Effect of Mn, Ni, Mo Contents on Microstructure Transition and Low Temperature Toughness of Weld Metal for K65 Hot Bending Pipe

doi:10.11900/0412.1961.2016.00403

Acta Metall Sin

2017, Vol. 53

Issue (6): 657-668 DOI: 10.11900/0412.1961.2016.00403

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Effect of Mn, Ni, Mo Contents on Microstructure Transition and Low Temperature Toughness of Weld Metal for K65 Hot Bending Pipe

Liming DONG^1,²(

),Li YANG¹,Jun DAI¹,Yu ZHANG²,Xuelin WANG³,Chengjia SHANG³

1 College of Automotive Engineering, Changshu Institute of Technology, Changshu 215500, China
2 Institute of Research of Iron and Steel, Sha-Steel, Zhangjiagang 215625, China
3 College of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China

Cite this article:

Liming DONG,Li YANG,Jun DAI,Yu ZHANG,Xuelin WANG,Chengjia SHANG. Effect of Mn, Ni, Mo Contents on Microstructure Transition and Low Temperature Toughness of Weld Metal for K65 Hot Bending Pipe. Acta Metall Sin, 2017, 53(6): 657-668.

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Abstract

To increase transport efficiency and to lower the costs of pipeline construction, longitudinally submerged arc welded (LSAW) pipes with larger diameters and thicker walls have been increasingly used by the pipeline industry. For example, in Russia, the LSAW pipeline in the Bovanenkovo-Ukhta project was recently constructed with K65 steel (the highest grade of the Russian natural gas pipeline), which is similar in specifications and yield strength requirement (550 MPa grade) to API X80 but has a stricter low temperature toughness value of 60 J at -40 ℃ (compared to -20 ℃ for API X80 grade) due to the extreme Arctic environment. Although weld metal with acicular ferrite (AF) has been developed to meet the requirement of low temperature toughness, the main objective of the present work was to clarify the microstructural evolution and the resulting changes in mechanical properties after the bending process. Hot bending pipes are necessary links in the construction of pipeline lying, which make more strin gent standards for the strength and low temperature toughness. That puts forward a challenge especially to the weld bead because of the deterioration of toughness during the hot bending process. In this work, submerged arc welding wire with high strength and toughness was developed for K65 hot bending pipes, and the alloying elements of Mn, Ni, Mo were considered to estimate the microstructure evolution and the effect of low temperature toughness for the weld metal. The results showed the low temperature toughness at -40 ℃ reached 90~185 J and 65~124 J for weld metal of straight seam pipe and hot bending pipe respectively, which reflect the excellent role of alloying elements of Mn, Ni, Mo. Microstructure characterization revealed that the weld metal, which originally consisted mainly of AF in the as-deposited condition, became predominantly composed of bainitic ferrite (BF) after hot bending. In addition, the large size cementite along the grain boundary was also the reason for the deterioration of toughness. It is found that reaustenisation caused a small austenite grain-sized matrix, which brought about a very high volume fraction of bainite. However, the low temperature toughness for hot bending pipe was improved to 124 J for the weld metal with 0.2%Mo, in which about 67.1% of high angle grain boundary were found. It is clear that the process of reaustenitisation during the bending process plays an important role in successful microstructural design for the steel weld metals.

Key words: pipe line steel submerged arc welding wire hot bending process weld metal low temperature toughness acicular ferrite

Received: 08 September 2016

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	Liming DONG
	Li YANG
	Jun DAI
	Yu ZHANG
	Xuelin WANG
	Chengjia SHANG

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https://www.ams.org.cn/EN/10.11900/0412.1961.2016.00403 OR https://www.ams.org.cn/EN/Y2017/V53/I6/657

Fig.1 Schematic of the industrial hot pipe bending process

Fig.2 Schematic of thermal simulation including bending and tempering for samples (WM-Q means the quenching process of the weld; WM-QT means the quenching and tempering process of the weld)

Table 1 Mechanical properties of K65 and X80 pipeline steels^[14]

Table 2 Submerged arc welding parameters

Fig.3 Macrograph of the weld joints and schematic of the positions for tests

Fig.4 OM image of K65 base metal

Table 3 Chemical compositions of the weld metals(mass fraction / %)

Table 4 Tensile properties of the weld metal and hardnesses at different conditions

Fig.5 Impact energies of WM and WM-QT at -40 ℃

Fig.6 OM images of 2# and 3# weld metals at different conditions (AF—acicular ferrite, BF—upper bainite ferrite, GBF—grain boundary ferrite, FSP—ferrite side-plate) (a) 2# WM (b) 3# WM (c) 2# WM-QT (d) 3# WM-QT

Fig.7 OM images of 3# weld metal for quenching (Q) condition and hot bending pipe (HBP)(a) 3# WM-Q (b) 3# WM-HBP

Fig.8 OM images of austenite grain boundaries in 3# weld metal(a) 3# WM(b) 3# WM-QT(c) 3# WM-HBP

Fig.9 OM images of martensite/austenite (M/A) in 3# weld metals(a) 3# WM (b) 3# WM-Q (c) 3# WM-QT (d) 3# WM-HBP

Fig.10 Volume fractions of M/A islands and their average sizes in samples of Fig.9

Fig.11 TEM images of 3# weld metal at quenching and tempering conditions(a) 3# WM (b) 3# WM-Q (c~e) 3# WM-QT

Fig.12 EBSD characterizations of 3# weld metal
(a) Euler map of 3# WM (b) Euler map of 3# WM-QT (c) distribution of boundary misorientation (d) distribution of effective grain size

Fig.13 Impact fracture SEM images of 3# weld metal at -40 ℃(a) 3# WM (b) EDS of the inclusion in Fig.13a (c) 3# WM-QT (d) 3# WM-HBP

Fig.14 OM images of the crack propagation(a) 3# WM (b) local enlarged image of the crack in Fig.14a(c) 3# WM-QT (d) local enlarged image of the crack in Fig.14c

Fig.15 Schematics indicating cleavage crack propagation and deflection(a) AF (b) AF+BF (c) BF

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