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Welding Carbon Steel Chassis for NASCAR

(Includes information on welding 4130 Chrome Moly).

The information below is abstracted from the April 2003 issue of The American Welding Society's Welding Journal. 

 

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Welding Carbon Steel Chassis for NASCAR etc.

By JERRY UTTRACHI

Race cars require hundreds of welds. Most welds join intersecting tubes that make up the frame and the all- important roll cage. NASCAR chassis are unique in racing circles. The cars have a minimum weight of 1542 kg (3400 lb), quite heavy by race car standards. NASCAR requires the frame and roll cage components be fabricated from mild steel. This allows the car to absorb the forces of a crash in a bend-before-break mode. Although the driver cannot be protected from every incident, considering the number of high-speed crashes encountered in NASCAR races, the drivers most often walk away unhurt. This is a tribute to the chassis design and the weld quality.

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 (shown in photo), 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 (SMAW, also know as "Stick Welding"), gas tungsten arc (GTAW; also known as TIG or HeliarcR, or gas metal arc (GMAW; also known as MIG) 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, GMAW 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 GMAW 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 GMAW 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 1/2 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. 

[Although not as common when welding mild steel race car chassis, making too small and too concave fillet welds can create problems.  This is especially true when using the TIG (or GTA) welding process.  CLICK  HERE to see the information and explanation of this picture in  "Welding 4130" .]

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 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.

Closing Advice
Be sure to employ the skills of a qualified welder.  Quality Short Circuiting MIG welding requires keeping the arc in front of the weld puddle to avoid lack of fusion or what is often referred to as "Cold Laps."  The weld may look OK on top but not be adequately fused to the base material. 

Small 110 volt input MIG welders may not have sufficient power or the proper arc characteristics to assure quality welds when making critical joints on material as heavy as structural tubing. 

 

See a Video  of Richard Petty and myself discussing the importance of welding in keeping a driver safe!!  Click here or on the move camera ICON

 

What about Welding 4130? (see summary elsewhere on this web site)

 

 

 

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