Evaluation of the fracture toughness of X70 pipeline steel with ferrite-bainite microstructure

The quasi-static fracture toughness KIC and J0.2BL(JIC), Charpy V-notch impact toughness CVN of ferrite-bainite (F/B) X70 pipeline steels with different phase content and morphologies were studied by experiments and numerical simulations at content temperature of −10 ℃. The stress-strain curves were obtained by experiments and unit cell model simulation. The results show that fracture toughness is strongly affected by bainite content. The fracture toughness increases with decreasing the bainite volume fracture fB and increasing aspect ratio Ra and neighboring factor Nf. Meantime, the aspect ratio Ra is more effective in improving the fracture toughness of tested steels compared with the neighboring factor Nf. For F/B X70 pipeline steel, two quantitative relationships have been derived, which correlates J0.2BL(JIC) and CVN with bainite content and morphologies in equations. It means that J0.2BL(JIC) can be calculated when CVN, fB, Ra and Nf of several typical F/B X70 pipeline steels are obtained. Although the quantification is specific to F/B X70 pipeline steels, the approach and model developed in this work can be applied to other metal structure materials.