Arc Termination Defect Analysis in Submerged Arc Strip Electrode Cladding

Literature Overview and Technical Context

This research by Wang Bin, Zhang Junling, Yang Fei, and Zhou Cui from Southwest Petroleum University and Atlantic Welding Materials Co., Ltd. was published in 2012 in the journal "Hot Working Technology" (热加工工艺). The study addresses a specific and frequently encountered quality issue in submerged arc welding (SAW) strip electrode cladding operations: arc termination defects. In the context of bimetal pressure vessel fabrication and clad plate production, arc termination quality directly impacts the integrity of the overlay layer and, consequently, the service reliability of the finished component.

Technical Background of Strip Electrode Cladding

Submerged arc strip electrode cladding (also known as submerged arc surfacing with strip electrode) is a widely used process for producing clad plates, particularly for stainless steel/carbon steel and nickel alloy/carbon steel combinations. The process employs a continuous strip of metal as the electrode, which is fed into the arc through a special torch assembly. The key advantages of this process include high deposition rates (typically 5-15 kg/h), excellent penetration, and the ability to produce uniform overlay layers with controlled dilution.

Process Configuration and Operating Parameters

Parameter	Typical Range	Notes
Arc Voltage	20-30 V	Depends on strip width and current density
Welding Current	400-1200 A	AC or DC, depending on application
Strip Width	10-50 mm	Wider strips for higher deposition rates
Travel Speed	100-400 mm/min	Higher speeds reduce dilution
Flux Coverage	10-25 mm	Ensures adequate shielding and slag formation
Preheat Temperature	100-300 °C	Depends on base material and dilution requirements
Interpass Temperature	< 350 °C	Critical for controlling dilution and microstructure

Arc Termination Defect Mechanism and Classification

Arc termination in strip electrode cladding is inherently more challenging than in conventional SAW processes due to the large molten pool volume, the geometry of the torch assembly, and the continuous feeding mechanism. When the arc is terminated, the molten pool must solidify in a controlled manner to avoid defects such as craters, shrinkage porosity, hot cracking, and lack of fusion at the termination end.

Defect Classification and Root Cause Analysis

Defect Type	Description	Primary Cause	Detection Method
Crater	Depressed area at arc termination	Rapid cooling and shrinkage	Visual inspection (VT), RT
Shrinkage Porosity	Gas pores in crater region	Insufficient solidification feeding	RT, UT
Hot Cracking	Transverse cracks in crater	High sulfur/phosphorus, rapid cooling	MT, PT, RT
Lack of Fusion	Incomplete bonding at termination	Insufficient heat input	UT, MT
Undercut	Groove along weld edge	Excessive arc energy concentration	VT, MT

Root Cause Analysis Using FMEA Approach

Applying Failure Mode and Effects Analysis (FMEA) to arc termination defects reveals several critical factors:

Arc Voltage Decay Rate: The rate at which arc voltage decreases during termination directly influences the cooling rate of the molten pool. A rapid voltage decrease leads to rapid solidification, increasing the risk of shrinkage porosity and hot cracking.
Current Tapering Profile: The manner in which welding current is reduced during termination is critical. Abrupt current cutoff creates a sudden loss of heat input, while gradual tapering allows controlled solidification.
Flux Distribution at Termination: As the torch moves away, the flux coverage at the termination end may become inadequate, exposing the molten pool to atmospheric contamination and accelerating cooling.
Travel Speed Variation: Any deviation in travel speed near the termination point affects the heat input distribution and can lead to uneven solidification.
Strip Electrode Feed Mechanism: The mechanical design of the strip feeding mechanism must ensure smooth and controlled withdrawal of the electrode during termination.

Defect Analysis and Countermeasures

Countermeasures for Crater and Shrinkage Porosity

The formation of craters and shrinkage porosity at arc termination can be mitigated through several approaches. First, implementing a controlled current tapering sequence—reducing the current gradually over a period of 2-5 seconds rather than abrupt cutoff—allows the molten pool to solidify more uniformly. Second, adding a "filler" element such as a small piece of backing plate or a specially designed termination block provides additional molten metal to compensate for shrinkage. Third, optimizing the flux composition to include deoxidizing agents and grain refiners can improve the feeding characteristics of the solidifying pool.

Countermeasures for Hot Cracking

Hot cracking at arc termination is primarily driven by the combination of high restraint stress, rapid cooling, and the presence of low-melting-point impurities. Countermeasures include:

Preheating the base material to reduce the thermal gradient and cooling rate at termination
Using consumables with low sulfur and phosphorus content (S < 0.01%, P < 0.02%)
Adding micro-alloying elements such as titanium, zirconium, or rare earth elements to refine the microstructure
Implementing post-termination heat treatment to relieve residual stresses

Countermeasures for Lack of Fusion

Lack of fusion at the termination end is often caused by insufficient heat input as the arc energy decreases. This can be addressed by:

Maintaining adequate arc voltage during the final portion of the weld
Ensuring proper flux coverage extends beyond the termination point
Using a slightly higher travel speed near the termination to concentrate heat in the termination zone
Implementing a "back-fill" technique where a short overlap weld is deposited to ensure complete fusion

Quality Control and Inspection Requirements

For critical applications such as pressure vessel cladding, arc termination defects must be detected and addressed before the component proceeds to the next manufacturing stage. The following inspection protocols are recommended:

Inspection Method	Application	Acceptance Criteria
Visual Testing (VT)	All welds	No visible craters, undercuts, or surface cracks
Magnetic Particle Testing (MT)	Surface and near-surface	No linear indications > 1 mm (per ASME Sec V)
Radiographic Testing (RT)	Critical welds	No porosity clusters > 3 mm (per ASME Sec V)
Ultrasonic Testing (UT)	Bond strength verification	No lack of fusion indications
Dye Penetrant Testing (PT)	Non-ferromagnetic overlays	No linear indications > 0.5 mm

Engineering Practice Implications

In the context of bimetal pressure vessel fabrication, arc termination defects in cladding welds can have serious consequences. A lack of fusion or cracking at the termination point can serve as a stress concentrator and potential failure initiation site under cyclic loading or corrosion conditions. Therefore, the arc termination quality must be controlled to the same standard as the main weld body. Engineers should incorporate arc termination quality checks into their quality control plans, including the use of witness coupons and periodic destructive testing of termination regions.

Study Insights and Conclusions

The systematic analysis of arc termination defects presented in this literature provides valuable guidance for improving the quality of strip electrode cladding operations. The key insight is that arc termination is not merely a procedural step but a critical process parameter that requires deliberate design and control. The FMEA-based approach to identifying root causes is particularly useful for developing targeted countermeasures rather than generic solutions. Engineers should adopt a proactive approach to arc termination quality by incorporating controlled current tapering, optimized flux distribution, and systematic inspection into their standard operating procedures. The findings of this study underscore the importance of process discipline in achieving reliable cladding quality, particularly for safety-critical applications such as pressure vessels and heat exchangers.