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

Abstract

Abstract. Safe and reliable transport of dense CO 2 by pipes needs a careful choice of the constitutive pipe materials to prevent brittle crack propagation after ductile or brittle failure initiation. This unexpected phenomenon can occur after failure or leak promoted by external interferences. In this case, the rapid decompression of dense CO 2 into gas leads to a very low local temperature of about -80°C. To prevent risk of brittle fracture initiation and propagation, the material must remain ductile at this temperature. In other terms, its ductile-brittle transition temperature (DBTT) has to be lower than -80°C minus a margin. It is admitted that the DBTT is not a material characteristic but depends on specimen geometry, loading rate and loading mode, i.e. on constraints. A loss in constraint leads to a lower brittle-ductile transition temperature. In order to select a steel for transportation of dense CO 2 , transition temperatures T t (from tensile test), T K27 and T K50 (from Charpy test) and T K100 (from fracture mechanics test) have been determined on an API 5L X65 pipeline 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 reference transition temperature.

Role of constraint on the shift of ductile–brittle transition temperature of subsize Charpy specimens

“…where α is a parameter that depends on the yield stress. A similar relation has been found for X65 pipe steel by Capelle et al between various transition temperatures T t ( T t , tensile , T 0 and T K 1/2 ) and critical effective T ‐stress ( T ef , c ). …”

“…Charpy impact tests have been performed on API 5L X65 pipe steel with standard Charpy specimens at temperature range [−196 to 20 °C] . Plotting the fracture energy Kc V (J) temperature and fitting the data according to Eq.…”

Study of low‐temperature effect on the fracture locus of a 420‐MPa structural steel with the edge tracing method

“…Ren 2 carried out tensile tests of a 420-MPa steel with temperature ranging from 0°C down to −90°C and found that the Lüders strain [3][4][5] increased as the temperature decreased. For most structural steels, as the temperature decreases continuously, the fracture behaviour will transform from ductile to brittle (DBT), [6][7][8][9][10][11][12] reducing the steels' ductility and fracture toughness. The DBT occurs when the temperature decreases down to the steel's DBT temperature.…”

Failure Assessment Diagram (FAD) analysis of fatigue test results for X65 welded joints

“…The yield strength and ultimate tensile strength of the X65 steel were obtained via relation to hardness [7] ( Y = 550 and u = 633 MPa), while the fracture toughness of 280 MPa √m was used [8].…”