CLADDING TECHNOLOGY SHANXI CO., LTDApplication of Strip Cladding Technology A Study Note
Overview and Core Significance
Strip cladding, also known as strip welding or strip overlay, has emerged as one of the most efficient and economical cladding processes in modern bimetal product manufacturing. The fundamental principle involves feeding a continuous strip of the desired cladding alloy into an arc furnace, where the strip is partially melted and deposited onto the base substrate to form a metallurgical bond. Unlike conventional wire-based overlay processes, strip cladding achieves deposition rates that are typically two to five times higher, making it particularly attractive for large-scale production of clad plate, pipe, and pressure vessel components.
The literature reviewed here focuses on the practical application of strip cladding technology across multiple industrial sectors, including chemical processing, oil and gas, power generation, and food processing. The core argument presented is that strip cladding can deliver superior metallurgical bond strength, lower dilution rates, and more uniform overlay thickness compared to traditional submerged arc welding (SAW) or flux-cored arc welding (FCAW) overlay methods, provided that process parameters are carefully controlled and the base material preparation meets stringent requirements.
Technical Principles and Process Parameters
Strip cladding operates on the principle of partial melting of the strip material, where only a portion of the strip cross-section is consumed by the arc energy. This partial melting mechanism is critical because it ensures that the remaining solid strip acts as a backing material, promoting rapid solidification and minimizing the diffusion of base metal into the cladding layer. The resulting dilution rate can be controlled within the range of 5 to 15 percent, depending on the alloy system and process configuration.
The following table summarizes the typical process parameters for strip cladding of stainless steel and nickel-based alloys:
| Parameter | Stainless Steel (304/316) | Nickel-Based Alloy (Inconel 625) | Carbon Steel Substrate |
|---|---|---|---|
| Arc Voltage | 22-28 V | 25-32 V | 22-28 V |
| Current Density | 150-250 A/mm² | 180-300 A/mm² | 150-250 A/mm² |
| Travel Speed | 300-600 mm/min | 200-400 mm/min | 300-600 mm/min |
| Strip Thickness | 0.5-1.5 mm | 0.5-1.5 mm | 0.5-1.5 mm |
| Shielding Gas | Ar + 2-5% CO₂ | Pure Ar or He | Ar + 2-5% CO₂ |
| Preheat Temperature | 100-200°C | 150-300°C | 50-150°C |
| Interpass Temperature | ≤300°C | ≤250°C | ≤250°C |
| Typical Dilution Rate | 8-15% | 5-12% | 10-20% |
The arc geometry in strip cladding is typically a vertical or slightly tilted configuration, with the strip fed horizontally into the arc pool. The shielding gas nozzle is positioned to provide complete coverage of the molten pool and the solidifying strip edge. Gas flow rates of 15 to 25 liters per minute of argon are typical for single-pass operations, while multi-pass configurations may require 30 to 40 liters per minute.
Metallurgical Bond Strength and Dilution Control
One of the most critical aspects of strip cladding is the quality of the metallurgical bond between the cladding layer and the base material. The bond strength is directly influenced by the dilution rate, which represents the percentage of base metal that has melted and mixed into the cladding layer. For austenitic stainless steel cladding on carbon steel substrates, a dilution rate of 10 to 15 percent is generally acceptable, as the resulting microstructure maintains adequate corrosion resistance and mechanical properties. However, for nickel-based alloy cladding, dilution rates exceeding 10 percent can significantly degrade the corrosion resistance of the overlay.
The metallurgical bond is evaluated through several methods, including the ring tensile test, the wedge bend test, and the microstructural examination of the bond interface. According to ASTM A263 and A264, the minimum bond strength for weld-overlay clad plate is typically specified at 150 MPa or higher, with no separation or cracking at the interface. The ring tensile test involves cutting a ring specimen from the clad plate and applying axial tensile load until the bond fails. Acceptable results show failure occurring within the cladding layer or the base metal, not at the interface.
Microstructurally, the bond interface in strip cladding typically exhibits a gradient of composition from the base metal to the cladding alloy. In the case of austenitic stainless steel on carbon steel, the transition zone may contain a mixture of ferrite and austenite, with the ferrite content increasing toward the base metal side. This gradient is beneficial because it provides a smooth transition of mechanical properties and reduces residual stresses at the interface.
Common Defects and Countermeasures
Despite its advantages, strip cladding is susceptible to several defects that must be identified and controlled during production. The following table presents the most common defects, their causes, and recommended countermeasures:
| Defect Type | Primary Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Cracking at Bond Interface | Excessive dilution, high carbon equivalent | MT, PT, microstructural examination | Reduce current, increase travel speed, preheat substrate |
| Porosity in Overlay | Contamination, inadequate shielding | RT, UT | Improve gas coverage, clean substrate surface |
| Incomplete Fusion | Low heat input, excessive travel speed | UT, TOFD | Increase current, reduce travel speed |
| Excessive Dilution | High heat input, slow travel speed | Metallographic analysis, hardness mapping | Optimize arc parameters, use lower current density |
| Surface Cracking | High cooling rate, hydrogen embrittlement | PT, MT | Post-weld heat treatment, control hydrogen content |
Cracking at the bond interface is the most critical defect in strip cladding, as it directly compromises the structural integrity and corrosion resistance of the clad component. The primary causes include excessive dilution leading to a brittle martensitic phase at the interface, high carbon equivalent of the base material, and inadequate preheating. To mitigate this risk, the base material should be preheated to a temperature of at least 100°C for carbon steel substrates and 150 to 300°C for high-alloy substrates. Additionally, the interpass temperature should be maintained below 300°C to prevent excessive grain growth and the formation of brittle phases.
