Arc-Out Defect Analysis in Submerged Arc Strip Electrode Overlay Welding

Literature Overview

This paper by Wang Bin and colleagues from Southwest Petroleum University and Atlantic Welding Materials Co., Ltd. investigates the arc-out (end-of-weld) defects in submerged arc strip electrode overlay welding processes. Published in Hot Working Technology in 2012, this work provides a detailed defect analysis combining non-destructive testing (PT), metallographic examination, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD) to characterize slag inclusion defects and their formation mechanisms.

Defect Characterization and Distribution

The researchers employed a systematic multi-scale analysis approach to characterize the arc-out defects. Penetrant testing (PT) was used to determine the macroscopic distribution of slag inclusions, revealing that defects concentrate in a narrow band at the end of the weld deposit. Subsequent metallographic examination showed that slag inclusions are primarily located within 1 mm of the overlay layer surface, indicating a surface-near distribution pattern.

The finding that slag inclusions have minimal effect on the δ-ferrite content of the overlay layer is significant because it indicates that the slag inclusions, while undesirable from a quality standpoint, do not significantly alter the metallurgical properties of the bulk weld metal. This suggests that the defects are primarily surface quality issues rather than bulk material degradation.

Root Cause Analysis

The EDS and XRD analyses conclusively identify the slag inclusions as residual flux material. The root cause analysis points to two primary factors: elevated silicon (Si) content in the flux and an excessively shallow molten pool. These two factors work synergistically to promote slag entrapment at the arc-out region.

The elevated Si content in the flux increases the viscosity of the molten slag, reducing its ability to flow away from the solidifying weld metal. When combined with a shallow molten pool, the reduced fluidity means that slag cannot adequately separate from the weld surface before solidification occurs. At the arc-out region, the welding parameters change rapidly as the arc is terminated, creating conditions particularly favorable for slag entrapment.

Process Mechanism of Arc-Out Defect Formation

During the normal welding process, the molten pool maintains sufficient depth and fluidity to allow slag to float to the surface and flow ahead of the solidification front. However, at the end of the weld run, several unfavorable conditions develop simultaneously: the heat input decreases as the arc approaches termination, the molten pool becomes shallower due to reduced energy input, and the relative velocity between the solidifying metal and the slag increases. These combined effects create a window where slag can become trapped beneath the solidifying surface.

The strip electrode configuration used in submerged arc welding provides high deposition rates but also creates a wider, flatter molten pool compared to wire electrode processes. This geometry, while beneficial for deposition efficiency, can exacerbate arc-out problems because the wider pool requires more time to solidify and slag separation, while the high deposition rate means less time per unit length for slag clearance.

Process Improvement Strategies

Based on the root cause analysis, several process improvement strategies can be implemented to minimize or eliminate arc-out slag inclusion defects. These include optimizing the flux composition to reduce Si content while maintaining adequate slag properties, adjusting welding parameters at the end of the run to maintain adequate molten pool depth, implementing arc-out crater filling techniques, and developing automated end-of-weld parameter control sequences.

Study Insights and Engineering Practice

This defect analysis work exemplifies the importance of systematic quality investigation in welding operations. The multi-scale analysis approach—combining macroscopic PT examination with microscopic SEM/EDS/XRD characterization—provides a comprehensive understanding of defect formation that cannot be achieved through any single analytical technique alone.

For engineering practice in pipe and fitting manufacturing, understanding arc-out defects is particularly important for overlay welding applications where surface quality is critical. In applications such as corrosion-resistant cladding of pipelines, wear-resistant overlay on valve components, and repair welding of critical equipment, surface slag inclusions can serve as corrosion initiation sites or stress concentrators that compromise component integrity.

The finding that slag inclusions are primarily surface phenomena within 1 mm of the overlay surface suggests that surface finishing operations (grinding or machining) can effectively remove these defects. However, this approach is only practical for applications where material removal is acceptable, and the underlying weld metal quality is not compromised.

In conclusion, this study provides a comprehensive understanding of arc-out slag inclusion defects in submerged arc strip electrode overlay welding, identifying elevated flux Si content and shallow molten pool as the primary contributing factors. The multi-scale analytical approach employed offers a methodology that can be applied to other welding defect investigations, and the process improvement strategies identified provide practical guidance for quality enhancement in industrial overlay welding operations.