Application of Strip Cladding Technology A Study Note on Process Fundamentals and Engineering Practice

Overview and Scope of the Topic

Strip cladding, also known as strip surfacing or strip weld overlay, is a mature and widely applied technique in the fabrication of bimetallic products and corrosion-resistant pressure vessels. The literature reviewed in this study focuses on the practical application of strip cladding technology, covering process parameters, consumable selection, quality assurance, and typical failure modes encountered in industrial settings. The study draws from both theoretical metallurgical principles and field-level engineering experience accumulated over years of working with clad plate products and overlay-welded pressure vessels.

Strip cladding differs fundamentally from other overlay methods in that it employs a continuous strip of overlay material—typically stainless steel, nickel-based alloy, or copper alloy—fed into the arc zone alongside a backing strip of matching or compatible material. The backing strip is removed after welding, leaving behind a high-quality, corrosion-resistant surface layer with excellent metallurgical bonding to the base metal. This technique is particularly advantageous for large-format flat plates, headers, and pressure vessel shells where uniform coverage and minimal dilution are critical.

Core Technical Principles

Metallurgical Bonding Mechanism

The metallurgical bond between the strip overlay and the base metal is achieved through a combination of mechanical interlocking and diffusion bonding during the solidification process. The key to achieving a sound bond lies in controlling the heat input and the composition of the first weld pass, which acts as the transition layer between the base metal and the overlay strip.

The first pass is typically deposited using a matching filler wire or electrode that contains a carefully balanced composition to minimize cracking susceptibility while ensuring adequate wetting of the base metal surface. Subsequent passes use the strip cladding method, where the strip material is melted and deposited in successive layers to build up the required overlay thickness. The dilution rate in strip cladding is generally lower than in conventional multi-pass overlay welding, typically ranging from 5% to 15% for the first pass and decreasing to less than 5% for subsequent passes.

Process Parameters and Their Influence

The interaction between arc voltage and travel speed is perhaps the most critical process variable. Higher arc voltages increase the melting rate of the strip material, which can lead to excessive dilution if the travel speed is not adjusted accordingly. Conversely, too high a travel speed results in incomplete melting of the strip, producing a rough surface and poor bond quality.

Common Defects and Countermeasures

Bond Defects and Delamination

Bond defects are the most serious failure mode in strip cladding applications, particularly in pressure vessel service where loss of the overlay layer can lead to catastrophic corrosion failure. The primary causes include:

1. Insufficient first-pass penetration — The transition layer does not achieve adequate fusion with the base metal, resulting in a weak interface.
2. Contamination of the base metal surface — Oxide scale, rust, or oil residues prevent proper wetting and bonding.
3. Excessive interpass temperature — Leads to grain coarsening and reduced bonding strength at the interface.

The countermeasures involve rigorous surface preparation using shot blasting or grinding to achieve a clean, slightly roughened surface with a minimum Ra of 6.3 μm. A bond strength test in accordance with NB/T 47014 or ASME IX qualification procedures should be performed on production welds to verify the metallurgical bond.

Cracking in the Overlay Layer

Cracking in the overlay layer can be categorized into hot cracking and cold cracking. Hot cracking is associated with the solidification of the austenitic overlay material and is influenced by sulfur and phosphorus content, grain boundary segregation, and restraint stress. Cold cracking is primarily a hydrogen-related phenomenon and is more common in high-strength base metals.

Integration with Engineering Practice

Application in Pressure Vessel Fabrication

In the fabrication of clad plate pressure vessels governed by GB/T 150 and ASME VIII Div. 1, strip cladding is commonly used for smaller-diameter vessels and headers where the geometry does not lend itself to explosive cladding or roll bonding. The strip cladding process must be qualified in accordance with NB/T 47014, which requires qualification of the welding procedure based on factors including base metal group, overlay material, thickness range, and process variables.

A typical engineering case involves the fabrication of a hydrogenation reactor shell using 16MnR base plate with a 316L stainless steel overlay layer of 3 mm total thickness. The strip cladding procedure employed a 2.5 mm thick 316L strip with a matching 316L backing strip. The first pass was deposited using a low-hydrogen E309L electrode to ensure adequate penetration and bonding, followed by three strip cladding passes to achieve the required overlay thickness. The resulting overlay layer exhibited less than 8% dilution in the first pass and less than 3% in subsequent passes, well within the acceptable limits specified by the design code.

Quality Assurance Considerations

The quality assurance program for strip cladding should include:

1. Visual inspection of the overlay surface for uniformity, undercut, and surface defects.
2. Penetrant testing (PT) or magnetic particle testing (MT) of the overlay surface to detect surface-breaking cracks.
3. Ultrasonic testing (UT) of the bond interface to detect lack of fusion and delamination.
4. Destructive bond strength testing on qualification coupons in accordance with the applicable standard.
5. Chemical analysis of the overlay layer to verify dilution and composition.

Key Reflections and Study Insights

The study of strip cladding technology reinforces the understanding that the first pass is the most critical element in the entire overlay process. A poorly executed first pass can compromise the entire overlay regardless of the quality of subsequent passes. This insight has direct implications for procedure qualification and welder certification programs, where the first-pass parameters should be given special attention during both qualification and production welding.

Furthermore, the economic analysis of strip cladding versus alternative overlay methods reveals that strip cladding offers the best balance of deposition rate, quality, and cost for large-format flat surfaces. However, for complex geometries such as small-diameter tubes or heavily contoured surfaces, alternative methods such as GTAW overlay or laser cladding may be more appropriate. The selection of the overlay method should always be driven by the specific requirements of the application, including the required overlay thickness, surface quality, and geometric complexity.

In conclusion, strip cladding technology remains a cornerstone of bimetal product manufacturing and pressure vessel fabrication, offering proven reliability when properly qualified and executed. The engineer's responsibility is to ensure that the welding procedure is robust, the consumables are traceable, and the quality assurance program is comprehensive enough to detect and prevent the defects that can compromise the long-term performance of the overlay layer.