Volgograd State Technical University, Volgograd, Russia

L. V. Palatkina, Cand Eng., Associate Prof., Dept. of Materials Technology, e-mail: lv.palatkina@yandex.ru
M. V. Matasova, Magister, Dept. of Materials Technology, e-mail: matasova.mary2016@mail.ru
M. V. Kirilichev, Head of Laboratory, Dept. of Materials Technology, e-mail: tecmat@vstu.ru

JSC “Volzhsky Pipe Plant“, Volzhsky, Russia

M. Yu. Chubukov, Cand. Eng., Head of the Central Plant Laboratory, e-mail: chubukovmu@vtz.ru

It is shown that the focus (foci) of metal destruction of corrosion-resistant high-strength casing pipes made of steel type 26KhGMF, when assessing resistance to sulfide stress corrosion cracking (SCR) according to method A of the NACE TM 0177 standard, is located on the surface of the sample and represents a pitting or a corrosion ulcer. For standard samples made of steel type 26KhGMF, a relationship has been established between the morphological features of the macro-relief of fractures and the development of fracture by the SCR mechanism. A sequential change of three main stages of destruction was recorded: I – the stage of brittle corrosion destruction with scars emanating from the fracture site; II – the stage of corrosion cracking with uneven furrowing according to the type of fatigue lines, without signs of plastic deformation of the metal; III – the stage of viscous destruction due to the combined action of plastic deformation (reduction of the cross section of the sample) and corrosion cracking (accumulation of hydrogen in places with high stress localization). In the metal of casing pipes made of steel type 26KhGMF (microalloyed Nb in an amount up to 0.03 % by weight) by the method of color etching in hot sodium picrate, phases such as MeС and Me(C,N) rich in niobium (Nb from 45.05 to 59.71 % by weight) were revealed. The lines (from 5.3 to 22.8 microns in length) consist of inclusions of various dispersities (from 0.3 to 3.8 microns) and shapes (oval, rounded, cubic and block). Based on a comprehensive analysis of the double Fe-Nb state diagram, a quasi-binary section of the eutectic Fe-NB type with a triple Fe-Nb-C system, a pseudobinar diagram of the state of 26KhGMF steel, a sequential change of solidification mechanisms and different solubility of Nb in δ-Fe and γ-Fe (for conditions of equilibrium crystallization of 26KhGMF microalloyed Nb steel in an amount up to 0.03 % by weight by weight) The possibility of formation of niobium-rich carbide and carbonitride phases in the melt is substantiated. The classification of the carbide (carbonitride) phase rich in Nb has also been clarified: primary, micron-sized niobium carbide (NbC)I is formed in the melt before the steel solidifies; secondary, eutectic, micron-sized niobium carbide (NbC)II is formed at the final stage of crystallization of steel as a result of a eutectic reaction; tertiary, nanoscale niobium carbide (NbC)III is isolated from a solid solution of austenite during heat treatment. Increasing the resistance to cracking in a hydrogen sulfide environment for microalloyed Nb steel of type 26KhGMF is possible by eliminating the very component of the nucleation of corrosion cracks and excluding areas of hydrogen accumulation with increased capacity in the metal volume, i.e., provided that the formation of micron-sized Nb-rich carbide and carbonitride phases is prevented.

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