• constraint effect;
  • elastoplastic fracture;
  • pipelines;
  • X-70 steel

Abstract:  The typical heat-affected zone developed in an X-70 steel, usually used in pipeline manufacture, has been simulated via thermal treatment. A non-equilibrium microstructure consisting of bainite, with ferrite and some martensite, has been produced and mechanically characterised, resulting in much greater hardness and strength and a lower ductility and toughness than the corresponding base metal. This also has a lower ductility. Different single-edge notched bending specimens with different crack lengths (a/W between 0.1 and 0.5) were experimentally tested to assess the fracture behaviour of this product under different degrees of constraint. The J–a resistance curves at room temperature were determined, and the obtained results were explained by the effect of constraint on ductile crack growth.