PSI - Issue 16

Available online at www.sciencedirect.com Available online at www.sciencedirect.com ScienceDirect StructuralIntegrity Procedia 00 (2019) 000 – 000 ScienceDirect StructuralIntegrity Procedia 00 (2019) 000 – 000

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Procedia Structural Integrity 16 (2019) 153–160 6 th International Conference “Fracture Mechanics of Materials and Structural Integrity” Feature of stress corrosion cracking of degraded gas pipeline steels Hryhoriy Nykyforchyn * , Halyna Krechkovska, Oleksandra Student, Olha Zvirko Karpenko Physico-Mechanical Institute of the National Academy of Sciences of Ukraine, 5 Naukova St., Lviv, 79060, Ukraine 6 th International Conference “Fracture Mechanics of Materials and Structural Integrity” Feature of stress corrosion cracking of degraded gas pipeline steels Hryhoriy Nykyforchyn * , Halyna Krechkovska, Oleksandra Student, Olha Zvirko Karpenko Physico-Mechanical Institute of the National Academy of Sciences of Ukraine, 5 Naukova St., Lviv, 79060, Ukraine Stress corrosion cracking (SCC) of steels can reduce the structural integrity of gas pipelines. To simulate in-service degradation of pipeline steels in laboratory the method of accelerated degradation consisted in subjecting specimens to electrolytic hydrogenation, to loading up the certain plastic deformation and heating of s pecimen at 250°C was recently developed. The purpose of this paper was to analyse mechanical and SCC behaviour of in-service and in-laboratory degraded gas pipeline steels and to reveal some fractographic features of SCC. Three pipeline steels of the different strength (17H1S, which is equivalent of API X52, API X60 and API X70) were investigated. The characteristics of the as-received pipeline steels with different strength were compared with the properties of pipeline steels after in-service and in-laboratory degradation. An influence of the NS4 solution on SCC resistance of 17H1S and API X60 steels in the as-received state and after the accelerated degradation, using slow strain rate tension method, was analysed. The noticeable decrease of plasticity for 17H1S and API X60 steels after long term operation was shown. Deep microdelaminations revealed in the central part of fracture surfaces for the operated steels can be considered as the signs of dissipated damaging in the metal caused by texture and hydrogen absorbed by metal. Comparison of the SCC tests results showed that the characteristics of both steels in the as-received state were insignificantly changed under the influence of the environment. At the same time, the degraded steels were characterized by a high sensitivity to SCC. It was shown fractographically that it associated with cracking along interfaces of ferrite and pearlite grains with secondary deep intergranular cracks formation and also by delamination between ferrite and cementite inside pearlite grains. The similar fracture mechanism at SCC tests was revealed for near the outer surface of the specimens and in the central part of the fracture surfaces of in-laboratory degraded specimens. These results demonstrated the key role of hydrogen during SCC and in-bulk cracking as well. © 2019 The Author(s). Published by Elsevier B.V. Peer- review under responsibility of the 6th International Conference “Fracture Mechanics of Materials and Structural Integrity” organizers © 2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the 6th International Conference “Fracture Mechanics of Materials and Structural Integrity” organizers. © 2019 The Author(s). Published by Elsevier B.V. Peer- review under responsibility of the 6th International Conference “Fracture Mechanics of Materials and Structural Integrity” organizers Abstract Stress corrosion cracking (SCC) of steels can reduce the structural integrity of gas pipelines. To simulate in-service degradation of pipeline steels in laboratory the method of accelerated degradation consisted in subjecting specimens to electrolytic hydrogenation, to loading up the certain plastic deformation and heating of s pecimen at 250°C was recently developed. The purpose of this paper was to analyse mechanical and SCC behaviour of in-service and in-laboratory degraded gas pipeline steels and to reveal some fractographic features of SCC. Three pipeline steels of the different strength (17H1S, which is equivalent of API X52, API X60 and API X70) were investigated. The characteristics of the as-received pipeline steels with different strength were compared with the properties of pipeline steels after in-service and in-laboratory degradation. An influence of the NS4 solution on SCC resistance of 17H1S and API X60 steels in the as-received state and after the accelerated degradation, using slow strain rate tension method, was analysed. The noticeable decrease of plasticity for 17H1S and API X60 steels after long term operation was shown. Deep microdelaminations revealed in the central part of fracture surfaces for the operated steels can be considered as the signs of dissipated damaging in the metal caused by texture and hydrogen absorbed by metal. Comparison of the SCC tests results showed that the characteristics of both steels in the as-received state were insignificantly changed under the influence of the environment. At the same time, the degraded steels were characterized by a high sensitivity to SCC. It was shown fractographically that it associated with cracking along interfaces of ferrite and pearlite grains with secondary deep intergranular cracks formation and also by delamination between ferrite and cementite inside pearlite grains. The similar fracture mechanism at SCC tests was revealed for near the outer surface of the specimens and in the central part of the fracture surfaces of in-laboratory degraded specimens. These results demonstrated the key role of hydrogen during SCC and in-bulk cracking as well. Abstract

* Corresponding author. Tel.: +38-032-263-2133. E-mail address: nykyfor@ipm.lviv.ua

2452-3216 © 2019 The Author(s). Published by Elsevier B.V. Peer- review under responsibility of the 6th International Conference “Fracture Mechanics of Materials and Structural Integrity” organizers 2452-3216 © 2019 The Author(s). Published by Elsevier B.V. Peer- review under responsibility of the 6th International Conference “Fracture Mechanics of Materials and Structural Integrity” organizers * Corresponding author. Tel.: +38-032-263-2133. E-mail address: nykyfor@ipm.lviv.ua

2452-3216  2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the 6th International Conference “Fracture Mechanics of Materials and Structural Integrity” organizers. 10.1016/j.prostr.2019.07.035