Zhuojin Pipe Fitting Co., LtdArc-End Slag Inclusion Defect Analysis in Submerged Arc Strip Electrode Overlay Welding
Literature Overview
This technical paper, published in Hot Working Technology (2012, Vol. 41, No. 19, pp. 139-141), investigates slag inclusion defects occurring at the arc termination (arc-end) region of submerged arc strip electrode overlay welds. The research was conducted jointly by Southwest Petroleum University (School of Materials Science and Engineering) and Atlantic Welding Materials Co., Ltd. The study provides a detailed root cause analysis of a recurring quality issue in industrial overlay welding production.
Submerged arc strip electrode (SASE) welding is a high-deposition-rate process widely used for overlay welding applications in the petroleum, chemical, and power industries. The process uses a continuous strip electrode instead of a consumable wire, offering higher productivity and consistent weld quality. However, like all arc welding processes, SASE overlay welding is susceptible to various defects, with slag inclusions being particularly problematic at the arc-end region.
Defect Identification and Characterization
Macroscopic Distribution
Penetrant testing (PT) was employed to map the macroscopic distribution of slag inclusion defects across the overlay weld surface. The analysis revealed that slag inclusions were concentrated in the arc-end region, primarily within a 1 mm depth from the overlay weld surface. This shallow distribution indicates that the slag inclusions are entrapped during the final stages of solidification rather than being deeply embedded in the weld metal.
Microscopic Morphology
Optical microscopy and scanning electron microscopy (SEM) examinations of the slag inclusions revealed the following characteristics:
- The inclusions appeared as irregularly shaped voids or pockets within the weld metal
- The morphology varied from small isolated particles to elongated stringer-like features
- The distribution was predominantly near-surface, consistent with the macroscopic PT findings
- The inclusions did not appear to significantly affect the δ-ferrite content of the overlay weld metal
Phase Identification
Energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analyses confirmed that the slag inclusions consisted primarily of residual flux material that had not been properly expelled from the molten weld pool before solidification. The chemical composition of the inclusions matched the flux composition, confirming their origin as entrapped flux rather than oxide or other contamination.
Root Cause Analysis
The study identified two primary contributing factors to the slag inclusion formation at the arc-end region:
Factor 1: Elevated Silicon Content in Flux
The flux used in the process contained a relatively high silicon content. During the welding process, the high Si content in the flux affects the slag viscosity and fluidity characteristics. At the arc-end, where the heat input decreases and the molten pool cools more rapidly, the high-viscosity slag becomes difficult to float and separate from the weld metal, leading to entrapment.
Factor 2: Shallow Molten Pool Geometry
The molten pool geometry during SASE welding is characterized by a relatively shallow depth-to-width ratio. At the arc-end, as the electrode is withdrawn and the arc is terminated, the molten pool becomes even shallower and cools more rapidly. This combination of shallow pool geometry and rapid cooling at the arc-end creates conditions where slag has insufficient time to rise and separate from the solidifying weld metal.
| Root Cause Factor | Mechanism | Effect on Slag Inclusion Formation |
|---|---|---|
| High Si content in flux | Increases slag viscosity, reduces fluidity | Slag cannot rise quickly enough to escape the solidifying pool |
| Shallow molten pool at arc-end | Reduced buoyancy driving force, rapid solidification | Slag is trapped before it can float to the surface |
| Rapid cooling at arc-end | Short liquid residence time | Incomplete slag-metal separation |
| Arc termination dynamics | Sudden change in heat input and pool shape | Disruption of normal slag expulsion mechanism |
Countermeasures and Process Optimization
Based on the root cause analysis, the following corrective actions are recommended:
- Flux composition modification: Reduce the silicon content in the flux to lower slag viscosity and improve slag fluidity, allowing better slag-metal separation.
- Arc-end process modification: Implement a controlled arc-end sequence that maintains adequate heat input during the final stages of welding, allowing the molten pool to remain liquid long enough for slag to float out.
- Welding parameter adjustment: Optimize the combination of current, voltage, and travel speed to create a molten pool geometry that facilitates slag expulsion, particularly at the arc-end region.
- Post-weld flux removal: Ensure thorough and timely removal of solidified flux from the weld surface to prevent re-entrainment during subsequent passes.
Engineering Practice Implications
For production operations using SASE overlay welding, this study provides valuable diagnostic guidance for addressing slag inclusion quality issues. The systematic approach of using PT for macroscopic mapping, followed by OM/SEM for morphological characterization, and EDS/XRD for phase identification represents a best-practice methodology for welding defect analysis.
The finding that slag inclusions are concentrated within 1 mm of the surface suggests that surface grinding or machining of the overlay weld could potentially remove these defects. However, this is only viable if the overlay thickness allows for material removal without compromising the functional coating thickness. Engineers should evaluate whether the defect depth is acceptable for the specific application or whether process modification is necessary to eliminate the root cause.
The interaction between flux chemistry and process parameters identified in this study highlights the importance of holistic process optimization. Changes to one parameter (such as flux composition) may have cascading effects on other aspects of weld quality, including weld metal chemistry, dilution, and mechanical properties. A comprehensive approach that considers all aspects of the welding process is essential for achieving consistent quality in SASE overlay welding production.
In summary, these five studies collectively illustrate the breadth of technical challenges and solutions in overlay welding technology, ranging from computational simulation tools and microstructure optimization through rare earth modification to post-weld heat treatment techniques and defect root cause analysis. Each paper contributes valuable insights that, when integrated, provide a comprehensive understanding of how to design, process, control, and quality-assure overlay welding operations for demanding industrial applications in the pipeline, rail, and equipment sectors.