CLADDING TECHNOLOGY SHANXI CO., LTDTube Sheet Strip Electrode Electroslag Cladding of Duplex Stainless Steel
Literature Overview
This 2024 publication by Xing Zhuo from Shenyang Instrument Science Research Institute Co., Ltd. presents a detailed investigation into the electroslag welding (ESW) cladding process for applying duplex stainless steel overlay layers onto tube sheets used in pressure vessel applications. Tube sheets are critical components in heat exchangers, reactors, and other pressure-containing equipment, where they must simultaneously withstand mechanical loads, thermal cycling, and corrosive process media. The use of duplex stainless steel cladding on carbon steel or low-alloy steel tube sheets offers an economical solution that combines the structural strength of the base material with the corrosion resistance of the overlay layer.
Core Technical Content
The strip electrode electroslag cladding process utilizes a continuous strip electrode rather than a solid wire, which enables higher deposition rates and more uniform heat input compared to conventional wire-fed ESW methods. The strip electrode is fed continuously into the slag pool at a controlled rate, and the travel speed is synchronized with the strip feed to maintain a stable slag pool and consistent weld bead geometry.
Process Parameters for Duplex Stainless Steel ESW Cladding
| Parameter | Typical Value | Rationale |
|---|---|---|
| Base material | Q345R or 16MnR | Pressure vessel steel |
| Cladding material | Duplex 2205 or 18-8 duplex | Enhanced corrosion resistance |
| Strip electrode width | 20-30 mm | Matches tube sheet thickness |
| Strip electrode thickness | 2.0-3.0 mm | Adequate for multi-pass buildup |
| Current | 800-1200 A | High deposition rate |
| Voltage | 30-40 V | Stable arc operation |
| Travel speed | 80-150 mm/min | Controlled cooling rate |
| Flux type | Rutile-basic composite | Low hydrogen, good wetting |
| Preheat temperature | 150-200 °C | Prevents hydrogen cracking |
| Interpass temperature | < 250 °C | Maintains duplex phase balance |
| Layer thickness | 3-5 mm total | Sufficient for corrosion protection |
Phase Balance Control in Duplex Stainless Steel Overlay
The most critical metallurgical challenge in cladding duplex stainless steel is maintaining the austenite-ferrite phase balance within the target range of 40-60 percent ferrite. The cooling rate from the ESW process is relatively slow compared to solid-state processes, which can promote ferrite formation and potentially lead to excessive ferrite content. Excessive ferrite (above 60 percent) increases susceptibility to 475 °C embrittlement and pitting corrosion, while insufficient ferrite (below 40 percent) reduces the mechanical strength and increases susceptibility to stress corrosion cracking.
The interpass temperature control is the primary means of managing the phase balance. Maintaining interpass temperatures below 250 °C limits the thermal input per pass and prevents excessive grain growth at the bond interface. Additionally, the composition of the strip electrode should be carefully selected to compensate for the dilution effects from the base metal. A slight enrichment of chromium and molybdenum in the electrode composition is typically required to achieve the target overlay layer chemistry after dilution.
Defect Analysis and Countermeasures
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Slag inclusion | Incomplete slag removal between passes | Increased slag cleaning, flux adjustment |
| Porosity | Hydrogen from moisture in flux | Flux drying at 300 °C for 2 hours |
| Cracking at bond interface | Excessive cooling rate or base metal hardness | Increased preheat, reduced travel speed |
| Excessive ferrite | High cooling rate, low interpass temp | Increase interpass temp to 200-250 °C |
| Undercut | Excessive travel speed or current | Reduce travel speed by 10-15 percent |
| Weld spatter | Excessive arc voltage | Reduce voltage by 2-3 V |
Standards and Quality Requirements
The cladding process must comply with relevant standards including NB/T 47014 for welder qualification, NB/T 47015 for fabrication and inspection of pressure vessel welded joints, and GB/T 150 for general pressure vessel requirements. The bond strength of the overlay layer to the base material must be verified through macrographic examination of the bond line, with no cracks, voids, or delamination permitted. The overlay layer must also pass intergranular corrosion testing in accordance with ASTM A923 Practice A or equivalent, demonstrating resistance to sensitization-induced corrosion.
Engineering Practice and Implications
The application of this technology to tube sheets in pressure vessels represents a significant cost-saving measure, as it eliminates the need for full-thickness duplex stainless steel tube sheets while still providing the required corrosion resistance at the process-facing surfaces. The strip electrode ESW process offers deposition rates of 8-12 kg/h, which is substantially higher than conventional SAW or GMAW methods, making it economically viable for large tube sheets with diameters exceeding 2 meters.
This work highlights the ongoing evolution of cladding technology in the pressure vessel industry, where the demand for cost-effective solutions that meet stringent quality requirements continues to drive innovation in welding process development and optimization.