Porosity in the overlay layer is another common defect that can significantly reduce the corrosion resistance and mechanical properties of the cladding. Porosity is typically caused by contamination of the substrate surface, inadequate shielding gas coverage, or the presence of moisture in the welding environment. Surface preparation is critical, and the substrate should be cleaned by grinding or shot blasting to a Sa 2.5 standard before cladding. The shielding gas nozzle should be positioned to provide complete coverage of the molten pool, with a gas flow rate sufficient to prevent air ingress.
Engineering Practice and Application Cases
Strip cladding has been successfully applied in numerous industrial applications, including the manufacture of clad plate for chemical reactors, heat exchangers, and storage tanks. One notable application involves the cladding of 16MnR carbon steel with 316L stainless steel for a hydrogenation reactor operating at temperatures up to 400°C and pressures up to 80 MPa. The strip cladding process was used to deposit a 2 mm thick overlay layer on the interior surface of the reactor shell, achieving a dilution rate of 12 percent and a bond strength exceeding 200 MPa.
In another application, strip cladding was used to repair a failed heat exchanger tube bundle in a petrochemical plant. The tubes were clad with Monel 400 using a strip cladding process, restoring the corrosion resistance of the tube surfaces and extending the service life of the heat exchanger by an estimated five years. The cladding was performed using a portable strip cladding machine with a vertical arc configuration, and the resulting overlay layer exhibited excellent corrosion resistance in a chloride-containing environment.
For pressure vessel fabrication, strip cladding is particularly advantageous for large-diameter vessels where the use of full cladding plate would be economically prohibitive. The process can be applied to the interior surface of a vessel shell, head, or nozzle to provide a corrosion-resistant barrier without the need for a separate cladding plate. The process parameters must be carefully optimized to ensure that the overlay layer meets the requirements of the applicable code, such as ASME VIII Div. 1 or GB/T 150.
Quality Control and Inspection
Quality control in strip cladding production involves a combination of process monitoring, non-destructive testing (NDT), and destructive testing. The process monitoring includes real-time measurement of arc voltage, current, travel speed, and shielding gas flow rate. Deviations from the specified parameter ranges are automatically flagged and can trigger an alarm or process stop.
NDT methods for strip cladding include magnetic particle testing (MT), penetrant testing (PT), ultrasonic testing (UT), and radiographic testing (RT). MT is used to detect surface and near-surface cracks in the overlay layer, while PT is used for non-ferromagnetic materials. UT is employed to detect subsurface defects such as incomplete fusion and porosity, and RT is used for volumetric inspection of the entire weld cross-section.
Destructive testing includes hardness mapping, microstructural examination, tensile testing, and intergranular corrosion testing. Hardness mapping is performed across the bond interface to verify that the hardness gradient is within acceptable limits. Microstructural examination involves metallographic preparation of cross-sections perpendicular to the bond interface, with etching and optical microscopy to identify phase distributions and potential defects. Tensile testing of the overlay layer is conducted to verify that the mechanical properties meet the specified requirements.
Key Questions and Reflections
A key question arising from this study is how to balance deposition rate with metallurgical quality in strip cladding. While higher deposition rates reduce production costs, they can also lead to increased dilution and the formation of undesirable microstructural phases. The optimal balance depends on the specific alloy system, substrate material, and service conditions. For example, in the cladding of austenitic stainless steel on carbon steel, a higher deposition rate may be acceptable because the dilution is less critical than in the case of nickel-based alloy cladding.
Another important consideration is the scalability of strip cladding for large production runs. The process is inherently suitable for high-volume production because of its high deposition rate and relatively simple equipment requirements. However, maintaining consistent quality over long production runs requires careful process control and regular inspection. Statistical process control (SPC) methods can be employed to monitor key parameters and detect trends that may indicate process drift.
The integration of strip cladding with other cladding processes is also worth considering. In some applications, strip cladding may be used as a first pass to achieve a thick overlay layer, followed by a finishing pass using hot-wire TIG or plasma transferred arc (PTA) cladding to improve surface finish and reduce dilution. This hybrid approach can provide the best of both worlds: high deposition rate from strip cladding and superior surface quality from the finishing process.
Study Insights and Implications
The study of strip cladding technology has several important implications for engineering practice. First, the technology offers a compelling alternative to traditional clad plate for applications where the cladding thickness is less than 5 mm and the substrate is relatively flat or cylindrical. Second, the process parameters must be carefully optimized for each specific application, taking into account the alloy system, substrate material, and service conditions. Third, quality control is essential, and a combination of process monitoring, NDT, and destructive testing should be employed to ensure that the overlay layer meets the specified requirements.
From a standards perspective, strip cladding is covered by several relevant codes and standards, including ASTM A263, ASTM A264, ASTM A265, EN 10028-7, and ASME VIII Div. 1. These standards provide requirements for the materials, fabrication, and inspection of clad plate and weld-overlay clad components. Engineers must ensure that the strip cladding process meets the requirements of the applicable standard, including the minimum bond strength, maximum dilution rate, and acceptable defect levels.
In conclusion, strip cladding technology represents a significant advancement in the field of bimetal product manufacturing, offering high deposition rates, excellent metallurgical bond strength, and low dilution rates. The technology is particularly well-suited for large-scale production of clad plate, pipe, and pressure vessel components, and its application is expanding across multiple industrial sectors. Engineers engaged in cladding and bimetal product manufacturing should familiarize themselves with the technical principles, process parameters, and quality control requirements of strip cladding to fully leverage its potential in their respective applications.