the fit must be really tight. Gaps greater
than 0.010 inch should be avoided, especially on thin tube. Whether the end is
cut, punched, or sheared, close the gap
by cleaning up burrs, grinding the faces
little by little, and frequently checking
The last weld prep step is surface
preparation. A clean surface is necessary
for a good weld. Using an abrasive pad
or sandpaper, clean up the steel in the
joint area plus an inch or so. Use a lacquer thinner or acetone to clean the area
immediately before welding to remove
all sources of hydrogen.
Preheating the weld area isn’t always
necessary as long as the ambient temperature is above 70 degrees Fahrenheit and
the wall thickness is less than 0.120 in.
Procedure settings and gas coverage for
a typical tube for an aircraft application,
0.035-in. wall thickness, are as follows:
• Gap: Not more than 0.010 in.
• Current and voltage settings: DC,
electrode negative; 20 to 40 amps; 9 to
• Electrode: 2 percent thoriated tungsten; 1/16 in. dia.; pointed
• Torch gas: Argon, 15 to 25 cubic
feet per minute; preflow 0.5 seconds,
postflow 15 sec.
A typical motorsport application likely requires a thicker-walled tube or a mix
of wall thicknesses, up to 0.120 in. The
rule of thumb for current setting is approximately 1 amp per 0.001 in. of wall
thickness. So for 0.080-in.-wall tubing,
80 amps is a good starting point.
How Much Heat
Is Too Much Heat?
Before striking the first arc, it’s critical
to understand what heat does to 4130.
“Some people think that 4130 always
needs preheating to slow the cooling
rate, but that’s not always true,” Hoes
said. Tubing with wall thickness below
about 0.120 in. does not usually require
preheat if the material and ambient tem-
perature is above 70 degrees F. The thin
material conducts the heat away from the
weld at a slow enough rate to prevent ex-
“On thicker sections, the weld and
heat-affected zone experience a much
faster cooling rate,” Hoes said. “It doesn’t
stay red-hot for long. By the time you lift
your helmet, the red heat is gone.”
The rapid cooling rate could result in
a hard brittle microstructure in the weld
and surrounding base metal. Preheating
these thicker base metals would slow the
cooling rate, resulting in a more ductile
Hoes cautioned, “If postweld heat
treatments such as annealing and stress
relief are required, they should be left
to the heat-treat shop, as improper torch
heat can do more harm than good.”
It is very important that the weld is of
high quality and of correct size for the
material that is being joined. The weld
should blend smoothly into the base
metal to avoid stress risers. Excessively
convex beads, undercuts, overlaps, and
lack of fusion can lead to fatigue failures
on high-strength thin-wall tubing.
When you are welding thin-wall tubing,
keep the parent material’s minimum temperature above 70 degrees F. Distribute
the heat as evenly as possible to prevent
loss of mechanical properties, burn-through, and distortion.
As the wall thickness rises, the cooling rate also rises; therefore, the need
for preheating the material may become
necessary. Preheating helps to control
the cooling rate, which is critical for a
successful, robust project.
Editor Eric Lundin can be reached at
Karl Hoes is a welding instructor for The
Lincoln Electric Welding Co., 22800 St.
Clair Ave., Cleveland, OH 44117, 216-