Effect of Textures and Microstructures on the Occurrence of Delamination during and after Fracture Toughness Tests of API X80 Steel Plates

Abstract

The API 5 L X80 is a high strength microalloyed steel, widely used in the gas and oil industry to fabricate pipelines. This steel presents a combination of elevated strength and toughness. In the present investigation, the microstructural features, fracture toughness and delamination occurrence of two X80 grade steel plates with different processing routes and chemical composition were studied. The first steel depicted a homogenous bainitic matrix and the second steel exhibited a banded microstructure composed of elongated ferrite grains, with macrosegregation in the mid-plane of the plates. Fracture toughness tests were conducted for both steels on 7-mm and 15-mm thick samples. The orientation distribution functions analysis revealed crystallographic intensity distribution of the austenite to ferrite transformation texture, especially the alpha-fiber (< 011 > || rolling direction) which explained the anisotropy and delamination occurrences. Both processed plates of steel presented the alpha-fiber due to hot-rolling of plates. Delaminations occurrences were further investigated and attributed to a strong {100} <011> orientation presence despite microstructure homogeneity. A schematic model was proposed, showing the source of delamination and the reason for the lowest toughness for 45° to the rolling direction.