Page 167 - CITS - Welder - Trade Theory

WELDER - CITS

Welding parameters for GMAW M.S and Alloy steels-

related Tables / data

Objectives: At the end of this lesson you shall be able to
• describe the welding parameter for GMAW
• explain the welding parameter for mild steel
• explain the welding parameter for alloy steels.

GMA welding process parameters/variables
The following parameters must be considered in the welding procedure of GMAW/CO2 welding.
Electrode size:
Rate of wire feed (Welding current) Arc voltage Stick out welding position Shielding gas Travel speed electrode
position
Electrode: Best results are obtained by using the proper size wire for the thickness of the metal to be welded and
the position in which the welding is to be done.

Electrode wires should be of the same composition as that of the material being welded.
Basic wire diameters are 0.8 mm, 1.0 mm, 1.2 mm, 1.6 mm and 2.4 mm.
Welding current: The wire feed speed will control the current. A wide range of current values can be used
with each wire diameter. This permits welding metal of various thicknesses without having to change the wire
diameter. The current selected should be high enough to secure the desired penetration and low enough to avoid
under-cutting or burn through.

The success of GMA welding is due to the concentration of high current density at the electrode tip.
General data on current selection is given in the table given below.
The current varies as the wire feed varies.
Ranges of wire feed rate in CO2 welding

(Current is shown in brackets)
Wire feed speed, m/min]
Wire dia. (mm) Spray type arcs (28 - 32 V) Short circuiting arcs (16-22 V)

0.8 5.0-15 (150-250 amps) 2.5-7.5 (60-160 amps)
1.2 5.0-15 (200-350 amps) 2.0-3.8 (100-175 amps)

1.6 5.0-8.8 (350-500 amps) 1.5-2.0 (120-180 amps)
Arc voltage: This is a very important variable in GMAW/ CO2 welding process, mainly because it determines the
type of metal transfer by influencing the rate of droplet transfer across the arc. The arc voltage to be used depends
on the base metal thickness, type of joint, electrode composition and size, shielding gas composition, welding
position, type of weld and other factors.
For details refer to the table of General Guide to Welding Conditions.
Arc travel speed: The linear rate at which the arc moves along the joint, termed arc travel speed, affects the weld
bead size and penetration
If the arc travel speed is lowered, the weld pool becomes larger and shallower. As the travel speed is increased,
the heat input rate of the arc is decreased; consequently, there is decreased penetration and narrower weld bead.
When the travel speed is excessive, undercutting occurs along the weld bead, because the deposition of the filler
metal is not sufficient to fill the paths melted by the arc.
Stick out: It is the distance between the end of the contact tube and the tip of the electrode. (Fig 1)

154

CITS : C G & M - Welder - Lesson 49 - 60