Design based on ductile-brittle transition temperature for API 5L X65 steel used for dense CO2 transport

Safe and reliable transport of dense carbon dioxide by pipes needs a careful choice of the constitutive pipe materials to prevent brittle crack propagation after ductile or brittle failure initiation. So the material must remain ductile at this temperature; its ductile-brittle transition temperature has to be lower than −80 °C minus a margin. This temperature is not a material characteristic but depends on specimen geometry, loading rate and loading mode, i.e. on constraints. Constraints can be estimated by different parameters: stress triaxiality, Q factor or T-stress. Constraints in a pipe under pressure are close to those given by a tensile specimen.