CLADDING TECHNOLOGY SHANXI CO., LTDResearch on the Application of Stainless Steel Strip Electrode Submerged Arc Cladding Technology
Research Background and Industrial Context
This study by Yu Hao, Ma Ming, and Li Wenyu from Harbin Boiler Works Co., Ltd. (published in 2012) investigates the application of strip electrode submerged arc cladding technology for stainless steel overlay welds. Strip electrode submerged arc welding (SAW) is a highly productive cladding process that uses continuous strip-shaped electrodes instead of conventional wire electrodes, enabling deposition rates 3–5 times higher than conventional wire SAW.
The research addresses the challenges and opportunities of applying this technology to stainless steel cladding applications in boiler and pressure vessel manufacturing, where large areas of overlay are required for corrosion resistance in aggressive service environments.
Technical Principles and Process Characteristics
Strip Electrode SAW Fundamentals
Strip electrode SAW differs from conventional wire SAW in several key aspects:
- Electrode form: Continuous strip (typically 10–25 mm wide, 1.5–3.0 mm thick) instead of wire
- Flux supply: Flux is fed directly to the arc zone through a flux feeder
- Weld pool: Wider and shallower than wire SAW
- Deposition rate: 3–5 times higher than wire SAW
- Dilution: Lower dilution due to reduced base metal penetration
The strip electrode SAW process is particularly suitable for:
- Large flat or slightly curved surfaces
- Thick overlay layers (5–30 mm)
- High production rate requirements
- Applications where dilution control is critical
Process Parameters
The key process parameters for strip electrode SAW stainless steel cladding include:
| Parameter | Typical Range | Effect on Weld Quality |
|---|---|---|
| Current | 300–600 A | Higher current = wider weld, higher dilution |
| Voltage | 25–35 V | Higher voltage = wider weld bead |
| Travel speed | 100–250 mm/min | Higher speed = thinner weld, lower deposition rate |
| Strip width | 10–25 mm | Wider strip = wider weld bead |
| Strip thickness | 1.5–3.0 mm | Thicker strip = higher deposition rate |
| Flux type | Rutile or basic | Affects weld chemistry and mechanical properties |
| Preheat temperature | 100–200°C | Reduces cracking susceptibility |
Weld Metal Composition Control
For stainless steel cladding, the weld metal composition must be carefully controlled to achieve the required corrosion resistance. The key alloying elements are:
- Chromium (Cr): 18–25% for 304/316-type composition
- Nickel (Ni): 8–12% for 304/316-type composition
- Molybdenum (Mo): 2–3% for 316-type composition
- Carbon (C): <0.03% for low-carbon grades (304L, 316L)
- Titanium (Ti): 5–10x C for stabilized grades (321, 347)
The dilution from the base metal (typically carbon steel or low-alloy steel) can reduce the chromium and nickel content of the weld metal. For strip electrode SAW, the dilution rate is typically 5–15%, which is lower than for wire SAW (15–30%).
Application in Boiler and Pressure Vessel Manufacturing
Typical Applications
Strip electrode SAW stainless steel cladding is widely used in:
- Boiler tubes: Overlay of austenitic stainless steel on carbon steel tubes for improved corrosion resistance in flue gas environments
- Pressure vessel shells: Large-area cladding of 304/316 stainless steel on carbon steel shells for chemical processing applications
- Heat exchanger tubesheets: Cladding of stainless steel on carbon steel tubesheets for improved resistance to tube leakage
- Reactor internals: Cladding of high-nickel alloys on carbon steel internals for nuclear applications
Welding Procedure Development
The development of a qualified welding procedure for strip electrode SAW stainless steel cladding involves:
- Prequalification testing: Determine the range of process parameters that produce acceptable weld quality
- WPS development: Establish the welding procedure specification with defined parameter ranges
- Welder qualification: Qualify welders to perform the procedure
- Production welding: Apply the qualified procedure in production
- Quality control: Implement inspection and testing procedures
The welding procedure must be qualified in accordance with applicable codes such as ASME IX, NB/T 47014, or ISO 15614.
Quality Control and Defect Analysis
Common Defects
The following defects are commonly encountered in strip electrode SAW stainless steel cladding:
| Defect Type | Cause | Prevention |
|---|---|---|
| Cracking | Excessive heat input, high carbon content | Control heat input, use low-carbon consumables |
| Porosity | Flux contamination, inadequate flux coverage | Use dry flux, ensure proper flux feeder operation |
| Incomplete fusion | Low current, high travel speed | Increase current, reduce travel speed |
| Excessive dilution | High current, low travel speed | Optimize current and travel speed |
| Weld undercut | Improper electrode alignment | Maintain proper electrode position |
| Crater cracks | Inadequate crater filling | Use proper crater filling technique |
Non-Destructive Testing
The following NDT methods are recommended for strip electrode SAW stainless steel cladding:
- Visual inspection (VT): Check for surface defects, weld shape, and dilution
- Magnetic particle testing (MT): Detect surface and near-surface cracks (note: austenitic stainless steel is non-magnetic, so MT may not be effective; use wet fluorescent penetrant testing instead)
- Penetrant testing (PT): Detect surface cracks and porosity
- Ultrasonic testing (UT): Detect subsurface defects and measure overlay thickness
- Radiographic testing (RT): Detect internal defects (limited by weld width)
Study Insights and Engineering Practice
The research by Yu Hao, Ma Ming, and Li Wenyu provides valuable insights into the practical application of strip electrode SAW for stainless steel cladding. A key finding is that the technology offers significant productivity advantages over conventional wire SAW, with deposition rates 3–5 times higher.
However, the study also highlights several challenges:
- Equipment requirements: Strip electrode SAW requires specialized equipment including strip electrode feeders, flux feeders, and electrode holders
- Process control: The wider weld pool requires careful control of travel speed and electrode alignment
- Weld metal composition: Dilution control is critical for maintaining the required corrosion resistance
- Inspection: NDT methods must be adapted for non-magnetic austenitic stainless steel welds
From a practical standpoint, the study demonstrates that strip electrode SAW is a viable and cost-effective technology for large-area stainless steel cladding in boiler and pressure vessel manufacturing. The key to successful implementation is careful process development, welder training, and quality control.
The integration of strip electrode SAW into production workflows requires consideration of the following factors:
- Initial equipment investment
- Consumable costs (strip electrode and flux)
- Training requirements for operators
- Inspection and testing procedures
- Maintenance of specialized equipment
This research contributes to the advancement of cladding technology by demonstrating the practical application of strip electrode SAW for stainless steel overlay welds. The findings provide a foundation for further development and optimization of this technology for various industrial applications.