CLADDING TECHNOLOGY SHANXI CO., LTDArc Termination Defect Analysis in Submerged Arc Strip Cladding
Introduction and Technical Context
Submerged arc welding (SAW) with strip electrodes is a widely used process for producing thick overlay layers on pressure vessels, heat exchangers, and other equipment requiring corrosion or wear resistance. The strip electrode, typically 200–400 mm wide and 1.5–4.0 mm thick, provides high deposition rates and good dilution control compared to wire electrodes. However, the arc termination (crater) region at the end of each weld pass is a known vulnerability where defects such as crater cracks, porosity, and insufficient fusion frequently occur. This literature provides a systematic analysis of arc termination defects in strip cladding, with emphasis on root cause identification and preventive measures.
Arc Termination Process Analysis
The arc termination phase in SAW strip cladding involves a complex sequence of events that differ significantly from the steady-state welding condition. As the electrode approaches the planned termination point, the operator or automated system must manage the transition from full deposition to arc extinction while maintaining adequate fusion and minimizing residual stress concentration at the crater. The critical parameters during termination include:
- Arc current tapering: The current must be reduced gradually to prevent crater formation while maintaining sufficient heat input for fusion.
- Flux level maintenance: The flux covering must remain consistent until arc extinction to prevent oxidation and spatter.
- Electrode positioning: The strip electrode must maintain proper alignment with the travel direction to avoid asymmetric crater geometry.
- Post-arc cooling control: The cooling rate after arc termination affects the microstructure and residual stress state of the crater region.
Typical Arc Termination Parameters
| Parameter | Steady-State | Termination Phase |
|---|---|---|
| Arc current (A) | 400–600 | 400–600 → 0 (taper over 1–3 s) |
| Arc voltage (V) | 28–34 | 28–34 → 0 (taper over 1–3 s) |
| Travel speed (mm/min) | 200–400 | 200–400 → 0 (taper over 2–5 s) |
| Flux level (mm) | 15–25 | 15–25 → 5–10 |
| Electrode angle (°) | 5–10 (drag) | 5–10 → 0 |
| Post-arc time (s) | — | 5–15 |
Defect Classification and Metallographic Analysis
The literature identifies four primary defect types at arc terminations in strip cladding, each with distinct metallurgical characteristics and formation mechanisms:
Crater Cracks
Crater cracks are transverse or radial cracks that initiate at the surface of the crater and propagate inward. Metallographic examination reveals that these cracks initiate at the last solidification site, where shrinkage porosity and segregation concentrate. The microstructure at the crack initiation site typically shows a coarse columnar grain structure with interdendritic microsegregation of sulfur and phosphorus. The crack propagation mechanism is solidification cracking, driven by shrinkage stress exceeding the low-temperature tensile strength of the interdendritic regions.
Crater Porosity
Gas porosity at arc terminations is caused by insufficient flux coverage during the taper-down phase. As the arc current decreases, the electromagnetic stirring of the molten pool weakens, reducing the ability of the pool to expel dissolved gases. Simultaneously, the reduced arc force decreases the flux penetration depth, allowing oxidation of the molten pool surface. The resulting porosity is typically spherical and distributed throughout the crater volume, with sizes ranging from 0.1 mm to 2.0 mm.
Insufficient Fusion
Incomplete fusion at the crater edges occurs when the current taper is too rapid or the travel speed is not properly synchronized with the current reduction. The result is a lack of fusion between the last deposited metal and the base metal or previous pass, creating a planar defect that may not be detected by surface inspection but can be revealed by ultrasonic testing.
Undercut and Profile Irregularities
Excessive current at the termination point, combined with inadequate flux coverage, can cause undercutting at the crater edges. This creates a stress concentration that may initiate fatigue cracks during service. The undercut depth typically ranges from 0.5 mm to 2.0 mm and is most severe at the transition from the steady-state weld profile to the crater.
