a cooling rate of 50℃/s were found to have the highest density of high angle grain boundaries resulting in the best combination of strength and ductile-brittle transition temperature. Upon intercritical annealing, the ductile-brittle transition temperature was significantly increased when a nearly continuous necklace of M/A formed on the prior austenite grain boundaries (for M/A ≥ 10%). This work presents a systematic study on the effect of the fraction of M/A constituents on the tensile stress-strain response and the ductile-brittle transition behaviour. The conclusions from this work have the potential to provide guidelines to linepipe steel producers (in terms of chemistry) and pipeline constructors on the input of different welding parameters on the properties in the heat affected zone of the base pipe.

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