Weld inspection and repair is a multi-step process that includes non-destructive testing, inspection data interpretation against the relevant code, weld metal excavation, follow-up non-destructive testing, weld repair per relevant construction code and final non-destructive testing. This article walks the reader through a steel weld that failed an inspection, was subsequently repaired, re-inspected and cleared for use.
Figure 1: Nondestructive testing and repair of structural steel.
The video shown above is representative of the steps required to return a failed weld to service. In this article it is discussed in the context of AWS D1.1 and 1.5. The first step is the standard ultrasonic testing weld inspection and is covered in many other articles in detail [1,2]. The results of this initial non-destructive test are often marked on the plate tested along with the inspection initials and date. In this example two indications were detected with ultrasonic testing; IND1 and IND2 as marked on the test surface. The length L, depth d, and Indication Rating D are marked on the piece. While not covered in this article and indication rating of 1.2 and 0.7 fail the requirements outline in AWS D1.1. The approximate location of the weld defect is identified by the inspector and presented to the welder. Accurate location relevant to the weld centerline and depth relative to the test surface are mandatory for a targeted weld excavation that minimizes the excavation labor and tooling as or the excavation and labor and welding materials for the repair.
After inspection, the weld repair process begins. The area of concern is excavated at the targeted area at least to the depth of the defect detected by ultrasonic testing. If testing using radiographic testing (RT) the depth is unknown and the weld will be excavated until the defect is visually observed. During weld excavation process, either with a grinding wheel or carbon arc gouging, the defect opening may be smeared or filled with filled metal. Additionally tight cracks or lack of fusions openings may be too slight to observe by the naked eye and back up non-destructive testing with magnetic particle testing will be required to confirm the presence of the defect. A magnetic particle testing indication is observed during the weld excavation process is shown in above video. Weld excavation must be continued until no visible indication is observed visually, with magnetic particles, or liquid penetrant testing.
Weld Repair per Structural Steel Welding Codes
The section describes the process to repair a complete joint penetration (CJP) single bevel groove field weld that is back gouged. Shielded metal arc welding (SMAW) was used to weld the joint vertical up with 7018 electrodes. The inspection described above was performed to AWS D1.1. This requires visual testing (VT), magnetic particle testing (MT), and ultrasonic testing (UT). The weld was air carbon arc gouged (CAC-A) and ground using and angle grinder with a flapper wheel. The ground surface was checked with wet florescent MT to verify defect removal. Five rounds of gouging and grinding while checking with MT were performed to confirm defect removal from the weld.
Figure 2: Example arc gouging weld repair
Understanding Carbon Arc Gouging
Air Carbon Arc gouging (CAC-A) is a widely used method in the welding and metalworking industries, primarily for cutting, gouging, and removing defects from metal workpieces. It uses a carbon electrode and a direct current (DC) power source to create a high-temperature arc, which melts the metal. Compressed air creates a jet stream under the electrode that ejects the metal from the workpiece. Figure 2 is a visual aid for this process of removing material. Figure 3 shows the air holes that direct the compressed air to the molten metal. Figure 4 shows this process from the worker’s view through filtered lenses on how the groove is created. This process is distinct from other cutting methods, such as oxy-fuel cutting or plasma cutting, in that it relies on an electric arc between a carbon electrode and the base metal to remove material. The high-intensity heat produced by the arc is used to melt the metal, while compressed air helps to blow away the molten metal, thus creating a groove.
The Process
The key components in carbon arc gouging include a power supply, a carbon electrode, and a compressed air system. The process begins with the creation of an electric arc between the carbon electrode (the positive terminal) and the bae metal (the negative terminal). This arc produces extremely high temperatures, typically exceeding 10,800°F (6,000°C), which is sufficient to melt most metals. The carbon electrode conducts electrical current to the base metal, and the intense heat generated at the arc melts a portion of the metal.
Simultaneously, compressed air is directed at the molten metal, blowing it away from the gouging area. The result is a clean, wide groove that can be used for applications like weld preparation, defect removal, or metal repair. The direction of the airflow can be adjusted to ensure that the molten metal is efficiently expelled from the work area, leaving a clean, smooth groove.
Equipment Used in Carbon Arc Gouging
Power Supply: Typically, a direct current (DC) power source is used for carbon arc gouging. The most common setup involves a DC reverse polarity, where the workpiece is the negative terminal, and the electrode is the positive terminal. This configuration helps produce a more stable and efficient arc.
Carbon Electrodes: These are the consumable components of the process. The carbon electrodes are often made from graphite or other forms of carbon, and they are available in various sizes and shapes depending on the application.
Air Supply: Compressed air is crucial for expelling molten material during gouging. The air is typically delivered through the air holes at the end of the electrode holder, which blows the molten metal away from the groove. In figure 3 the air holes are shown in the electrode holder.
Electrode Holder: The electrode holder is a device that grips the carbon electrode and conducts electricity to it. It also provides the operator with a means to maneuver the electrode during gouging.
Advantages of Carbon Arc Gouging
One of the key benefits of carbon arc gouging is its versatility. It can be used to cut and gouge various metals, including steel, aluminum, and other alloys. It's especially useful for preparing weld grooves, removing faulty welds, or cleaning up weld joints. The gouging process also produces minimal heat-affected zones compared to other methods like flame cutting, which can distort the workpiece. Moreover, carbon arc gouging provides a good level of precision in cutting and gouging, which helps to reduce material wastage.
Limitations and Safety Considerations
Although carbon arc gouging is an efficient and effective method, there are some limitations to consider. For weld preparation the gouged surface has a deposit of carbon from the carbon electrode. If the surface is not cleaned with a grinder before welding carbon embrittlement can occur leading to cracking. The process produces significant noise and can generate a considerable amount of fumes, especially when working with materials like steel or cast iron. Adequate ventilation and safety measures are necessary to mitigate these risks.
Another limitation of carbon arc gouging is that it may not be suitable for very fine or intricate cuts due to the relatively large size of the arc and the groove it creates. It is more effective for rougher cuts and removing large amounts of material. This is why grinding after gouging is required to check with MT. If the surface is not moderately smooth indications will be harder to see.
Conclusion
In conclusion, the process of weld inspection, excavation, repair, and re-inspection is critical to ensuring the integrity of structural steel welds. By following the procedures outlined in AWS D1.1, including ultrasonic testing (UT), magnetic particle testing (MT), and visual inspection (VT), defects can be accurately detected, located, and addressed. Air carbon arc gouging (CAC-A) is an effective method for removing defective material, followed by grinding and further testing to confirm defect removal. This multi-step process, when executed properly, guarantees that the repaired welds meet the required standards for safety and performance.
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