Instructions / Assembly

34
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GMAW
Selecting Carbon and Low Alloy Steel Electrodes
Selecting the proper filler metal for use with GMAW is similar to
t
he process that must be employed when determining the
a
pplicable electrode for any welding process:
•
The electrode must deposit welds that will have the mechanical
a
nd chemical properties needed for the proper performance
of the welded connection.
•
The electrode must be suitable to address the unique
circumstances associated with the particular application.
• The electrode must meet the welder’s expectations in terms
of arc action and puddle control.
• The selected electrode should result in the total overall lowest
cost of welding. This is not to say that the lowest cost
electrode will always lead to the total overall lowest cost; in
many cases, a higher purchase priced electrode will result in
decreased overall costs.
A Two Component System
GMAW is a two-component welding process, in terms of the
consumables that will be used: both a gas and an electrode are
required. The electrode selection process cannot be made
separate from consideration of the type of shielding gas that will
be used. The most efficient means of selection of the proper
filler metal for GMAW is to consider electrode/gas combinations
when making comparisons. For example, a more expensive
shielding gas, with a less expensive electrode, may or may not
be more economical than a low cost gas and more expensive
electrode. Thus, the electrode/gas combination must be
considered.
The gas selection can have an effect on mechanical properties
as well. Changes in shielding gas may increase or decrease
Charpy V-Notch properties of the deposit, for example. The
effect of shielding gas on weld properties is more significant for
the higher strength, and more alloyed, filler metals.
Electrode Diameter
As with other processes, the diameter of the electrode used with
GMAW is an important decision. Considerations have to be
made for the various welding positions, the material thickness
and the selected mode of metal transfer.
Production Conditions
The type of conditions that are encountered in production
welding influences the electrode selection. A key factor is
material cleanliness: the more oxide on the base metal surface,
the higher the need for deoxidizers in the electrode.
Electrode Selection Checklist
The following are suggestions for selecting the proper GMAW
steel electrode for a given application:
A. Determine the required mechanical property requirement
The primary mechanical properties of interest are the yield and
tensile strength, and Charpy V-Notch. For most steels with a
t
ensile strength of 70 ksi or less, matching strength filler metal
will be used for all welds. For higher strength steels, and
especially when the tensile strength exceeds 100 ksi, under
matching filler metal may be selected for fillet and partial joint
p
enetration groove welds.
The Charpy V-Notch (CVN) properties may be specified in
contract documents, or in applicable codes.
When the mechanical property requirements have been
determined, available filler metals can be compared to
requirements. Typically, the AWS classification properties are
compared to the application requirements.
Tables 3 and 4 on page 36 can be helpful in determining the
suitable electrodes for matching tensile strength requirements.
B. Determine the weld deposit chemistry requirements
Under some conditions, it is important for the deposited weld
metal to have a specific chemistry in order to meet service
conditions. For example, when the weld is to have atmospheric
corrosion resistance similar to "weathering" steels (such as A588),
the weld is typically required to have a nickel content of 1%.
C. Evaluate the production conditions
The surface condition is a key condition to consider. Mill scale,
rust, and other surface contaminants may justify the use of an
electrode with a higher level of deoxidizers.
D. Determine final weld appearance requirements
Where toe wetting, flat weld faces, low levels of spatter and
other visual criteria are important, higher levels of silicon in the
electrode may be helpful.
E. Determine the appropriate electrode diameter
Before the electrode diameter can be determined, a basic
understanding of the welding procedure variables must be
known. Larger diameter electrodes usually cost less, feed better,
and can carry more current. Smaller diameters are appropriate
for minimizing melt through on thinner materials, as well as to
maximize deposition rates per amp of output.
GMAW of Carbon and Low Alloy Steels