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The Basic's of Welding 4130 Chrome Moly (Chromoly) Steel are Presented on This Web Page

For other 4130 welding info

Click "How To Pronounce" CHROME MOLY
CLICK to See Side Bar Below Regarding the Strength of Welds Made with ER70S-2 versus ER80S-D2

The Editor of the American Welding Societies Monthly Journal Asked Me To Write and Article on Motorsports Welding for the March 2018 Issue.

This is a PDF of what was published. It compares differences in NHRA with NASCAR chassis construction. Where NHRA top classes use high strength 4130 tubing, NASCAR belives in a "Bend Before Break" design. They specify mild steel tubing and the min wall thickness that can be used!

CLICK PDF ICON RIGHT or pic to Download a PDF.

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Video-Welding 4130 CrMoly

4130 was widely used in WWII airplanes! It was welded with oxyacetylene as TIG welding had just been invented! With oxyacetylene the material is being preheated and cooling is slow so the relatively high airplane ptcarbon 4130 was considered very weldable. With TIG or MIG there is is usually no preheat used and cooling is very rapid leading to possible hard and brittle welds without employing precautions.

Of interest, in WWII about 40% of the workforce were women. This is a picture from War Archives is of Ann Rosner oxyacetylene welding an aircraft supercharger duct at Ford's Willow Run plant.

When writing "Advanced Automotive Welding" for CarTech I talked with Gene Golliet about his welding of 4130 aircraft tubing in shops in Southern California during the War. Some experimental aircraft still use and prefer that welding process.

Welding 4130 Chrome Moly Steel for Race Cars

During the WW II era 4130 high strength steel was used for some aircraft components.  At that time oxy-acetylene was the welding process of choice for many of these items.  The preheat and slow cooling inherent with that process made welding the nominal 0.30 carbon steel relatively straight forward (assuming one could oxyacetylene weld!).  However with more modern welding process like TIG and MIG, the cooling rates can be much faster and care must be taken to avoid forming high hardness and brittle Martensite on cooling transformation.  On heavier sections preheat and post weld heat treatment should be used.  With the proper post weld heat treatment strengths of 200,000 psi can be achieved with reasonable toughness by tempering the Martensite that forms in the heat treating process.  However when welding race car tubing, preheat is not often used nor are the parts post weld heat treated.  

Most of the tubing used for race car construction is referred to as normalized.  This refers to the heat treatment and cooling rate the tubing was subjected to in manufacture.  Most normalized tubing will range in tensile strength from 95,000 to 110,000 psi.  This can be welded with the proper filler metals to achieve similar strengths.  Although there are more weldable grades of steel (those with lower carbon content from 0.06 to 0.15)  in the 100,000 to 115,000 psi tensile strength range readily available for plate and sheet, 4130 remains a commonly used grade for tubing.  Just be sure to take the precautions noted when welding.

The following is extracted from an article I wrote for the American Welding Societies technical journal called "The Welding Journal".  It has additional information to that presented in the publication.  You can see the article in the 2003 April issue of the AWS Welding Journal:


In the mid 1970’s, while managing an R&D group for a leading welding shielding gas/filler metals manufacturer, a phone call was received from a dragster chassis builder.  They wanted to weld 4130 tubing and needed a filler metal suggestion.  Being a “car buff,” a number of alternatives were considered to provide the optimum solution.  After careful review of their requirements and desired welding practices, the solution was defined.  They were welding 4130 normalized tubing, it would not be heat treated after welding, preheat was not desirable and most of the weld joints were intersecting tubes that required fillet welds.  The best filler material to use was a low carbon alloy now called ESAB Spoolarc 65 (meeting an American Welding Society (AWS) ER70S-2 specification).  The main objective is to produce porosity and crack free weld deposits. This welding alloy has a very low carbon content, nominally 0.06, which can handle dilution into the relatively high (in terms of weld metal), 0.30 carbon in the 4130.  The resulting diluted weld deposit has a tensile strength of approximately 590 to 620 MPa ( 85,000 to 90,000 psi.) The actual strength will depend on the amount of dilution with the 4130, weld bead size and material thickness.  This is usually an under match for the 4130 tubing which could have a 760 to 800 MPa (100,000 to 115,000 psi) tensile strength depending on how the material was processed.  [Added Note: some normalized 4130 tubing may be only have a 90,000 psi tensile strength, it depends on the manufacturer]   However, if extra joint strength is required, a slightly larger fillet size or gussets can be employed.  In addition, this welding wire contains small amounts of aluminum, titanium and zirconium.  Although these elements were initially added to handle welding over mill scale, they also contribute to a less fluid weld puddle.  The benefit to the welder is, it is easier to make out of position welds.  Note, it is suggested all welding on 4130 be performed on ground surfaces free of oil or grease (to keep the hydrogen levels as low as possible).

Several years after making this suggestion, when looking at a catalog from the dragster chassis manufacturer, it was interesting to note they were advertising their use of the ER70S-2 filler metal for their 4130 welding.  In fact, they were offering it for sale for those customers purchasing frame parts and doing their own welding!

The Internet was searched to see what current recommendations were being made for joining 4130 tubing.  Several hundred sites were found that recommend the ER70S-2 welding rod/wire alloy.  It was the predominant recommendation.  Typical of the Internet however, there were many improper descriptions of why this alloy should be used and several incorrect recommendations.

