13.3 Application and Techniques
Stephanie Oostman
General Uses
Carbon arc cutting can be found in several different types of welding environments. From fabrication shops, to pipeline work, construction trades, even in underwater construction and demolition.
If a weld joint has shallow penetration, then the root can be back-gouged from underneath, cleaned with a grinder, then rewelded to add a better joint penetration. Back-gouging is the process in which the root of a joint is carved away via CAC-A from the backside. This allows the welder to then add a new root pass from the underside of the weld, adding reinforcement. Carbon arc cutting is also used in superficial situations such as washing to remove material from a wide surface area. In washing, a welder will weave the electrode side to side across the metal surface, maintaining a shallow depth as he/she pushes and weaves the electrode across. This process will sweep or wash away superficial material in the electrode’s path. This process is often used when the area needs to undergo hard facing.
Another term you may hear in conjunction with CAC-A is scarfing. Scarfing is another shallow material removal that does not damage the base metal like removing a backing bar from a weld test sample. The intention is to remove the metal backing strip without nicking or gouging into the base plate. The difference between gouging and scarfing is a matter of the depth of the cut, as scarfing does not dig into the base material, but only removes surface items.
If a welder is trying to repair a crack, they will need to dig out the crack first via gouging. In this scenario, the welder will need to carve into the base material as deep as necessary to remove the crack or discontinuity such as porosity. Because carbon arc cutting leaves surface carbon behind, the welder will then need to grind and clean the area after, prior to making a repair weld.
Carbon arc cutting can also be utilized to cut material, and also put a bevel on a groove in the field. For example a joint fit up calling for a J-groove or a double U-groove, while this could be machined into the joint in some fabrication shops, it’s not always economical, practical, timely or accessible to do so. In which case the welder can use round CAC-A electrodes to assist in carving the needed bevel into the joint to be welded.
Common operation techniques
CAC-A can be performed in any position, however there are some basic techniques or considerations when operating a carbon arc cutting machine. When a vertical cut or gouge needs to be made in a work area, it is recommended to move the electrode in the vertical down position aka downhand. This ensures that the molten slag stays out of the path and moves downward more easily with the assist of gravity. Start at the top of the joint or area to be gouged and move towards the bottom. This will also reduce the amount of sparks produced.
If the user is making a deep cut or gouge then they will have to slow their speeds to keep the cut effective and maintain control, whereas a shallow gouge will allow for the welder to move more quickly. The gouge left behind is typically ⅛” wider than the electrode used. Keep this in mind when making a cut so as to not exceed any tolerance callouts.
We have already learned that the electrode should be kept at a distance of 2”-7” from the end of the holder, however when cutting aluminum it is recommended to keep the electrode slightly closer 2”-4” from the electrode holder. The user must continuously adjust the rod position as it is consumed in the cutting process.
Initial gouges should not be deeper than 1 ½ times the diameter of the carbon electrode being used. If a deeper cut is needed the user will need to complete multiple passes until they reach the desired depth, and cleaning between passes may or may not be required.
Special considerations
Carbon arc cutting is a very loud process, capable of producing 108-120dB. When subjected to this level of noise, both the welder and surrounding workers are at risk of hearing loss or damage. It is recommended that anyone in the area adorn appropriate ear protection and PPE. Carbon arc cutting also produces fumes such as carbon monoxide, and the electrodes contain irritants such as copper, graphite and silica crystalline. Some metals such as stainless steels and metals containing zinc release toxic fumes when welded or cut. All of which pose health risks. Welders should wear a properly fitted respirator and safety glasses during cutting and gouging. CAC-A is an electric arc cutting process and therefore a welding helmet with the appropriate shade is needed to protect the user’s eyes from flash burns and potential blindness. As with all welding and cutting processes, operators should use flash screens to protect people and equipment from flying sparks, dross and debris. Never point the debris at gas cylinders and or electric cabling. Keep the tops of your shoes covered to avoid molten metal from dropping down the tops of your work boots, uncuff pant legs and sleeves if applicable, and only wear natural fiber clothing to avoid severe burns.
Attributions
- Figure 13.9: Carbon arc uses in the field by Stephanie Oostman, for WA Open ProfTech, © SBCTC, CC BY 4.0
the process in which the root of a joint is carved away via CAC-A from the backside.
to remove material from a wide surface area
is another shallow material removal that does not damage the base metal