Chapter 8 Manual Welding Techniques
135
Welding Steels Using DCEN
(Straight) Polarity
DCEN polarity is used for welding steels because
the higher concentration of heat input into the base
material results in deeper penetration. DCEN polarity
is the best option for obtaining full penetration welds
and controllable weld bead profi les. With DCEN
polarity, two-thirds of the arc’s heat is concentrated
on the workpiece and one-third is concentrated on
the electrode. Since the majority of the heat is concen-
trated on the work and not on the electrode, smaller
electrodes can be used without worrying about elec-
trode deterioration.
Electrodes Used on Steels
For a long time, 1% or 2% thoriated electrodes
were recommended for welding steel. When thorium
is added to tungsten in small amounts, it increases the
electrode’s current-carrying capabilities. Thoriated
electrodes do not work well with ac current and are
not recommended for use with the newer inverter-
type machines. If thoriated electrodes are overheated,
the end of the electrode can begin to vaporize and spit
small amounts of the electrode into the weld. Grinding
thoriated tungsten electrodes results in radioactive
dust that can be hazardous. These electrodes usually
have a yellow or red color band near the top of the
electrode, depending on the percentage of thorium.
Today, electrodes with more desirable character-
istics are available. Lanthanated tungsten electrodes
are relatively new to the US, but are becoming more
popular. Lanthanum is a “rare earth” material that is
not radioactive. Lanthanated electrodes work well for
ac or dc currents, have good current-carrying capaci-
ties, and can be used universally, eliminating the
confusion of selecting an electrode. Lanthanated elec-
trodes usually have a black color band near the top of
the electrode.
Other types of electrodes, such as ceriated, zirco-
niated, and pure tungsten, all have some desirable
traits. Ceriated tungstens usually have an orange color
band, zinconiated tungstens usually have a brown
color band, and pure tungstens usually have a green
color band. Research the base materials to be welded
to determine which electrode is best suited for your
application. More detailed information about different
electrode types and when they should be used was
presented in Chapter 3, Auxiliary Equipment and Systems.
Amperage ranges must be considered when
selecting the size of the electrode. Higher welding
amperages require larger electrodes.
When preparing an electrode for welding steel,
keep in mind that different tip confi gurations will
result in different bead profi les. For welding steel, it
is generally recommended to sharpen the electrode to
a point and make a slight fl attened tip. Pointed tips
direct the arc in a more concentrated area. Flattening
the very tip of the electrode helps concentrate the arc
even more.
Shielding Gases for GTAW on Steels
Pure argon gas can be used when welding metal
up to 1/8″ (3.2 mm) thick. When thicker materials
are being welded, helium should be added to the
shielding gas to improve weld pool control and pene-
tration. Helium increases the temperature of the arc,
resulting in deeper penetration. The following stan-
dard shielding gas mixes can be used:
• 75% argon–25% helium
• 50% argon–50% helium
• 25% argon–75% helium
Pure helium gas can be used for automatic
welding of steel or steel alloys. Gas purity must be
suffi cient to prevent hydrogen pickup. Hydrogen will
cause porosity and cracking.
Techniques for Welding Steels and
Steel Alloys
1. Clean the oxide scale from the weld joint imme-
diately prior to welding. Clean the entire weld
area to bright metal.
2. Select a welding rod that has the required prop-
erties. When welding porosity-prone material,
always use a welding rod that contains deoxi-
dizers to prevent porosity.
3. Preheat low-alloy steels and tool steels to
prevent cracking in the weld and heat-affected
zones.
4. Weld porosity-prone materials by maintaining
the arc on the molten metal. Do not hold the arc
on the base metal. On crack-sensitive materials,
do not make concave welds. These welds are
prone to cracking through the centerline.
5. On multipass welds, always remove any scale on
the surface of each pass to reduce the possibility
of oxide entrapment.
6. Always fi ll craters on crack-sensitive materials.
7. When welding tool steels, make small stringer
beads, (Figure 8-10A), to reduce the expansion
of the base metal and prevent cracking. Imme-
diately after welding, the weld metal can be
peened to reduce shrinkage as the metal cools.