PSI - Issue 3

M.A. Beltrán et al. / Procedia Structural Integrity 3 (2017) 57–67

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Author name / Structural Integrity Procedia 00 (2017) 000–000

results are unpredictable because they can result in overestimates of resistance to Fracture generating oversizing and cost overruns. In real conditions the cracks occur in all possible directions and have a high degree of anisotropy. In the particular case of steel, it has Joo et al. (2012) demonstrated that the anisotropy of the cracking is influenced by the orientation of the microstructure, being of particular interest the banding of the perlite and the size, morphology and distribution of the nonmetallic inclusions. However, it has not been determined how the orientation of these microstructural features with respect to the path of the crack affect its propagation. In the particular case of pipes and containers at internal pressure the behavior of the steel is essentially anisotropic and directional, which originates in the manufacturing process (hot and normalized rolling) of the material. The microstructural characteristics of these steels have been shown to exert a significant effect on the propagation of a crack. On the other hand, previous studies on pipeline steel suggest that crystallographic textures have an important function on toughness anisotropy, as stablished Zong et al. (2013) or Jang et al. (2007); however, detailed discussions about the relationship between crystallographic textures and anisotropic fracture toughness are rare. Fracture behavior has an important function in safety design of pipelines, although the API-5L steel only must meet minimum standard specifications of its mechanical properties and it’s microstructural characteristics are not described widely, hence the presence of mechanical anisotropy can compromise the safety of the component. Therefore, it is necessary to understand the mechanical behavior of API-5L steel integrally and adequately describe it, in order to produce safe structures and pipelines of low cost and high efficiency. The objective of this work is to determine the influence of the microstructure orientation on the anisotropy of the mechanical properties in order to correlate them quantitatively. .

Nomenclature Ω 12

microstructure orientation

A i

banding degree

C

circumferential direction

C v Charpy impact energy CVN Charpy V-notch specimen EBSD electron backscatter diffraction HAGB high angle grain boundary K IC-CV fracture toughness obtained by Charpy impact energy correlations L longitudinal direction LAGB low angle grain boundary N normal direction OM optical microscopy r rv coefficient of relative variation UTS ultimate tensile strength YS yield strength

1. Materials and methods 1.1 Chemical composition

The assessed material was extracted from an API X46 steel tube with a nominal diameter of 24 in and a wall thickness of 0.875 in. The experimental chemical composition obtained can be seen in Table 1, altogether with the reference chemical composition for API-5L steel.