PSI - Issue 36

Halyna Krechkovska et al. / Procedia Structural Integrity 36 (2022) 334–341 Halyna Krechkovska, Volodymyr Kulyk, Volodymyr Vira et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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(2020), Krechkovska 2,3 et al. (2015, 2017), Nykyforchyn 4 et al. (2021)), to assess the degradation the impact tests were used in the temperature range from +20 to 60 ° C. The values of impact toughness KCV were determined on the transverse Charpy specimens with a V-shaped concentrator with a radius of 0.25 mm. The direction of specimen fracture occurred in the direction of pipe rolling. Fractographic studies were accompanied by a quantitative assessment of the revealed fractographic features of steel degradation using software developed by Kosarevych et al. (2013), by computer analysis of half-tone images of fracture surfaces with automatic recognition of research objects and subsequent determination of their geometric parameters. 3. Results and discussion After analysing the mechanical properties of 17H1S steel in the initial state, it was found (Fig.1a) that on the specimens cut out in the outer pipe surface vicinity (designated by I), the average values of both strength characteristics were σ UTS = 558 MPa, σ YS = 378 MPa, and near inner pipe surface (designated by II) σ UTS = 560 MPa, σ YS = 423 MPa. Thus, for both investigated steel versions, the characteristics turned out to be higher than the values regulated for this steel ( σ UTS ≥ 510 MPa, and σ YS ≥ 363 MPa). After long- term operation of steel on the main gas pipeline, its ultimate strength, σ UTS , decreased (Fig. 1b), but remained within acceptable limits (except for the value σ UTS = 500 MPa, obtained on specimens from the lower part of the pipe in the vicinit y of its outer surface). As for the yield strength σ YS , this indicator decreased and became below the minimum permissible value σ YS = 363 MPa. Moreover, regardless of the part of the pipe (upper or lower) from which the specimens were cut, the σ YS values determined on the specimens cut out from the outer surface of the pipe were lower than those of the inner one. Moreover, the negative effect of degradation was more pronounced on the metal from the lower part of the pipe. Since for the metal in the vicinity of the outer surface of the pipe in its lower part, the maximum decrease in both strength characteristics was recorded, this was interpreted as evidence of the highest intensity of degradation processes in this place. Such steel softening effect was connected to the high intensity hydrogenation of steel from the side of its free contact with groundwater. According to the scheme proposed by Nykyforchyn 4 et al. (2021) the process of degradation of steels is broken into stages accounting tendencies in changing of their properties. Since after operation both strength characteristics of steel (especially the yield stress) decreased, and then according to this scheme, the state of the analysed steel corresponds to the stage of merging of the damage accumulated during operation.

a

b

50

600

600

50

elongation

− elongation

 UTS

 YS

−  UTS

−  YS

500

500

 r

 r

UTS

UTS

40

40

400

400

 r

elongation  

elongation  

 r

 UTS ,  YS , MPa

 UTS ,  YS , MPa

YS

YS

300

300

30

30

200

200

100

20

100

20

I

I

II

II

II

I

The top of the pipe

The bottom of the pipe

Fig. 1. Characteristics of strength (σ UTS and σ YS ) and plasticity (elongation) of 17H1S steel in the initial state (a) and after 31 years of operation on the main gas pipeline (b), determined on specimens cut from the top and bottom parts of pipes near their outer (I ) and inner (II) surfaces. The elongation of steel in the initial state and after operation was higher in the specimens cut from the outer surface of the pipes as compared to their inner surface. As a feature, it was noted that against the background of the lowest strength of exploited steel in the vicinity of the outer surface of the pipe in its top part, at the same time the maximum elongation was recorded here. This is an atypical nature of changes in these indicators, associated