CLADDING TECHNOLOGY SHANXI CO., LTDFerrite Content Control in Automatic TIG Overlay Welding
Literature Overview
This 2019 paper by Wang Yinli from the Training Center of PetroChina Daqing Petrochemical Company addresses a critical metallurgical aspect of overlay welding: the control of ferrite content in duplex stainless steel overlay layers deposited by automatic TIG (gas tungsten arc welding) welding. Ferrite content is a key microstructural parameter that directly influences the mechanical properties, corrosion resistance, and weldability of duplex stainless steel welds. The paper provides practical guidance on process parameter optimization for achieving target ferrite content in overlay applications.
Core Technical Content
Duplex stainless steel overlay layers are widely used in petrochemical and refinery applications where combined strength and corrosion resistance are required. The ferrite-austenite microstructure of duplex stainless steels provides excellent resistance to chloride stress corrosion cracking while maintaining mechanical properties comparable to austenitic stainless steels. However, the ferrite content must be carefully controlled to achieve optimal performance.
Ferrite Content Significance
The ferrite content in duplex stainless steel welds directly influences several critical properties:
| Ferrite Content (FN) | Mechanical Properties | Corrosion Resistance | Weldability |
|---|---|---|---|
| < 10 FN | Low strength, high ductility | Susceptible to pitting and SCC | Good |
| 10–35 FN | Optimal strength-ductility balance | Excellent pitting and SCC resistance | Good |
| 35–50 FN | High strength, reduced ductility | Moderate pitting resistance | Moderate |
| > 50 FN | High strength, brittle | Susceptible to intergranular attack | Poor |
The target ferrite content for most duplex stainless steel overlay applications is 35–45 FN, which provides an optimal balance of mechanical properties and corrosion resistance. The paper emphasizes that ferrite content must be controlled within this range throughout the entire overlay thickness, as variations in ferrite content between layers can create localized regions of reduced corrosion resistance.
Process Parameters Influencing Ferrite Content
The paper identifies several process parameters that influence ferrite content in automatic TIG overlay welding:
| Parameter | Effect on Ferrite Content | Recommended Range |
|---|---|---|
| Current | Higher current → higher ferrite | 180–250 A |
| Travel speed | Higher speed → lower ferrite | 200–300 mm/min |
| Heat input | Higher heat input → higher ferrite | 0.8–1.2 kJ/mm |
| Shielding gas composition | Higher Ar% → lower ferrite | 98–100% Ar |
| Interpass temperature | Higher temperature → lower ferrite | < 150 °C |
| Electrode diameter | Larger diameter → higher ferrite | 2.4–3.2 mm |
| Pulse parameters | Pulse frequency affects grain structure | 50–100 Hz |
Heat Input Control
Heat input is the primary parameter controlling ferrite content in TIG overlay welding. The paper describes the relationship between heat input and ferrite content as follows:
- Low heat input (< 0.6 kJ/mm): Results in high cooling rates, promoting austenite formation and low ferrite content (< 20 FN). The rapid cooling also promotes martensite formation in some compositions, which can reduce ductility.
- Medium heat input (0.8–1.2 kJ/mm): Provides optimal cooling rates for achieving target ferrite content (35–45 FN). The moderate cooling rate allows adequate time for austenite-to-ferrite transformation while preventing excessive grain growth.
- High heat input (> 1.4 kJ/mm): Results in slow cooling rates, promoting ferrite formation and potentially leading to excessive ferrite content (> 50 FN). High heat input also promotes grain growth and may lead to sensitization and intergranular corrosion susceptibility.
Shielding Gas Effects
The shielding gas composition significantly influences ferrite content through its effect on weld pool dynamics and cooling rates:
| Gas Composition | Ferrite Content (FN) | Notes |
|---|---|---|
| 100% Ar | 35–45 | Baseline, moderate heat input |
| 98% Ar + 2% O₂ | 40–50 | Slightly higher ferrite, improved wetting |
| 95% Ar + 5% CO₂ | 45–55 | Higher ferrite, increased spatter |
| 90% Ar + 10% He | 30–40 | Lower ferrite, deeper penetration |
| 100% He | 25–35 | Lowest ferrite, high cost |
The paper recommends using 100% argon as the primary shielding gas for duplex stainless steel overlay welding, with optional addition of small amounts of oxygen (1–2%) to improve weld pool fluidity without significantly affecting ferrite content.
Layer-by-Layer Ferrite Control
The paper emphasizes the importance of monitoring and controlling ferrite content on a layer-by-layer basis, as the first layer of overlay typically experiences higher dilution from the base material, resulting in different ferrite content compared to subsequent layers.
| Layer | Expected Ferrite Content | Control Measures |
|---|---|---|
| First layer | 40–55 FN (higher dilution) | Lower heat input, smaller bead width |
| Second layer | 35–45 FN | Standard parameters |
| Subsequent layers | 35–45 FN | Standard parameters |
| Final layer | 35–45 FN | Verify with ferrite meter |
The paper recommends using a handheld ferrite meter for real-time monitoring of ferrite content during welding, with measurements taken on each completed layer before proceeding to the next. This layer-by-layer control ensures that the final overlay achieves uniform ferrite content throughout its thickness.
Quality Control Procedures
The paper describes a comprehensive quality control program for ferrite content verification:
- Pre-weld verification: Verify base material composition and heat treatment condition to ensure predictable dilution behavior
- In-process monitoring: Use handheld ferrite meter to measure ferrite content on each completed layer, recording values for trend analysis
- Post-weld verification: Perform ferrite number measurement on the completed overlay at multiple locations across the surface to verify uniformity
- Metallographic examination: Prepare metallographic samples from representative locations for optical microscopy examination of microstructure and grain structure
- Mechanical property testing: Perform hardness and impact testing to verify that mechanical properties are consistent with the measured ferrite content
Engineering Practice Integration
The ferrite content control procedures described in this paper are consistent with industry standards including AWS D16.0 (Stainless Steel Welding) and ISO 13919 (Duplex stainless steel welding). The paper's emphasis on layer-by-layer monitoring is particularly important for critical applications where corrosion resistance is paramount.
The paper also addresses the relationship between ferrite content and post-weld heat treatment. Duplex stainless steel overlay layers with ferrite content in the 35–45 FN range typically do not require post-weld heat treatment, as the as-welded microstructure provides adequate corrosion resistance. However, if ferrite content exceeds 50 FN, a solution heat treatment at 1050–1100 °C followed by rapid cooling may be required to dissolve sigma phase and restore corrosion resistance.
The paper's practical guidance on process parameter selection is particularly valuable for field applications where real-time ferrite monitoring may not be available. By establishing baseline parameter sets that achieve target ferrite content under standard conditions, engineers can implement control charts to monitor process performance and detect deviations before they result in nonconforming overlay.
Study Reflections
This paper addresses a fundamental metallurgical challenge in duplex stainless steel overlay welding: achieving and maintaining target ferrite content through process parameter control. The paper's systematic approach to parameter optimization, combined with practical guidance on in-process monitoring, provides engineers with a comprehensive framework for ferrite content control in overlay welding applications. The emphasis on layer-by-layer monitoring reflects the understanding that ferrite content can vary significantly between layers, particularly in the first layer where base metal dilution is highest. For engineers working on duplex stainless steel overlay projects, this paper provides essential guidance on process parameter selection, quality control procedures, and the metallurgical implications of ferrite content variation. The paper's practical orientation, grounded in field experience at a major petrochemical company, makes it a valuable resource for both process development and production welding operations.