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Acta Metall Sin  2016, Vol. 52 Issue (6): 649-660    DOI: 10.11900/0412.1961.2015.00453
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EFFECT OF Mn, Ni, Mo PROPORTION ON MICRO-STRUCTURE AND MECHANICAL PROPERTIESOF WELD METAL OF K65 PIPELINE STEEL
Xuelin WANG1,Liming DONG2,Weiwei YANG3,Yu ZHANG2,Xuemin WANG1,Chengjia SHANG1()
1 School of Materials Science and Technology, University of Science and Technology Beijing, Beijing 100083, China
2 Institute of Research of Iron and Steel of Shasteel, Zhangjiagang 215625, China
3 CNPC Bohai Equipment Steel Pipe Research Institute, Cangzhou 062658, China
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Abstract  

Longitudinal submerged arc welding pipeline steels with heavy caliber and large wall thickness are widely applied in the oil gas transmission to enhance the transmission efficiency and save cost. K65 pipeline steels are the main material for the Bovanenkove-Ukhta oil & gas transmission project. It is required that the -40 ℃ low temperature toughness of weld metal and heat affected zone (HAZ) are over 60 J for K65 pipelines. This standard is much stricter than that of X80 pipelines. The pipeline with superior low temperature toughness is seldom investigated. In this work, the Mn-Ni-Mo-Ti-B alloy submerged arc welding wire with high strength and high tough ness was designed, which was favorable to obtain excellent low temperature toughness. The results showed that the weld metal had a good combination of strength and low temperature toughness, the yield strength was 583~689 MPa, the tensile strength was 714~768 MPa, and the impact absorbed energy at -40 ℃ was over 90 J. The wire with a diameter of 4.0 mm was suitable for double-sided submerged arc welding with four wires, and the -40 ℃ impact energy of HAZ was over 100 J. The microstructure of weld metal was primarily comprised of fine acicular ferrite (AF), proeutectoid grain boundary ferrite (GBF), ferrite side plates (FSP) and small martensite/austenite (M/A) constituents. The weld metal with 0.2%Mo can effectively restrain the formation of GBF and FSP, significantly refining the grain size. The increased Mn and Ni contents enhanced the low temperature toughness of weld metal by increasing the amount of acicular ferrite. However, the concentration of Mn and Ni should be controlled under a critical value; much more Mn and Ni additions would promote the formation of martensite or other low temperature microstructural features, which is detrimental to weld metal toughness. The optimum combination of alloying element content was (1.5%~2.0%)Mn, (0.9%~1.2%)Ni, (0.2%~0.25%)Mo. Excellent strength and toughness can be obtained through replacing Ni by Mn in the terms of the concentration of Mn and Ni being above the Ms line.

Key words:  K65 pipeline steel      weld metal      heat affected zone (HAZ)      acicular ferrite      martensite/austenite (M/A) constituent      low temperature impact toughness     
Received:  24 August 2015     
Fund: Supported by National Natural Science Foundation of China (No.51371001)

Cite this article: 

Xuelin WANG,Liming DONG,Weiwei YANG,Yu ZHANG,Xuemin WANG,Chengjia SHANG. EFFECT OF Mn, Ni, Mo PROPORTION ON MICRO-STRUCTURE AND MECHANICAL PROPERTIESOF WELD METAL OF K65 PIPELINE STEEL. Acta Metall Sin, 2016, 52(6): 649-660.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2015.00453     OR     https://www.ams.org.cn/EN/Y2016/V52/I6/649

Weld pass Wire-1 Wire-2 Wire-3 Wire-4 Velocity Heat input
Current Voltage Current Voltage Current Voltage Current Voltage cmmin-1 (η=0.9)
A V A V A V A V kJcm-1
Inside 950 33 850 36 750 40 600 42 110 57.5
Outside 1200 33 900 36 800 40 650 40 120 58.5
Table 1  Parameters of submerged arc welding
Weld metal C Si Mn Ni Mo P S Others
No.1 0.063 0.21 1.60 1.19 0.132 0.010 0.0046 0.305
No.2 0.063 0.21 1.60 1.45 0.127 0.011 0.0050 0.307
No.3 0.067 0.22 1.81 0.93 0.256 0.011 0.0054 0.301
No.4 0.068 0.23 1.99 1.17 0.191 0.011 0.0057 0.313
Table 2  Chemical compositions of the weld metal
Fig.1  Location of the measurement of mechanical properties in the test specimens (WM—weld metal, FL—fusion line, HAZ—heat affected zone, CGHAZ—coarse grained HAZ, FGHAZ—fine grained HAZ, ICHAZ—inter critical HAZ, ICCGHAZ—inter critical coarse grained HAZ, d—measuring distance)
Fig.2  OM image of hot-rolled K65 pipeline metal
Fig.3  OM images of weld metals of No.1 (a), No.2 (b), No.3 (c) and No.4 (d) (GBF—grain boundary ferrite, PCGB—prior columnar grain boundary, AF—acicular ferrite, FSP—ferrite sideplates)
Fig.4  OM images of M/A in weld metals of No.1 (a), No.2 (b), No.3 (c) and No.4 (d) (M/A—martensite/austenite)
Fig.5  Quantitative analysis of the weld metal microstructure
Fig.6  TEM images of AF (a, c, d, g), M/A (b, f, i, j, k), BF (e, h) in weld metals of No.1 (a, b), No.2 (c), No.3 (d~f), No.4 (g~k), and EDS analysis of inclusion in No.4 shown in Fig.6g (l) (BF—bainitic ferrite)
Fig.7  EBSD images of weld metals of No.1 (a) and No.2 (b), distributions of effective grain size (EGS) (c) and misorientation angle (d)
Fig.8  OM images of FL (a), CGHAZ (b), FGHAZ (c), ICHAZ (d), ICCGHAZ (e) and magnified view of ICCGHAZ (f) in weld joint of weld metal No.3
Fig.9  OM images of M/A constituents in CGHAZ (a), FGHAZ (b), ICHAZ (c) and ICCGHAZ (d) in weld joint of weld metal No.3
Weld metal Hardness / HV Yield strength / MPa Tensile strength / MPa Total elongation / %
No.1 231 583 723 21.8
No.2 238 606 722 23.5
No.3 244 647 714 22.0
No.4 250 689 768 21.7
Table 3  Mechanical properties and hardness of weld metals
Fig.10  Hardness distribution of weld joint of weld metal No.3 (BM—base metal)
Fig.11  Effect of Mn/Ni/Mo proportion on strength and toughness of weld metal
Fig.12  Ductile-brittle transition temperature (DBTT) of weld metals
Fig.13  SEM images of macro fracture (a), dimple (b) and quasi cleavage (c) of weld metal No.3 after Charpy impact at -80 ℃
Fig.14  Low temperature impact toughness of weld joint of weld metal No.3
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