PSI - Issue 42

Aleksander Omholt Myhre et al. / Procedia Structural Integrity 42 (2022) 935–942 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Figure 1 shows the microstructures of the four steels from the mid area in the longitudinal direction of the pipe, after polishing and etching in a Nital solution. Materials A, B and D consist of alternating layers of ferrite and pearlite, while Material C is composed by fine-grained homogeneously distributed ferrite and bainite. The banding structure changes throughout the thickness and becomes coarser in the mid-section of the pipe. The same is true for ferrite grains. Another microstructural feature of the vintage steels is the granular bainite presented within the pearlite bands. Smooth tensile specimens with a gauge length of 30 mm and a diameter of 4 mm, were machined from the longitudinal and transversal direction of the pipes. Details of the specimen geometry and the schematic sketch of the location of extraction of the specimens and their identification numbers are shown in Figure 2.

Table 1: Chemical composition for the pipeline steels investigated. The American Welding Society carbon equivalent (CE) is given for all Materials.

Element

C

Si

Mn

P

S

Cu

Cr

Ni

Mo

V

N

CE

Material A 0.06

0.40

0.1

1.63

0.011

0.002

0.17

0.03

0.23

0.01

0.06

-

Material B

<0.01 0.56

0.1

0.15 <1.6 <0.025 <0.015 <0.25 <0.25 <0.25 <0.05

<0.1

Material C 0.07 0.23 1.17

0.39

0.01

0.002

0.14

0.17

0.15

0.13

0.03

0.07

Material D 0.12 0.26 1.36

0.43

0.020

0.003

0.010 0.022

0.25

0.002 0.065

-

Figure 1: Optical micrographs from the mid thickness position in the longitudinal direction of the pipe for a) Material A, b) Material B, c) Material C and d) Material D

Figure 2: Sample geometry and locations a) Specimen geometry used for slow strain rate tensile testing in air and in-situ hydrogen charging. B) Sketch showing the specimen locations. Positions 1, 2 and 3 refer to the outer, mid, and inner locations in the cross section of the pipe in the longitudinal direction, respectively, and the specimen in position 5 represents the transversal direction of the pipe.