CLADDING TECHNOLOGY SHANXI CO., LTDCladding Process Research for Chromium-Nickel Austenitic Stainless Steel Materials
Research Background and Significance
The study by Zhang Yixia, Gao Junsong, and Chen Juhui, published in "Material Development and Application" in 2010, investigates weld overlay processes for chromium-nickel austenitic stainless steel materials. The research was conducted collaboratively by China State Construction Second Engineering Bureau Co., Ltd. and the 725th Research Institute of China Shipbuilding Industry Corporation, combining construction industry application experience with naval metallurgical research expertise. This collaboration is significant because it addresses the practical challenges of cladding austenitic stainless steels in large-scale engineering structures, where the base metal thickness, joint geometry, and thermal mass present unique metallurgical challenges.
Austenitic stainless steels such as 304, 316, 321, and 347 are widely used in chemical processing, food processing, and marine environments due to their excellent corrosion resistance. However, when these materials require surface repair or when dissimilar metal joints need corrosion protection, weld overlay becomes necessary. The challenge lies in maintaining the austenitic microstructure and corrosion resistance of the overlay layer while ensuring sound metallurgical bonding with the base metal.
Process Comparison and Technical Parameters
The authors evaluate multiple cladding processes for austenitic stainless steel substrates, with particular attention to gas tungsten arc welding (GTAW/TIG) and plasma transferred arc (PTA) welding. The following table summarizes the process parameters and their effects:
| Process | Shielding Gas | Typical Current | Travel Speed | Layer Thickness | Dilution Rate |
|---|---|---|---|---|---|
| GTAW/TIG | Ar (99.99%) | 80-150 A | 30-80 mm/min | 0.5-1.5 mm | 15-25% |
| PTA | Ar + 5% N2 | 200-400 A | 50-150 mm/min | 1.0-2.5 mm | 10-18% |
| GMAW | Ar + 2% CO2 | 150-250 A | 60-120 mm/min | 1.0-3.0 mm | 20-35% |
| FCAW | Self-shielded or gas-shielded | 180-280 A | 70-130 mm/min | 1.5-3.5 mm | 25-40% |
The study finds that GTAW and PTA offer the lowest dilution rates and the best control over overlay microstructure, making them preferred for critical corrosion service applications. However, their lower deposition rates make them less economical for large-area cladding. GMAW and FCAW provide higher productivity but require careful control of parameters to minimize dilution and maintain overlay composition.
Microstructural Analysis and Corrosion Performance
A key contribution of this study is the correlation between overlay microstructure and corrosion resistance. The authors conducted metallographic examination and intergranular corrosion testing (per ASTM A262 Practice E, using the 65% boiling HNO3 method) on overlay samples produced by different processes. The findings are summarized as follows:
- GTAW overlay layers exhibited fully austenitic microstructure with grain size of 8-10 per ASTM E112, showing no intergranular corrosion attack after 24-hour exposure in boiling HNO3.
- PTA overlay layers showed a similar austenitic structure but with slightly finer grains (9-11 per ASTM E112), also passing the intergranular corrosion test.
- GMAW overlay layers showed minor ferrite content (5-8% delta ferrite) due to higher dilution, which slightly reduced corrosion resistance but still met acceptance criteria.
- FCAW overlay layers exhibited higher ferrite content (10-15%) and occasional carbide precipitation at grain boundaries, resulting in mild intergranular corrosion attack.
These results clearly demonstrate that dilution control is the primary factor governing overlay corrosion performance, and that processes with lower dilution inherently produce superior overlay layers.
Engineering Application Considerations
For practical cladding of austenitic stainless steel structures, the study recommends a multi-layer approach where the first layer (tack coat) uses a filler metal with slightly higher alloy content to compensate for base metal dilution, followed by subsequent layers with the target composition. The preheat temperature should be limited to below 100°C for austenitic stainless steels to avoid excessive sensitization in the heat-affected zone. Post-weld heat treatment is generally not recommended for austenitic stainless steel overlays, as solution annealing above 1050°C would be required to restore full corrosion resistance, which is impractical for large structures.
The authors also address the issue of hot cracking susceptibility in austenitic weld metals, noting that sulfur and phosphorus content in the filler metal should be controlled below 0.015% and 0.020% respectively. The addition of niobium or titanium as grain refiners in the filler metal helps suppress hot cracking and improves the mechanical properties of the overlay.
Study Insights and Professional Reflection
This paper provides a well-structured comparison of cladding processes for austenitic stainless steel that is directly applicable to engineering practice. The emphasis on dilution control as the governing factor for overlay quality is a fundamental principle that should be communicated to all engineers involved in cladding specification. The intergranular corrosion testing results serve as a clear benchmark for evaluating overlay quality in critical service environments. For engineers working in the chemical processing or marine industries, this research supports the selection of GTAW or PTA for critical overlay applications and GMAW for less demanding service conditions, always with appropriate procedure qualification and non-destructive testing verification.