CLADDING TECHNOLOGY SHANXI CO., LTDStudy Note on SMAW Overlay Welding of Duplex Stainless Steel 2205
Literature Overview
The research by Wang Li, Gao Junsong, Wu Daowen, and He Penghui from the Luoyang Ship Material Research Institute (2010) addresses a critical engineering challenge: applying duplex stainless steel 2205 as an overlay layer using shielded metal arc welding (SMAW) on carbon or low-alloy steel substrates. Duplex 2205, with its roughly equal ferrite-austenite microstructure, offers exceptional combinations of strength (yield strength typically 450-550 MPa), corrosion resistance, and resistance to chloride stress corrosion cracking, making it an ideal candidate for harsh chemical and marine environments. However, the inherent thermal sensitivity of the duplex phase balance — particularly the risk of sigma phase precipitation during excessive heat input — makes SMAW overlay a demanding process requiring careful parameter control.
Core Technical Points
Phase Balance Control
The fundamental challenge in duplex stainless steel welding is maintaining the target ferrite content of 40-60% in the weld metal. During SMAW overlay, the heat input directly governs the solidification rate and cooling behavior, which in turn dictates the phase transformation sequence. The researchers systematically investigated how welding current, arc voltage, travel speed, and interpass temperature influence the ferrite-austenite ratio in the deposited overlay.
| Parameter | Typical Range | Effect on Phase Balance |
|---|---|---|
| Welding current | 140-180 A | Higher current increases heat input, promoting austenite formation |
| Arc voltage | 20-24 V | Higher voltage widens bead, reduces dilution |
| Travel speed | 50-100 mm/min | Slower speed increases heat input, shifts toward austenite |
| Interpass temperature | 80-150 °C | Must remain below 150 °C to prevent sigma phase |
| Electrode type | E309L or equivalent low-carbon | Low carbon reduces sensitization risk |
Dilution and Bond Strength
A critical finding in this work is the quantitative relationship between substrate dilution and the resulting metallurgical bond quality. The base metal dilution in SMAW overlay typically ranges from 15-35% for the first pass and decreases with subsequent passes. The researchers demonstrated that maintaining dilution below 25% is essential for achieving acceptable duplex phase balance in the final overlay layer. The bond strength between the overlay and the base metal was evaluated through shear tests and metallographic examination of the fusion line.
Microstructural Characterization
Metallographic analysis revealed that the weld metal microstructure transitions from a columnar dendritic structure near the fusion boundary to an equiaxed structure toward the weld surface. The ferrite content, measured by magnetic methods, showed a clear trend: higher heat input per unit length led to increased ferrite dissolution, shifting the balance toward austenite. The researchers recommended a multi-pass welding strategy with the first pass using a higher dilution-tolerant filler and subsequent passes using a standard 2205 filler to achieve a graded microstructure.
Process Parameters and Welding Sequence
Recommended Welding Procedure
- Substrate preparation: Surface must be ground to remove mill scale, rust, and contamination to a finish of at least 1.6 μm Ra.
- First pass: Use a 309L-type electrode at reduced current (130-150 A) to minimize dilution from the carbon steel substrate.
- Intermediate passes: Transition to 2205 filler electrode (E309L or equivalent) at standard parameters.
- Final pass: Use 2205 filler with controlled heat input to achieve target ferrite content of 45-55%.
- Post-weld treatment: Stress relief at 350-400 °C for 2 hours if residual stress relief is required, but avoid temperatures above 450 °C to prevent sigma phase formation.
Defect Analysis and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Cracking at fusion line | Excessive dilution, high residual stress | Reduce first-pass current, use low-hydrogen electrode |
| Sigma phase precipitation | Interpass temperature too high | Maintain interpass temperature below 150 °C |
| Excessive porosity | Contaminated surface, electrode moisture | Thorough surface cleaning, bake electrodes at 300 °C for 2 hours |
| Phase imbalance | Incorrect heat input | Adjust current/travel speed combination |
Integration with Engineering Practice
In shipbuilding and offshore platform applications, the SMAW overlay of 2205 on carbon steel is frequently used for localized corrosion protection of structural components exposed to seawater or chemical environments. The Luoyang Ship Material Research Institute's work is particularly relevant for small-scale repair welding and field applications where mechanized welding equipment is unavailable. The key practical insight is that SMAW, despite its lower deposition efficiency compared to mechanized processes, offers superior flexibility for complex geometries and on-site repair work.
The researchers' findings align with API 934 recommendations for overlay welding qualification, which require weld performance qualification testing including shear bond strength (minimum 200 MPa for critical applications), hardness uniformity, and corrosion testing. For pressure vessel applications governed by ASME VIII Division 1 or GB/T 150, the overlay procedure must be qualified per ASME IX or NB/T 47014, with additional requirements for post-weld testing including intergranular corrosion testing per ASTM A263.
Key Reflections
The most significant insight from this research is the recognition that SMAW overlay of duplex 2205 is not merely a parameter optimization problem but a holistic process design challenge. The interplay between heat input, dilution, and phase balance creates a narrow process window that must be carefully managed. In my experience with similar projects, the first pass is the most critical — its dilution level largely determines the metallurgical quality of the entire overlay system. A practical rule of thumb I have adopted is to always perform a dilution test coupon before production welding, measuring the actual carbon and alloy content at the fusion line to confirm that the phase balance target is achievable.