Microstructure and mechanical properties analysis of X70 pipeline steel with polygonal ferrite plus granular bainite microstructure

Microstructure and mechanical properties of X70 pipeline steel with polygonal ferrite plus granular bainite were characterized using tensile tests, Charpy V-notch impact tests, drop weight tear tests, hardness tests and scanning electron microscopy. The results of experiment indicated that X70 pipeline steel with polygonal ferrite plus granular bainite showed an excellent combination of high strength and toughness. The base metal with polygonal ferrite plus granular bainite microstructure exhibited perfect mechanical properties in terms of the transverse yield ratio of 0.81, elongation of 46%, an impact energy of 335 J at -10 degrees C and a shear area of 90% at 0 degrees C in the drop weight tear test. The heat affected zone contained coarse grain zone and fine grain zone, which exhibited good low temperature toughness of 216 J at -10 degrees C. The weld metal primarily consisted of intragranularly nucleated acicular ferrites which led to the high strength and toughness.