PSI - Issue 16

Yevstakhiy Kryzhanivskyy et al. / Procedia Structural Integrity 16 (2019) 237–244 Yevstakhiy Kryzhanivskyy et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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simultaneously. The specific area of fracture of the main zones of specimens was determined (in %) – i.e., the base metal and the thermal effects zones (W). Then, a comparative analysis of their fracture mechanisms was performed. The study of macro-sections of the W showed high quality of welded joints. The defects of the weld (pores, slag inclusions, non-melting, bad welding, etc.) have not been found, which indicates the high quality of the welds, Vuherer et al. (2012). Parameters of specimen areas (in %) evaluated by the analysis of images for different test temperatures are systematized in Table 2.

Table 2. Quantitative characteristics of fracture zones of Charpy specimens tested for impact toughness of pipe steel of X70 strength class

Area S (in %) of fracture, for different test temperatures t, (in o C) +20 0 -20 -40 -60 BM

Zones on fracture surface

Crack start zone (І)

6.1

9.0

7.8

5.1

17.5 63.9

Crack propagation zone (ІІ)

67.2 20.9

51.3 20.9 18.8

72.1 11.0

42.4 48.6

Shear lips (ІІI)

6.7

Rupture area (IV)

5.8

9.1

3.9

11.9

Fusion zone (W1)

Crack start zone (І)

13.7 43.6 23.6 19.1

13.3 57.2

12.9 50.8 15.2 21.1 11.2 62.1 16.4 10.3

20.5 54.6 12.7 12.2 10.9 50.5 19.5 19.1

7.3

Crack propagation zone (ІІ)

80.4

Shear lips (ІІI)

16

7.2 5.1

Rupture area (IV)

13.5

Fusion zone (W2)

Crack start zone (І)

8.1

10.2 55.5 23.0 11.3

7.5

Crack propagation zone (ІІ)

78.2

78.2

Shear lips (ІІI)

9.6 4.1

8.3 6.0

Rupture area (IV)

It should be noted separately that there were no stratifications, plots of multiple brittle cracks parallel to the rolling direction of the sheet, from which the pipe was made. Based on the data analysis, the geometric parameters of the fracture surface were systematized in Table 2. Crack start zone (І). This area had a sufficiently large scatter of values for all the specimens studied. Although it was known that the local weakness of weld tends to reduce the energy intensity of the crack nucleation, this was not observed in our case. In particular, at 20 ° C, the fracture area of BM was 6.1%, that of W1 was 13.7%, and that of W2 was 8.1%. At -40 ° C, the fracture area of BM was 5.1%, that of W1 was 20.5%, and that of W2 was 10.9%. These data do not correlate with the data presented in Table 2, where the crack start in the BM was more energy intensive. However, in our opinion, this is because a part of work in BM was spent on fracture of the specimen, which had a greater margin of plasticity. For other test temperatures, the data fluctuated, but did not exceed the specified values. Crack propagation zone (ІІ). The ability to inhibit significant ductile fracturing depends on the volume and intensity of the plastic deformation of the material in front of the propagating crack tip. Both for the BM and weld specimens, this area was the largest. At 20 ° C, the fracture area of BM was 67.2%, that of W1 was 43.6%, and that of W2 was 78.2%. At -40 ° C, the fracture area of BM was 42.4%, that of W1 was 54.6%, and that of W2 was 50.5%. It should be noted that at -40 ° C, it was the lowest value of the crack propagation area. For other test temperatures, this value increased.