Weldcraft -

4130 chrome-moly steel. It’s heavier than aluminum and less corrosion resistant than stainless steel. So why use it? In a word: strength. In another word: formability.

4130 chrome-moly steel offers approximately 70,000 psi of tensile strength down to one-eighth inch wall thickness, but is malleable enough to bend to the needs of critical tubing applications used in certain racing and experimental aircraft applications. It is also a material that continues to be surrounded with mystery when it comes to welding.

For many years welding operators have used the oxy-fuel process to weld 4130 chrome-moly steel tubing, while others avoided it altogether, thinking it too exotic and difficult to weld. However, welding operators are learning that it is not so different to weld than mild steel, and in some instances TIG welding has replaced oxy-fuel as a preferred method of joining.

As with most materials, heat is the number one enemy of 4130 chrome-moly steel. Fortunately, TIG welding creates a narrow heat affected zone (HAZ) to minimize weak joints or cracking that can result from excessive heat. The lower heat also helps prevent carbide precipitation, a discontinuity or weakening caused when excessive heat pulls the carbon from the weld.

4130 chrome-moly steel contains approximately 0.28- to 0.33-percent carbon, 20-percent molybdenum and 0.8- to 1.0-percent chromium. While the latter amounts of chromium make it significantly less corrosion resistant than stainless steel, the amount of carbon it contains (though higher) makes it similarly weldable and conductive as mild steel.

Start to Finish with Success
Dirt, oil and rust can weaken or contaminate 4130 chrome-moly steel welds, and therefore, the tubing must be thoroughly cleaned prior to TIG welding. Remove any rust with either a 3M Scotch-Brite™ pad and/or an emery cloth and wipe the tubing with a clean cloth (Figure 1). A stainless steel brush designated for the purpose also works

Figure 1

Next, choose the appropriate filler rod for the desired weld results. For example, more rigid welds would be best made with an ER80S-D2 classification filler rod (due to its higher tensile strength), while an ER70S-2 classification filler rod would be better suited for welds requiring more flexibility or ductility. In either instance, both filler rods offer overmatching strength (more than the 70,000 psi of the 4130 chrome-moly tubing itself). A 3/32-inch diameter filler rod works well for 4130 chrome-moly tubing with a wall thickness of one-eighth inch thick (a common thickness for certain racing applications). Thinner tubing (1/16-inch) would require a 1/16-inch diameter filler rod.

As with the tubing itself, clean the filler rod with an emery cloth or Scotch Brite pad to remove any contaminates that may have been picked up from a worktable or another filler rod.

Note, stainless steel filler rods, despite their corrosion resistance, are not recommended.

A pointed 3/32-inch ceriated tungsten provides good arc transfer at a wide range of amperages and is an appropriate choice most of the time when TIG welding 4130 chrome-moly tubing. Grind the tungsten with an electric grinder (Figure 2) or on a grinding wheel designated for this purpose. When using a grinding wheel, grind the tungsten straight versus at a 90-degree angle to ensure that the grind marks run the length of the electrode, which in turn helps reduce arc wandering.

Figure 2

Set the tungsten no further than the distance of the inside diameter of the cup being used. For example, the tungsten extension with number four cup should be approximately 1/4 inch.

A DC power source with 100- to 200-amp welding capabilities is preferred for TIG welding 4130 chrome-moly tubing, as most of the welding takes place in the range of 80 to 120 amps. Match the power source with a 200-amp air-cooled TIG torch.

It is not necessary to pre-heat or post-heat 4130 chrome-moly unless the wall thickness is greater than one-eighth inch. Instead set the power source amperage higher than needed so there is more heat when first beginning to weld on the cold material. As tubing becomes warmer, the welding operator should decrease the amount of heat required to complete the weld, as overheating can result in cracking. Use only the amount of heat required to obtain complete penetration of the joint and to maintain uniform bead width.

4130 chrome-moly steel requires pure argon shielding gas and the gas flow should be set according to the cup size used. For example, when welding indoors using a number four cup 10 CFH would be adequate, whereas a cup as large as a number eight would require gas flow closer to 20 CFH. When TIG welding outdoors or in an area that is prone to breezes, increase the gas flow to approximately 20 to 30 CFH as proportionate to the cup size; this higher rate helps ensure proper coverage of the weld puddle and prevents discontinuities such as cracking.

Figure 3

Backpurging the tubing is not required, however, for critical applications it is recommended. Back purging ensures that the underside of the weld is protected from atmospheric elements and can be done with commercial apparatuses or individually manufactured methods.

For complex or tight welding joints, such as a cluster where three to five tubes meet (Figure 3), using a gas lens is recommended. A gas lens replaces the collet body that is standard in a TIG torch and provides more laminar, less turbulent shielding gas flow to better protect the weld puddle. It also allows the welding operator to move the nozzle further away from the joint and extend the tungsten electrode past the nozzle by one inch or more. This extension helps minimize tungsten inclusions and improves visibility of the arc and the weld puddle.

Taking Your Mark
Often 4130 chrome-moly tubing reaches the welding operator with the joints already prepared. Commonly, this tubing has a ‘saddle joint’ that is cut either by an end mill or a die and cutter and it must be tacked to avoid shifting and creating gaps during welding.

To tack the tubing, first secure it with a clamp, find a comfortable position and be certain there is enough room around the work area to move unobstructed during welding. Remember, TIG welding tubing requires a certain amount of agility on the part of the welding operator, so comfort is key.

Using either a ‘pencil grip’, or a ‘standard grip’ hold the torch in a manner that provides the most control, and angle it at a 90-degree angle to the tubing (Figure 4).

Figure 4

Place the filler rod at a 45-degree angle and weld four short tacks (approximately one-eighth inch long) on the tubing, one in each quadrant: top, bottom, left and right. These tacks prevent the tubing from shifting and also eliminate gaps that can lead to lack of fusion during welding. They also serve as good starting and stopping points during welding. For example, it is recommended to weld between the nine-o-clock to twelve-o-clock position first, stop, and then weld on the opposite side.

To establish a weld puddle, a good rule of thumb is to increase the welding amperage (via a foot petal or fingertip control) until the puddle is approximately twice the diameter of the filler rod. For example, when using a 3/32-inch diameter filler rod, establish a puddle that is around three-sixteenths of an inch in diameter.

Because 4130 chrome-moly steel is susceptible to carbide precipitation, care should be taken to minimize heat input. The best way to do this is to maintain the amperage and travel speed necessary to keep the weld puddle uniform at twice the diameter of the filler rod.

There are two methods to adding the filler metal to the weld puddle. Some welding operators prefer to dab the filler rod into the weld puddle at a steady rate, as is required when TIG welding aluminum. Others prefer to rest the filler rod on the work piece, keeping it in constant contact with the weld puddle.

The method chosen is primarily a matter of preference and practice, but in both cases, maintaining a shielding gas post-flow of approximately 10 to 15 seconds after completing the weld is recommended. Doing so helps protect the weld puddle from contaminants.

A Final Word
Remember, as when welding any other material, becoming proficient at TIG welding chrome-moly tubing requires practice. Beginning welding operators may find that using less heat and slower travel speeds is necessary until they have had more experience TIG welding on 4130 chrome-moly tubing, while the more experienced welding operators may choose to weld faster and hotter.

In either instance, the goal is the same: minimize heat, and with it the HAZ, to create sound, quality welds.