Need a higher strength deposit? If a higher strength weld is required for perhaps a butt weld that cannot be reinforced, strengthened with a gusset, or put in a less critically stressed area, there are  possible solutions.  The use of Spoolarc 83, which contains 0.50 Moly, will provide a weld deposit with higher strength.  When diluted into the 4130 base material a weld tensile level of 760 to 800 MPa (110,000 to 115,000 psi) can be achieved. If this higher strength welding wire is employed, a minimum preheat of 65 degrees C (150 degrees F) is suggested.    Weld strength can increase to a level slightly higher than the normalized 4130 with Spoolarc 83 (AWS ER 80D-2).    Do not use an austenitic stainless steel such as an ER308L, (which is recommended on some Internet sites).  Diluting this or similar austenitic stainless alloys with 4130 can lead to cracks.  Also, consider that providing a higher strength weld deposit cannot compensate for the reduction in strength that will occur in the parent metal immediately next to the weld deposit. 

 For Parts to be "Heat Treated" (meaning carefully raising the part to 1600 F; Quenching in water; then Reheating precisely to 1000 F; then Slow Cooling) After Welding: Note: modified with additional information obtained since writing this article.  Click to See Detailed Explanation:)

If the part is to be used for potentially high impact, structural applications and will be heat-treated after welding, a matching chemistry or matching hardenability filler metal to the 4130 should be employed.  This may be a filler metal with somewhat lower carbon but increased moly, chrome, manganese etc. Because of the relatively high carbon, a minimum of 200 degrees C, (400 degrees F) preheat and very slow cooling after welding should be used to avoid cracking.  Heat treating after welding refers to the finished welded part being heated to 870 degrees C (1600 degrees F), quenched in oil or water then tempered back to say 540 degrees C (1000 degrees F).  A complex cycle, but this will result in a tensile strength of approximately 1035 MPa (150,000 psi).  Since the weld is the same chemistry as the base material, it and the heat affected zone will have the similar properties as the base material when heat-treated.  All critical welds of this type should be inspected for internal soundness to assure they are free from cracks. 

End Of Abstracted Article


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 contain cracks!

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 suggestion. 

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 Blodgett)

Bottom line is use larger flat  fillets to assure less dilution with the 4130 and a less crack sensitive shape.

Side Bar

Had an interesting conversation with two folks who have extensive experience welding 4130 tubing for race cars.  One said his company was asked to help a  large racecar team solve welding problems.  He found they were welding with a special stainless rod "Super Missile Weld" (a 312 stainless steel rod with a fancy name.)  Because of the small TIG fillet welds they were making with very little added filler rod, they were not getting an austenitic  stainless steel weld deposit because of excess dilution with the base material.  Instead they were getting a high chrome, brittle ferritic deposit!  He took in some carbon steel rod (the one we suggest, ER70S-2) but told them this was a "very special" rod that would solve their problem.  It did! " Just because it costs more does not make it better!"

The other fellow who has fabricated a number of sportsman and other drag cars and  weld repaired a number of  other 4130 race cars.  He  saw a number of weld failures on some chassis caused by the very small concave fillets that had been used .  He also said some welds were made with poor, cold MIG Short Arc welds that were not fused properly to the tubing.  Both very bad welding practice. 


The following discusses AWS TIG/MIG Filler Metal Specifications that are often misinterpreted in Welding Forum comments:

  1. The Specification that defines carbon steel wires and rods, including ER70S-2, for MIG and TIG welding is AWS 5.18.  ER80S-D2 is covered by AWS A5.28, for Welding Low Alloy Steel.

  2. Both Specifications require reporting the tensile strength from a weld made in a ¾-inch thick, mild steel weld joint having a 45-degree V bevel angle, gapped ½-inch on a steel backing bar.  It requires many weld passes to fill this joint minimizing any effects of the base material on weld properties.  Each weld pass is made at a defined (by the Specifications) amperage, voltage and travel speed.

  3. These welds do NOT increase in strength by mixing with the relatively high carbon 4130 (by welding standards, it's high) as occurs when making single pass fillet or butt welds in that material.  The weld strength in a single pass diluted weld in 4130 will be higher.  The exact strength achieved depends on the amount of rod added to the melted 4130, joint type and thickness, welding parameters used etc.

  4. Both Specifications define a minimum tensile strength, i.e. ER70S requires a minimum of 70,000 psi (note 1,000 psi = 1 ksi) and for ER80S - a minimum of 80 ksi.  It does not say what weld strength will be produced; just the minimum to be classified in that category.  The specific chemical range for a particular alloy (S2, S6, S7 etc)  is also specified.

  5. The following data are AWS Specification weld test results from ESAB (data is from Fig. 4-52, in my recently published book Advanced Automotive Welding that also discusses the implications in detail):

TIG, MIG, or Base

Rod, Wire or (Base)

Tensile Strength,




Impact Strength;


TIG Weld ER70S-2 82 31% 170
MIG Weld ER70S-2 82 28% 45
MIG Weld ER80S-D2 110 22% 85
Base Material (Normalized 4130) ~90 ~25% ~60

Bottom Line:

Even when not diluted with 4130, ER70S-2 has a tensile strength just 9% under typical Normalized 4130.  ER80S-D2 has a strength, with no  dilution with  the higher carbon 4130, that is 22% higher than Normalized 4130.  Significantly higher strength weld metal than base metal may cause a weldment, when stressed under load, to yield excessively or initiate failures in the heat affected zone.

The elongation of Normalized 4130 (a measure of ductility) is less than the very ductile ER70S-2 weld.  The higher strength ER80S-D2 weld has a lower elongation than the base material.

The impact properties of a TIG weld made with ER70S-2 are excellent at 170 ft-lbs.  Note, the reason TIG welds have higher impact toughness than MIG welds made with the same alloy filler metal is discussed in the Advanced Automotive Welding book.


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 25,000 registered.  In fact many of them are members of the  65,000 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 major importance.

Closing Suggestion
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.

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The Basic's of Welding 4130

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