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

170

5

Table 1. Standard mechanical properties of pipe steels in air Steel Metal state

σ UTS , М P а

σ y , М P а

δ , % 26,0 20,2 23,5 18,5 22,3 23,0

RA, %

as received

565 627 585 633 615 641

483 502 485 502 521 547

78,0 64,0 82,0 71,1 73,4 74,5

17H1S

after 30 years of operation

as received

Х60

after 25 years of operation

as received

X70

after 37 years of operation

Susceptibility of steels 17H1S , Х60 and Х70 to degradation was analyzed taking into account changes in their resistance to brittle fracture. Values of impact toughness KCV obtained on longitudinal specimens are presented in Table 2. It is shown earlier by K rechkovs’ka and Student (2017) that characteristics of brittle fracture resistance are depended essentially on specimen orientation and stress concentrator geometry, especially in a case of long term operated steels. To prove this statement, impact toughness was additionally determined for the steel X60 using transversal specimens, the obtained KCV values for the specimens this orientation was 3.26 and 2.25 MJ/m 2 for the as received and the operated steel respectively, which were noticeably less than that for longitudinal specimens. This could be explained by the difference in a length of perlite grain strips separated by ferrite grains in specimens of different orientation. In the case of longitudinal specimens testing, the fracture plane intersects almost uninterrupted strips of pearlite and ferrite grains that leads to a higher resistance to brittle fracture in this direction. In contrast, fracture of transverse specimens realizes by detachment along structural elements of the texture (delamination). Since the continuous pearlite strips in this plane are considerably shorter, hence, a fracture path with the lowest brittle fracture resistance most likely to be implemented on transverse specimens.

Table 2. Impact toughness of pipeline steels (longitudinal specimen orientation relative to rolling)

17H1S

Х60

Х70

Parameter

Gas pipeline #1

Gas pipeline #2

Operation time, years

0

30

51

0

28

29

31

38

40

0

25

0

37

KCV, MJ/m 2

2.55

1.29

0.33

2.06 1.65 1.38 1.15 1.54 1.25

3.42 2.63

2.37 2.1

The rate of loss of brittle fracture resistance for steels due to their operation was estimated as follows: V = (KCV 0 – KCV op ) / τ , where KCV 0 – impact toughness of as received steels, and KCV op – after their operation. A clear correlation of this parameter on the operation time τ was not found (Fig. 3а) due to essential data scattering caused by different operational conditions for pipelines, using specimens of different orientation etc. However, estimation of areas of brittle fracture fragments on fracture surfaces of specimens after the Charpy testing for all tested steels gave us the parameter  definitely  dependent on the parameter V (Fig. 3b). The dependence can be expressed as V = 4,24 – 4,32 exp (   where  = S brittle / S ductile . Thus, the degree of damaging for the operated metal was determined by the area of brittle elements S brittle at the fracture surface. Fractofraphic elements of brittle fracture were identified as non-structured smooth areas (delaminations) against the background of a typical relief of ductile fracture due to microvoid coalescence (Fig. 4a, b), as transgranular cleavage with typical river patterns (Fig. 4c), and as their combinations observed at fracture surfaces of Charpy specimens. The parameter  a ratio of the area of brittle fracture elements S brittle on the fracture surface to the area of generally ductile relief S ductile is proposed to consider as the quantitative fractographic parameter of degree of steel operational degradation.