The repair of welds is a process that acquires great importance when it comes to replacing defective plates or eliminating imperfections from previous welds.
Traditionally, the first repairs were made mechanically, using techniques such as grinding, hand milling and deburring. Although still in use, particularly in machine shops, these methods have always been slow, noisy, and extremely expensive because they use dedicated labor.
It was in World War II, during the construction of the huge US Navy ships, that the impracticality of repairing welds using these techniques became apparent. Therefore, an effective method proposed by the welding engineer Myron Stepath and carried out between two operators was tested: one held a carbon electrode with a clamp and the other followed the arc with a compressed air nozzle. This rudimentary test would mark the beginning of a new technique that was launched in 1949 by Arcair, Stepath’s own company, and which is known to this day by the name of repelled, or more properly, arc gouging with carbon electrode.
Carbon arc gouging is therefore the process of cutting/removing metal by means of the intense heat generated by a carbon arc. When using a carbon electrode, compressed air, and a welding power source (constant current, constant voltage, AC, or DC), the arc created between the carbon electrode and the workpiece melts the material, while the compressed air drags the molten slag leaving a groove or groove; hence the name “slotted”. The method can be used on mild steel, cast iron, copper, aluminum, and nickel alloys. However, it produces smoke and gases in excess, and can deposit inclusions in the base metal.
Over time, more and more efficient and versatile grooving methods have emerged. Thus, the gouging with oxygen and fuel gas it added portability and significantly reduced noise levels, but as it is a combustion-based technique it can only be applied to carbon steels.
In these last two decades, plasma arc cutting technology has been presenting its variant of plasma arc gouging as a highly effective alternative that attempts to maximize the volume of metal removal. The process is relatively quiet and smoke free, can be used on mild steel, stainless steel, aluminum and almost any ferrous or non-ferrous alloy, and can use different gases to optimize groove quality and surface conditions.
How is a plasma arc gouging kit made up?
In this article we provide details about the elements that make up a plasma cutting equipment. In the case of plasma gouging, the equipment is very similar, with the difference that the torch is provided with other types of consumables to produce a wider and shallower arc, so as not to cut the piece, but only produce a groove. Many systems of plasma cutting currently marketed can be used for grooving with minimal modification or little additional expense.
The main components are otherwise the same: a power supply, a gas supply, a torch with associated hoses, and consumables. In the following table we mention the main characteristics of each one.
What are the plasma gouging techniques?
Obtaining different profiles and slot sizes takes place using different techniques. Generally, when the pilot arc is formed and the arc is transferred to the plate, the torch is tilted at an angle of 40-60 degrees relative to the work piece. The operator then draws the arc into the throat by moving the torch forward across the plate. Greater arc penetration is achieved with steeper angles and lower speeds. Conversely, less removal, and therefore a shallower groove, is achieved with smaller angles and higher speeds. This means that by varying the speed, the torch angle and the distance from the torch to the part, different profiles can be obtained, as the following table exemplifies.
In addition to variation of grooving parameters, different techniques can also be applied to achieve different types of groove. In the figure below we see examples of techniques for straight slotted, side Y zigzag.
Single pass straight grooving produces a relatively narrow, parabolic groove. For grooving wider and deeper sections side or zigzag grooving can be used.
Plasma Arc Gouging Applications
What emerged as a technique dictated by the constraints of an era to achieve weld repairs in combat ships, today has been extended to a variety of fields that, in addition to including shipbuilding, extends to the maintenance, manufacture and repair of heavy equipment, as well as the manufacture of truck bodies, tanks and steel structures.
In addition to grooving, the technique of opposite side grooving, which removes back metal from arc-welded joints to eliminate defects and improve strength. Thus, the defects welding such as cracks, porosity and lack of fusion could be removed by gouging with a plasma torch and then repaired with a new blemish free weld.
Other applications such as removal of thick backing beads, lifting lugs, temporary supports, spot welds and rivets allow plasma gouging to be used in demolition or salvage operations to disassemble welded structures, as well as in foundries to remove excess of casting material and anywhere where there is excess metal, excess welding, or defects that must be removed.
Like any powerful technology, plasma gouging has some drawbacks, mainly translated into a certain limitation in accessibility and gouging depth depending on the torch used, the need to train the operator, the risk of electric shock due to high voltages workload, the use of protective clothing, and the cost and poor portability of equipment relative to other gouging processes.
However, the advantages of the process far outweigh the drawbacks, as plasma gouging is capable of:
- Generate a considerable increase in production and the possibility of varying the speed of removal with the power of the equipment, which allows precise digging of the material.
- Provide automation, as the speed and rate of removal can be increased by virtue of the continuous advance, which maintains the discharge of material, a characteristic impossible to reproduce manually. On the other hand, due to the low heat provided, the slag is always lodged on one side without welding with the base material, so it becomes self-removing.
- Achieve an optimal surface appearance, because as carbon is not deposited on the base material, subsequent cleaning is not required. The uniformity and good appearance create an ideal surface for automated welding, without running the risk of discontinuities due to dislocation of the bead.
- Lower overall costs, thanks to higher removal rates, faster process speeds, and longer consumable life than other processes, such as carbon arc gouging, for example, where carbon needs to be changed much more frequently .
An example of plasma gouging with mechanized equipment shows us, in the following video, the excellent groove quality achieved.