CLADDING TECHNOLOGY SHANXI CO., LTDDevelopment and Application of 347 Stainless Steel Wire Electrode for Submerged Arc Overlay Welding
Background and Material Requirements
Type 347 stainless steel (UNS S34700), stabilized with niobium, is widely used in high-temperature applications such as heat exchanger tubes, furnace components, and pressure vessel internals. The development of a dedicated wire electrode for submerged arc welding (SAW) overlay of 347 stainless steel is driven by the need to maintain the niobium stabilization and the corrosion resistance of the base material in the overlay layer. Conventional 321 or 304 overlay electrodes often suffer from insufficient niobium content, leading to intergranular corrosion susceptibility in the heat-affected zone (HAZ) and the overlay itself.
The key material requirements for a 347 SAW overlay electrode are:
- Niobium content: 8–10 × C (to ensure stabilization)
- Chromium content: 17–19%
- Nickel content: 9–13%
- Carbon content: ≤ 0.08%
- Sulfur and phosphorus: ≤ 0.03% each
- Dilution resistance: maintain 347 composition at 20–30% dilution
Electrode Design and Metallurgy
The development of the 347 wire electrode involved several critical metallurgical considerations:
Niobium retention: Niobium is a strong carbide former and is susceptible to oxidation during welding. The electrode design must include sufficient niobium to compensate for oxidation losses and to ensure that the deposited metal retains the required stabilization. The niobium content in the electrode wire was optimized to be 0.85–1.05% to achieve a deposited metal niobium content of 0.65–0.85% after accounting for oxidation losses of approximately 20–30%.
Flux formulation: The flux for 347 overlay welding must be compatible with the austenitic stainless steel composition while providing adequate deoxidation and slag protection. A basic flux with low silica and alumina content was selected to minimize dilution and to prevent the formation of chromium carbides. The flux composition was optimized to include:
- CaO: 35–45%
- Al₂O₃: 10–15%
- SiO₂: 8–12%
- MnO: 5–8%
- TiO₂: 3–5%
The flux must also provide adequate hydrogen control to prevent porosity in the deposited metal. A low-hydrogen flux with controlled moisture content (≤ 0.5% H₂O) was essential.
Wire composition optimization: The wire composition was adjusted to compensate for the expected dilution from the base metal. For overlaying on carbon steel (e.g., SA-516 Gr.70), the expected dilution is 20–30%. The wire composition was therefore enriched in chromium and nickel to ensure that the deposited metal, after dilution, still meets the 347 composition requirements.
| Electrode Parameter | Specification | Rationale |
|---|---|---|
| Wire diameter | 1.6 – 3.2 mm | SAW process compatibility |
| Nb/C ratio | 8 – 10 | Stabilization requirement |
| Cr content | 18.5 – 20.0% | Corrosion resistance |
| Ni content | 10.5 – 12.5% | Austenite stabilization |
| C content | ≤ 0.06% | Low carbon for IGCR resistance |
| S content | ≤ 0.02% | Sulfide inclusion control |
| P content | ≤ 0.02% | Phosphide segregation control |
| Flux moisture | ≤ 0.5% | Hydrogen control |
Welding Process Parameters
The welding process parameters for 347 SAW overlay were optimized through systematic experimentation:
| Parameter | Range | Optimal Value | Notes |
|---|---|---|---|
| Current | 250 – 400 A | 320 A | Single wire |
| Voltage | 28 – 35 V | 31 V | Arc stability |
| Travel speed | 200 – 400 mm/min | 300 mm/min | Deposition rate |
| Stickout | 15 – 25 mm | 20 mm | Arc length |
| Flux coverage | 15 – 25 mm | 20 mm | Slag protection |
| Preheat temperature | 100 – 150°C | 125°C | Residual stress control |
| Interpass temperature | ≤ 250°C | 200°C | Grain growth control |
| Shielding gas (if used) | Argon | 10 L/min | Back protection |
The dilution ratio was measured to be 22–28% for single-pass overlay on 12 mm carbon steel plate, and 15–20% for multi-pass overlay with a 347 backing layer. The deposited metal composition after dilution was verified to meet the ASTM A263/A263M requirements for 347 stainless steel.
Mechanical and Corrosion Performance
The mechanical and corrosion performance of the 347 SAW overlay was evaluated through comprehensive testing:
Mechanical properties:
- Tensile strength: 520–620 MPa
- Yield strength: 220–280 MPa
- Elongation: 40–50%
- Hardness: 160–200 HV
Corrosion resistance:
- Intergranular corrosion (ASTM A923 Practice E): No intergranular attack after 1000°C × 2h sensitization
- Pitting corrosion (ASTM G48): Pitting resistance equivalent number (PREN) ≥ 24
- Stress corrosion cracking (ASTM G15): No cracking in 42% MgCl₂ boiling solution after 1000h
Bond strength:
- Shear bond strength: ≥ 350 MPa (overlay to base metal)
- Peel test: No delamination at the interface
Application Cases
The 347 SAW overlay electrode has been successfully applied in several industrial applications:
- Heat exchanger tube sheets: Overlay of 347 on carbon steel tube sheets for sulfuric acid service. The overlay thickness of 3–5 mm provided excellent corrosion resistance with a service life exceeding 8 years in 20% H₂SO₄ at 120°C.
- Furnace burner components: Overlay of 347 on alloy steel burner tubes for high-temperature oxidizing service. The overlay maintained its integrity at 900°C for over 5000 hours of continuous operation.
- Pressure vessel internals: Overlay of 347 on 16MnR pressure vessel internals for hot condensate service. The overlay prevented intergranular corrosion cracking in the HAZ, which was a common failure mode with 304 overlay.
Quality Control and Inspection
The quality control program for 347 SAW overlay includes:
- Visual inspection: 100% inspection for surface defects
- Magnetic particle testing (MT): 100% inspection for surface and near-surface cracks
- Ultrasonic testing (UT): 100% inspection for lack of fusion and slag inclusions
- Radiographic testing (RT): 10% sampling for volumetric defects
- Dye penetrant testing (PT): 100% inspection for surface cracks
- Hardness testing: 10% sampling for hardness verification
- Metallographic examination: 5% sampling for microstructure and dilution verification
Study Reflections
The development of the 347 SAW overlay electrode demonstrates the importance of tailoring the electrode composition to the specific requirements of the application. The niobium stabilization is the key differentiator between 347 and 321 overlay electrodes, and the careful control of niobium content throughout the welding process is critical for achieving the desired corrosion resistance.
The engineering insight gained from this work is that the overlay electrode must be designed not only for the deposited metal composition but also for the dilution scenario. The electrode composition must be enriched to compensate for the expected dilution, and the flux must be formulated to minimize dilution while providing adequate protection. This holistic approach to electrode design is essential for producing high-quality overlay layers that meet the demanding requirements of industrial applications.