Zhuojin Pipe Fitting Co., LtdFerrite Content Control in Automatic TIG Overlay Welding on 12Cr2Mo1R Steel
Literature Overview
This paper by Wang Yinli, published in Refining and Chemical Engineering (2019, Vol. 30, No. 3, pp. 19-20), investigates the control of ferrite content in overlay weld deposits on 12Cr2Mo1R steel using automatic TIG (Tungsten Inert Gas) welding. The author is affiliated with the Training Center of PetroChina Daqing Petrochemical Company. The study focuses on the critical relationship between ferrite content, welding parameters, and hot cracking susceptibility in stainless steel overlay welds used in hydrogenation reactors and threaded locking ring heat exchangers.
Core Technical Content
Ferrite content is a critical quality indicator for austenitic stainless steel overlay welds, particularly in high-temperature and high-pressure applications such as hydrogenation reactors and threaded locking ring heat exchangers. The ferrite content directly influences:
- Hot cracking resistance: Higher ferrite content (up to a certain level) improves resistance to solidification cracking by providing a ductile phase that accommodates shrinkage stresses.
- Corrosion resistance: Excessive ferrite content (> 15%) can reduce corrosion resistance in chloride-containing environments due to the lower pitting resistance of ferrite compared to austenite.
- Mechanical properties: Ferrite content affects hardness, strength, and ductility of the overlay layer.
The optimal ferrite content for most applications is 5-10% (by volume), as measured by the Ferritscope or magnetic permeability method.
Experimental Setup
The study examines overlay welding of ER309L and ER347 consumables on 12Cr2Mo1R steel plates using automatic TIG welding. The 12Cr2Mo1R steel is a 2.25Cr-1Mo low-alloy steel commonly used for high-pressure hydrogen service vessels.
| Parameter | ER309L | ER347 |
|---|---|---|
| Composition (approx.) | C ≤ 0.04%, Cr 23-25%, Ni 12-14% | C ≤ 0.04%, Cr 21-23%, Ni 10-12%, Nb 0.65-1.10% |
| Weld metal type | Ferrite-austenite | Ferrite-austenite |
| Typical ferrite content | 10-15% | 8-12% |
| Application | High dilution service | Stabilized for high temperature |
Welding Parameters Investigated
| Parameter | Range Tested | Effect on Ferrite |
|---|---|---|
| Current (A) | 150-250 | Higher current → lower ferrite (more dilution) |
| Travel speed (mm/min) | 100-300 | Higher speed → higher ferrite (less dilution) |
| Wire feed speed (m/min) | 3-8 | Higher feed → lower ferrite (more overlay material) |
| Shielding gas | Ar, Ar+2%O₂, Ar+2%CO₂ | O₂ addition → lower ferrite |
| Preheat temperature (°C) | 50-200 | Higher preheat → slightly lower ferrite |
| Number of passes | 2-4 | More passes → lower ferrite (cumulative dilution) |
Key Findings
Effect of Current and Travel Speed
The study found that the ratio of current to travel speed (heat input per unit length) is the primary factor controlling ferrite content:
- High heat input (high current, low travel speed): Increases dilution of the base metal into the weld pool. Since 12Cr2Mo1R is a ferritic steel, increased dilution introduces more iron into the weld metal, which promotes ferrite formation. However, the increased heat input also promotes grain growth and more complete melting, which can reduce ferrite through enhanced austenite stabilization.
- Low heat input (low current, high travel speed): Reduces dilution, resulting in weld metal composition closer to the consumable. ER309L typically produces 10-15% ferrite, while ER347 produces 8-12% ferrite.
Effect of Shielding Gas Composition
The addition of 2% O₂ to the shielding gas was found to significantly reduce ferrite content. Oxygen acts as a deoxidizer, promoting the formation of oxide inclusions that serve as nucleation sites for austenite. Additionally, oxygen increases the fluidity of the weld pool, promoting more uniform mixing and reducing segregation that can promote ferrite formation.
Effect of Number of Passes
Multi-pass welding was found to reduce ferrite content compared to single-pass welding. This is attributed to the cumulative dilution effect and the thermal cycling of previously deposited layers, which promotes phase transformation and ferrite dissolution.
Optimal Process Parameters
Based on the experimental results, the following parameters were identified as optimal for achieving 5-10% ferrite content:
| Parameter | ER309L Optimal | ER347 Optimal |
|---|---|---|
| Current | 200-220 A | 180-200 A |
| Travel speed | 150-200 mm/min | 150-200 mm/min |
| Wire feed speed | 5-6 m/min | 5-6 m/min |
| Shielding gas | Ar + 2% O₂ | Ar + 2% O₂ |
| Preheat | 100-150 °C | 100-150 °C |
| Number of passes | 2-3 | 2-3 |
| Resulting ferrite | 6-9% | 5-8% |
Engineering Practice Implications
The findings have direct implications for the manufacturing of hydrogenation reactors and threaded locking ring heat exchangers:
- Hydrogenation reactors: These vessels operate at high temperatures (350-450 °C) and high pressures (15-25 MPa) in hydrogen-rich environments. The overlay layer must resist hydrogen attack, corrosion, and thermal fatigue. Ferrite content above 15% can promote hydrogen-induced cracking, while below 5% increases susceptibility to solidification cracking during welding.
- Threaded locking ring heat exchangers: These operate under cyclic thermal loading, and the overlay layer must maintain integrity over thousands of thermal cycles. Excessive ferrite can lead to thermal fatigue cracking due to the different thermal expansion coefficients of ferrite and austenite.
Quality Inspection Requirements
| Test Method | Standard | Acceptance Criteria |
|---|---|---|
| Ferrite measurement | ASTM E491 | 5-15% (typical) |
| Chemical analysis | ASTM E415 | Per consumable specification |
| Hardness | ASTM B231 | ≤ 250 HV (overlay) |
| Tensile test | ASTM E8 | UTS ≥ 515 MPa |
| Bend test | ASTM A262 | No cracking |
| Corrosion test | ASTM A262 Practice E | No intergranular corrosion |
Key Reflections
This study demonstrates the importance of systematic parameter optimization in overlay welding, particularly when the goal is to achieve a specific microstructural composition rather than simply depositing a corrosion-resistant layer. The ferrite content, while seemingly a secondary characteristic, is actually a primary determinant of the overlay's performance in high-temperature and high-pressure service.
The finding that shielding gas composition significantly affects ferrite content is particularly practical. Adding 2% O₂ to the shielding gas is a simple and cost-effective measure that can be implemented without modifying the welding equipment or consumables. This makes it an attractive option for existing production facilities.
The study also highlights the importance of understanding the metallurgical interactions between the base metal and the overlay material. The 12Cr2Mo1R steel, being a ferritic material, has a strong influence on the weld metal composition through dilution. This is a fundamental consideration in all overlay welding applications involving dissimilar metals.
The practical significance of this work extends beyond the specific application studied. The methodology of systematically varying welding parameters and measuring their effect on ferrite content can be applied to any overlay welding operation where microstructural control is critical.