PSI - Issue 13

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 13 (2018) 1215–1220 Hryhoriy Nykyforchyn / Structural Integrity Procedia 00 (2018) 000 – 000

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3. Test results and discussion

A new approach for evaluation of in-service degradation of long-term operated pipeline steels based on electro chemical analysis of fracture surface has been grounded on experimental observations of changes in microstructure of long-term operated steels of transit gas pipelines consisted in precipitation of carbides at grain boundaries and refining the grains, as it was reported by Chuvil’deev (2006). Presence of carbides at grain boundaries leads to a reduction of brittle fracture resistance and stress corrosion cracking resistance of long-term operated pipeline steels. According to the calculations made by Chuvil’deev (2006), the limiting stage of the formation of carbides at grain boundaries is diffusion of carbon from grain bulk to intergranular boundaries: for steel produced by controlled rolling this period is cca 24 years, and time of formation of carbides is about 1 year. That is, the calculated time is proportional to the operation time of pipeline steels, after which a significant decrease in resistance to brittle fracture is often observed. It was suggested by Nechaev (2008) that carbohydride-like and other nanosegregated structures at dislocations and grain boundaries are formed during long-term operation of transit gas pipelines made of ferrite-pearlite pipeline steels. This phenomenon is associated with intergranular fracture of gas pipelines. In such circumstances for formation of the carbides network at grain boundaries it is enough diffusion of carbon from the grains body to their boundaries at a distance of cca 1 micron only during long-term (10 – 20 years) operation of gas pipelines, as it was calculated by Nechaev (2008). In this paper, it is assumed that diffusion of carbon not only to grain boundaries, but also to various defects inside grains (boundaries of sub-grains or other defects) during long-term operation led to formation of nanoparticles of carbides, which resulted in intergranular cracking of operated pipeline steels under service and their transgranular cracking under impact toughness testing. In such circumstances diffusion of carbon can be intensified by hydrogen being absorbed by steel. Precipitation of carbide nanoparticles at grain boundaries and defects inside grains caused steel embrittlement and, consequently, led to brittle fracture and reduction in fracture energy. Intergranular cracking of pipeline steels under service and transgranular cracking of post-operated pipeline steels under impact toughness testing are associated with this phenomenon. Under such a mechanism of in-service degradation of metal, fracture surface should be enriched by carbon compounds (carbide-type), and electrochemical characteristics should be sensitive to this. Based on the assumption it was proposed to apply electrochemical method to evaluation of in-service degradation of metal by fracture surface analysis. Taking into account the fact that certain electrochemical parameters, namely corrosion current density and polarization resistance, depend on surface area, which is difficult to estimate in the case of fracture surface, the study was focused on determination of potential of fracture surface of steel sample, which is sensitive to electrochemical micro-heterogeneity of steel, including different content of carbon. In order to detect sensitivity of electrochemical properties, in particular, potential, to carbon content in steels, a series of electrochemical experiments were carried out in 0.3% NaCl solution. Carbon non-alloyed steels with different carbon content C C from 0.2 to 1.2% were investigated to distinguish the influence of carbon content on the value of potential E pol . Time dependencies of potential E pol of the polished steel surface of carbon steels are presented in Fig. 1. Based on research results, the dependence between electrode potential Е pol of the polished steel surface and carbon con tent in steel was revealed (Fig. 2, the value of potential Е pol corresponds to 500 sec of exposure of the metal to electro lyte). General trend of a negative shifting potential with increase in carbon content С C in carbon steel was confirmed by this dependence. Using regression analysis, the dependence was approximated by the following expression: -0.044 C ; 0.476 E C pol    . . R 0 997   (1) Potential E pol of the polished surface of the X52 pipeline steel in the as-received state and after long-term operation was measured in 0.3% NaCl solution, Table 1. Impact toughness values experimentally determined for X52 pipeline steel in dependence on its state are presented in Table 1. Based on the obtained results, it should be noted that long-term operation of gas transit pipelines caused significant decrease in brittle fracture resistance of steels. Following the experimental procedure described above, time dependencies of potential E fr of the fracture surface of specimens made of X52 pipeline steel in different states (in the as-received state and after 30 years of operation) fractured after impact toughness testing were recorded in 0.3% NaCl solution, Fig. 3.