An integrated approach to identifying causes and mechanisms of destruction of steel tubing couplings

UDK: 622.276.5.05.004.6
DOI: 10.24887/0028-2448-2022-6-48-51
Key words: sulfide stress corrosion cracking, ferritic steel, X-ray diffraction analysis, crystallographic texture, anisotropy
Authors: V.D. Sitdikov (RN-BashNIPInef LLC, RF, Ufa), A.A. Nikolaev (RN-BashNIPInef LLC, RF, Ufa), A.K. Makatrov (RN-BashNIPInef LLC, RF, Ufa), A.V. Malinin (RN-BashNIPInef LLC, RF, Ufa), I.M. Filyaeva (RN-BashNIPInef LLC, RF, Ufa), I.V. Mironov (RN-BashNIPInef LLC, RF, Ufa)

The article presents the developments of RN-BashNIPIneft LLC in researches of microstructure, crystallographic texture, level and anisotropy of material strength properties, the integral analysis of which helps to establish the destruction causes and mechanisms of pipes used in the oil industry. To solve these tasks, it is suggested to combine the methods of scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), X-ray fluorescence analysis and computer modeling. As a result of the comparative analysis of the information on the internal structure of the material in the areas exposed to corrosion, the mechanisms and causes of their destruction are revealed. This work shows that on examining the fracture and microcracks nature, determining the size and type of non-metallic inclusions, the average size and mutual orientation of grains, the elemental composition of corrosion products by the SEM method established the destruction of a pump and compressor pipe (tubing) by stress corrosion cracking (SCC) mechanism. The use of X-ray method determines that the preferred grain orientations formed during tubing forming in the fracture zone and in the crack-free zone are significantly different. As a result of analyzing the texture formation processes, it was found that in the upper layers of the tubing near the fracture zone, there are the traces of restoring and recrystallization processes, which led to local softening of the tubing while molding. At the same time, the violations of the technological modes of molding were detected, which led to the formation of a homogeneous type of crystallographic texture up to half the thickness of the tubing wall, which led to a resistance decrease of the material and the spread of microcracks. In other areas where there are no traces of SCC, the various types of crystallographic texture along the entire thickness of the tubing wall were revealed. As a part of the analysis of texture coefficients and making 2D projections of the yield surfaces, a relatively low value of the yield strength in the fracture zone in the direction of the longitudinal axis of the tubing was established. As a result, the authors make a conclusion that the local softening of the material, selectively homogeneous crystallographic texture along the pipe wall thickness, as well as a low level and strong anisotropy of the strength properties on the outer surface of the wall, led to the appearance and propagation of the cracks along the longitudinal axis of the tubing being under pressure.

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