The same author referenced above who wrote in the American Welding Society about process and filler also wrote this which is pertinent to the discussion regarding TIG (sorry for the crazy spacing, couldn't get it to paste without it!).
(material sourced from http://www.netwelding.com/Welding%204130.htm
OTHER PROBLEMS ENCOUNTERED
In addition to the filler metal selection issues mentioned, some additional
cautions should be followed. Many fabricators use TIG welding
and make very small, concave fillet welds. There seems to be a
feeling that the smaller the better. This raises several concerns.
First there is little filler metal used to make these very small welds.
Therefore the weld consists mostly of the high (by welding standards) carbon
from the 4130 base material. This can cause cracking since there is no preheat
or postweld heat treatment being used. Also cooling rates for these small welds,
especially when using TIG, can be quite high. Therefore one suggestion I
had made in the article (removed from this abstract) was that some
stainless steels filler materials could be used. This is also mentioned on
a number of Internet sites. However with these small fillet welds there is
only a small amount of stainless filler in the deposit and possibly a
significant amount of the high carbon base material. This combination can
lead to a crack sensitive deposit. It is suggested stainless filler metals
not be used for welding 4130.
Making an analysis of the resulting weld chemistry for varying amounts of filler
metal dilution creates a scary scenario at low amounts of any stainless
filler alloy. When I discussed the use of stainless filler metal making
these small fillets in 4130 tubing with a friend who is an acknowledged
"worldwide stainless welding expert," he cringed! As he said, the
suggestion that 312 stainless filler be used is based on at least 40 to 50%
filler metal diluted in the high carbon material. If you make almost an
autogenous TIG weld (no filler metal) and add just 20% of even 312 stainless you
get a Martensitic deposit. You do not obtain the desired
microstructure on which folks base their recommendation for a particular
stainless alloy rod being acceptable. I have had race car fabricators say
they like to use stainless filler because it makes the weld stand out and look
good on unpainted frames they sell! Not a good reason since it could also
With only small additions of these filler alloys to the weld deposit there
is a high percentage admixture of 4130. In these very small deposits
this can create a crack sensitive metallurgical structure. In fact for
these small welds the use of ER70S-2 becomes even more of a preferred
ER70S-2 with its low carbon and leaner Manganese and Silicon alloy than some
other of the rods/wires often recommended as usable such as ER70S-6, creates
less of a dilution problem.
Small cracks and the presence of a brittle Martensitic structure in
these welds can lead to failure or can cause a brittle fracture when
subjected to a crash. See the welds in the photo of the dragster
chassis. I don't know what filler wire was used to weld these joints,
what little there was, but the fillets are very small. It
does not appear very much if any bending took place in the structure before they failed!
Another problem created with small concave fillet welds is when they cool the
surface is put in tension. This makes it susceptible to cracks especially near
the toe of the weld where it is very thin. (Sketches from article by Omar
Bottom line is use larger flat fillets to assure less dilution with
the 4130 and a less crack sensitive shape.
CHECK WELD QUALITY
It is very important to check
weld quality and understand the types of defects that could be encountered.
Check your weld procedures and keep them consistent. You should make some
sample welds and bend them to destruction to assure failure occurs only after
considerable bending has taken place. Look for porosity or cracks that may
have been present in the weld. It would be a wise investment to hire the
services of an American Welding Society (AWS)
Certified Welding Inspector (CWI). There are some 20,000 registered.
In fact many of them are members of the 50,000 member AWS. They can
advise on procedures and what to check for such as small undercuts at the weld
toe of fillet welds that can lead to premature failure.
Consistently following the
proper weld procedures and knowing how to check for possible weld problems is of
When welding 4130 chrome moly in the
normalized condition, AWS ER70S-2 filler metal, with its low carbon content is
the proper choice. Make sufficiently large fillets and make them flat, not
concave. If the part is to be heat-treated after welding, then a
filler metal matching the 4130 chemistry should be employed. This requires
preheat and special precautions to avoid cracking.
Be sure to employ the skills of a qualified
welder who has experience welding this material. Also inspection of the
final welds by an Certified Welding Inspector (Certified by The American Welding
Society) is highly recommended.