PSI - Issue 22

Li Yan-hua et al. / Procedia Structural Integrity 22 (2019) 118–124 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

119

2

Keywords: X90; line pipe; strain aging; aging sensitive coefficient; microstructure

1. Introduction Strain aging refers to the phenomenon that when subjected to plastic deformation and heated to a certain temperature, the strength and hardness of low carbon steel would increase, and the ductility and toughness would decrease[1]. A certain amount of plastic deformation would occur to line pipe during the production process of submerged arc welded pipe. At the same time, local deformation and natural aging would occur when the actual using area of line pipe are earthquake zone or permafrost region where geological conditions is bad. According to the literature [2, 3], For every 1% increase in plastic deformation, the ductile-brittle transition temperature of pipeline steel would increase by more than 5 ℃ . Three polyethylene or epoxy powder coating should be coated on the surface of the pipe body. During the coating process, the pipe surface needs to be heated to 200 ℃ to 250 ℃ , and then strain aging would occur. Strain aging have significant effect on the mechanical properties of line pipe for inducing increase of yield strength and yield ratio, so as to affect the safe use. As a new line pipe product, the research on X90 line pipe is very limited. As a result, it is quite necessary to study the strain aging phenomenon of line pipe. 2. Experimental materials Taking three X90 SAW pipes as the experimental material, the size of the pipe is Φ1219mm×16.3mm, and the chemical composition analysis result of the pipe is shown in Table 1.

Table 1. Chemical composition analysis result of the X90 SAW pipes (Wt/%) Chemical composition C Nb V Ti

Cu

N

A

0.056

0.08

0.033

0.017

0.25

0.0041

B

0.045

0.098

0.013

0.014

0.24

0.0037

C

0.041

0.085

0.0055

0.015

0.24

0.0040

3. Charpy impact test The relationship between aging conditions and Charpy impact test result of pipe body in transversal direction is shown in Table 2.

Table 2. Charpy impact test result of pipe body in transversal direction corresponding to different aging conditions

Aging at 200 ℃ for 5

Aging at 250 ℃ for 1h

Aging conditions Without aging

Mins

CVN /J (-10 ℃ )

321

298

307

CVN/J (-10 ℃ )

334

307

310

CVN /J (-10 ℃ )

307

285

292

It can be obtained from Table 2 that the impact energy would decrease significantly after the introduction of