PSI - Issue 40

S.V. Danilov et al. / Procedia Structural Integrity 40 (2022) 112–117

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S.V. Danilov at al. / Structural Integrity Procedia 00 (2022) 000 – 000

RD ND TD

rolling direction normal direction transverse direction

ODF

orientation distribution function

YS

yield strength

UTS

ultimate tensile strength

1. Introduction The development of oil and gas sector requires pipeline construction and reconstruction in Eastern Siberia and Far East – the regions with challenging climatic and geological conditions (Pyshmintsev and Smirnov (2019)). High-strength steels suitable for adverse climatic conditions are used in pipeline construction. This allows to increase pipe operation pressure making pipelines more cost-efficient. Implementation of high-strength pipes with thinner walls also results in decreased metal consumption. Development and adoption of thermo-mechanical controlled processing (TMCP) that comprises controlled rolling and subsequent controlled cooling led to a breakthrough improvement of the structural strength in low carbon, low-alloyed steels in the middle of the 1970s (Pyshmintsev and Smirnov (2019) or Nishioka and Ichikawa (2012)). However, running fracture formation poses the greatest threat in the operation of pipelines (Zhu (2015) and Zhuang and O'Donoghue (2000)). State of the plates for pipe production plays a crucial role in pipe resistance against running ductile fracture formation. Enhancement of steel strength characteristics (grades X70, X80 and higher) has had a considerable negative effect on the ability of semi-finished and finished pipe products to resist trunkline crack formation (Pyshmintsev et al. (2016) or Lobanov et al. (2020) or Gervasyev et al. (2020) or Shtremel et al. (2020)). Field pneumatic pipe rapture test is the only viable practical method for pipe crack resistance estimation in case of X80 and higher grades (Pyshmintsev and Smirnov (2019) and Joo et al. (2012)). Plates for pipe production exhibit mechanical property anisotropy due to their specific structural and textural state (Pyshmintsev et al. (2016) and Gervasyev et al. (2020)). Texture can affect steel strength and crack resistance differently, depending on the measurement direction. Both theoretical calculations and experimental data provide reliable evidence to conclude that the cleavage brittle fractures in α -Fe i.e., in metallic material with BCC lattice, run along {001} crystallographic planes (Lobanov et al. (2020) or Hara et al. (2006) or Farber et al. (2015)). Texture formation during TMCP was thoroughly studied in Gervasyev et al. (2020). The results of these studies demonstrate that extended structural fields with a texture containing a “dangerous” {001} plane parallel to an axis of a pipe are formed in pipe steels after TMCP. Pipe steel crack resistance is commonly investigated by Charpy impact testing. However, the direction of the specimen blanking and the notch do not coincide with the plane containing {001} orientation grains. As a result, separations occur on the fracture surface of the specimens (Pyshmintsev and Smirnov (2019) or Lobanov et al. (2020) or Arabei et al. (2009)). In general, these separations can be described as brittle cracks perpendicular to the main surface of the ductile fracture. They form near a propagating tip of a ductile crack as a result of triaxial stress and due to the fact that under certain conditions a material can be ductile along one direction and brittle along another, thus exhibiting anisotropy of brittleness. Cleavage fractures in pipe steels commonly occur along the initial rolling plane of the plate (Tesař and Jäger (2014)) . Therefore, it is necessary to enhance the methods for pipe specimen tests, in order to improve the content quality of their results and to minimize the quantity of resources required to carry them out. In this case the advantage of the tensile testing that is normally carried out in order to determine standard mechanical properties of metallic materials is conditioned by the guaranteed specimen failure. Moreover, contemporary fracture zone (reduced section) monitoring methods should provide valuable information on the characteristics of the ductile fracture of the analyzed textured materials, if the rolling direction and plane are precisely established. This paper focuses on studying fracture by deformation during tensile testing and its relation to crystallographic texture of low-carbon, low-alloyed pipe steel after TMCP.