other characteristics that change steel’s
properties. Even in small amounts, chromium increases steel’s strength when
exposed to high temperatures, hardenability, toughness, and wear resistance.
When combined with manganese, it
changes quenching properties, making
air cooling an effective alternative.
Another metal, molybdenum, does
quite a bit to enhance steel. It increases
steel’s strength, elastic limit, wear resistance, impact resistance, and hardenability. Molybdenum restrains grain
growth, which increases its fracture resistance, and it makes chromium steel less
susceptible to embrittlement. It also improves the steel’s creep strength in high-temperature applications. Finally, like
chromium, molybdenum contributes to
stainless steel’s corrosion resistance and
What would happen if steel with a
nominal 0.30 percent weight carbon had
just a small amount of chromium (around
1 percent), a bit of manganese (approxi-
mately 0.5 percent), and molybdenum at
about 0.20 percent? The resulting alloy
wouldn’t have much more corrosion re-
sistance than common carbon steel, but
it would be much stronger while main-
taining good ductility. Typical AISI 4130
chrome-moly tubing in the normalized
heat-treated condition yields more than
70,000 PSI and fails under tension at
more than 90,000 PSI.
Because 4130 tubing is much stronger than common steel tubing, designers
can specify thinner wall thickness. It’s
well-suited to applications that rely on a
robust material, particularly aircraft, motorsports, and recreational equipment.
It’s not difficult to weld, but because
4130 is neither carbon steel nor stainless
steel, best practices for welding differ
from those of carbon and stainless steel.
Bear in mind that 4130 is a heat-treat-able material, so its mechanical characteristics can be changed by annealing,
hardening, or tempering or even the
heating and cooling cycles induced by
welding. The information in this article
applies to normalized 4130, condition N.
Joining Chrome-Moly Steel
When making an assembly of chrome-moly tubes, one of the first concerns is
selecting which welding process to use.
Thin-wall 4130 tubing is most commonly
welded with GTAW as this process, in
the hands of a skilled welder, will usually
achieve the most consistent-quality welds
on this high-strength material. Weld dis-
continuities such as undercuts, burn-
through, cold laps, and bead profiles that
lead to concentration of stress should be
especially avoided on these thin-walled,
dynamically loaded materials.
Filler metal selection is another often
debated subject. Welding rod made from
4130 is available, but it is recommended
only when postweld heat treatment is
part of the weld procedure. Matching
the base metal with a 4130 filler metal is
usually not recommended for most motorsports and experimental aircraft fabrication. The higher carbon and alloy content of these filler metals could result in
welds with less-than-optimum ductility.
For most uses, three lower-strength
products—ER80S-D2, ER70S- 2, and
ER70S- 6—are recommended as filler
metals (see Figure 1 and Figure 2). Of the
three, ER80S-D2 should produce welds
that are closest in strength and ductility
to the parent material. ER 70-S2/6 also
makes welds with acceptable strength
with a slight improvement in ductility.
All of these carbon and alloy steel filler
metals pick up alloy from the base metal,
which increases weld metal strength. For
best results, select a filler rod diameter
that is as close as possible to the material
The next concern is joint preparation.
Regardless of the joint type or angle,
AISI 4130 0.28 -
0.4 - 0.6 0.035 0.04 1.15 -
0.35 - 0.40* 0.15* 0.50*
ER80S-D2 0.07 -
0.025* 0.025* 0.50 -
0.80 - 0.40* 0.15* 0.50*
ER70S- 2 0.07* 0.90 -
0.0025* 0.035* 0.40 -
ER70S- 6 0.06 -
0.025* 0.035* 0.80 -
** Indicates a combined maximum of 0.50 percent
The three carbon-steel filler metals recommended for chrome-moly vary somewhat in chemistry, strength, and ductility. Note this this
table reflects the major constituents. For a complete listing of all of the alloying elements, see the AWS guidelines or the manufacturer’s