Correlation of Microstructure with Anisotropy of Low-Temperature Toughness in Two API X70 Pipeline Steels

This study aims to investigate the correlation between the microstructures and the anisotropy of low-temperature toughness for two high-strength API X70 pipeline steels fabricated at different coiling temperatures. The microstructures are characterized using an optical microscope, a scanning electron microscope, and an electron backscattered diffraction analysis, with tensile and Charpy V-notch impact tests also conducted on the steel specimens in various directions relative to the rolling direction. Some pearlites formed by a higher coiling temperature increase the ductile-to-brittle transition temperature (DBTT) by 38 degrees C in the T-L (transverse-longitudinal) direction. On the other hand, the DBTT of the specimens with the T-L and L-T (longitudinal-transverse) directions (-106.6 and -109.3 degrees C, respectively) exhibits excellent low-temperature toughness, but the specimen with the D-D (diagonal-diagonal) direction shows the highest DBTT (-65.2 degrees C). The resulting anisotropy in the low-temperature toughness of the API X70 pipeline steel is discussed from the standpoint of an orientation distribution function analysis in this study. It is suggested the anisotropy of the low-temperature toughness is mainly attributed to the texture components of RD (rolling direction) fibers originating from deformed austenite.

Automated summary

This study investigates the correlation between microstructures and anisotropy of low-temperature toughness in two high-strength API X70 pipeline steels fabricated at different coiling temperatures. It is found that higher coiling temperatures lead to the formation of pearlites, which increase the ductile-to-brittle transition temperature. The specimens tested in the transverse-longitudinal and longitudinal-transverse directions exhibit excellent low-temperature toughness, while the specimen tested in the diagonal-diagonal direction shows the highest ductile-to-brittle transition temperature.