1. Welding Institute, Central Iron & Steel Research Institute, Beijing 100081, China 2. Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China
Cite this article:
Tongbang AN,Jinshan WEI,Jiguo SHAN,Zhiling TIAN. Influence of Shielding Gas Composition on Microstructure Characteristics of 1000 MPa Grade Deposited Metals. Acta Metall Sin, 2019, 55(5): 575-584.
In recent years, high and ultra-high strength steels have been developed and used in light-weight constructions such as the structural members of mobile equipment in order to reduce weight and fabrication costs and to enhance the performance. Welding of steels with yield strength of more than 900 MPa is particularly challenging because of the toughness requirements for the weld metal, which calls for welding consumables of high strength and good toughness. Weld metals have been produced for a variety of welding methods with yield strength up to or above 1000 MPa, but their impact toughness remained only at medium yield strengths. Proper microstructure is the key to meeting this requirement, and its final microstructure depends on the chemical composition and cooling rate. For the deposited metal produced by gas metal arc welding (GMAW), the composition is dependent on the welding wire and shielding gas. The cooling rate of the weld metal is controlled by a combination of heat input and heat extraction. It is known that the addition of CO2 to argon based shielding gas is effective for improvement of productivity in GMAW welding of steel. Through the chemical reaction in the welding arc, CO2 in the shielding gas can affect the chemical composition of the weld metal, and its microstructure. The 1000 MPa grade deposited metals was welded with GMAW, and the effects of shielding gas composition (Ar+(5%~30%)CO2, volume fraction) on the general compositional and microstructural characteristics of deposited metals, including nonmetallic inclusions, were experimentally characterized with SEM, EBSD and TEM. The microstructure of the deposited metals is mainly composed of martensite and bainite. With the increase of CO2 content (5%~30%), the strength of the deposited metals decrease slightly and the impact toughness increases first and then decreases. Meanwhile, the transformation range (B50-Ms) of the deposited metal increases, the bainite content increases (8%~29.6%) with the quantity of inclusions that are suitable for bainite nucleation increases, and the nucleation position changes from the original austenite grain boundary to the common nucleation on the original austenite grain boundary and inclusions within the grain. At the same time, the microstructure morphology of the deposited metal changes from parallel to intertexture, which presented an intersected configuration and microstructure refinement.
Table 1 Chemical compositions of deposited metals with different shielding gases(mass fraction / %)
Shielding gas
Rm
MPa
Rp0.2
MPa
Charpy absorbed energy / J
Vickers hardness
kg·mm-2
γ
RT
-40 ℃
95%Ar+5%CO2
1173
1038
49.3
17.3
404.25
0.88
90%Ar+10%CO2
1101
942
55.0
33.0
381.47
0.86
80%Ar+20%CO2
1160
980
72.6
52.0
366.71
0.84
70%Ar+30%CO2
1044
921
57.6
47.6
358.39
0.88
Fig.1 SEM images of the as-deposited top beads with different shielding gases (Bc—coalesced bainite)(a) 95%Ar+5%CO2 (b) 90%Ar+10%CO2 (c) 80%Ar+20%CO2 (d) 70%Ar+30%CO2
Fig.2 Bright-field TEM image of martensite/bainite lath structure of deposited metal by gas metal arc welding (GMAW) with 95%Ar+5%CO2 (a), bright-field TEM image of residual austenite in deposited metals by GMAW with 80%Ar+20%CO2 (b) and dark-field TEM image of residual austenite in deposited metals by GMAW with 80%Ar+20%CO2 (Inset is the corresponding select area electron diffraction (SAED) pattern of residual austenite) (c)
Fig.3 EBSD orientation maps of the martensite/bainite block substructure in the deposited metal with different shielding gases and inverse pole figure (insets)Color online(a) 95%Ar+5%CO2 (b) 90%Ar+10%CO2 (c) 80%Ar+20%CO2 (d) 70%Ar+30%CO2
Fig.4 BSE-SEM images of inclusions in the deposited metal with different shielding gases(a) 95%Ar+5%CO2 (b) 90%Ar+10%CO2 (c) 80%Ar+20%CO2 (d) 70%Ar+30%CO2
Shielding gas
Average diameter
μm
Maximum size
μm
Number density
104 mm-2
Area fraction
%
95%Ar+5%CO2
0.3316
1.118
10.0
0.08
90%Ar+10%CO2
0.4023
1.365
9.8
0.13
80%Ar+20%CO2
0.4137
1.408
12.3
0.20
70%Ar+30%CO2
0.4338
1.447
12.9
0.22
Table 3 General characteristics of inclusions observed in deposited metals with different shielding gases
Shielding gas
Position
O
Al
Si
S
Ti
Mn
95%Ar+5%CO2
Center
16.53
12.82
3.14
2.11
44.04
21.32
Verge
18.25
8.42
8.83
4.06
30.75
29.66
90%Ar+10%CO2
Center
17.83
9.08
7.93
1.13
34.68
29.32
Verge
18.51
6.01
11.64
4.52
20.18
39.11
80%Ar+20%CO2
Center
20.36
8.58
2.89
1.21
45.63
20.32
Verge
15.17
6.04
15.56
4.21
17.38
41.63
70%Ar+30%CO2
Center
14.39
3.94
6.31
1.02
38.06
36.25
Verge
14.86
2.66
15.68
4.18
17.63
44.98
Table 4 EDS analyses of chemical compositions of inclusions in deposited metals with different shielding gases(mass fraction / %)
Fig.5 Color OM images of the as-deposited top beads with different shielding gasesColor online(a) 95%Ar+5%CO2 (b) 90%Ar+10%CO2 (c) 80%Ar+20%CO2 (d) 70%Ar+30%CO2
Fig.6 Variation of B50 and Ms of deposited metals as function of CO2 content in the shielding gas (B50—temperature above which bainitic transformation never proceeds over 50% in volume, Ms—starting temperature of martensitic transformation)
Fig.7 Schematics of microstructure transformation mechanism of deposited metals with different shielding gases(a) vertical cross-section phase diagrams of deposited metal(b) schematic of solidification model of weld pool metal(c) 95%Ar+5%CO2 (d) 90%Ar+10%CO2 (e) 80%Ar+20%CO2 (f) 70%Ar+30%CO2
Fig.8 SEM (a, b) and TEM (c) images of bainite plates growing from one inclusion(a) 90%Ar+10%CO2 (b, c) 80%Ar+20%CO2
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