Machinery's Handbook, 31st Edition
Gas Metal Arc Welding 1579 Shielding Gases for GMAW Welding of Stainless Steels.— The major problems encoun- tered when using GMAW on stainless steels of thinner than 14 gage include controlling potential melt-through, controlling distortion, and black oxidation on the weld surface. These three welding problems have a common denominator, which is heat. The key to welding thin stainless steel is to minimize the potential heat when welding, by appropriate choice of gas mixture. A popular gas mixture that is often recommended for GMAW welding of thin-gage stainless steel is the three-part helium gas mixture containing helium 90 + argon 7.5 + CO 2 2.5 percent. In contrast to gas mixtures without helium, the helium tri-mixture requires the use of higher voltages to sustain the arc, which adds unnecessary heat to the heat-sensitive thin-gage welds. A practical and lower-cost alternative for GMAW short-circuit transfer on stainless steels is an argon mixture with 2 to 4 percent CO 2 . The argon + CO 2 mixture allows use of lower voltages than is practical with argon/helium mixtures, and the lower voltages resulting from the argon + CO 2 mixture will help to reduce distortion and oxidation, and decrease the melt-through potential. The mixture that works with short-circuit transfer is also a logical practical choice for spray transfer welding of stainless steel because it is less oxidizing than argon/oxygen mixtures. Table 3 provides practical gas mixture recommendations for specific applications. Table 3. Shielding Gases for Welding Carbon Steels and Stainless Steels
Gas mixtures
Argon + CO 2 + Oxygen
Argon + 2–4% CO 2
Argon + 6–10% CO 2
Argon + 13–20% CO 2
Argon + 25% CO 2
Argon + Oxygen
Application
Short-circuit melt- through problems; less than 20 gage Short-circuit 18 to 11 gage Spray if mill scale or surface problems; carbon steels Spray if low energy required; carbon steel Spray, best impact strengths, lowest porosity; carbon steels Best single gas mixture for carbon steels Spray; stainless steels Best single gas mixture for stainless and duplex steels Short-circuit; stainless steels
1
1
1
1
2
3
… … … … 1
1
… … … … 1
2
1
1
1
1
… …
… … … … 1
…
… … … … 1
…
… … 1
… … … … … … … … …
2
… 1
… … 1
Preferred choice of shielding gas is 1, followed by 2 and 3. For GMAW spray transfer welding of stainless steels thicker than 11 gage, the traditional GMAW shielding gas has been argon 98 + oxygen 2 percent. The argon + oxygen mixture provides excellent, stable, spray transfer, but the oxygen promotes oxidation, leaving the weld with a black surface. To reduce the oxidation, the 2 percent oxygen can be replaced with the less oxidizing 2–4 percent CO 2 . Shielding Gases for GMAW Welding of Aluminum.— For GMAW welding of alumi num, helium is added to argon to provide additional weld energy, increasing penetration
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