PSI - Issue 5

Jan Kec et al. / Procedia Structural Integrity 5 (2017) 340–346 Jan Kec, Ivo Cerny / Structural Integrity Procedia 00 (2017) 000 – 000

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geometry of the dents found. Further, the stress-strain behaviour of two dents, the dent response to 10 000 pressure cycles at the cycle assymetry R = 0 and pressure p max =6,3 MPa as well as the deformations during the burst test are described. The results of mechanical tests (tensile test and impact bend test) of the standardized specimens, metallographical investigation of structure and the fractographical analysis complete the work.

2.1. Material analysis

The analysis of chemical composition of the pipe was carried out with the help of SPEKTROMAXx optical-emmission spectrometer. The amounts of the principal elements are given in Table 1. It follows from the analysis that the material is an unalloyed low-carbon killed steel with low phosphorus content. It is 11 353 (St35) steel suitable for the manufacture of seamless tubes for transportation systems.

Table 1 Chemical composition of the pipe C [wt. %]

Si [wt. %]

Mn [wt. %]

P [wt. %]

S [wt. %]

DN 300

0,121

0,291

0,349

0,013

0,024

Structure of the pipeline was evaluated in the circumferential and longitudinal direction. The specimens were brushed and polished. The structure was revealed by etching in 2 % Nital (2 ml HCl + 98 ml ethanol). No difference between the structure in the longitudinal and circumferential direction was found. The structure is ferritic-pearlitic with the uniform size of feritic grains. Pearlite colonies have typical lamellar morphology with alternating ferrite and cementite lamellas. In the centre of the specimen, a row-like structure was observed in the rolling direction. Tensile test at room temperature according to ISO 6892-1 Standard was performed using EUS 40 testing machine on two specimens taken in the longitudinal direction and on two ones taken in the circumferential direction. The DDA 50 – EU extensometer with L 0 = 85 mm was used for measuring the deformations. The values of yield point (σ y ), ultimate tensile strength (σ uts ) and elongation are summarized in Table 2 and meet the 11 353 (St35) steel requirements. Impact bend test according to ISO 148-1 Standard was performed using PSWO 30 impact testing machine at 20°C and 0°C on three specimens taken in the longitudinal direction and on three specimens taken in the circumferential direction (see Table 2). The evaluation of impact bend test was carried out according to ISO 3183 Standard (Steel tubes for pipeline transportation systems) in which the minimum values of the impact bend test are specified. Although this Standard does not mention 11 353 steel and its equivalent St35, it is quite sufficient for the evaluation of the minimum absorbed energy as the steel belongs to the category of steels with the yield point between 175 MPa and 415 MPa and nominal external diameter up to 508 mm. For these steels, the minimum absorbed energy (CVN) 27 (20) J at 0 °C, which is the limit of their techical application, is determined. Therefore, the values of absorbed energy (CVN) 82 (78) J in the longitudinal direction and 159 (155) J in the circumferential direction quite meet the ISO 3183 Standard requirements. Table 2 Mechanical properties of the pipe

σ uts [MPa]

σ y [MPa]

Elong. [%]

CVN 20°C [J]

CVN 0°C [J]

Direction of specimens

longitudinal

299 271

424 432

31

91,8

81,8

circumferential

26,5

160,8

158,5

2.2. Measuring dimension of the dents and location of the strain gauge

Measurement of dents profiles was carried out in two directions (circumferential and longitudinal perpendicular to the pipe axis) with the help of digital slide caliper. The measurement of the depth of dent profile began always at the edge between the dent and unaffected surface. Measurements were carried out in both directions in points distant 10 mm from each other. The highest value of the depth of dent A (13,11 mm) was found in its centre – Fig. 2. To measure the deformations on the dent A, six strain gauges, circumferentially oriented and depicted in Fig. 3, were used. Strain gauge 19 was situated across the crack indicated by the non-destructive testing. Strain gauges 20 – 22 were situated inside the dent profile and distant 20 mm from each other. Strain gauge 23 was situated on the edge between the dent and unaffected surface. Strain gauge 24 was situated about 100 mm from strain gauge 23 to measure reference circumferential deformations.