CLADDING TECHNOLOGY SHANXI CO., LTDMIG Vertical-Down Overlay Welding of Inconel 625 Process Study
Literature Overview and Research Background
This study investigates the gas metal arc welding (GMAW/MIG) vertical-down overlay welding process for depositing Inconel 625 cladding on carbon steel and low-alloy steel substrates. Vertical-down welding is a specialized technique where the weld is deposited on a vertical surface with the arc moving from top to bottom, relying on gravity to assist metal deposition rather than fighting against it. This technique is particularly valuable for repairing and cladding large-diameter pressure vessels, storage tanks, heat exchanger shells, and columnar structures where conventional vertical-up or horizontal welding would require extensive fixtures and multi-pass procedures.
Inconel 625 (UNS N06625) is a nickel-chromium-molybdenum alloy with excellent resistance to corrosion, oxidation, and stress corrosion cracking. It is widely used as an overlay material for pressure vessels and heat exchangers in the chemical, petrochemical, and nuclear industries. However, Inconel 625 overlay welding presents unique challenges: high thermal conductivity of the nickel-based filler leads to excessive heat input requirements, the narrow solidification range makes it susceptible to hot cracking, and the high dilution with carbon steel substrate can lead to intermetallic compound formation and reduced corrosion resistance of the overlay.
The combination of vertical-down welding with Inconel 625 overlay creates additional process challenges related to weld pool control, spatter management, and deposition efficiency in the vertical-down orientation.
Core Technical Points and Process Parameters
The vertical-down MIG overlay process requires careful optimization of welding parameters to achieve sound, continuous deposits with acceptable dilution and microstructure. The process typically uses a solid Inconel 625 wire (ERNiCrMo-3) or a flux-cored wire with Inconel 625 composition, with argon or argon/CO₂ shielding gas.
| Parameter | Typical Range | Effect on Weld Quality |
|---|---|---|
| Welding current | 180–280 A | Higher current → deeper penetration, higher dilution |
| Arc voltage | 22–28 V | Controls arc length and wire feed stability |
| Travel speed | 15–40 cm/min | Lower speed → higher heat input, thicker beads |
| Wire feed speed | 4–8 m/min | Must match current for stable arc |
| Stick-out length | 12–18 mm | Affects arc stability and gas shielding |
| Shielding gas | 100% Ar or 98% Ar + 2% CO₂ | Ar gives better wetting; CO₂ increases spatter |
| Preheat temperature | 100–200°C | Reduces cracking risk, increases dilution |
| Interpass temperature | ≤250°C | Prevents excessive grain growth in overlay |
The dilution rate in vertical-down Inconel 625 overlay is a critical quality parameter. Typical dilution rates range from 20–40% for single-pass deposits, which can be reduced to 15–25% by using stringer beads with lower current and higher travel speed. The dilution rate directly affects the corrosion resistance of the overlay: higher dilution introduces more iron and carbon into the nickel matrix, promoting the formation of chromium carbides (M₂₃C₆) and reducing the pitting resistance equivalent number (PREN).
The weld pool in vertical-down orientation behaves differently from horizontal or vertical-up positions. Gravity assists the molten metal in flowing downward along the substrate, which can lead to:
- Thicker deposits at the bottom of the weld pool due to gravity accumulation
- Reduced spatter compared to vertical-up (gravity pulls spatter away from the arc)
- Better wetting of the substrate surface due to downward metal flow
- Risk of sagging if the current is too high or travel speed too low
Microstructure and Properties of Inconel 625 Overlay
The microstructure of the Inconel 625 overlay deposited by vertical-down MIG typically consists of a columnar dendrite structure growing from the fusion line, with equiaxed grains in the center of thicker deposits. The primary phases are FCC γ-Ni solid solution and δ-Ni₃Nb intermetallic. The δ phase forms preferentially at grain boundaries and can reach 5–15 vol% depending on the cooling rate and dilution.
