Microstructure and mechanical properties of two Api steels for iron ore pipelines

Abstract

This research compares the mechanical behavior of two API steels (X60 and X70) used in the longest pipeline in the world for the conveyance of iron ore. Tensile tests, Charpy impact tests, CTOD tests and fatigue crack growth tests are performed at ambient temperature. Metallographic examination showed a banded microstructure consisting of polygonal ferrite and pearlite in both steels, with smaller grain size and the presence of a small quantity of bainite in the X70 steel. All the mechanical tests revealed a ductile behavior for the two steels. The X70 steel is preferable for the pipeline project, due its better mechanical resistance, with no significant loss of fracture toughness and fatigue resistance. Its performance could be even better, if an appropriate combination of thermomechanical processing parameters were able to produce a microstructure with minor amount of pearlite, where acicular ferrite/bainite are present.

Fusion-bonded epoxy composite coatings on chemically functionalized API steel surfaces for potential deep-water petroleum exploration

“…The white regions in the OM results are associated with perlite, which is hard and brittle, and the dark areas are the soft and ductile ferrite [20]. This ferrite-perlite microstructure is in agreement with the low carbon content of the sample, as determined by chemical analysis (0.13 wt%, Table 3) [21]. The ferrite grain size was obtained by OM following the Fig.…”

“…1c, revealing an average grain size of 6 ± 1 μm, which corresponds to an equivalent, G, ASTM grain size number of 11. This is characteristic of a very fine grain size that improves the strength of the material [21,22]. However, there is no direct relationship between the grain size and API 5L grade steels with the mechanical properties of a steel as they depend on several aspects such as fabrication route, grain size, and presence of inclusions [22].…”

Evaluation of Microstructure and Mechanical Properties of Seamless Steel Pipes API 5L Type Obtained by Different Processes of Heat Treatments

“…However, these methods are not based on fracture mechanics concepts to evaluate fracture toughness and fatigue resistance, and their data have large deviations because they largely depend on the specimen size and geometry. Little information is available on fracture toughness [13][14][15][16][17][18][19] and fatigue crack growth resistance [20][21][22] , mainly on near-threshold fatigue crack growth behavior. It is also important to note that the publications are mainly concentrated on welded microalloyed steel pipes, obtained from thermomechanical operations (controlled rolling), with a characteristically complex microstructure.…”

Influence of Niobium and Molybdenum on Mechanical Strength and Wear Resistance of Microalloyed Steels

“…These pipelines operate at great depths and withstand pressures up to 350 bar in the presence of chlorides, sulfides and other dissolved ions 1 . API steels are also used in the ore transportation; however, the inner walls of the pipes have low wear resistance, when in contact with slurries and abrasive particles 2,3 . The conventional processing of API steels by rolling and forging allow a wide range of thicknesses with control of temperature and plastic deformation during the mechanical forming stages 4 .…”