Study of phase transformations In API 5L X80 Steel in order to increase its fracture toughness

Abstract

Phase transformations in API 5L X80 steel were studied in different thermomechanical sequences with a view to increasing the fracture toughness of this steel. Dilatometry tests performed on the quenched steel detected a phase transformation occurred, during heating, in the temperature range 593-618 K. This phase transformation was identified as the dissolution of M-A islands. Based on preliminary dilatometric tests, ten thermal and thermomechanical treatments were performed on X80 steel samples. Initially, the material was hot rolled and quenched and only quenched. On the material without deformation, aging was also performed at temperatures of 603, 673, 723, 773, 823 and 873 K. These treatments resulted in the formation of the acicular ferrite constituent, among others. Tensile tests showed that the aging treatments produced reductions in yield strength and increases in the elongation and toughness of X80 steel. All the treatments resulted in an increase in the tensile strength of steel.

The Analysis of the Microstructure and Mechanical Properties of Low Carbon Microalloyed Steels after Ultra Fast Cooling

“…This is in rather good agreement to the other authors' findings 20 . As mentioned above, the size of secondary phases decreases with decreasing finish rolling temperature (Fig.…”

“…The AF nucleates at intragranular sites and it is characterized by an assemblage of interwoven non-parallel ferrite laths with high density of tangled dislocations, where the most parts of the neighboring lath (subgrain) boundaries have dissimilar orientations 19 . A microstructure of acicular ferrite has the potential of combining high strength and high toughness, because a crack would have to follow a more tortuous path through a microstructure of acicular ferrite 20 . As mentioned above, the structure of steel A presents a smaller amount of acicular ferrite than steel B.…”

“…It is well known that MA is mainly generated at rapid cooling rates and low finish cooling temperatures 16 . The volumetric fraction of M-A islands increases because of higher cooling rates 20 . The size of MA tended to decrease with increasing finish cooling temperature.…”

Effect of Rolling in the Recrystallization Temperature Region Associated with a Post-Heat Treatment on the Microstructure, Crystal Orientation, and Mechanical Properties of API 5L X70 Pipeline Steel

“…The development of the (112) [1] and (111) [10] texture components in the ACN and WQ700 samples improved the mechanical properties. -…”

“…These steels offer excellent mechanical properties, the highest safety in pipeline operation, and low costs [1][2][3] . Thermomechanical control processing (TMCP) is used industrially to produce high-strength low-alloy (HSLA) steels.…”