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Pipes used in the construction of long-distance oil and gas pipelines often have to undergo hot-bending when the required bend radius is very small. The present work discusses the effect of the thermal cycling applied during the induction-heated bending process, on the microstructure and the mechanical properties of an API 5L X80 steel pipe fabricated from plates produced by controlled rolling, as well as the effects of post-bending heat treatments applied to the pipes. The curved pipe was evaluated at several positions; the straight section (not affected by the bending process), the bend area and the transition zones (between the straight and the curved regions). Microhardness measurements, tensile tests and Charpy V-notch impact tests were performed. After bending, significant differences were observed between the microstructures of the straight and curved sections, and consequently a reduction in yield stress to a value below that required by the industry standard. A significant reduction in impact transition temperature was also observed, indicating a toughness improvement. Water cooling, applied only to the external radius of the tube during bending, provoked further microstructural modification across the wall-thickness. All regions investigated were characterised by optical and scanning electron microscopy. The results shows that the 500oC tempering heat treatment after bending, promotes the recovery of the yield strength, to values above the minimum required by the standard, resulting in adequate properties for the tube submitted to the bending process.

AVALIAÇÃO DA MICROESTRUTURA E PROPRIEDADES MECÂNICAS DE TUBO API 5L X80 SUBMETIDO A CURVAMENTO POR INDUÇÃO