®COPYRIGHT
2000 THE ESAB GROUP, INC. LESSON
VI 6.3
WIRE SELECTION FOR GAS SHIELDED ARC
WELDING
6.3.0.1 When
selecting the wire or filler metal for either the GMAW or GTAW process,
several things must be considered.
1. Mechanical
Properties - The wire
chosen must produce weld metal having approxi- mately
the same mechanical properties as the base metal whether it is carbon steel or
low alloy high tensile steel.
2. Shielding
Gas - In Lesson II, we
learned that the shielding gases used in GTAW of carbon
steel are pure argon or argon helium mixtures. In GMAW, shielding gases
may be pure CO2,
or mixtures of argon, helium, CO2
and oxygen. The gas mixtures contain- ing
oxygen or CO2 will
exhibit oxidizing characteristics which, if they combine with
carbon, will form carbon monoxide gas
porosity in the weld metal. a. The
most common shielding gases used for welding mild and low alloy steels may
be classified in terms of their oxidizing
effect as shown in Figure 1. b. Each
of the following variables should be considered when selecting the proper gas
for a specific job:
• MATERIAL
TYPES •
WELD METAL MECHANICAL
- Carbon,
Stainless, Aluminum, etc. PROPERTIES
• MATERIAL
CONDITION •
JOB REQUIREMENT
- Rusty,
Oily, Primed, etc. -
Fit-Up •
TYPES OF METAL TRANSFER
- Penetration
- Short
Circuit, Spray, Pulse, etc. -
Spatter Levels
OXIDATION POTENTIAL OF COMMONLY USED
SHIELDING GASES FIGURE
1 Pure Argon or
98% Argon 75%
Argon Argon -
Helium 2% O2
25% CO2
Pure CO2
Mixtures Process
GTAW GMAW
GMAW GMAW
Degree of Non-
Slightly More
Most Oxidation
Oxidizing Oxidizing
Oxidizing Oxidizing
