PSI - Issue 26

Olha Zvirko et al. / Procedia Structural Integrity 26 (2020) 219–224 Zvirko et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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properties, first of all, mechanical ones. Degradation of the mechanical properties is intensified due to corrosion, which is one of the most active and dangerous damage mechanisms. Hydrogen evolution and penetration into a metal during corrosion causes its embrittlement. One of the most dangerous consequence of hydrogen embrittlement of pipeline steel during long-term operation is significant reduction of its resistance to brittle fracture, as demonstrated by Nykyforchyn et al. (2011), Maruschak et al. (2014), Meshkov et al. (2015), Tsyrulnyk et al. (2018), Zvirko et al. (2018, 2019a) and others, which may lead to pipeline failure. Long-term mutual action of corrosion, hydrogenation and working stresses during operation, which intensifies each other, is one the most important reasons of in-service degradation of gas pipeline steels. Since mechanical properties associated with a safe operation of pipelines, which were put in engineering calculations at pipeline design stage, are deteriorated during operation, pipeline should be periodically assessed and timely repaired. When solving reliability and residual lifetime problems of pipelines, methods of accurate evaluation of current technical state of pipeline steel are of great importance (Dzioba and Tsyrul’nyk (2009), Andreikiv et al. (2012)). These measurements must, as a rule, be performed without interruption of operation. For this purpose various monitoring and discrete non-destructive inspection and testing techniques are used. Most of non-destructive testing methods only detect and size defects and damages. Information concerning degradation degree of steel under operation can be provided by a usage of several non-destructive methods only. In particular, different methods based on indentation techniques are developed (Jang et al. (2005), Bolzon et al. (2015, 2018) and Bolzon and Zvirko (2017)). Recently, an electrochemical method (Nykyforchyn et al. (2017a, 2018, 2019), Zvirko (2017) and Zvirko et al. (2019b)) for evaluation of degradation degree of pipeline steels was proposed, as it was demonstrated, that electrochemical characteristics of a metal are sensitive to change in its state caused by in-service degradation. In this paper some peculiarities of estimation of degradation degree of pipeline steels are considered and possibilities of usage of non-destructive methods for evaluation of operated pipeline steel state considering degradation stage are analysed. Degradation of pipeline steels is usually evidenced by mechanical testing of steel samples machined from pipe, comparing properties of as-received metal and operated one. For analysis of changes in mechanical behaviour of metal caused by operation a ratio of a certain parameter of operated steel to that of as-received steel, P deg /P as-rec , was used. Relative changes in basic mechanical properties and impact toughness of low-carbon ferrite-pearlite pipeline steels with different strength caused by in-service degradation are presented in Fig. 1. The 17H1S steel (API 5L X52 strength grade), API 5L X60 steel, and API 5L X70 steel were operated for 30, 25 and 37 years, respectively. As it can be seen from Fig. 1, the steels were strengthened during operation. Concerning plasticity, it was reduced for steels with lower strength (17H1S and X60); however for the high-strength X70 steel slight increase in both plasticity characteristics was observed. Impact toughness of the 17H1S and X60 steels was also decreased (Fig 1 a and b). However, increase in impact toughness after operation was revealed for the X70 steel. These peculiarities of mechanical properties changes are in accordance with that previous demonstrated for pipeline steels after different operation time. As it was reported by Nykyforchyn et al. (2011), two stages of in-service degradation of pipeline steels were distinguished, based on regularities of mechanical properties degradation: stage I – deformation aging, and stage II – dissipated damaging. According to these regularities, strength and hardness of steels are increased, and plasticity and resistance to brittle fracture are decreased during deformation aging. At the same time, further operation of the degraded steel leads to in-bulk material damage accumulation: strength and hardness of the steel is decreased, but elongation can be increased at this stage. Increasing in impact toughness observed for the X70 steel due to operation can be, obviously, associated with difference in pipe manufacturing process for different pipes or its intensive dissipated damaging, which can imply an increase in fracture energy, as shown by Nykyforchyn et al. (2017b) and Zvirko et al. (2018). In-service degradation of ferrite-pearlite pipeline steels can be accompanied by a decrease of cohesion between the structure fibers and development of damages orientated along rolling direction, which influence on fracture energy of specimens at mechanical characterization testing, in particular, at impact toughness testing. Despite of the fact that impact toughness of pipeline steels, determined on longitudinal specimens, can be increased due to decreasing cohesion between fibers of structure and developing delamination and microcracks orientated along rolling direction, increasing fracture energy, their fracture toughness is always decreased during the whole operation time of pipeline, as it demonstrated by Nykyforchyn et al. (2011) and Andreikiv et al. (2012). 2. In-service degradation of gas transit pipeline steels