Study Notes on Arc-Out Defect Analysis in Submerged Arc Strip Cladding

Defect Classification and Significance

The paper "Arc-Out Defect Analysis in Submerged Arc Strip Cladding" addresses a critical quality issue in the manufacture of clad plate and weld overlay pressure vessels. Arc-out defects, which occur at the termination of a cladding pass, are among the most common and problematic defects in submerged arc welding (SAW) strip cladding. These defects can compromise the bond strength between the overlay and the base metal, the corrosion resistance of the overlay, and the fatigue life of the cladding.

The significance of arc-out defects cannot be overstated. In a pressure vessel application, an arc-out defect can serve as a crack initiation site, leading to catastrophic failure under cyclic loading. In a corrosion-resistant application, an arc-out defect can provide a pathway for corrosive media to penetrate the overlay and attack the base metal. The cost of repairing or replacing a vessel with arc-out defects can be substantial, making the prevention of these defects a priority for the manufacturer.

The paper presents a systematic analysis of arc-out defects using the 5W2H methodology (What, Why, Where, When, Who, How, How much), which provides a structured framework for identifying the root causes of the defects and developing effective countermeasures.

Root Cause Analysis

The root cause analysis identifies several factors that contribute to arc-out defects. The first is the welding current and voltage, which must be maintained at a stable level throughout the welding process. A sudden decrease in current or voltage at the end of a pass can cause the arc to extinguish prematurely, resulting in an incomplete fusion and a lack of penetration.

The second factor is the travel speed, which must be constant throughout the pass. A sudden increase in travel speed at the end of a pass can cause the molten pool to cool too rapidly, resulting in a shrinkage crack or a lack of fusion. The third factor is the flux coverage, which must be maintained at a uniform depth throughout the pass. A reduction in flux depth at the end of a pass can cause the arc to become unstable, resulting in spatter and porosity.

The fourth factor is the magnetic field, which can cause arc blow in SAW strip cladding. Arc blow is a distortion of the arc caused by the interaction between the welding current and the magnetic field generated by the base metal. This distortion can cause the arc to shift away from the intended weld position, resulting in an incomplete fusion and a lack of penetration.

The fifth factor is the operator technique, which is often the most difficult to control. An experienced operator can recognize the signs of an impending arc-out defect and take corrective action, such as adjusting the current or the travel speed. However, operator fatigue and inattention can lead to defects that are difficult to detect and prevent.

Countermeasures and Quality Control

The paper proposes several countermeasures to prevent arc-out defects. The first is the use of a welding current and voltage that is optimized for the specific cladding application. The paper presents a table of recommended parameters for a range of cladding thicknesses and base metals, which provides a starting point for the optimization of the welding process.

The second countermeasure is the use of a constant travel speed, which can be achieved by the use of an automatic welding machine with a speed control system. The paper demonstrates that the use of an automatic machine reduces the frequency of arc-out defects by 60 % compared to manual welding.

The third countermeasure is the use of a flux coverage system that maintains a uniform flux depth throughout the pass. The paper describes a system that uses a flux feeder to maintain a constant flux depth, which reduces the frequency of porosity and spatter.

The fourth countermeasure is the use of a magnetic field compensation system, which can be used to reduce the effect of arc blow. The paper describes a system that uses a set of magnets to create a compensating magnetic field that counteracts the magnetic field generated by the base metal.

The fifth countermeasure is the use of a quality control system that includes non-destructive testing (NDT) of the cladding. The paper recommends the use of magnetic particle testing (MT) and ultrasonic testing (UT) to detect arc-out defects, which can then be repaired by grinding and re-welding.

Engineering Practice and FMEA Integration

In my own experience with clad plate manufacturing, the use of Failure Mode and Effects Analysis (FMEA) has been invaluable in identifying and preventing arc-out defects. The FMEA process involves identifying the potential failure modes, assessing the severity, occurrence, and detection of each failure mode, and developing countermeasures to reduce the risk.

For arc-out defects, the severity is typically high, as the defects can compromise the structural integrity of the cladding. The occurrence is moderate, as the defects are common but not universal. The detection is low, as the defects are often difficult to detect without NDT. The resulting risk priority number (RPN) is high, which justifies the implementation of the countermeasures described above.

The FMEA process also provides a framework for continuous improvement. By regularly reviewing the FMEA and updating the risk assessment, the manufacturer can identify new failure modes and develop new countermeasures. This approach has been successful in reducing the frequency of arc-out defects in our own production, with a reduction of 80 % over a period of three years.

Study Insights and Outlook

The paper presents a comprehensive analysis of arc-out defects in SAW strip cladding, which is a valuable resource for the practicing engineer. The use of the 5W2H methodology and the FMEA process provides a structured approach to defect analysis and prevention, which can be applied to other welding defects and other manufacturing processes.

The key takeaway is that quality control in cladding manufacturing is not a one-time activity but a continuous process that requires a systematic approach to defect analysis and prevention. The paper demonstrates that a combination of process optimization, equipment improvement, and quality control can significantly reduce the frequency of arc-out defects and improve the quality of the cladding.

For the practicing engineer, the lesson is that the prevention of defects is always more cost-effective than the repair of defects. The investment in process optimization and quality control pays for itself many times over in reduced rework and improved product quality.