Analysis of Incomplete Fusion Defects in Submerged Arc Strip Electrode Cladding of In-Service High-Pressure Vessels

Literature Overview

This technical article, published in 2022 in the journal Petroleum and Chemical Equipment by Hu Juanjuan from Hefei Jinshu Special Equipment Technology Service Co., Ltd., addresses a critical and recurring problem in the maintenance and repair of in-service high-pressure vessels. The paper focuses on incomplete fusion defects encountered during submerged arc welding (SAW) strip electrode cladding operations on operating pressure vessels, analyzing their root causes and proposing practical countermeasures. The study is particularly significant because in-service vessels face constraints that new fabrication does not, including limited access, residual stress from prior service, and the inability to perform full thermal treatments post-cladding.

Core Technical Analysis

Incomplete fusion in strip electrode cladding arises from a complex interplay of thermal, mechanical, and process factors. The fundamental mechanism involves insufficient heat input to achieve complete melting of the base metal surface prior to or during the deposition of the cladding layer. In strip electrode SAW, the electrode is a solid strip of the cladding alloy rather than a consumable wire, and the arc is struck between the strip and the base metal. The thermal profile differs substantially from conventional wire SAW, requiring careful control of parameters to ensure adequate base metal melting.

The root causes identified include several categories. First, thermal factors: the strip electrode process generates a relatively shallow heat-affected zone compared to wire SAW, and if the current density is insufficient or the travel speed too high, the base metal surface may not reach the melting point required for metallurgical bonding. Second, mechanical factors: in in-service vessels, surface oxidation, scale, or contamination from prior operation can act as a barrier to fusion. Third, process factors: improper flux coverage, incorrect electrode angle, or inconsistent gap control between the strip and the base metal surface.

The article emphasizes that in-service conditions introduce additional challenges not present in greenfield fabrication. Residual stresses from the vessel's operating history, including cyclic thermal loading and pressure cycling, can create micro-cracks or porosity in the base metal surface that are not visible to the naked eye but impede fusion. Furthermore, the inability to perform full post-weld heat treatment (PWHT) on an operating vessel means that any hydrogen retained in the weld zone cannot be fully diffused, increasing the risk of hydrogen-induced cracking alongside incomplete fusion.

Defect Identification and Characterization

The detection of incomplete fusion in strip electrode cladding typically relies on ultrasonic testing (UT) or phased array ultrasonic testing (PAUT) in accordance with standards such as NB/T 47013 or ASME V. Incomplete fusion appears as a planar indication parallel to the weld surface, distinguishable from porosity or slag inclusions by its linear orientation and consistent depth. The article notes that magnetic particle testing (MT) can detect surface-breaking incomplete fusion but cannot identify subsurface defects, making UT the primary method for quality assurance.

A key insight from the literature is that incomplete fusion defects in strip electrode cladding often follow a characteristic pattern related to the welding sequence. When multiple passes are applied, the first pass is most susceptible because it establishes the initial bond with the base metal. Subsequent passes, while they may have their own fusion issues, often mask the first-pass defects, making them difficult to detect without thorough NDT coverage of each pass.

Countermeasures and Engineering Practice

The countermeasures proposed can be organized using a PDCA framework. In the Plan phase, thorough pre-weld surface preparation is paramount: mechanical grinding to remove all oxidation, scale, and contamination down to bare metal, followed by solvent cleaning within a defined time window before welding. The welding procedure specification (WPS) must be optimized for the specific base metal condition, with particular attention to current density and travel speed to ensure adequate base metal melting.

In the Do phase, process control parameters must be maintained with tight tolerances. The following table summarizes typical parameter windows for strip electrode cladding on carbon steel bases with stainless steel or nickel alloy cladding:

In the Check phase, interpass NDT is essential. UT scanning should be performed after each pass, not only after the final pass, to catch incomplete fusion early. The acceptance criteria should follow NB/T 47014 or the relevant section of ASME IX, with particular strictness for critical service applications.

In the Act phase, any identified incomplete fusion requires removal by grinding or machining back to sound metal, followed by re-cladding with a revised procedure that addresses the root cause. The article stresses that simply re-depositing material without addressing the underlying cause leads to recurrence.

Study Insights and Engineering Implications

This literature provides valuable practical guidance for engineers working on in-service vessel repairs. The key takeaway is that strip electrode cladding of operating vessels demands a more conservative approach to process parameters than new fabrication, because the inability to perform full PWHT and the presence of service-induced defects in the base metal create a less forgiving environment. The emphasis on interpass NDT and thorough surface preparation represents a fundamental shift from the traditional end-of-weld inspection approach, and should be adopted as standard practice for all in-service cladding operations. The PDCA framework applied to defect prevention offers a systematic methodology that can be integrated into quality management systems for pressure vessel repair operations.