CLADDING TECHNOLOGY SHANXI CO., LTDHigh Efficiency Strip Cladding of Low Carbon Steel
Overview and Motivation
Strip cladding, also known as strip metal arc welding (SAM), represents one of the most productive methods for depositing corrosion-resistant or wear-resistant overlay layers onto carbon and low-alloy steel substrates. The fundamental appeal of this technique lies in its ability to achieve deposition rates exceeding 10 kg/h with minimal dilution, making it ideal for large-area cladding operations on pressure vessels, heat exchanger shells, and storage tanks. This study note examines the process parameters, metallurgical considerations, and quality control measures that govern the successful execution of high-efficiency strip cladding on low carbon steel substrates.
Process Parameters and Welding Window
The core of strip cladding efficiency resides in optimizing the interplay between current, voltage, travel speed, and strip feed rate. The following table summarizes typical parameter ranges observed in industrial practice for a 316L stainless steel strip deposited on a Q235 low carbon steel plate.
| Parameter | Typical Range | Recommended Value |
|---|---|---|
| Current (A) | 1200–2000 | 1500–1800 |
| Voltage (V) | 22–30 | 25–27 |
| Travel Speed (m/h) | 6–15 | 8–12 |
| Strip Feed Rate (m/min) | 0.3–0.6 | 0.4–0.5 |
| Backing Gas Flow (L/min) | 10–20 | 15 |
| Shielding Gas | CO₂ or Ar+20%CO₂ | Ar+20%CO₂ |
| Strip Width (mm) | 25–50 | 35 |
| Strip Thickness (mm) | 1.2–2.0 | 1.5 |
A critical observation from the literature is that increasing current beyond 1800 A without proportionally raising travel speed leads to excessive dilution, often pushing the dilution rate above 15% for the first pass. This dilution degrades the corrosion resistance of the overlay and may introduce intermetallic phases at the fusion line. The recommended approach is to maintain a current-to-speed ratio of approximately 150:1 for the first pass and gradually increase current for subsequent passes once the strip-to-strip dilution becomes the dominant mechanism rather than strip-to-substrate dilution.
Metallurgical Considerations
The fusion zone in strip cladding on low carbon steel is characterized by a gradient composition from the base metal through the dilution zone into the sound overlay. Metallographic analysis typically reveals a narrow transition region approximately 0.3–0.8 mm wide where chromium and nickel content drops precipitously. This region is particularly susceptible to intergranular corrosion in chloride-containing environments. To mitigate this, a common strategy is to apply a transition layer of 309L or 310L before the final 316L overlay. The 309L layer, with its higher chromium and nickel content (23% Cr, 13% Ni), acts as a buffer that reduces the dilution gradient and prevents the formation of a low-alloy band at the substrate interface.
The microstructure of the overlay itself, when properly executed, consists of a fine-grained austenitic or austenite-ferrite dual-phase structure. The ferrite content should ideally be maintained between 5% and 15% to prevent hot cracking while ensuring adequate corrosion resistance. Excessive ferrite above 20% may introduce 400-series sensitization risk, while ferrite-free structures are prone to solidification cracking under high travel speeds.
Common Defects and Countermeasures
Defect identification and prevention constitute a major portion of the study material. The following table outlines the most frequently encountered defects and their root causes.
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Undercut | Excessive current, insufficient overlap | Reduce current by 10%, increase overlap to 30% |
| Excessive Dilution | Low travel speed, high current | Raise speed, reduce current, add transition layer |
| Porosity | Inadequate shielding, contaminated strip | Increase backing gas flow, clean strip surface |
| Cracking at Fusion Line | High dilution, low alloy content | Apply 309L transition layer, preheat to 150°C |
| Uneven Surface Profile | Uneven strip feed, operator drift | Use wire tracking system, maintain constant tension |
Preheating the substrate to 100–150°C is recommended for plates thicker than 12 mm to reduce thermal stresses and minimize the risk of hydrogen-induced cracking in the heat-affected zone. Interpass temperature should be controlled below 200°C to preserve the microstructure of previously deposited layers.
Quality Control and Inspection
Post-weld inspection of strip cladding typically involves a combination of visual examination, magnetic particle testing (MT), and ultrasonic testing (UT) for bond integrity. For critical applications such as hydrogenation reactor shells, penetrant testing (PT) of the final surface is mandatory to detect any surface-breaking cracks. Bond strength testing per ASTM A263 or API 934 provides quantitative assessment of the metallurgical bond between overlay and substrate, with acceptable values typically exceeding 350 MPa for austenitic overlays on carbon steel.
The study emphasized the importance of establishing a documented welding procedure qualification per NB/T 47014 or ASME IX before production welding commences. Parameter variation beyond the qualified range—particularly in current and travel speed—can lead to unacceptable dilution and bond strength degradation that may not be detectable by conventional NDT methods.
Study Insights and Engineering Implications
The most valuable insight from this literature is the recognition that strip cladding efficiency cannot be pursued in isolation from metallurgical quality. The temptation to maximize deposition rate often leads to process windows that compromise overlay integrity. A disciplined approach that prioritizes dilution control, transition layer application, and rigorous parameter qualification yields long-term reliability far superior to aggressive speed-oriented strategies. For pressure vessel fabrication, where overlay failures can lead to catastrophic leakage in corrosive service, the engineering judgment to select conservative parameters is paramount. The literature reinforces the principle that the cost of a properly qualified and executed strip cladding operation is always justified by the avoidance of in-service failures and unplanned shutdowns.