CLADDING TECHNOLOGY SHANXI CO., LTDArc Surface Modification of TIG Weld Joints in 1Cr18Ni9Ti Stainless Steel
Literature Overview
The study by Wang Qiming and Luo Wei, conducted jointly by Hangzhou Oxygen Plant Group and Zhejiang University in 1999, addresses a critical practical challenge in the fabrication of pressure vessels and heat exchangers using 1Cr18Ni9Ti (equivalent to ASTM SA-321 or UNS S32100). This austenitic stainless steel, stabilized with titanium to prevent intergranular sensitization, is widely employed in cryogenic oxygen service, petrochemical reactors, and hydrogenation equipment where both corrosion resistance and structural integrity are paramount. The research focuses on using TIG arc processing to modify the surface characteristics of completed weld joints, improving their resistance to localized corrosion and stress corrosion cracking.
Core Technical Content
The fundamental premise of this work is that weld joints in stabilized austenitic stainless steels, while structurally sound, may exhibit microstructural inhomogeneities that compromise long-term corrosion performance. The weld metal, heat-affected zone, and base metal transition regions can develop chromium depletion, carbide precipitation at grain boundaries, and residual stress concentrations that serve as initiation sites for pitting, crevice corrosion, and chloride stress corrosion cracking.
The TIG arc surface modification technique involves re-melting the surface layer of a completed weld joint under controlled parameters to achieve the following objectives:
| Modification Parameter | Typical Range | Purpose |
|---|---|---|
| Arc current | 80–150 A | Controlled surface remelting without excessive penetration |
| Travel speed | 3–8 mm/min | Ensure adequate heat input for homogenization |
| Shielding gas flow | 12–20 L/min (Ar) | Prevent oxidation of the remelted surface |
| Pulse frequency | 50–150 Hz (if pulsed) | Control heat input and bead geometry |
| Surface remelt depth | 0.3–1.0 mm | Target the critical surface zone without affecting bulk properties |
The modification process effectively dissolves any chromium carbide precipitates that may have formed during the original welding cycle, redistributes alloying elements to a more homogeneous state, and relieves near-surface residual stresses through thermal cycling. The titanium stabilizer in 1Cr18Ni9Ti forms TiC preferentially over Cr23C6, but at elevated temperatures near the weld, some chromium depletion can still occur, particularly in the coarse-grained HAZ.
Engineering Practice and Standards Compliance
In the context of pressure vessel fabrication governed by GB/T 150 or ASME Section VIII Division 1, the surface modification of weld joints serves as a supplementary treatment rather than a replacement for proper weld procedure qualification. The following considerations are essential:
- Procedure qualification: The surface modification process must be qualified under NB/T 47014 or ASME IX, with a separate welding procedure specification (WPS) documenting the modification parameters.
- Non-destructive examination: Post-modification inspection via dye penetrant testing (PT) or magnetic particle testing (MT) is mandatory to detect any surface cracks or lack of fusion introduced during the remelting process.
- Mechanical property verification: The modified surface layer must retain adequate hardness (typically below 250 HV for 321 stainless steel) and ductility to avoid introducing brittle phases.
- Corrosion testing: Intergranular corrosion testing per ASTM A263 or ASTM A276 should be performed on coupon samples to verify that the modification has not exacerbated sensitization.
A notable engineering challenge is maintaining the dimensional accuracy of the vessel shell after surface modification. In thin-walled cryogenic vessels, where wall thicknesses may range from 3 to 8 mm, excessive heat input during modification can cause distortion. The recommended approach is to use low-current, high-speed pulsed TIG with precise gas shielding to limit the thermal affected zone to less than 2 mm from the original weld bead surface.
Key Insights and Reflections
This 1999 study predates the widespread adoption of advanced surface treatment technologies such as laser remelting and plasma arc surfacing. However, the fundamental metallurgical principles remain valid and are still applied in practice today. The use of TIG arc modification is particularly cost-effective for field repair applications and for treating weld joints in existing pressure vessels where access is limited. The approach aligns with the PDCA cycle: Plan the modification parameters based on metallurgical analysis, Do the modification under controlled conditions, Check the results through NDE and corrosion testing, and Act by adjusting parameters for subsequent joints.
The research also highlights an important distinction between weld repair and weld modification. Repair addresses defects, while modification enhances performance characteristics. In pressure vessel codes, these are treated differently, and the distinction must be clearly documented in the fabrication records. The engineering value of this work lies in its demonstration that a simple, low-cost arc process can significantly improve the corrosion resistance of critical weld joints without requiring specialized equipment beyond a standard TIG welding station.