PSI - Issue 21

M. Hredil et al. / Procedia Structural Integrity 21 (2019) 166–172 M. Hredil, H. Krechkovska, O. Student, I. Kurnat / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 1. Microstructure of the steel Х70 (axial cross section) (а) as received and (b) after 37 years of operation.

Peculiarities of degradation of 17H1S steel were analysed in detail. Its structure clearly reveals texture in both axial (Fig. 2 а, с) and circumferential (Fig. 2b, d) directions. Some common features were traced in its structure in as-received state and after 30 years of operation, namely, a polygonal form of ferritic grains in relatively wide strips (up to 30 μ m) and thin interlayers of pearlite grains (up to 12 μ m) alternating with strips of ferrite . Interlayers of pearlite grains in the steel in as-received state (axial direction) are almost interrupted and extended up to 2 mm in length (Fig. 2а), whereas in circumferential direction this continuity is disturbed by separate ferritic grains, here the length of pearlite interlayers not exceed 150 μ m . Herewith, both ferritic and pearlite grains are polygonal, and this is more pronounced in the circumferential cross section of the pipes. Here also a grain size distribution is more nonuniform, in ferrite layers and in pearlite ones as well . Besides, long chains (100 – 200 μ m) with very thin (up to 1 μ m) non-metallic inclusions of manganese sulphides are observed in the pipe axial section (Fig. 2e) almost in a whole pipe wall. They preferentially intersect the strips of ferritic grains. It was shown by Chan and Charles (1986) in the example of pure Fe-C alloys with ferritic-pearlite microstructure that grain boundaries between ferrite and pearlite or between neighboring pearlite are effective traps for hydrogen whereas boundaries ferrite – cementite (constituents of pearlite) absorb hydrogen weakly but suppress hydrogen diffusion through pearlite colonies. Basing on the analysis of the researches Chan and Charles (1986), Ichitani and M. Kanno (2003) it was suggested that the network of interphase boundaries inside pearlite (as a mechanical mixture of finely dispersed cementite with ferrite lamellae) formed into almost uninterrupted stripes (especially in axial direction relative to rolling) which are the bariers for hydrogen diffusion, promote its nonuniforn distribution around numerous ferrite – cementite borders. As a result, pearlite became porous with a well-developed network of damages along these borders (especially along extended borders with ferrite bands), that caused partial elimination of damaged pearlite grains at the specimen surface during its polishing and etching. This resulted in formation of almost uninterrupted thin and long (up to 500 µm) strips with spalled pearlite constituents, most noticeable in axial direction of the operated pipe. (Fig. 2с). Similar features of pearlite loosening in circumferential direction appeared in a form of discontinuous strips no longer than 40 µm interrupted by ferrite. These ferrite partitions served as a path for hydrogen which can diffuse in this way through the whole pipe wall (Fig. 2d). Thereby, hydrogen absorbed by steel during its long term operation on a gas pipeline leads to formation of narrow (according to the size of strips with damaged pearlite in circumferential cross-section of a pipe), long (according to pearlite strip length with damaged pearlite in longitudinal direction) and almost uninterrupted chains of defects in a form of delamination oriented along the pipe axis. If this suggestion is correct, then fairly long delamination can form along pearlite strips in a pipe wall under influence of hydrogen, which inevitably occurs during the long-term operation of gas pipelines, as shown by Hredil and Tsyrulnyk (2010). Merging together, these delaminations can reach 3 – 25 mm in the operated 17H1S steel. Taking into account the fact that in-service defects usually occur in pipelines along the pipe axis (especially under their spontaneous propagation), the revealed orientation of structure-concerned damages, attributed to the texture in the axial direction of the pipe, undoubtedly, weakens its transversal cross-section and facilitates the fracture process.