Cyclic Corrosion Crack Resistance of an Exploited Welded Joint of 17G1S Pipe Steel
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CYCLIC CORROSION CRACK RESISTANCE OF AN EXPLOITED WELDED JOINT OF 17G1S PIPE STEEL V. A. Voloshyn
UDC 620.193: 620.17
We analyze the influence of numerous factors, such as an NS4 model soil medium, the load ratio ( R = 0.1 and 0.9), and the potential of cathodic protection (– 0.98 V) on the cyclic crack-growth resistance of the base metal and the metal of circular butt weld of 17G1S pipe steel in the intact state and after 30 years of the operation in a gas main. It is shown that the long-term operation of steel maximizes the negative influence of the soil medium on its cyclic crack-growth resistance under the combined action of the high load ratio and high potential of cathodic protection. Keywords: gas pipeline, weld metal, crack growth rate, soil medium, cathodic polarization.
Introduction The long-term (for more than 30 yr) operation of the main networks of oil-and-gas pipelines sharpens the problem of the degradation of pipe steels. The indicated degradation manifests itself, first of all, in a sharp drop of the brittle-fracture resistance of steel and its resistance to hydrogen-corrosion cracking [1–4]. It is believed that the role of the main factor responsible for the deterioration of these properties is played by microdefects dispersed in the volume of the pipe wall [5]. Moreover, the transported hydrocarbons may promote the hydrogenation of pipe walls on the side of its inner surface as a result of the electrochemical reactions of water condensate with the metal, which would intensify the development of defects under the combined action of the mechanical load and hydrogen [6]. This also explains the observed in-service lowering of the cyclic crack-growth resistance of pipe steels [7, 8]. Welded joints (WJ) of the main pipelines prove to be especially vulnerable from the viewpoint of their integrity under the operating conditions and the action of hydrogenating corrosive media [9–11]. It is known that WJ are characterized by high degrees of electrochemical heterogeneity. This leads to the initiation of local corrosion processes facilitating the processes of crack initiation and propagation. In what follows, we study the influence of various in-service factors on the cyclic crack-growth resistance of butt WJ of the main pipelines. Materials and Experimental Methods We studied a field butt WJ of 17G1S steel. It was cut out from a pipe of the main pipeline with an outer diameter of 1220 mm and a thickness of 12 mm after operation for 30 yr. For comparison, we also studied a WJ of the metal of stock pipes produced according to the generally accepted technology of welding of Karpenko Physicomechanical Institute, Ukrainian National Academy of Sciences, Lviv, Ukraine; e-mail: [email protected]. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 56, No. 1, pp. 112–117, January–February, 2020. Original article submitted January 10, 2020. 1068-820X/20/5601–0119
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Springer Science+Business Media, LLC
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V. A. VOLOSHYN
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the pipes under the conditions of field assembling. We compared the properties
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