Defect Severity Assessment
| Defect Type | Detection Method | Acceptance Criteria (ASME VIII Div.1) | Rejection Criteria |
|---|---|---|---|
| Crater cracks | MT / PT / UT | None allowed | Any crack indication |
| Crater porosity | RT / UT | ≤1.5 mm diameter, ≤3 per 100 mm | >1.5 mm or >3 per 100 mm |
| Insufficient fusion | UT / RT | None allowed | Any indication |
| Undercut | Visual / MT | ≤1 mm depth for overlay | >1 mm depth |
Root Cause Analysis and FMEA Application
A Failure Mode and Effects Analysis (FMEA) was conducted to systematically identify all potential causes of arc termination defects. The analysis assigned severity, occurrence, and detection ratings to each failure mode, resulting in a risk priority number (RPN) for prioritization.
| Failure Mode | Severity | Occurrence | Detection | RPN | Root Cause |
|---|---|---|---|---|---|
| Crater crack | 10 | 4 | 5 | 200 | Insufficient current taper time |
| Crater porosity | 8 | 5 | 4 | 160 | Premature flux withdrawal |
| Insufficient fusion | 9 | 3 | 6 | 162 | Travel speed not synchronized |
| Undercut | 6 | 5 | 3 | 90 | Excessive current at termination |
| Crater shrinkage | 7 | 4 | 5 | 140 | Inadequate post-arc cooling |
The FMEA results identify crater cracks as the highest-risk defect, followed by insufficient fusion and crater porosity. The root causes cluster around three categories: process parameter control, flux management, and equipment synchronization.
Preventive Measures and Best Practices
Based on the defect analysis, the following preventive measures are recommended:
Process Parameter Optimization
- Current taper profile: Implement a two-stage taper where current is reduced to 70 % of full value over the first 1.0 second, then to zero over the next 1.5 seconds. This provides a gentler transition than linear tapering.
- Travel speed synchronization: The travel speed must be reduced to zero at the same time as the current reaches zero, with a tolerance of ±0.5 seconds. Automated systems should use encoder feedback for precise synchronization.
- Flux level management: The flux level should be maintained at the steady-state value until the current reaches 30 % of full value, then reduced gradually. A flux level of at least 10 mm should be maintained until arc extinction.
- Post-arc hold time: A minimum of 10 seconds of flux coverage should be maintained after arc extinction to allow the crater to solidify under protective atmosphere.
Equipment and Procedure Enhancements
- Automated termination sequences: Use programmable controllers to execute precise current, voltage, and travel speed profiles during termination.
- Flux level sensors: Install real-time flux level monitoring with automatic adjustment to maintain consistent coverage.
- Crater fill procedures: For critical applications, use a small filler wire to fill the crater after arc termination, providing additional fusion and reducing shrinkage.
- Termination pad plates: Use sacrificial pad plates at the end of each pass to allow the arc to terminate on a non-critical area, eliminating crater defects from the production weld.
Engineering Practice Cases
The literature presents two case studies illustrating the practical application of these principles:
Case 1: Hydrogenation reactor cladding — A 300 mm thick overlay layer was deposited using 300 mm wide strip electrodes. The original procedure used a 1-second linear current taper, resulting in crater cracks found in 18 % of pass terminations. After implementing a two-stage taper with 2.5-second total duration and 15-second post-arc hold, the crack rate was reduced to zero over 200 subsequent passes.
Case 2: Heat exchanger tube sheet cladding — Crater porosity was detected in 12 % of pass terminations during PAUT inspection. Root cause analysis revealed that the flux level dropped below 8 mm during the taper phase due to inadequate flux feed rate. Correcting the flux feed to maintain 15 mm minimum level eliminated porosity entirely.
Study Insights and Reflections
The arc termination region in strip cladding represents a transition from a steady-state process to a transient process, and this transition is where most defects originate. The key insight is that the termination phase requires its own independent qualification and optimization, separate from the steady-state welding parameters. Many fabrication shops treat termination as a minor detail, but this literature demonstrates that it is a critical quality gate that demands systematic attention.
The FMEA approach proved particularly valuable in this context because it provided a structured framework for identifying and prioritizing failure modes. The RPN analysis directed engineering resources to the highest-risk defects first, resulting in efficient use of development time. I would recommend that all cladding operations adopt FMEA as a standard practice for process qualification and ongoing quality improvement.
The case studies also highlight an important practical consideration: the effectiveness of preventive measures depends on consistent implementation. Even the best-designed termination procedure will fail if operators deviate from the specified parameters. Training, procedural documentation, and automated controls are all necessary to ensure reliable execution.