CLADDING TECHNOLOGY SHANXI CO., LTDTIG Welding Process Application of Cr22 S31803 Duplex Stainless Steel
Literature Overview
This 2017 study by Sun Wantian from Sinopec Fifth Construction Company examines the application of TIG welding processes for Cr22 type S31803 duplex stainless steel. The research addresses practical welding challenges encountered in petrochemical construction where duplex stainless steels are increasingly specified for their superior corrosion resistance and mechanical strength compared to conventional austenitic stainless steels. The work provides valuable field-level insights into process parameter selection and quality control for this demanding material.
Material Characteristics and Welding Challenges
S31803 (also known as UNS S31803 or equivalent grades such as 2507) is a super duplex stainless steel containing approximately 22% chromium, 7% nickel, and 3% molybdenum with nitrogen additions. This composition produces a dual-phase microstructure consisting of austenite and ferrite phases in roughly equal proportions, providing exceptional resistance to chloride stress corrosion cracking, pitting, and crevice corrosion while maintaining high yield strength values exceeding 550 MPa.
The welding of duplex stainless steels presents unique challenges that require careful process control. The primary concern is maintaining the austenite-ferrite phase balance in the weld metal and heat-affected zone. Excessive cooling rates can promote the formation of intermetallic phases such as sigma phase (Cr2N), which embrittles the material and severely degrades corrosion resistance. Conversely, excessive heat input can lead to excessive ferrite dissolution and the formation of Laves phase or other detrimental precipitates.
TIG Welding Process Parameters
The study establishes optimal TIG welding parameters for S31803 duplex stainless steel based on practical fabrication experience in petrochemical construction. The recommended parameters emphasize controlled heat input and rapid cooling to maintain the desired phase balance.
| Parameter | Recommended Value | Rationale |
|---|---|---|
| Arc Current | 100-180 A | Controls heat input; prevents excessive HAZ width |
| Travel Speed | 6-10 cm/min | Ensures adequate penetration without excessive dilution |
| Gas Flow Rate | 15-20 L/min Argon | Provides complete shielding; prevents nitrogen pick-up |
| Electrode | EWPur (pure tungsten) | Minimizes tungsten inclusion in weld metal |
| Filler Metal | ER2209 or equivalent | Matches base metal composition; maintains phase balance |
| Preheat Temperature | 50-100°C | Reduces cracking susceptibility; controls cooling rate |
| Interpass Temperature | Below 150°C | Prevents excessive grain growth and phase transformation |
The selection of filler metal is critical for duplex stainless steel welding. ER2209 or equivalent grades with slightly higher nickel content than the base metal are typically specified to compensate for the preferential dissolution of austenite-forming elements during welding. This ensures that the weld metal achieves a ferrite content in the target range of 40-60% by volume fraction.
Quality Control and Defect Prevention
The study emphasizes several quality control measures essential for duplex stainless steel TIG welding. First, the ferrite content of the weld metal must be verified through magnetic ferrite measurements or metallographic examination. Ferrite numbers outside the 40-60 FN range indicate potential corrosion resistance or ductility issues. Second, the weld surface must be inspected for any signs of intermetallic phase formation, which can be detected through proper metallographic preparation and optical microscopy.
Common defects encountered in duplex stainless steel welding include hot cracking due to low sulfur and phosphorus content, cold cracking from hydrogen embrittlement in thick sections, and phase imbalance leading to localized corrosion. The TIG welding process is particularly well-suited for duplex stainless steels because it provides excellent arc stability, precise heat input control, and minimal spatter that could contaminate the weld surface.
Engineering Practice Reflections
This study provides practical guidance for field welders and engineers working with duplex stainless steels in petrochemical applications. The emphasis on controlled heat input and rapid cooling reflects the fundamental metallurgical requirements of maintaining a balanced duplex microstructure. Engineers should note that the TIG welding parameters established here serve as a starting point; actual parameters must be qualified through welding procedure qualification in accordance with applicable codes such as ASME Section IX or NB/T 47014.
The research also highlights the importance of welder skill and technique in duplex stainless steel fabrication. Maintaining consistent travel speed, proper torch angle, and stable arc length throughout the weld are essential for achieving uniform phase balance and mechanical properties. Training programs for welders working with duplex stainless steels should include specific instruction on these critical technique elements.