CLADDING TECHNOLOGY SHANXI CO., LTDDeep Penetration TIG Welding Process for Super Duplex Stainless Steel S32750
Literature Overview
This study, published in Petrochemical Equipment in 2024 by Chen Jing, Xu Cheng, Jin Yanfeng, Wang Yingbo, Wei Junrui, and Wei Yiqi from Shanghai Lanbin Petrochemical Equipment and Gansu Lanke Petrochemical High-Tech Equipment, focuses on the deep penetration TIG welding process for super duplex stainless steel S32750. The research addresses a significant engineering challenge in the petrochemical and oil and gas industries, where super duplex stainless steels are increasingly used for high-pressure, high-temperature, and highly corrosive service conditions.
Super duplex stainless steel S32750 (UNS S32750) is characterized by a microstructure consisting of approximately 50 percent austenite and 50 percent ferrite, with a PREN (pitting resistance equivalent number) exceeding 40. This gives it exceptional resistance to chloride-induced pitting and crevice corrosion, as well as superior mechanical strength compared to conventional duplex stainless steels. However, the welding of super duplex stainless steels presents unique challenges related to microstructural evolution, cracking susceptibility, and mechanical property degradation in the heat-affected zone (HAZ).
Core Technical Content and Welding Process Parameters
The deep penetration TIG welding process, also known as keyhole TIG welding or deep penetration plasma welding, utilizes a high-energy-density arc to achieve deep penetration with relatively low travel speeds. This process is particularly suitable for welding super duplex stainless steel components in the petrochemical industry, where thick-section welding with excellent mechanical and corrosion properties is required.
The key welding parameters investigated in this study include:
| Parameter | Range | Unit | Notes |
|---|---|---|---|
| Arc current | 180 - 350 | A | Higher currents for thicker sections |
| Arc voltage | 14 - 22 | V | Depends on current and electrode |
| Travel speed | 150 - 600 | mm/min | Inversely related to current |
| Shielding gas | Argon + 5% CO2 or pure Argon | L/min | Flow rate 12-20 |
| Backing gas | Pure Argon | L/min | Flow rate 5-10 |
| Electrode material | Thoriated tungsten (WTh2) | mm | Diameter 2.4-4.0 |
| Filler metal | ER2209 or ER2594 | mm | Wire diameter 1.0-2.0 |
| Preheat temperature | 50 - 150 | °C | To reduce cracking susceptibility |
| Interpass temperature | < 150 | °C | Strictly controlled |
The deep penetration TIG welding process achieves penetration depths of 6 to 10 times the plate thickness under optimal conditions, compared to 1 to 2 times the plate thickness for conventional TIG welding. This is achieved through the formation of a keyhole in the molten weld pool, where the arc pressure and plasma jet force penetrate the workpiece surface and create a deep, narrow weld groove.
Microstructural Control in the Weld and HAZ
The microstructure of super duplex stainless steel S32750 is highly sensitive to thermal cycling during welding. The key metallurgical concerns include:
- Ferrite/austenite balance: The weld metal and HAZ must maintain a ferrite/austenite ratio of approximately 40-60 percent to ensure adequate toughness and corrosion resistance. Excessive ferrite promotes intergranular corrosion, while excessive austenite reduces strength and promotes sigma phase formation.
- Sigma phase formation: The intermetallic sigma phase (Cr,Ni-sigma) can form during prolonged exposure to temperatures between 600 and 870 °C. This phase is hard and brittle, severely reducing ductility and corrosion resistance. The welding process must be designed to minimize the time spent in the sigma phase formation temperature range.
- Chromium carbide precipitation: At temperatures between 450 and 850 °C, chromium carbides can precipitate at grain boundaries, leading to chromium depletion and reduced corrosion resistance. This is particularly relevant for the HAZ, where the thermal cycle may include prolonged exposure to this temperature range.
- Hot cracking susceptibility: Super duplex stainless steels have a relatively low hot cracking susceptibility compared to austenitic stainless steels, but it can still occur under adverse conditions such as high sulfur content in the filler metal, excessive restraint, or rapid solidification.
Welding Procedure Specification
A typical welding procedure specification (WPS) for deep penetration TIG welding of S32750 super duplex stainless steel would include the following elements:
| WPS Element | Specification |
|---|---|
| Base material | UNS S32750 / 1.4501 |
| Filler metal | ER2209 (UNS E2209) or ER2594 (UNS E2594) |
| Process | Deep penetration TIG (GTAW) |
| Electrode | WTh2, 2.4-4.0 mm diameter |
| Shielding gas | 100% Ar or Ar + 5% CO2 |
| Backing gas | 100% Ar |
| Current range | 180-350 A |
| Travel speed | 150-600 mm/min |
| Preheat | 50-150 °C |
| Interpass temperature | Maximum 150 °C |
| Post-weld treatment | Solution annealing at 1050-1100 °C, water quench |
| NDT methods | PT, UT, RT (as applicable) |
Quality Control and Non-Destructive Testing
The quality control program for deep penetration TIG welding of super duplex stainless steel S32750 must be comprehensive and rigorous, given the criticality of these components in petrochemical applications. The following quality control measures are recommended:
Pre-Weld Quality Control
- Material verification: Verify the chemical composition and mechanical properties of the base material and filler metal through mill certificates and, if necessary, supplementary testing.
- Surface preparation: Clean the welding surfaces to remove oil, grease, oxide scale, and other contaminants using mechanical methods (grinding, brushing) and chemical cleaning (degreasing).
