In addition to the main chassis members, some welds are made in 4130 chrome moly (molybdenum) tubing used for such things as radiator supports. Suspension and steering components must also be welded. Using the proper welding process and filler metals is very important to ensure superior quality.

ESAB Welding and Cutting Products, Florence, S.C., recently provided welding training to Petty Enterprises at its Level Cross, S.C., facility. Below are some key points Bob Bitzky, an ESAB welding engineer with 25 years of experience in the welding industry, outlined for the team fabricators.

Pick the Process
The first consideration for mild steel welding, which is the predominant material joined, is deciding what welding process to use. Shielded metal arc (SMA), gas tungsten arc (GTA), or gas metal arc (GMA) welding are the three processes usually considered. Shielded metal arc welding offers few benefits for in-shop use. The process presents slag removal and possible slag entrapment issues. Gas tungsten arc welding can produce excellent quality welds but so can gas metal arc welding. For the majority of the fillet welds required, GMA welding is faster and may produce less heat input for lower distortion. This process also makes it easier to produce consistent-quality welds. There is a misconception in the race car and street rod circles that GMA welding is not usable for critical welds. In fact, the GMAW process is used extensively in industry to make very high-quality, critical welds in items such as submarine hulls. Submarine hulls are made from high-strength steel and are predominantly welded with the GMAW process. Pulsed gas metal arc welding (GMAW-P) can provide welds without any spatter, similar to GTA welding, as well as a controllable, hot arc to ensure the weld is fused to the base metal. Short circuit GMA welding (GMAW-S) is most often used on thinner materials such as tubing and provides excellent quality in the hands of a skilled welder.

Select the Proper Filler Metals
Selecting the proper filler metal requires an understanding of the mechanical properties desired and weld appearance considerations. Table 1 presents the chemical composition of typical mild steel tubing and several welding wires that can be used to join it.

As seen in the table, to achieve the required strength, welding wires contain less carbon than the base material and more of the alloying elements, manganese and silicon. These differences, and low levels of impurities in the welding wire, help provide crack-free and porosity-free welds. Note that the manganese-to-silicon ratio in ER70S-7 is significantly higher than ER70S-3 or ER70S-6. This higher ratio gives weld bead wetting and makes it easier to produce undercut-free welds. ER 70S-7 is the preferred alloy for welding mild steel. As noted, in general, the welds will be at least as strong as the mild steel tubing.

Maintaining Welding Parameters
After selecting the welding process and filler metal, the proper welding parameters must be maintained. Wire feed speed, voltage, and travel speed are the key parameters to set and maintain. Welding current is a dependent variable and is controlled by wire feed speed and electrode extension. This extension is a critical variable. This is the distance from the end of the welding gun contact tip to the workpiece. In tight confines, it may be desirable to use a longer contact tip to ensure this value does not exceed about 1Ú2 in. If the electrode extension becomes excessive, welding current will automatically reduce, resulting in a colder weld with reduced penetration into the base metal. It is important to keep the arc on the leading edge of the weld pool to assure proper tie-in to the base material.

Special Cases
Problems encountered can often be fixed. For example, a small weld made on a heavy steel part cracked immediately after welding. The cause was attributed to the very high restraint being placed on the small weld bead. Also, a high-strength stainless steel alloy was used as the filler metal, which further stressed the weld joint. The solution was to use a lower strength carbon steel filler metal and a larger, more convex weld bead.

In another case, a crack occurred when welding on a small threaded part. A chemical analysis performed on the part indicated it was made from free-machining steel. This particular alloy used a high sulfur content to aid the machinability. Some free-machine alloys also use additions of lead or phosphorous. High sulfur, phosphorous, or lead additions can only lead to poor quality welds. The solution for this application is simply to not weld free-machining steels. The machine shop should pick another alloy.

Summary
Selecting the proper process is the first task of ensuring quality welds. Gas metal arc welding can be used very effectively to achieve the desired results. Picking the proper filler metal is also critical. An AWS ER70S-7 welding wire is a good choice for welding mild steel.

For GMAW, selecting and maintaining the proper welding voltage, wire feed speed, and electrode extension are very important to achieving quality welds. The resulting welds should be checked and verified to be sure they meet the requirements. As a minimum, all welds should be visually inspected for undercut and smooth transition to base metal. When satisfactory welds are produced, record the machine settings for future reference.

What about Welding 4130?
In the mid 1970s, while managing an R&D group for a welding filler metals manufacturer, I received a phone call from a dragster chassis builder. The company wanted to weld 4130 tubing and needed a filler metal recommendation. After careful review of the requirements and desired welding practices, the solution was defined. The company was welding 4130 normalized tubing. It would not be heat-treated after welding, and preheat was not desirable. Most of the weld joints were intersecting tubes that required fillet welds.

Filler Metal Choice
The main objective was to produce porosity- and crack-free weld deposits. The best filler material to use was a low-carbon alloy, AWS ER70S-2. 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 lb/in.2) 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 760 to 800 MPa (100,000 to 115,000 lb/in.2) tensile strength, depending on how the material was processed. 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 pool. The benefit to the welder is easier out-of-position welding. Note: It is recommended all welding on 4130 be performed on ground surfaces free of oil or grease.

Several years after making this recommendation, when looking at a catalog from the dragster chassis manufacturer, it was interesting to note it advertising its use of the ER70S-2 filler metal for their 4130 welding. In fact, 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 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.

Go for Higher Strength
If a higher strength weld is required for perhaps a butt-joint weld that cannot be reinforced, strengthened with a gusset, or put in a less critically stressed area, there are several possible solutions. The use of AWS ER80S-D2, 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 lb/in.2) can be achieved. If this higher strength welding wire is employed, a minimum preheat of 65¡C (150¡F) is recommended. It is also possible to use an AWS ER312 stainless steel welding wire. Weld strength can increase to a level slightly higher than with AWS ER80S-D2.

Generally, the use of this high chrome stainless alloy is only needed when welding stainless to steel. Do not use an austenitic stainless steel such as an ER308L, which is, unfortunately, sometimes recommended. 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 most likely occur in the base metal immediately next to the weld deposit. To achieve the higher strength, the base metal was heat-treated, reducing the weld heat-affected zone area hardness.

If the part is heat-treated after welding to achieve very high strength, a matching chemistry filler metal to the 4130 must be employed. Because of the relatively high carbon content, a minimum of 200¡C, (400¡F) preheat and very slow cooling after welding should be used to avoid cracking. After welding, the part can be heated to 870¡C (1600¡F), quenched in oil or water then tempered back to 370¡C (700¡F). This might be considered a complex cycle, but it will result in a tensile strength of approximately 1380 MPa (200,000 lb/in.2). Since the weld is the same chemistry as the base metal, it and the heat-affected zone will have properties similar to the base metal when heat-treated. All critical welds of this type should be inspected for internal soundness to assure they are free from cracks.

Closing Advice
When welding 4130 chrome moly in the normalized condition, AWS ER70S-2 filler metal, with its low carbon content; is the proper choice. 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.