The intercritical heat treatment effect on the mechanical properties of the API 5L X65 steel, microalloyed with Ti and Nb, widely used in welded structures, was investigated. The embrittlement, observed by several authors in the intercritical region of the heat affected zone of welds performed in HSLA steels, is closely related to the volume fraction of M-A constituent, formed in the cooling. Just upon the homogenization and normalizing heat treatments, the specimens were submitted to different intercritical heat treatments, performed at 780 °C for 30 minutes and cooled with various Δt8/5 rates, with the purpose of varying the microstructure. High fracture toughness was obtained with the lower cooling rate. Also, it was verified that the low M-A constituent volume fraction was not enough to promote reduction on the fracture toughness.

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