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. 2. Microstructure of 17H1S steel (а, b) as received and (c, d) after 30 years of operation on the main pipeline in (a, c) axial and (b, d) circumferential cross section of the pipes at a distance of 3 mm from their external surface and (е ) inclusions of manganese sulphides in the pipe axial section. Mechanical properties of steels. It is derived from the analysis of both characteristics of strength that all tested steels strengthened during their operation (Table 1) while their plasticity decreased. Namely, reduction of plasticity characteristics is most considerable for 17H1S steel (elongation δ by 2 2%, а reduction in area RA by 18%), somewhat less effect of long-term operation for the steel X60 – by 21% and 13% respectively, and in a case of the steel X70 plasticity became even higher (although the change did not exceed 3% which is negligible). The structure peculiarities of steels described above (first of all, damages in pearlite interlayers) are mostly characterized 17H1S steel and to a lesser extent Х70 steel. This is in a good agreement with results of mechanical testing according to which the steel Х70 is almost unsusceptible to operational degradation.