PSI - Issue 16

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 16 (2019) 153–160 Hryhoriy Nykyforchyn et al. / StructuralIntegrity Procedia 00 (2019) 000 – 000

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Keywords: Gas pipeline, in-service degradation, stress corrosion cracking, hydrogen cracking, texture, microfratography, delamination.

1. Introduction

Stress corrosion cracking (SCC) of gas pipeline steels is often responsible for their failure (Parkins (2000), Zvirko et al. (2016)). SCC from the soil side is widely studied topic. However, it was shown by Tsyrulnyk et al. (2008) that transported natural gas can serve as a source of hydrogenation of pipe wall due the specific electrochemical reactions in the surface condensed water inside the pipeline. Therefore, pipe steel can be hydrogenated from both external pipe surface in local sites of insulation damages and internal pipe surface on large areas along pipeline. It means that hydrogen can play double role in the SCC process: in hydrogen mechanism of SCC and corrosion crack growth in hydrogenated metal. It complicates understanding of fracture mechanics approaches to SCC (Nazarchuk and Nykyforchyn (2018)) concerning fracture of gas main steels. Moreover, long-term operation of gas pipelines caused a degradation of the steels properties, including SCC resistance, which defined their serviceability, as it was demonstrated in numerous issues by Nykyforchyn et al. (2008, 2009), Maruschak et al. (2014a, 2014b), Zvirko et al. (2016), Bolzon et al. (2017). The main peculiarity of such degradation consists in a decrease in plasticity, corrosion resistance, impact strength, fracture toughness, SCC resistance, and corrosion fatigue crack growth of steels (Mil’man et al. (2012), Kharchenko et al. (2017). Two main stages in degradation of steels properties were distinguished by Nykyforchyn et al. (2010, 2017): deformation aging and dissipated damaging. The first one is accompanied by strengthening of material and corresponded loss of plasticity. The peculiarity of the second one is a sharp decrease of brittle fracture resistance accompanied by insignificant change in material strength or hardness. It explains the possible high degree of in service degradation of low strength steels (Nykyforchyn et al. (2009)). Hydrogenation of pipeline steels plays an important role in a development of dissipated damaging (Student (1998)); hydrogen has a significant effect on mechanical properties of steel, especially at cyclic loading (Nykyforchyn and Student (2001)), therefore degradation is a result of the simultaneous effect of stresses and hydrogen on material. Combination of these factors, which caused degradation, was a base for a development of in laboratory methods for accelerated degradation of steels simulating a case of possible hydrogenation of steel during service ( Tsyrul’nyk et al. (2018)) . Fractography was successfully used for revealing of fractographic signs of in-service degradation of welded joints of oil mains and steam pipelines (Student et al. (1999), Krechkovs’ka et al. (2015), Student and Krechkovs’ka (2016)). The development of computer programs for the processing of images opens up possibilities for quantitative analysis of fractographic signs of degradation of operated steels (Student et al. (2003), Kosarevych et al. (2013), Marushchak et al. (2016)). This research was aimed to analyse mechanical and SCC behaviour of in-service and in-laboratory degraded gas pipeline steels and to reveal some fractographic features of SCC, developing the research work performed by Zvirko et al. (2016). Pipeline steels of the different strength were studied: 17H1S (equivalent of API X52), API X60 and API X70, in as-received state, after operation (30, 25 and 37 years correspondingly) and after in-laboratory accelerated degradation. The method of in-laboratory degradation was based on the simultaneous effect of deformation and hydrogenation of steel on its state. The procedure consisted in a preliminary subjecting of cylindrical specimens to electrolytic hydrogen charging in an aqueous solution of H 2 SO 4 (pH2) at current density of 20 mA/cm 2 for 95 h, then loading by tension to 2.8% deformation and, finally, a heating at 250 °C for 1 h (Zvirko et al. (2016)). The chemical composition of the investigated steels in as-received state and after long-term operation evaluated using of the spark optical atomic-emission spectrometer SPECTROMAX LMF 0.5 is presented in Table 1. Cylindrical tensile specimens with a 25 mm gauge length and 5 mm gauge diameter were cut in longitudinal direction (rolling direction) from pipes. One set of tests was loaded by tension in air at a strain rate of 3∙10 – 3 с – 1 to determine tensile properties of the steels: ultimate strength σ UTS , yield strength σ YS , reduction in area ψ and 2. Materials and methods