PSI - Issue 17

Jan Kec et al. / Procedia Structural Integrity 17 (2019) 230–237 Jan Kec / Structural Integrity Procedia 00 (2019) 000 – 000

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SPECTROMAXx conformably to the internal test procedure ZP 04-31:2015. The specimens for microstructural analysis were taken in circumferential and longitudinal direction with regard to the pipeline position. All specimens were hot embedded into conductive bakelite resin with carbon filler. After the embedding, the specimens were prepared by machine grinding and polishing. Structure was revealed by etching in 2 % Nital (98 ml ethanol + 2 ml HNO 3 ). The microstructure was examined by optical microscope (LM) Zeiss Axio Observer Z1M (SEM) with the motorized table and scanning electron microscope (SEM) Zeiss EVO MA 10 equipped with Bruker energy dispersion X-ray spectrometer (EDXS). Tensile test at room temperature according ČSN EN ISO 6892 -1:2017 Standard was carried out on EUS 40 testing machine. Deformations were measured with the help of DDA 50-EU = 100 mm strain gauge. Loading rate during the test was 20 MPa/s. Charpy V- notch impact test was carried out in compliance with ČSN EN ISO 148 -1:2017 Standard by means of PSWO 30 pendulum impact testing machine at temperatures 40, 20, 0, -20, -40 and -60 °C. 2.2. Results and discussion The results of analysis of the chemical composition of the steel are given in Table 1. They show that there is a low carbon steel alloyed with Si and Mn. A somewhat higher carbon content suggests that the steel was hot rolled and normalized. The steels used for gas pipelines are usually alloyed with 0,1 wt. % to 0,5 wt. % Si, owing to deoxidization and solid solution strengthening. On the other hand, higher amount of silicon causes the decrease of the absorbed energy and increase of DBTT. The strengthening effect of Mn is smaller, but, at higher amounts, it causes a favorable decrease of DBTT. Higher amounts of Mn lead to the formation of bainitic structures required for high strength gas pipelines – e.g. X70M – X100M (DeArdo at el. 2009).

Table 1. Chemical composition of base material and weld metal Specimen

Element content [wt. %]

C

Si

Mn

V

Nb

Ti

P

S

Base material Weld metal

0,18 0,14

0,41 0,38

1,47 1,46

0,10 0,06

0,03 0,02

- -

0,020 0,023 max. 0,025

0,007 0,010 max. 0,015

Composition of X60 steel according to ČSN EN ISO 3183:2014 Standard

max. 0,24 max. 0,45 max. 1,40 max. 0,1 max. 0,05 max. 0,04

The investigated X60 gas pipeline steel did not show discontinuous yielding during the tensile test; from this reason, the yield points at 0,2 % plastic deformation and 0,5 % total deformation (the latter is usually considered in the gas industry) were evaluated. It is generally known that the discontinuous yielding in steels results from dislocation-locking or carbon-locking mechanism. Discontinuous yielding also shows that the dislocation density in this steel is very low (Wang at el. 2001) and, consequently, the mobility of dislocation can be high and enables a higher deformation increment in the course of room temperature creep. Chen et al. 2002 even suggest that the exhausted creep deformation can positively increase the resistance to SCC growth. Nevertheless, the measured values of yield point, ultimate strength and ductility meet the requirements for X60 steel.

Table 2. Results of the tensile test Specimen designation

R t0,5 [MPa]

R m [MPa]

A [%]

Z [%]

R p0,2 [MPa]

Specimen orientation

T1 T2 T3 T4

411 440 438 444

411 440 474 444 415

625 621 631 636 520

27 25 25 25 19

57 56 57 54

Longitudinal

Circumferential

Strength properties of X60 steel according ČSN EN ISO 3183:2014 Standard

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