Figure 8 allowed to determine a transition temperature relative to the investigated component, i.e., a transition temperature corresponding to the same constraint as the component at failure. For that purpose, it is necessary to obtain the “material master curve” which is the relationship between fracture toughness and constraint and effective T-stress (Tef) at critical pressure for a test according to the procedure described in [13]. Unfortunately, these data are not yet available for X 65 and an estimation has been evaluated on the basis of the API 5L X52 pipe steel and a pipe diameter of 610 mm and thickness equal to 5.8 mm. The pipe exhibits a surface notch with a notch angle ϕ = 0, a notch radius ρ = 0.25 mm and a notch depth (a) to thickness (t) ratio equal to a/t = 0.5. From that, the constraint range is estimated between -450 MPa and -550 MPa. The average component transition temperature Tcomp is deduced from the master curve and evaluated to 150 K (see Figure 6). This temperature is lower than TK27 transition temperature of 24 K; this value represents the degree of conservatism of using transition temperature deduced from Charpy specimens. The safety temperature range between the depressurisation temperature plus a temperature allowance and the component temperature is 35 K, which is enough, allowing to consider that API 5L X65 could be used for dense CO2 transportation. Figure 6. Estimation of component transition temperature from the master curve (Tt = f (constraint)). 4. CONCLUSION In order to select a steel for transportation of dense CO2, transition temperatures Tt (from tensile test), TK27 and TK50 (from Charpy test)and TK100 (from fracture mechanics test) have been determined on an API 5L X65 steel. These transition temperatures have been reported versus a constraint parameter, e.g. T-stress, in a master curve. Differences between different brittle-ductile transition temperatures and temperature corresponding to T-stress acting in a pipe submitted to internal pressure on the master curve, give an estimation of the conservatism of the chosen transition temperature. Based on this methodology the selected API 5L X65 pipeline steel could be used for dense CO2 transportation since the experimental and calculated transition temperatures are lower than the expected -80 °C following a rapid decompression of dense CO2 pipeline rupture. The most conservative transition temperature was obtained by Charpy impact test TK27, which is lower than depressurisation temperature plus a temperature allowance.
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