Back Gouging for Full Penetration Welds

Many times for critical strength members or applications that involve pressure vessels, we need to complete a full penetration weld to meet the requirements of the project or job. Complete Joint Penetration ( CJP) is the term many use in industry. Often we are required to complete that weld from one side only, involving some type of root pass on a open root joint. Other applications where we have complete access to the backside of the weld may not involve an open root pass type of weld. When we need to complete a full penetration weld where we are going to weld on both sides, many times we need to back gouge out the root of the first weld before applying any filler metal on the backside of the weld joint. How much has to be removed, what type of material, and often the position or location of the weld will determine how we go about gouging out the weld.

If the material is not very thick, is not very long, or does not involve very much gouging, sometimes an air or electric grinder is a good tool for back gouging. Depending on the joint configuration, a thin or thick abrasive disc may be the right tool for the job. We need to be careful when using the grinder in a groove. If the wheel binds in the joint, it can splinter a break in the blink of an eye. Safety glasses and a face shield are the minimum face protection required. Also, many grinding wheels are not intended to be used on the edge, make sure your wheel is ok for this application. Safety first always.

If the material is thicker, larger, or the gouge is long or in a tight location, the grinder may not be practical. In that case, we need to think about a thermal process for removing the material. An oxy-fuel gouging tip, air carbon arc gouging, or plasma arc gouging are the options in many fabricator's tool box.

The oxy-fuel gouging tip works only on carbon steel. It requires time for the base material to preheat and the process proceeds slow compared to other operations. The process is portable since we only need an oxygen tank, fuel gas tank, some regulators, hoses and a torch to get us going.

Air carbon arc gouging and plasma arc gouging require some access to electrical power. If we have access to a gas or diesel powered generator the process is more portable. Air carbon arc gouging can be done using a stick welding or shielded metal arc welding machine and some compressed air. Plasma arc gouging generally has a special power source dedicated just for that process. Both processes can remove a large amount of material in a short amount of time in the hand of a skilled and knowledgeable operator. The consumables for carbon arc gouging consist of copper coated carbon electrodes. The consumables for plasma arc cutting consist of the electrode and the copper constricting cup. Of the two processes, carbon arc gouging is more portable in the sense that the electrode lead from the power source is the only item that needs to be strung to the location of the work. Usually a plasma arc gouging torch is not more than 25 feet long. Because of that, the power source usually has to be located not far from the work. Inverter power sources have made plasma cutting and gouging a simpler and easier operation with smaller size power sources. These inverter power sources size and weight have added to plasma gouging portability.

All these gouging processes generate a lot of sparks that can fly a significant distance. We must consider that when we get ready to do any type of gouging that involves one of these processes. Making sure the area is free of combustibles and a fire extinguisher close by is the minimum prep required. Often these processes use more amperage then generally used for welding. With that in mind we should use a darker shade lens in our welding hood that equals what is required for the amperage we are using. ANSI Z49.1 has the chart of appropriate shades. Hearing protection is strongly encouraged since most of these processes generate a significant amount of noise. Fresh air, and proper ventilation is also an important consideration.

Regardless of the process the goal is to remove unwelded materials to the point we reach sound metal. Once the material has been removed, welding can proceed to fill up the void. We may remove material from the back of a butt joint, the seam on a pressure vessel, or even a crack in a cast-iron part. Regardless of the application, gouging is the opposite of welding.

Check out the Longevity website (www.longevity-inc.com) or YouTube channel (www.youtube.com/longevitywelding) for more details and information about equipment for different welding and cutting processes. Longevity has the right machine for your exact application, so take a look and choose what is the best fit for your materials, product and needs.