Influence of Microstructure on Impact Toughness of X80 Pipeline Steel

The charpy impact properties,distribution and composition of precipitates in X80 pipeline steel with different microstructures have been studied by means of optical microscopy,transmission electron microscopy and drop weight impact testing.The results show that the pipeline steel with acicular ferrite or polygonal ferrite microstructure had higher impact energy,fracture fiber rate and crack propagation energy than the steel with granular ferrite microstructure.Precipitations with size range of 50-140 nm were observed in all steels with different microstructure,they had small harmful effect on the toughness of the steel.The acicular ferrite is suitable for X80 pipeline steel with 22 mm thickness considering strength and toughness.