The dilution zone (the region where Inconel 625 mixes with the base metal) is of particular concern. At dilution rates above 30%, the following detrimental phases may form:
- M₂₃C₆ chromium carbide: Forms when carbon from the substrate diffuses into the overlay, depleting chromium and reducing corrosion resistance.
- Laves phase (Mo-rich): Forms at high dilution rates, particularly with low-alloy steels containing Mo.
- Sigma phase (Fe-Ni-Cr): Forms during prolonged exposure at 600–900°C, embrittling the overlay.
The mechanical properties of the Inconel 625 overlay are:
| Property | Overlay (Low Dilution) | Overlay (High Dilution) | Substrate (16Mn) |
|---|---|---|---|
| Hardness (HV) | 220–280 | 280–350 | 180–220 |
| Tensile strength (MPa) | 620–720 | 550–650 | 450–550 |
| Yield strength (MPa) | 310–380 | 280–350 | 270–345 |
| Elongation (%) | 35–50 | 25–40 | 20–25 |
| PREN | 24–26 | 18–22 | 5–8 |
Engineering Practice and Quality Control
For pressure vessel applications, the Inconel 625 vertical-down overlay must comply with relevant standards:
- NB/T 47014: Weld procedure qualification requires demonstration of mechanical properties, macrostructure, and corrosion resistance of the overlay.
- GB/T 150: The overlay thickness, coverage, and bond strength must meet the design requirements.
- ASME VIII Div.1 / API 934: For API 934 compliance, the overlay must meet specific thickness, coverage, and inspection requirements.
- JB/T 4730: NDT requirements include UT for overlay thickness and bond integrity, and PT or MT for surface defects.
The quality control procedure for vertical-down Inconel 625 overlay includes:
- Pre-weld inspection: Surface preparation (grinding to bare metal), substrate cleanliness verification, and preheat application.
- In-process monitoring: Visual inspection of bead geometry, spatter level, and arc stability. Welding parameters should be recorded and maintained within qualified ranges.
- Post-weld inspection: UT scanning for bond integrity and overlay thickness, PT for surface cracks, and mechanical testing of qualification coupons.
- Final verification: Hydrostatic pressure test at 1.25× design pressure for pressure vessels.
Common defects in vertical-down Inconel 625 overlay include:
- Hot cracking: Caused by the narrow solidification range of Inconel 625 and the high sulfur/phosphorus content of the substrate. Countermeasures include limiting substrate S and P content, using low-carbon filler wire, and maintaining appropriate interpass temperature.
- Undercut: Occurs at the trailing edge of the vertical-down weld due to gravity-assisted metal flow. Countermeasures include reducing current, increasing travel speed, or using a backing bar.
- Excessive spatter: Can be reduced by using pure argon shielding, optimizing stick-out length, and using a slightly higher travel speed.
- Porosity: Caused by inadequate gas shielding or contamination. Countermeasures include increasing gas flow rate, ensuring proper shielding coverage, and cleaning the substrate thoroughly.
Study Insights and Implications
The vertical-down MIG overlay of Inconel 625 represents a practical solution for cladding large vertical surfaces on pressure vessels and storage tanks where conventional welding positions are impractical. The key to success lies in managing the dilution rate through parameter optimization and stringer bead techniques, while ensuring adequate gas shielding and maintaining interpass temperatures below 250°C. The process offers significant advantages in terms of deposition efficiency and reduced labor compared to vertical-up or horizontal welding, but requires careful attention to weld pool control and dilution management. For critical applications such as hydrogenation reactors or acid storage tanks, the overlay qualification should include intergranular corrosion testing (ASTM A263/A264) and pitting resistance testing (ASTM G48) to verify that the dilution-controlled overlay maintains the required corrosion resistance. The study reinforces the principle that process parameter optimization must always consider the interplay between dilution control, microstructure evolution, and final service performance.