PSI - Issue 20

Nikolay I. Golikov / Procedia Structural Integrity 20 (2019) 161–166 Nikolay I. Golikov / Structural Integrity Procedia 00 (2019) 000 – 000

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inner surface and tensile RWS is on the outer surface of the pipe. The level of RWS increases with an increase in heat input of welding. Pipes mainly used in the construction of gas pipelines have a ratio of the radius to the thickness of the pipe wall (r/t) in the range from 20 to 45. Thus, in the inner near-surface layers of the field circumferential butt welds of the pipes, tensile RWS is formed in the root weld zone. This is confirmed by experimental studies conducted in by Golikov and Dmitriev (2012) and by Golikov and Sidorov (2012). Herein, the effect of residual welding stress on the destruction of girth welds of sections of the main gas pipeline was studied. 2. Materials and methods Fragments of destroyed girth welds of sections of the main gas pipeline with a diameter of 530 mm were investigated. Low-alloyed steels of 09Mn2Si, 17Mn1Si, and 13Mn1Si-U grades were used in the construction of the pipeline (Table 1). Field joints are obtained by manual arc welding. Electrodes with the appropriate strength class were used in welding pipes. In all cases, the initiation of the destruction of welded joints was located in heat affected zones.

Table 1. Chemical composition and mechanical properties of steel pipes. Steel grades C Si Mn Ni S P

σ ts , MPa

Cr

Cu

Fe

δ, %

09Mn2Si 17Mn1Si

0,1

0,48 0,47 0,50

1,61

0,057 0,029

0,028 0,024

0,025 0,018

0,064

0,08 0,04 0,03

97,54 97,92 97,65

530 525 565

25 21 26

0,13 0,11

1,1

0,04 0,04

13Mn1Si-U

1,45

0,02

0,01

0,02

The determination of RWS was performed using the x-ray method based on the measurement of microstrains of the material crystal lattice caused by their action. The technique of measuring macrostresses using x-rays is given in the work of Vasiliev and Trofimov (1988). The residual stress in welded joints was determined using a portable x ray machine. In recent years, the X-ray method has been successfully used to study RWS by Dong et al. (2016), by Gurova et al. (2017), by Hemmesi et al. (2016), by Launert et al. (2017). Residual stress was measured in the σ z and σ θ directions at points located at different distances from the center of the seam. Mechanical tests on impact bending of the welded joint metal were accomplished according to GOST 6996-66. The impact strength was determined at test temperatures of (+20) °C, ( - 15) °C and ( - 40) °C on the Amsler RKP 450 pendulum impact testing machine. The samples had V-notches in the weld metal zone, the fusion line and the base metal. 3. Results and discussion Consider the destruction of the girth weld of the linear part of the underground gas pipeline with a diameter of 530 mm. A metal opening was detected along the pipeline from the outside with numerous branching cracks at the site of the field circumferential seam. After the accident, fragments of destruction were collected with a total length of 2160 mm. The girth weld is split across the seam into four separate sections: 1010, 235, 315, 127 mm, respectively. The total length of the weld around the perimeter is 1687 mm. The destruction was explosive without ignition. The propagation of cracks proceeded by the mechanism of separation at the site of the crack arrest, which changes to a shear mechanism with plastic components. The study of the fracture surface of the pipe destruction revealed that the site of the destruction is located on the inner side of the pipe perpendicular to the girth weld in the heat-affected zone. The fracture surface indicates long term crack development. Since the initiation of the destruction is located on the inner side of the pipe perpendicular to the girth weld in the heat affected zone, RWS of the inner near-surface layers of the pipe were investigated. The upper part of the pipeline underwent the destruction, so a sample of 600x700 mm was cut out of the section with the girth seam that