Originally Posted by
nixrox
Well you guys certainly are extremely sensitive and very easy to get a rise out of.
However, in the interests of clearing the air, I decided to go straight to the horses mouth - Miller Welding.
I found an article that covers best practices for welding 4130 chrome-moly, low alloy tubing.
Surprise, surprise - if you follow this article carefully, you should notice that the 'novice' home built welders on this forum have been giving you totally wrong information.
So if you want the correct information you need to go to the specialist who actually do this for a living.
Enjoy the article.
Best Practices for TIG Welding of 4130 Chrome-Moly Tubing
In General Motorsports and Aerospace Applications
Chrome-moly steel offers a good blend of strength, weight and ductility
for many motorsports and aerospace applications. The following article
discusses some of the recommended procedures and equipment for
successfully TIG welding 4130 chrome-moly steel.
Just as a NHRA (National Hot Rod Association) drag racing team depends
on blend of team talent to win races, the material selected for a
particular application requires a blend of properties to withstand the
stresses involved. For a drag racing chassis, and for many other
motorsports and aerospace applications, that material is 4130
chromium-molybdenum, or chrome-moly, which is selected for its blend of
ductility, strength, weight and fabrication advantages.
The 4130 grade of chrome-moly is a high-strength low-alloy (HSLA) steel
that contains molybdenum (0.15 - 0.25 percent by weight) and chromium
(0.8 - 1.1 percent by weight) as strengthening agents. However, it has
relatively low carbon content (nominally 0.30 percent), so it welds,
machines and bends almost as easily as 1018 DOM mild steel tubing (which
has an 0.18 percent carbon content).
*Material* *Tensile Strength* *Yield Strength *Hardness, Rockwell B* *Elongation*
4130 chrome-moly 97,200 psi 63,100 psi 92 25.5%
Chrome-moly is not lighter than steel, a common misconception. Both
weigh about 491 lbs. per cubic foot.
However, chrome-moly offers a better strength-to-weight ratio and better
elongation (a measure of ductility), which enables designers to use
thinner wall and/or smaller diameter tubing to reduce overall weight.
Because many motorsports, aerospace and sporting goods applications
involve welding normalized 4130 chrome-moly tubing with a wall
thicknesses of less than .125-in., this article will focus on best
practices for these applications.
Welding Consumables and Variables
Material 4130 chrome-moly tubing (normalized).
Thickness < 0.125 in. Most chassis tubing has a wall thickness ranging
from 0.035 – 0.083 in. and diameters from 1/4in. - 1-5/8in.
Amperage 1 amp per .001 in. of wall thickness.
Polarity DC Electrode Negative. HF for arc starts only.
Pulsing Optional.
Filler Material ER70S-2 or ER80S-D2.
Filler Diameter 0.030 – 1/8-in. Generally, do not use a rod larger than the thickness of the base metal.
Tungsten Type 2% type (Ceriated is first choice, then Thoriated)
Tungsten Diameter 1/16 – 3/32 (smaller diameter for thinner wall).
Tungsten Prep Pointed.
Arc Length
Less than or equal to the electrode diameter. Generally, the tighter
the better, as shorter arc lengths reduce heat input.
Electrode Stickout
No further than the distance of the inside diameter
of the cup being used. However using a gas lens can extend this distance.
Gas Lens Not required, but helpful if a tight joint configuration
demands a longer stickout or involves multiple tubes. Keep the screen
free of debris and spatter.
Shielding Gas
100% argon, 15 – 20 CFH. More is not better, as
turbulence could suck atmosphere into the weld.
Pre-flow
.4 to .6 seconds.
Post-flow
10 – 15 seconds.
Backing Gas
Follow applicable codes/standards, if any; not required for NHRA applications.
It is required for aircraft to reduce potetial contamination on the backside of the weld.
Pre-heat
Not required as long as tubing is above 60 – 70 degrees F.
Stress Relief
Not required on material < 0.125 inch; simply allow the weld to air-cool.
Tack Welds
Four tacks made 90 degrees apart; tacks ideally should be longer than wide.
Joint Preparation
Tubing notcher for coping, drum sander for final
fit-up, deburring for edge preparation, Scotch-Brite™ or 120 grit sand
paper to clean about 1 in. back from the joint, and final clean with
acetone, lacquer thinner or similar solvent to remove oil.
Joint Gap
None! Realistically, gaps smaller than 0.010 are permissible; larger
gaps promote poor quality.
Weld Size
Keep welds to within their specified size: a weld needs to be no larger
than its thinnest section, which will be the “weakest link” in the
chain. Larger-than-necessary welds add excess heat and waste gas, filler
rod and time.
Weld Technique
Weld in one continuous motion, pulsing the foot control
and adding filler rod to create the “stack of dimes” appearance (or use
the machine’s pulsing controls). Do not stack separate puddles on top of
each other, as this may lead to incomplete fusion.
End-of-Weld Procedure
Avoid pinholes by tapering off heat input at the end of the weld and
maintaining a constant distance between the tungsten and the weldment.
Weld Appearance
A good weld looks shiny and has a bluish tint. A dirty, gray-looking
weld may indicate poor shielding gas coverage or excess heat.
