Strip Cladding and Electroslag Weld Overlay Operator Qualification

Overview and Process Characteristics

Strip cladding and electroslag welding (ESW) overlay are specialized processes used for large-area, high-volume overlay applications where the deposition rate requirements exceed the capabilities of conventional arc welding methods. Strip cladding involves the use of a continuous strip electrode that is fed into the arc or slag pool, while ESW overlay utilizes a submerged electrode with a flux-covered slag pool to achieve very high deposition rates. The qualification of operators for these processes is distinct from conventional GTAW or GMAW overlay qualification and requires specialized training and certification.

Strip cladding is typically performed using a submerged arc welding (SAW) machine equipped with a strip electrode feed mechanism. The strip electrode is usually 10 to 20 mm wide and 1.5 to 3.0 mm thick, with compositions ranging from austenitic stainless steels (309L, 312L, 321) to nickel-based alloys (NiCr-3, Ni27, Ni60). The deposition rate for strip cladding can reach 15 to 30 kg/h, making it highly efficient for large-area overlay work such as hydrogenation reactor shell cladding and large-diameter column internal linings.

ESW overlay, on the other hand, utilizes a submerged electrode that melts in a molten slag pool, with the electrode wire fed through the slag pool to achieve deposition rates of 20 to 50 kg/h. ESW overlay is particularly suitable for very thick overlay layers (greater than 10 mm) and is commonly used for the cladding of large pressure vessel shells and heat exchanger tubesheets.

Qualification Requirements and Essential Variables

The qualification requirements for strip cladding and ESW overlay operators are governed by ASME IX, ISO 9606-1, and NB/T 47014, with additional requirements specific to the process. The table below summarizes the key essential variables for strip cladding and ESW overlay qualification.

The flux type and composition is a critical essential variable for both strip cladding and ESW overlay. The flux serves multiple functions including arc stabilization, slag pool formation, alloying, and deoxidation. Changes in flux composition can significantly affect the dilution level, weld metal chemistry, and mechanical properties of the overlay layer.

Typical Flux Compositions for Strip Cladding and ESW Overlay

Qualification Testing Procedures

The qualification test for strip cladding and ESW overlay operators must be performed on a coupon that replicates the production conditions as closely as possible. The coupon preparation should include the following elements:

• Base plate: minimum 300 mm x 150 mm x 10 mm, matching the production P-No.
• Overlay layers: minimum 3 passes, total overlay thickness of at least 5 mm for strip cladding and at least 8 mm for ESW overlay
• Strip/wire: matching the production F-No. and dimensions
• Flux: matching the production flux type and composition
• Testing: visual inspection, dilution analysis, hardness traverse, bond strength test, and metallographic examination

The metallographic examination is particularly important for strip cladding and ESW overlay qualification because these processes involve high heat input and rapid cooling, which can lead to microstructural issues such as columnar grain growth, microsegregation, and hot cracking susceptibility. The examination should include the following observations:

• Grain structure at the overlay/base material interface
• Presence of columnar or dendritic grain structures
• Microsegregation patterns and their extent
• Any indications of hot cracking or cold cracking
• Dilution gradient from overlay through base material

Specialized Training and Authorization

Strip cladding and ESW overlay operations require specialized training that goes beyond conventional arc welding skills. The training program should include the following components:

• Process principles: understanding of strip cladding and ESW overlay metallurgy, heat input effects, and dilution control
• Equipment operation: familiarization with the specific machine configuration, including strip feed mechanism, flux handling system, and slag pool monitoring
• Parameter control: ability to adjust and maintain process parameters within the qualified range
• Defect identification: recognition of common defects such as insufficient fusion, porosity, hot cracking, and flux inclusions
• Safety procedures: awareness of the specific hazards associated with high-current, high-heat-input processes

For facilities that use proprietary or proprietary machine configurations for strip cladding or ESW overlay, the operator qualification may require additional factory authorization beyond the standard code qualification. This authorization typically involves a combination of theoretical examination and practical assessment performed by the equipment manufacturer or an authorized representative.

Study Insights and Engineering Implications

The qualification of strip cladding and ESW overlay operators is a critical quality assurance activity that requires specialized knowledge of the process metallurgy and equipment operation. The high deposition rates and heat inputs associated with these processes introduce unique challenges related to dilution control, microstructural integrity, and defect prevention that must be addressed through rigorous qualification testing and ongoing training.

Engineers should also consider the impact of strip cladding and ESW overlay on the overall fabrication schedule and cost. The high deposition rates can significantly reduce the fabrication time for large-area overlay applications, but the qualification requirements and specialized equipment needs can add to the initial project cost. A careful cost-benefit analysis should be performed before selecting these processes for a specific application.

In summary, the strip cladding and ESW overlay operator qualification program is a specialized quality assurance activity that requires careful attention to process metallurgy, equipment operation, and essential variable coverage to ensure reliable, code-compliant overlay welds with controlled dilution and adequate metallurgical integrity.