The ductile-brittle transition temperature (DBTT), nil ductility temperature (NDT), or nil ductility transition temperature (NDTT) of a metal represents the point at which the fracture energy passes below a pre-determined value [2]. Design against brittle fracture considers that the material exhibits at service temperature, a sufficient ductility to prevent cleavage initiation and sudden fracture with an important elastic energy release. Concretely, this means that service temperature Ts is higher than transition temperature Tt: t sT T≥ (1) Service temperature is conventionally defined by codes or laws according to the country where the structure or the component is built or installed. A Fracture Mechanics based design ensures that stress intensity factor for design is lower than admissible fracture toughness and fracture toughness is greater than 100 MPa√m (i.e. the reglementary service temperature defined is above the reference temperature). This additional criterion is expressed by: T RT T t s ≥ +Δ (2) where ΔT is the uncertainty on reference temperature. This reference temperature RTi varies according to codes (RTNDT or RTT100). In this paper, a selected pipeline steel API 5L X65 is controlled by several mechanical tests in order to check if the material is eligible for dense CO2 transportation. The material has to satisfy a design based on transition temperature with a service temperature Ts equal to -80°C minus a ΔT, where ΔT is a safety margin estimated to 8°C [3]. Due to the fact that different fracture tests give different transition temperatures, the choice of the most adequate test to provide a value close to the “structure or component” transition temperature Tstruct is an open question. In addition, traditional design is based on transition temperature given by Charpy tests. Thus, it is necessary to know the degree of conservatism of this approach. For this purpose, three transition temperatures have been determined: • transition temperature for elastic to elastoplastic behaviour in tension Tt, • brittle to ductile transition temperature for Charpy V test TK27, • fracture toughness transition temperature T100. It is generally admitted that fracture resistance is sensitive to constraint. Consequently transition temperatures are affected as well. In order to evaluate the sensitivity to constraint of the abovementioned transition temperature, each transition temperature has been expressed as a function of T-stress [4], which is often used as a constraint parameter and determined by a master curve. This master curve allows determining the structure transition temperature (Tstruct) and consequently the degree of conservatism of each abovementioned transition temperature. 2. MATERIAL AND METHODS The investigated material is a pipeline steel API 5L X65 grade supplied as seamless tube with wall thickness equal to 19 mm and external diameter of 355 mm. The typical chemical composition is given in Table 1, mechanical properties at room temperature are given in Table 2. Table 1. Typical chemical composition of pipe steel API 5L X65 (wt %) [6]. C Si Mn P S Mo Ni Al Cu V Nb min. 0.05 0.15 1.00 - - - - 0.01 - - - max. 0.14 0.35 1.50 0.020 0.005 0.25 0.25 0.04 0.080 0.080 0.040
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