- Fit-up inspection: Verify the joint preparation, including groove geometry, root gap, and alignment. For deep penetration TIG welding, the root gap should be controlled within ±0.5 mm of the specified value.
- Backface protection: Install backing bars or backing rings and apply backing gas to prevent oxidation of the weld root.
In-Process Quality Control
- Parameter monitoring: Continuously monitor arc current, voltage, and travel speed to ensure they remain within the qualified range.
- Visual inspection: Inspect the weld bead appearance for uniformity, proper fusion, and absence of surface defects.
- Thermal monitoring: Use infrared thermography or thermocouples to monitor the interpass temperature and ensure it does not exceed 150 °C.
Post-Weld Quality Control
| NDT Method | Application | Acceptance Criteria |
|---|---|---|
| Visual Testing (VT) | All welds | No surface defects exceeding acceptance criteria |
| Dye Penetrant Testing (PT) | Surface cracks | No linear indications exceeding 3 mm |
| Ultrasonic Testing (UT) | Internal defects | No indications exceeding acceptance criteria per ASME V |
| Radiographic Testing (RT) | Internal defects | No porosity exceeding 1 mm, no cracks |
| Magnetic Particle Testing (MT) | Not applicable | S32750 is non-magnetic |
| Hardness Testing | HAZ and weld metal | Within specified range (typically 200-350 HV) |
| Intergranular Corrosion Test | Weld metal and HAZ | No intergranular attack per ASTM A262 Practice E |
Mechanical Property Requirements
The weld metal and HAZ must meet the following mechanical property requirements:
| Property | Requirement |
|---|---|
| Tensile strength | Minimum 620 MPa |
| Yield strength | Minimum 450 MPa |
| Elongation | Minimum 15% |
| Impact energy (Charpy V-notch) | Minimum 47 J at -40 °C |
| Hardness | 200-350 HV |
| Ferrite content (weld metal) | 35-65 F% |
Engineering Practice and Case Studies
The application of deep penetration TIG welding for super duplex stainless steel S32750 has been demonstrated in several petrochemical applications, including:
- Hydrogenation reactor internals: High-pressure, high-temperature hydrogen service requiring excellent resistance to hydrogen embrittlement and high-temperature corrosion.
- Sulphuric acid concentrators: Highly corrosive sulfuric acid service at elevated temperatures, requiring excellent resistance to intergranular corrosion.
- Chlor-alkali plant components: Chloride-containing environments requiring resistance to chloride-induced stress corrosion cracking.
- Offshore oil and gas platforms: Seawater exposure requiring resistance to chloride-induced pitting and crevice corrosion.
A notable case study involves the welding of a 25 mm thick S32750 super duplex stainless steel flange for a high-pressure hydrogenation reactor. The welding procedure involved a multi-pass deep penetration TIG weld with a root pass followed by fill and cap passes. The root pass was performed using deep penetration TIG with an arc current of 280 A, travel speed of 300 mm/min, and a root gap of 1.0 mm. The fill passes were performed using conventional TIG with an arc current of 220 A and travel speed of 200 mm/min. The cap pass was performed using TIG with an arc current of 180 A and travel speed of 150 mm/min.
The resulting weld exhibited a penetration depth of 22 mm (88% of the plate thickness) in the root pass, demonstrating the effectiveness of the deep penetration TIG process. The weld metal and HAZ microstructure showed a ferrite/austenite ratio of 45/55, which is within the acceptable range. Mechanical testing confirmed that the weld metal and HAZ met all specified requirements, including a Charpy impact energy of 62 J at -40 °C.
Study Insights and Reflections
The deep penetration TIG welding process for super duplex stainless steel S32750 represents a significant advancement in welding technology for high-performance stainless steel applications. The process offers several advantages over conventional TIG welding, including reduced number of passes, lower heat input, and improved penetration, all of which contribute to better control of the microstructure and mechanical properties.
One of the most important insights from this study is the critical importance of thermal management in welding super duplex stainless steels. The interpass temperature must be strictly controlled below 150 °C to prevent sigma phase formation and chromium carbide precipitation. This requires careful planning of the welding sequence and the use of thermal monitoring techniques such as infrared thermography or thermocouples embedded in the workpiece.
Another important consideration is the selection of the filler metal. While ER2209 (UNS E2209) is the most commonly used filler metal for S32750, it may not always be the optimal choice. ER2594 (UNS E2594) offers higher chromium and molybdenum content, providing superior corrosion resistance in highly aggressive environments, but it may be more susceptible to hot cracking. The selection of the filler metal should be based on a careful evaluation of the service conditions, mechanical requirements, and weldability considerations.
The study also highlights the importance of post-weld heat treatment (PWHT) for thick-section welds. Solution annealing at 1050-1100 °C followed by water quenching is recommended to dissolve any sigma phase or chromium carbides that may have formed during welding, restoring the full corrosion resistance and mechanical properties of the weld. However, PWHT must be performed with care to avoid excessive grain growth, which can reduce toughness.
Summary
This research on deep penetration TIG welding of super duplex stainless steel S32750 provides valuable guidance for engineers working on petrochemical and oil and gas applications that require high-performance stainless steel components. The study demonstrates that deep penetration TIG welding can achieve excellent weld quality with reduced heat input and fewer passes, provided that the welding parameters are carefully optimized and the thermal management is rigorously controlled. The key to successful application lies in the careful selection of filler metal, strict control of interpass temperature, comprehensive non-destructive testing, and appropriate post-weld heat treatment. For engineers designing and fabricating super duplex stainless steel components, this research offers a practical and proven welding technology that can meet the demanding requirements of modern petrochemical and energy industries.