Zhuojin Pipe Fitting Co., LtdSMAW Surfacing Process Study on Duplex Stainless Steel 2205
Literature Overview
This research by Wang Li, Gao Junsong, Wu Daowen, and Hei Penghui from the Luoyang Ship Material Research Institute, published in Electric Welding Machine (2010, Vol. 40, Issue 2), addresses the challenging task of surfacing duplex stainless steel 2205 using shielded metal arc welding (SMAW). Duplex stainless steels, characterized by their roughly equal ferrite and austenite phase content, offer excellent combinations of strength, corrosion resistance, and toughness. However, maintaining the proper phase balance during welding and surfacing is difficult due to the sensitivity of the ferrite-austenite ratio to composition and thermal cycles. The researchers developed a two-layer surfacing procedure using E309MoL as a transition layer and E2209 as the cap layer, with specific process parameters designed to preserve the duplex microstructure.
Two-Layer Surfacing Strategy
The two-layer approach is a well-established strategy for surfacing duplex stainless steels. The transition layer serves several critical functions:
- Dilution control: The austenitic E309MoL composition is designed to compensate for dilution from the base material, ensuring that the subsequent cap layer achieves the correct composition
- Crack resistance: Austenitic weld metal has excellent ductility and resistance to solidification and hot cracking
- Composition buffering: The transition layer provides a composition gradient between the base material and the final duplex cap layer
The cap layer, deposited with E2209 electrode, is specifically designed to produce a duplex microstructure with approximately 40-60% ferrite. The researchers achieved a ferrite area fraction of 47.14%, which falls within the acceptable range for duplex stainless steels and indicates good phase balance.
| Layer | Electrode Type | Function | Key Property |
|---|---|---|---|
| Transition layer | E309MoL (φ4mm) | Dilution compensation, crack resistance | Austenitic, high ductility |
| Cap layer | E2209 (φ4mm) | Duplex microstructure, corrosion resistance | 47.14% ferrite, 23.8-24.7 HRC |
Process Parameter Optimization
The researchers employed specific process parameters to control the thermal cycle and preserve the duplex microstructure:
- Low current: Minimizes heat input and reduces the risk of excessive ferrite dissolution or sigma phase formation
- Fast welding speed: Further limits heat input and promotes rapid solidification, which favors the formation of both ferrite and austenite phases
- Water cooling: Applied to the base material to accelerate cooling and reduce the time spent in temperature ranges where harmful phases can form
The combination of low current, fast travel speed, and water cooling represents a deliberate strategy to minimize heat input and control the thermal cycle. This is critical for duplex stainless steels because excessive heat input can lead to:
- Dissolution of ferrite, resulting in an austenitic microstructure with reduced strength
- Formation of sigma phase (FeCr) at 700-900°C, which severely reduces toughness
- Grain coarsening, which reduces both strength and corrosion resistance
- Excessive dilution, which shifts the composition away from the duplex range
The use of water cooling is a practical technique that can be implemented in field conditions, though it requires careful management to avoid quench cracking in the base material. The effectiveness of this approach depends on the thickness and thermal mass of the base material.
Corrosion Resistance Verification
The corrosion performance of the surfaced 2205 layer was evaluated using two standard methods:
- FeCl3 solution pitting test: This test simulates aggressive chloride environments and is sensitive to the presence of sensitized regions or phase imbalances
- NaOH solution electrolytic corrosion test: This test evaluates resistance to alkaline corrosion, which is relevant for certain industrial environments
The results showed no metal precipitates or deposits, indicating that the surfacing layer maintained its corrosion resistance and did not exhibit signs of localized corrosion attack. The chemical composition met the requirements for E2209-type electrodes, confirming that the composition was within the duplex stainless steel range.
The hardness of 23.8-24.7 HRC is consistent with typical duplex stainless steel values, reflecting the balanced microstructure of ferrite and austenite. This hardness level provides adequate wear resistance while maintaining the toughness and corrosion resistance characteristic of duplex alloys.
Quality Control and Inspection
For surfacing applications involving duplex stainless steels, the following quality control measures are recommended:
- Ferrite number measurement: Using a ferrite gauge to verify the phase balance in the cap layer (target: 40-60% ferrite)
- Chemical analysis: Confirming that the composition meets the specified requirements for the electrode type
- Hardness testing: Verifying that the hardness is within the expected range for the microstructure
- Corrosion testing: Conducting pitting resistance tests (e.g., ASTM G48) to confirm corrosion performance
- NDT: Visual inspection, magnetic particle testing, or liquid penetrant testing to detect surface defects
- Metallographic examination: Verifying the microstructure and checking for harmful phases such as sigma phase or chi phase
Engineering Practice and Application
The successful surfacing of duplex stainless steel 2205 using SMAW demonstrates that this alloy can be applied as a wear- and corrosion-resistant coating in industrial settings where high-performance materials are required. Typical applications include:
- Marine and offshore equipment subject to chloride corrosion
- Chemical processing equipment requiring resistance to aggressive media
- Oil and gas industry components exposed to sour service
- Pulp and paper industry equipment subject to corrosive and abrasive conditions
The two-layer approach with E309MoL transition and E2209 cap is a proven strategy that can be adapted to various base materials and service conditions. Engineers should develop specific welding procedures for each application, taking into account the base material composition, thickness, and the specific service environment.
This research provides valuable guidance for the practical application of duplex stainless steel surfacing, demonstrating that careful process control and appropriate electrode selection can achieve the desired microstructure and performance. The results confirm that SMAW is a viable process for duplex stainless steel surfacing when the thermal cycle is properly managed, offering a practical alternative to more expensive processes such as TIG or laser cladding for many industrial applications.