Zhuojin Pipe Fitting Co., LtdHot Wire TIG Hardfacing of UNS S32750 Super Duplex Stainless Steel
Literature Overview
This 2021 paper by Wang Chun and colleagues, published in Pressure Vessel journal, investigates the feasibility and methodology of hot wire TIG (HW-TIG) hardfacing for UNS S32750 super duplex stainless steel (SDSS). The collaboration between Harbin Weil Welding and Lanzhou Lanshi Heavy Equipment, supported by the Gansu Provincial Key Laboratory for Special Material Welding of Pressure Vessels, addresses a critical need in the pressure vessel and process equipment industry—repair and overlay of SDSS components that operate in aggressive chloride-containing environments.
Material Background and Welding Challenges
UNS S32750 (equivalent to Zeron 100 or Sandvik 254 SMO in some contexts, though strictly UNS S32750 refers to a specific super duplex grade) represents the highest-performing duplex stainless steel available commercially. Its nominal composition includes approximately 25% Cr, 7% Ni, 3% Mo, and 0.25% N, providing exceptional resistance to chloride stress corrosion cracking (SCC), pitting, and crevice corrosion.
The welding of super duplex stainless steels presents unique challenges:
| Challenge | Technical Requirement | Consequence if Not Met |
|---|---|---|
| Phase balance maintenance | 40–60% ferrite/austenite ratio | Excess ferrite leads to 475°C embrittlement; excess austenite increases SCC susceptibility |
| Nitrogen retention | Minimize nitrogen loss during welding | Loss of N reduces pitting resistance number (PREN) |
| Heat input control | Moderate to low heat input | Excessive heat input causes intermetallic phase precipitation (sigma, chi) |
| Dilution management | Control base metal dilution | High dilution can shift phase balance unfavorably |
Hot Wire TIG Process Characteristics
The hot wire TIG process introduces a consumable wire directly into the arc zone while the tungsten electrode remains non-consumable. This configuration offers several advantages for SDSS hardfacing:
- Higher deposition rates: The preheated wire melts more efficiently, achieving deposition rates 1.5–2.5 times greater than conventional GTAW.
- Reduced heat input to base metal: Despite higher deposition rates, the base metal receives less total heat because energy is concentrated on melting the wire rather than the workpiece.
- Improved weld bead geometry: The continuous wire feed into the arc produces smoother, more uniform bead profiles with reduced spatter.
- Better phase balance control: The lower and more concentrated heat input allows better retention of the 40–60% ferrite ratio critical for SDSS properties.
Welding Procedure Development
The welding parameters developed for UNS S32750 HW-TIG hardfacing include:
| Parameter | Typical Value | Rationale |
|---|---|---|
| Arc current | 100–180 A | Sufficient for wire melting without excessive base metal penetration |
| Wire feed rate | 3.5–5.5 m/min | Matched to arc current for complete wire melting |
| Travel speed | 150–250 mm/min | Controls heat input and bead width |
| Shielding gas | 99.99% Ar or Ar/2% N2 | High purity required; small N2 addition may help retain N in deposit |
| Wire preheat temperature | 200–350°C | Ensures stable arc and efficient melting |
| Interpass temperature | ≤150°C | Prevents excessive grain growth and phase transformation |
| Post-weld heat treatment | Solution anneal at 1050°C, water quench | Restores equilibrium phase balance |
Performance Verification
The comprehensive testing program evaluated:
- Chemical composition: Confirmed that the deposited alloy composition matches UNS S32750 specification, with nitrogen content maintained above 0.20% to ensure adequate PREN (≥42).
- Mechanical properties: Tensile strength exceeding 620 MPa, yield strength above 450 MPa, and elongation greater than 25%, meeting ASTM A928/A928M requirements for SDSS.
- Metallographic examination: Ferrite content maintained within the 40–60% range, with no evidence of intermetallic phase precipitation (sigma, chi, or Laves phase) in the weld metal or HAZ.
- Corrosion resistance: Pitting corrosion potential (Epit) exceeding +600 mV vs. SCE in 3.5% NaCl solution, demonstrating superior resistance to chloride attack. Linear polarization resistance (LPR) testing confirmed adequate general corrosion resistance.
Engineering Practice Integration
For pressure vessel manufacturers working with SDSS components, this research provides a validated welding procedure that addresses several practical concerns:
- Repair applications: When SDSS pressure vessel components suffer localized corrosion damage, HW-TIG hardfacing provides an effective repair method that maintains the material's full corrosion resistance without requiring complete component replacement.
- Wear-resistant overlays: In process equipment where SDSS components also experience mechanical wear (e.g., pump internals in chlorinated service), hardfacing with SDSS provides both corrosion and wear protection in a single overlay.
- Joint qualification: The procedure developed can serve as the basis for welding procedure qualification (WPQ) under ASME Section IX or EN ISO 15614-1, enabling production welding with confidence.
- Cost considerations: HW-TIG hardfacing of SDSS is significantly more economical than replacing entire components, particularly for large-diameter pressure vessels where the cost of replacement can exceed hundreds of thousands of dollars.
Key Questions and Reflections
The research raises important questions for further investigation. The long-term stability of the phase balance in HW-TIG deposited SDSS under service conditions (particularly at temperatures between 300–500°C where 475°C embrittlement can occur) warrants extended aging studies. Additionally, the effect of multiple hardfacing layers on the cumulative heat input and subsequent phase balance requires systematic investigation for thick overlay applications.
Study Insights and Implications
This work demonstrates that HW-TIG is a technically viable and economically attractive method for hardfacing UNS S32750 super duplex stainless steel. The process's ability to maintain the critical 40–60% phase balance while achieving high deposition rates makes it particularly suitable for repair and overlay applications in the pressure vessel industry. For engineers specifying welding procedures for SDSS components, this research validates HW-TIG as a preferred process, provided that post-weld solution heat treatment is available to ensure final phase equilibrium. The integration of this technology into production workflows requires investment in specialized HW-TIG equipment and training, but the resulting quality improvements and cost savings in repair operations justify the capital expenditure for organizations with significant SDSS component inventories.