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.
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.
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)
Steel
Rp0.5
Rm
Z
Impact energy / J
MPa
MPa
%
T
BM
WM
℃
Single
Average
Single
Average
K65
555~665
≥640
≥18
-40
≥150
≥200
≥42
≥ 56
X80
555~690
≥625
-
-10
≥140
≥180
≥80
≥ 90
Table 1 Mechanical properties of K65 and X80 pipeline steels[14]
Bead
Wire-1
Wire-2
Wire-3
Wire-4
Welding
Heat input
Current
Voltage
Current
Voltage
Current
Voltage
Current
Voltage
speed
(η=0.9)
A
V
A
V
A
V
A
V
cmmin-1
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 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
No.
C
Si
Mn
Ni
Mo
P
S
Others
Fe
1#
0.063
0.21
1.60
1.19
0.132
0.010
0.0046
0.305
Bal.
2#
0.063
0.21
1.60
1.39
0.127
0.011
0.0050
0.307
Bal.
3#
0.067
0.22
1.81
1.13
0.256
0.011
0.0054
0.301
Bal.
4#
0.068
0.23
1.99
1.17
0.191
0.011
0.0057
0.313
Bal.
Table 3 Chemical compositions of the weld metals(mass fraction / %)
No.
Rp0.5
Rm
Z
Hardness / HV10
WM
WM-QT
MPa
MPa
%
1#
583
723
21.8
231
230
2#
606
722
23.5
238
240
3#
647
714
22.0
244
253
4#
689
768
21.7
250
259
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
[1]
Gao H L.The challenges for pipeline projects & development trend of pipeline steel[J]. Weld. Pipe Tube, 2010, 33(10): 5
[1]
(高惠临. 管道工程面临的挑战与管线钢的发展趋势[J]. 焊管, 2010, 33(10): 5)
[2]
Stalheim D G.The use of high temperature processing (HTP) for high strength oil and gas transmission pipeline application [A]. Proceedings of the 5th steels Conference[C]. Iron Steel, 2005, 40: 699
[3]
Niu J, Liu Y L, Feng Y R, et al.Low temperature embrittlement of X80 steel weld after heat treatment[J]. Hot Work. Technol., 2010, 39(19): 15
Keehan E, Karlsson L, Andren H O, et al. New developments with C-Mn-Ni high-strength steel weld metals, Part A——Microstructure [J]. Weld. J., 2006,85: 200.s
[5]
Keehan E, Karlsson L, Andrén H O.Influence of carbon, manganese and nickel on microstructure and properties of strong steel weld metals: Part 1——Effect of nickel content[J]. Sci. Technol. Weld. Join., 2006, 11: 1
[6]
Keehan E, Karlsson L, Andrén H O, et al.Influence of carbon, manganese and nickel on microstructure and properties of strong steel weld metals: Part 2——Impact toughness gain resulting from manganese reductions[J]. Sci. Technol. Weld. Join., 2006, 11: 9
[7]
Bhole S D, Nemade J B, Collins L, et al.Effect of nickel and molybdenum additions on weld metal toughness in a submerged arc welded HSLA line-pipe steel[J]. J. Mater. Process. Technol., 2006, 173: 92
[8]
Zhang M, Yao C W, Fu C, et al.Submerged arc welding wire matched with X80 pipeline steel[J]. Trans. China Weld. Inst., 2006, 27(4): 64
Pan X, Wang Y B, Zhang Y.Development of submerged arc welding wire for third generation pipeline X90 [A]. The National Metal Products Information Network Twenty-Third Annual Meeting and the 2013 Metal Products Industry Information Technology Symposium[C]. Wuxi: The Chinese Society for Metals, 2013
Bi Z Y, Liu H Z, Jing X T, et al.Research on submerged arc welding wire for X100 pipeline steel[J]. China Weld., 2011, 20(2): 56
[11]
Zhang X L, Liu Y L, Feng Y R, et al.Relationship of microstructure and toughness index of reheated high grade pipeline steels[J]. Dev. Appl. Mater., 2008, 23(1): 1
[12]
Arai Y, Kondo K, Hirata H, et al.Metallurgical design of newly developed material for seamless pipes of X80-X100 grades [A]. ASME 2007 26th International Conference on Offshore Mechanics and Arctic Engineering Volume 4: Materials Technology; Ocean Engineering[C]. San Diego, California, USA: ASME, 2007, 37
[13]
Wu D Y, Han X L, Tian H T, et al.Microstructural characterization and mechanical properties analysis of weld metals with two Ni contents during post-weld heat treatments[J]. Metall. Mater. Trans., 2015, 46A: 1973
[14]
Yang W W, Zhao J, Jiao B, et al.Analysis and comparison of the K65 steel grade standards[J]. Weld. Pipe Tube, 2013, 36(7): 67
Wang X L, Dong L M, Yang W W, et al.Effect of Mn, Ni, Mo proportion on micro-structure and mechanical properties of weld metal of K65 pipeline steel[J]. Acta Metall. Sin., 2016, 52: 649
Abson D J, Pargeter R J.Factors influencing as-deposited strength, microstructure, and toughness of manual metal arc welds suitable for C-Mn steel fabrications[J]. Int. Met. Rev., 1986, 31: 141
[18]
Babu S S.The mechanism of acicular ferrite in weld deposits[J]. Curr. Opin. Solid State Mater. Sci., 2004, 8: 267
[19]
Li Y, Baker T N.Effect of morphology of martensite-austenite phase on fracture of weld heat affected zone in vanadium and niobium microalloyed steels[J]. Mater. Sci. Technol., 2010, 26: 1029
[20]
Thomas G.Retained austenite and tempered martensite embrittlement[J]. Metall. Mater. Trans., 1978, 9A: 439
[21]
Hwang B, Kin Y G, Lee S, et al.Effective grain size and charpy impact properties of high-toughness X70 pipeline steels[J]. Metall. Mater. Trans., 2005, 36A: 2107
[22]
Padap A K, Chaudhari G P, Pancholi V, et al.Microstructural evolution and mechanical behavior of warm multi-axially forged HSLA steel[J]. J. Mater. Sci., 2012, 47: 7894
[23]
Yan W, Zhu L, Sha W, et al.Change of tensile behavior of a high-strength low-alloy steel with tempering temperature[J]. Mater. Sci. Eng., 2009, A517: 369
[24]
Edmonds D V, He K, Rizzo F C, et al. Quenching and partitioning martensite——A novel steel heat treatment [J]. Mater. Sci. Eng., 2006, A438-440: 25
[25]
Yang J R, Yang C C, Huang C Y.The coexistence of acicular fe-rrite and bainite in an alloy-steel weld metal[J]. J. Mater. Sci. Lett., 1992, 11: 1547
[26]
Yang J R, Huang C Y, Huang C F, et al.Influence of acicular fe-rrite and bainite microstructures on toughness for an ultra-low-carbon alloy steel weld metal[J]. J. Mater. Sci. Lett., 1993, 12: 1290
