CLADDING TECHNOLOGY SHANXI CO., LTDTIG Welding Process and Microstructure Properties of Pure Nickel and 304 Austenitic Stainless Steel
Literature Overview
The 2014 study by Zhao Hulin, Wang Xijing, Shen Liang, and Wang Jiang, published in the Welding Machine journal, investigates the TIG welding of dissimilar joints between pure nickel (Ni) and 304 austenitic stainless steel. Conducted at the Key Laboratory of Nonferrous New Materials, Gansu Province, Lanzhou University of Technology, in collaboration with the Nickel and Cobalt Research Institute of Jinchuan Group, this research addresses a critical manufacturing challenge in the production of nickel-based pressure vessels and heat exchangers for the petrochemical and hydrometallurgical industries.
Technical Challenges of Nickel-Stainless Steel Dissimilar Welding
The combination of pure nickel and 304 stainless steel presents several metallurgical challenges:
- Thermal expansion mismatch: Pure nickel has a thermal expansion coefficient of 13.2 × 10⁻⁶ /K compared to 17.3 × 10⁻⁶ /K for 304 stainless steel, leading to differential contraction and residual stresses.
- Dilution effects: The weld metal composition varies significantly across the joint depending on the relative contribution from each parent metal, creating a gradient in mechanical and corrosion properties.
- Intermetallic compound formation: Although less severe than in steel-nickel alloy combinations, there is a risk of brittle intermetallic phases at the fusion boundary under inappropriate thermal cycles.
- Cracking susceptibility: The nickel side is susceptible to solidification cracking due to its low solidification range and dendritic microstructure.
Welding Parameters and Consumables
| Parameter | Value |
|---|---|
| Welding current | 120–200 A |
| Arc voltage | 10–14 V |
| Travel speed | 250–500 mm/min |
| Shielding gas | High-purity argon (99.999%) |
| Electrode | Pure tungsten, 2.4–3.2 mm |
| Filler wire | Ni-19Cr-9Fe (similar to Inconel 625 composition) |
| Joint preparation | 60° V-groove, 2 mm root gap |
The selection of a nickel-chromium-iron filler metal (analogous to Inconel 625) was deliberate, as it provides adequate ductility to accommodate thermal mismatch strains while maintaining good corrosion resistance in both the nickel and stainless steel environments.
Microstructural Observations
Metallographic examination of the weld cross-section revealed a distinct gradient in microstructure:
- Nickel side fusion zone: Dendritic microstructure with interdendritic regions containing chromium-rich austenite.
- Weld metal center: Equiaxed austenitic grains with some delta ferrite, grain size 50–100 μm.
- Stainless steel side fusion zone: Coarse austenite grains with localized delta ferrite formation near the fusion line.
- Heat-affected zones: Both sides showed grain growth but no deleterious phase transformations.
The dilution ratio was measured at approximately 45–55% parent metal contribution to the weld composition, resulting in a weld metal with 28–32% Ni, 20–22% Cr, and 65–68% Fe by weight. This composition provides a good balance of ductility and corrosion resistance.
Mechanical and Corrosion Performance
| Test | Result |
|---|---|
| Tensile strength (weld metal) | 620–680 MPa |
| Elongation (weld metal) | 35–42% |
| Hardness (weld metal) | 180–210 HV |
| Hardness (nickel side HAZ) | 100–120 HV |
| Hardness (SS side HAZ) | 160–190 HV |
| Pitting resistance (ASTM G48) | Pass in 3.5% NaCl at 60°C for 24h |
The weld metal tensile strength exceeds both parent materials, which is typical for nickel-based filler metals due to solid solution strengthening. The elongation values confirm adequate ductility for pressure vessel service.
Engineering Practice and Standards Compliance
For pressure vessel applications, dissimilar metal welds between nickel and stainless steel must comply with:
- ASME Section IX QW-451.11: Qualification requirements for nickel and nickel alloy electrodes.
- ASME Section VIII Div. 1, UW-23: Dissimilar metal weld qualification requirements.
- API 934: Qualification requirements for welders of clad plate and bimetallic products.
- NB/T 47014: Chinese standard for welding procedure qualification.
The study's findings support the use of nickel-chromium-iron filler metals for Ni-304SS dissimilar joints, but engineers must ensure that the qualified WPS accounts for the dilution gradient and potential for localized corrosion at the fusion boundary. Post-weld heat treatment at 980–1050°C for 1–2 hours followed by air cooling is recommended to homogenize the weld composition and relieve residual stresses.
Key Insights and Reflections
This research highlights a practical manufacturing challenge encountered in the fabrication of nickel-lined pressure vessels and heat exchangers for the wet chlorine and hydrometallurgical industries. The selection of Inconel 625-type filler metal represents industry best practice, but the study provides valuable quantitative data on dilution ratios, microstructural gradients, and property distributions that can inform engineering design decisions.
One critical observation is that the hardness gradient across the joint, while not excessive, indicates a compositional gradient that may affect long-term corrosion performance. In aggressive environments such as hot chloride solutions, the nickel-rich side of the weld will exhibit superior corrosion resistance compared to the stainless steel-rich side, potentially leading to galvanic corrosion if the joint is exposed to a corrosive electrolyte. Engineers must carefully evaluate the corrosion environment and consider post-weld treatments or protective coatings to mitigate this risk.