Zhuojin Pipe Fitting Co., LtdArc Termination Defect Analysis in Submerged Arc Strip Electrode Overlay Welding
Literature Overview
This paper by Wang Bin, Zhang Junling, Yang Fei, and Zhou Cui, published in Hot Working Technology in 2012 (Volume 41, Issue 19, pages 139-141), investigates slag inclusion defects occurring at arc termination in submerged arc strip electrode overlay welding. The authors from Southwest Petroleum University and Atlantic Welding Materials Co., Ltd. employed PT inspection, optical microscopy, scanning electron microscopy, energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) to characterize the defect morphology, distribution, and composition. The findings identify high SiO₂ content in flux and excessively shallow weld pools as the primary causes of slag inclusion at arc termination.
Defect Characterization and Distribution
The study systematically characterizes slag inclusion defects using multiple analytical techniques, establishing a comprehensive understanding of defect morphology and spatial distribution. The PT inspection reveals the macroscopic distribution pattern of slag inclusions, while optical microscopy and SEM provide microstructural details of individual defect features.
Defect Characterization Summary
| Characterization Method | Key Finding | Significance |
|---|---|---|
| PT inspection | Macroscopic distribution pattern | Identifies defect location and extent |
| Optical microscopy | Slag inclusions within 1 mm of surface | Defines defect depth range |
| SEM analysis | Microscopic morphology of slag | Reveals slag particle characteristics |
| EDS analysis | Elemental composition of slag | Confirms slag composition |
| XRD analysis | Phase identification | Confirms slag as residual molten flux |
A critical finding is that slag inclusions are confined primarily within the 1 mm range from the overlay layer surface. This shallow distribution has important implications for surface finish requirements and post-weld machining allowances in overlay welding applications. The defect depth limitation also suggests that surface grinding or machining can effectively remove the affected zone without significant material removal.
Root Cause Analysis
The investigation identifies two primary contributing factors to slag inclusion formation at arc termination:
Primary Cause Factors
| Factor | Mechanism | Effect on Defect Formation |
|---|---|---|
| High SiO₂ in flux | Increased slag viscosity | Slag cannot flow away before solidification |
| Shallow weld pool | Insufficient slag flotation time | Slag trapped before rising to surface |
The high SiO₂ content in the flux increases slag viscosity, reducing its fluidity and ability to separate from the molten weld metal. At arc termination, when the heat input ceases, the cooling rate increases rapidly. If the slag viscosity is too high, the slag cannot flow to the surface and float free before the weld metal solidifies, resulting in trapped slag inclusions.
The shallow weld pool condition exacerbates the problem by reducing the time available for slag flotation. A deeper weld pool provides a larger volume of molten metal with extended solidification time, allowing viscous slag to rise to the surface through buoyancy forces. At arc termination, the reduced heat input creates a shallow pool with rapid cooling, compounding the slag entrapment tendency.
FMEA for Arc Termination Defects
| Failure Mode | Cause | Effect | Detection | Prevention |
|---|---|---|---|---|
| Slag inclusion | High SiO₂ flux + shallow pool | Surface defect within 1 mm | PT inspection | Flux composition control |
| Surface slag entrapment | Insufficient cooling rate control | Overlay surface quality degradation | Visual + PT | Arc termination procedure |
| δ-ferrite variation | Slag inclusion presence | Minimal impact (< threshold) | Metallographic analysis | Not critical for this defect |
An important secondary finding is that slag inclusion does not significantly affect the δ-ferrite content of the overlay layer. This observation is metallurgically significant because it indicates that the slag inclusion is a surface defect rather than a bulk microstructural issue. The δ-ferrite content, which governs cracking susceptibility in austenitic overlay welds, remains within acceptable limits despite the presence of slag inclusions.
Engineering Practice and Process Control
Submerged arc strip electrode overlay welding is widely used for depositing thick overlay layers on large components such as pump casings, valve bodies, and heavy-duty wear parts. The strip electrode provides high deposition rates and uniform bead profiles, making it ideal for building up significant material thickness efficiently.
The arc termination defect is a common challenge in all submerged arc welding processes, but it is particularly problematic in overlay welding where surface quality is critical. Unlike structural welding where defects can be ground out and the component continues to function, overlay weld surface quality directly affects the performance of the deposited layer.
Process Control Recommendations
- Flux composition control: Reduce SiO₂ content in flux to improve slag fluidity and reduce inclusion tendency. The optimal SiO₂ level must balance slag fluidity against other flux functions such as alloying and deoxidation.
- Arc termination procedure: Implement controlled arc termination procedures that include trailing current or backing electrode techniques to maintain a deep weld pool during termination.
- Parameter optimization: Adjust welding current, voltage, and travel speed to maintain adequate weld pool depth throughout the welding sequence, including at termination.
- Post-weld machining: Plan for surface finishing operations that remove at least 1 mm from the overlay surface to eliminate the defect zone, though this is a remedial rather than preventive measure.
The collaboration between Southwest Petroleum University and Atlantic Welding Materials Co., Ltd. provides practical insight into how academic research and industrial practice can address real production problems. The defect analysis methodology — combining macroscopic inspection with microscopic characterization and compositional analysis — represents a rigorous approach to welding defect investigation that is directly applicable to quality assurance programs in overlay welding operations.
Study Insights and Quality Control Implications
This paper demonstrates the value of systematic defect analysis in improving welding process quality. The multi-technique characterization approach provides comprehensive understanding of defect formation mechanisms, enabling targeted process improvements rather than trial-and-error parameter adjustments.
For quality control programs in overlay welding operations, the findings suggest that arc termination procedures should be specifically addressed in welding procedure specifications. The shallow defect depth (1 mm) provides clear acceptance criteria for surface finish requirements, and the minimal impact on δ-ferrite content offers reassurance that the defect does not compromise the fundamental metallurgical properties of the overlay layer.
The work highlights the importance of flux composition optimization as a fundamental process control lever. Flux chemistry directly influences slag properties, which in turn govern defect formation during critical process events such as arc termination. This insight extends to other welding processes where flux or flux-cored consumables are employed, reinforcing the principle that consumable selection is a critical quality control parameter.
The research contributes practical knowledge to the welding community regarding a specific but common defect mode in submerged arc overlay welding, providing both diagnostic criteria and corrective measures that can be immediately implemented in industrial settings. The systematic approach to defect investigation serves as a model for quality improvement programs in welding operations.