How the equipment and techniques you use affect welding results
August 1, 2013
Gas metal arc welding is an effective method for joining thin-gauge materials-- from 24-gauge to 0.1875-in. carbon steel and 18-gauge to 0.1875-in. stainless steel. But extra attention should be paid to the equipment and the welding technique to minimize burn-through and distortion.
Gas metal arc welding (GMAW) is an effective method for joining light-gauge metals such as carbon steel and austenitic stainless steel. Thin-gauge materials—24-ga. to 0.1875-in. carbon steel and 18-ga. to 0.1875-in. stainless steel—are used in applications such as HVAC, automotive, mobile machinery guards and cabs, and small parts, as well as stainless steel food service and processing equipment, sanitation applications, and corrosive environments.
Minimizing burn-through and distortion is essential to obtaining a good weld in thin-gauge materials. For this reason, the short-circuit transfer (SCT) mode, with its low heat input, often is used. The pulse mode of transfer also is used when the travel speed can be increased enough to prevent burn-through and the heat input [(Amps x Volts) x 60/Travel Speed = Joules per Inch] is comparable to SCT.
Newer machines can be programmed to pulse-on-pulse or pulse-on-short-circuit, so that two different weld conditions can be programmed for a preset time, and the power supply alternates between the two sets of weld parameters. However, if the heat input isn’t comparable to the SCT heat input, distortion can occur.
Welding in a sequence that limits the amount of heat can help minimize distortion in thin material. For instance, when producing a 3-ft. weld in the vertical-down position, weld 6 in. and then stop to allow the weld to cool down before restarting again. On larger thin-gauge material, spread out several smaller welds over the joint.
The use of a power supply with slope or inductance control can offer improved weld bead appearance, especially when welding with stainless steel filler metals. Because of their lower thermal conductivity, stainless steel filler metals do not transfer heat as well as carbon steel, so the weld bead could have a higher amount of spatter, and the bead itself may not be as flat as a carbon steel weld made with an ER 70S-6 electrode using the same settings (see Figure 1).
Slope limits the short-circuit current, while inductance slows down the response time of the power supply, resulting in a slower, more controlled wire burnoff during the short circuit. Depending on the power supply and the welding amperage in the short-circuit transfer, you may have from 20 to more than 200 short circuits per second. The number increases with the wire feed speed.
Some newer equipment provides feedback from the arc to the power supply and automatically adjusts the voltage to deliver what is considered the optimum for the feedback received. These machines are suitable for less experienced welders and also maintain the flexibility to allow for adjustments, if necessary.
The shielding gas also can affect the current level, spatter, and bead appearance. The most common gases used for SCT are 75 percent argon/25 percent CO2 for carbon steel and 90 percent helium/7.5 percent argon/2.5 percent CO2 for stainless steel. The parameters listed in Table 1 were developed for these gas mixtures.
When using other gases, joint types, or configurations for SCT, you will need to make minor adjustments.
Torch angle also is a critical variable when welding thin-gauge materials. Typically, a pushing torch angle is employed when welding in the flat, horizontal, and overhead positions, as this generally helps prevent burning through the material. When welding vertically on thinner materials in the SCT mode, your progression most often is from the top to the bottom of the joint.
When welding in the SCT mode, it is important to use a relatively short contact-tip-to-work distance, generally 0.25 to 0.5 in. maximum. Excessive contact-tip-to-work distance results in an unstable arc because of the greater resistance between the workpiece and the contact tip.
Typical wire diameters for short-circuit welding of carbon steel include 0.023, 0.030, 0.035, and 0.045 in. While the stainless steel wire diameters used for SCT are predominantly 0.030, 0.035, and 0.045 in., you can use a smaller diameter when welding thin materials. However, you will need a higher wire feed speed to match the current level of the larger wire.