Effect of Active Gas on Weld Shape and Microstructure of Advanced A-TIG-Welded Stainless Steel

doi:10.1007/s40195-016-0381-6

Abstract

Abstract:

Advanced A-TIG method was conducted to increase the weld penetration and compared with the conventional TIG welding process. A two-pipeline setup was designed to apply Ar + CO₂ mixed gas as the outer layer, while pure argon was applied as the inner layer to prevent any consumption of the tungsten electrode. The results indicate that the presence of active gas in the molten pool led to the change in the temperature coefficient of surface tension so that the Marangoni convection turns inward and forms a deep weld zone. The increase in gas flow rate causes a decrease in the weld efficiency which is attributed to the increase in oxygen content in the weld pool and the formation of a thicker oxide layer on the weld surface. Moreover, the stir and the temperature fluctuation, led by double shielding gas, create more homogeneous nucleation sites in the molten pool so that a fine grain microstructure was obtained.

Key words: Advanced A-TIG welding, Weld metal, Active gas, Microstructure, Marangoni convection

Reza Nakhaei, Alireza Khodabandeh, Hamidreza Najafi. Effect of Active Gas on Weld Shape and Microstructure of Advanced A-TIG-Welded Stainless Steel[J]. Acta Metallurgica Sinica (English Letters), 2016, 29(3): 295-300.

Figures/Tables 10

Table 1 Chemical composition of base alloy SS304L (in wt%)

C	Cr	Ni	Mn	Mo	Si	P	N	Trace elements	Fe
0.014	18.27	9.26	1.43	0.22	0.4	0.02	0.06	<0.05	Bal.

Fig. 1 Schematic diagram of advanced A-TIG welding process using a specific torch

Table 2 Welding parameters for different samples

Sample code	Welding current (A)	Inner gas	Outer gas	Gas flow rate (L/min)
1	130	Ar	-	5
2	150	Ar	-	5
3	130	Ar	Ar + 0.5 vol% CO₂	5
4	150	Ar	Ar + 0.5 vol% CO₂	5
5	130	Ar	Ar + 2.0 vol% CO₂	5
6	150	Ar	Ar + 2.0 vol% CO₂	5
7	150	Ar	Ar + 2.0 vol% CO₂	8

Fig. 2 Geometric characteristics of weld metal: a sample 1; b sample 2; c sample 3; d sample 4; e sample 5; fsample 6; g sample 7

Table 3 Weld dimensions of different samples (in mm)

Sample code	Weld depth	Weld width	Weld D/W ratio
1	2	7.8	0.26
2	2.5	9.2	0.28
3	6	5.1	1.17
4	6	6.8	0.88
5	2.8	5.9	0.47
6	6	8.6	0.7
7	2.9	9.7	0.3

Fig. 3 Oxygen content in the welded samples

Fig. 4 Microstructure of the fusion zone profile a lathy ferrite, b skeletal ferrite made by conventional TIG

Fig. 5 Effect of active gas on the microstructure of weld metal: a sample 4, Ar + 0.5 vol% CO2. b Sample 6, Ar + 2.0 vol% CO2

Fig. 6 Ferrite fraction in the weld metal of different samples

Fig. 7 a Schematic of different regions according microhardness measurement of the joint zone and bmicrohardness profiles of the welded samples

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