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Acta Metall Sin  2019, Vol. 55 Issue (5): 575-584    DOI: 10.11900/0412.1961.2018.00375
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Influence of Shielding Gas Composition on Microstructure Characteristics of 1000 MPa Grade Deposited Metals
Tongbang AN1,2(),Jinshan WEI1,Jiguo SHAN2,Zhiling TIAN1
1. Welding Institute, Central Iron & Steel Research Institute, Beijing 100081, China
2. Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China
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Abstract  

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.

Key words:  shielding gas composition      1000 MPa grade deposited metal      oxide inclusion      microstructure characteristics     
Received:  16 August 2018     
ZTFLH:  TG442.3  
Fund: National Key Research and Development Program of China(2017YFB0305100);National Key Research and Development Program of China(2017YFB0305105)
Corresponding Authors:  Tongbang AN     E-mail:  antongbang@cisri.com.cn

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.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2018.00375     OR     https://www.ams.org.cn/EN/Y2019/V55/I5/575

Shielding gasCSiMnNi+Cr+MoTiONFe
95%Ar+5%CO20.0970.521.673.880.0700.0210.0034Bal.
90%Ar+10%CO20.0890.481.543.830.0650.0270.0032Bal.
80%Ar+20%CO20.0890.471.463.800.0450.0320.0036Bal.
70%Ar+30%CO20.0870.431.383.690.0430.0400.0034Bal.
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%CO21173103849.317.3404.250.88
90%Ar+10%CO2110194255.033.0381.470.86
80%Ar+20%CO2116098072.652.0366.710.84
70%Ar+30%CO2104492157.647.6358.390.88
  
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%CO20.33161.11810.00.08
90%Ar+10%CO20.40231.3659.80.13
80%Ar+20%CO20.41371.40812.30.20
70%Ar+30%CO20.43381.44712.90.22
Table 3  General characteristics of inclusions observed in deposited metals with different shielding gases
Shielding gasPositionOAlSiSTiMn

95%Ar+5%CO2

Center16.5312.823.142.1144.0421.32
Verge18.258.428.834.0630.7529.66

90%Ar+10%CO2

Center17.839.087.931.1334.6829.32
Verge18.516.0111.644.5220.1839.11

80%Ar+20%CO2

Center20.368.582.891.2145.6320.32
Verge15.176.0415.564.2117.3841.63

70%Ar+30%CO2

Center14.393.946.311.0238.0636.25
Verge14.862.6615.684.1817.6344.98
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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