Perfect Fit-Up
When welding thin-wall tubing, whether chrome-moly or other metals, the
welders at Cagnazzi Racing joke that their tolerances range from perfect
to almost not perfect. That is, if the parts don’t fit perfectly, they
start over because thin-wall tubing does not have sufficient mass to
absorb excess heat.
The usual “trick” to filling gaps with GTAW is to use a larger diameter
filler rod. However, larger rods require more heat and excess heat
promotes burn-through, warping and embrittlement. Using a larger rod
might be an acceptable solution in a non-critical application, but it’s
a poor practice when welding chrome-moly.
Consistency
Cagnazzi uses hundreds of jigs and fixtures bolted to a surface that is
flat to within a few thousandths of an inch for fabricating even the
smallest items to help ensure tight fit-up and consistent tube
placement, which provides repeatability
Chrome Moly Tubing Consistency
By producing a new chassis nearly identical to the previous, the crew
chief can hone his craft of gear ratio and clutch management without
worrying about a new chassis introducing unknown variables. Many
fabricators believe that they cannot afford to build fixtures for all of
their components or for larger weldments. However, if repeatability and
accuracy are important, good fixturing is mandatory.
Coping Mechanism
Most of Cagnazzi’s jigs have a go/no-go type fit, which enables
“sneaking up” on a perfect fit by making small incremental adjustments.
After cutting to approximate length with a band saw, Cagnazzi uses a
tube notcher for rough coping. The fabricator will then check
the length and use a drum sander to sneak up on a perfect fit by slowly
sanding away excess metal from the mouth of the tube. Before
welding, the fabricator will deburr the edge , clean back 1 in.
from edge using Scotch-Brite or 120 grit sandpaper and remove oils or
other contaminants with a solvent. Be sure to wear nitrile
gloves, as the natural oils from your fingers can ruin a weld just as
thoroughly as grease or cutting fluid. Don’t forget to use the sandpaper
and solvent on the filler rod, too.
Filler Metal Selection
In many motorsports and aerospace applications, engineers want some
degree of ductility in the weld to help absorb impacts and prevent
cracking. For this reason, most NHRA fabricators intentionally dilute
the strength of the parent material by selecting ER70S-2 for filler for
roll cages, chassis and other applications requiring more flexibility.
While actual tensile strength of the weld will vary and depend on other
factors, 4130 diluted with ER70S-2 filler likely produces a weld with a
tensile strength in the 80,000 to 82,000 psi range.
For areas requiring higher strength, such as spindles and upper and
lower control arms, fabricators select ER80S-D2 filler, which produces
welds with a high tensile strength (as a side note, consider that S-2
fillers clean impurities better than D-2 fillers). In any event, do not
use 4130 filler, as the weld will not have sufficient ductility unless
it undergoes stress relief.
As for filler rod diameter, use a rod diameter that matches the
thickness of the base metal. Trying to weld 0.035-in. tubing with a
1/16th inch filler (0.063-in.) is a bad idea because the tubing wall
will melt before the filler rod is up to temperature. Cagnazzi
predominantly uses 0.030-, 0.045- and 0.063-in. (1/16th) filler rods
, with .045 and .063 being the most common. For thicker, larger
diameter tubing they use 3/32- and 1/8-in. rods.
Heat Control
Successfully welding 4130 requires preserving its mechanical properties
by heating and cooling the weld in a controlled manner. Excess heat
causes carbide precipitation, and cooling too quickly causes
embrittlement. Fortunately, the GTAW process provides sufficient heat
control. Welds made on tubing 1/8-inch or thinner do not require
pre-heating or post-weld stress relief, yet they will have sufficient
penetration as long operators follow the general rule of the thumb by
using 1 amp per 0.001 in. of metal thickness.
Note that if the tubing is below 60 degrees F, use a small propone torch
to heat the base metal to up to 300 degrees F. Otherwise, the metal
could cool too quickly and become brittle. Welding cold metal may also
promote hydrogen cracking, so that’s another reason to preheat 4130 if
it’s cold.
Puddle Size and Arc Length
While welding too slowly increases overall heat input, operators should
not necessarily focus on welding travel speed. Rather, they should focus
on controlling their body (Fig. 11), controlling puddle size by making
the puddle only as wide as necessary and holding a tight arc length of
1/8-in. or less.
Note that a longer arc length, or tungsten tip-to-work distance,
increases overall heat input, because a GTAW power source automatically
increases voltage when arc length increases. If the joint configuration
limits access because of cup size, do not try to weld with a longer arc.
Rather, use a smaller cup or a gas lens and extend the tungsten.
To better control heat input, and to enable repositioning the body to
better control torch movement, do not weld the circumference of a tube
in one pass. Rather, weld it in four quarters. Weld only two of the
quarters (on opposite side of the tube) then move to another joint. When
the first joint cools, come back and complete the remaining sections.
No Slacking Allowed — EVER
The authors of this article weld 4130 tubing in critical applications,
and they have a combined 50 years of welding experience. As such, they
take every step of the fabrication process seriously.
“Close enough” is simply not good enough for NHRA or AEROSPACE work,
nor should it be, for a child’s go-kart or bicycle.
Reply With Quote
