MIG Vertical-Down Overlay Welding of Inconel 625

Literature Overview

This study addresses the challenges of depositing Inconel 625 (UNS N06625) overlay using gas metal arc welding (GMAW/MIG) in the vertical-down position. The vertical-down position is inherently challenging because gravity causes the molten weld pool to sag, leading to poor bead profile, excessive dilution, and increased susceptibility to porosity. The study demonstrates that with careful parameter selection and technique, high-quality Inconel 625 overlays can be achieved in this position, opening up new possibilities for repairing large vertical components without repositioning.

Core Technical Points

Inconel 625 Material Properties

Inconel 625 is a nickel-chromium-molybdenum alloy with exceptional corrosion resistance, high strength at elevated temperatures, and excellent weldability. Its key characteristics are:

The low thermal conductivity of Inconel 625 (approximately 12 W/m·K compared to 50 W/m·K for carbon steel) means that heat accumulates in the weld zone, leading to higher temperatures and longer solidification times. This is particularly problematic in the vertical-down position, where the combination of low thermal conductivity and gravitational sagging creates severe challenges.

Process Challenges in Vertical-Down Position

The vertical-down position presents several unique challenges for Inconel 625 overlay welding:

1. Gravitational sagging: The molten weld pool tends to flow downward, creating an elongated, sagging bead profile. This is exacerbated by the low thermal conductivity of Inconel 625, which slows heat dissipation and keeps the pool molten for longer.
2. Excessive dilution: The sagging pool increases the contact time between the molten overlay and the base metal, leading to higher dilution. For Inconel 625 overlays, dilution above 30% can significantly reduce corrosion resistance and mechanical properties.
3. Porosity: The vertical-down position traps gas bubbles in the molten pool as they attempt to escape against gravity. This leads to increased porosity, particularly in the lower portion of the weld bead.
4. Heat input control: Maintaining consistent heat input is difficult because the weld pool geometry changes continuously as the weld progresses downward.

Welding Parameter Optimization

The study systematically varies welding parameters to identify the optimal window for vertical-down Inconel 625 overlay welding.

The key finding is that the heat input must be kept below 0.9 kJ/mm to prevent excessive sagging and porosity. This is achieved through a combination of moderate current, moderate voltage, and relatively fast travel speed. The use of a 1.0 mm wire instead of the more common 1.2 mm wire reduces the deposited metal rate and allows better control of the molten pool size.

Shielding Gas Selection

The selection of shielding gas is critical for vertical-down welding. The study evaluates three gas mixtures:

The addition of 2% CO2 to the shielding gas mixture serves multiple purposes: it increases the arc stability, promotes a more compact arc, and enhances the wetting characteristics of the weld pool. The CO2 also slightly increases the dilution due to the increased arc energy density, but this is offset by the improved bead profile and reduced porosity.

Welding Technique

The vertical-down welding technique requires specific skills and attention to detail:

1. Gun angle: The torch is held at a 5–15 degree leading angle, pointing slightly upward. This directs the arc force upward, counteracting the gravitational sagging of the molten pool.
2. Travel speed: A consistent travel speed of 6–7 cm/min is maintained. The welder must resist the temptation to slow down at the top of the weld where the pool appears to be sagging.
3. Weave pattern: A slight weave (2–3 mm amplitude) is used to distribute heat evenly across the joint. The weave is synchronized with the travel speed to maintain a consistent bead width.
4. Arc length control: The arc length is maintained at 3–4 mm (1.2–1.6 times the wire diameter). A longer arc increases porosity and dilution, while a shorter arc causes spatter and poor penetration.
5. Start and stop technique: Starting and stopping are the most critical moments. The welder must apply slightly higher current at the start to ensure good penetration, then reduce to the normal current. At the stop, the current is reduced gradually to prevent a crater crack.

Microstructural and Mechanical Characterization

Microstructure

The microstructure of the vertical-down Inconel 625 overlay exhibits a columnar dendritic structure growing from the fusion boundary toward the surface. The dendrite arm spacing is approximately 20–35 μm, which is slightly coarser than in the horizontal or flat position due to the lower cooling rate associated with the vertical-down position.

The interdendritic regions contain Nb-rich carbides (NbC) and Laves phase (Ni2MoSi), which are characteristic of Inconel 625 welds. The volume fraction of these phases is approximately 5–8% in the as-welded condition. No significant porosity is observed in the optimized welds, confirming the effectiveness of the parameter selection.

At the overlay-base interface, a thin transition zone of 50–100 μm is observed. This zone contains a mixture of Inconel 625 dendrites and base metal phases, with a gradual composition gradient. The transition zone is free of cracks and indicates good metallurgical bonding.

Mechanical Properties

The hardness gradient from the surface to the interface reflects the dilution profile. The surface hardness of 200–230 HV is within the expected range for Inconel 625, while the interface hardness of 250–300 HV indicates some dilution from the base metal. The peel test results confirm good bond strength between the overlay and the base metal.

Dilution Analysis

The dilution profile shows that the first 0.6 mm of the overlay maintains Ni and Cr content above 50% and 18% respectively, which is sufficient for good corrosion resistance. The dilution increases with depth, reaching 35% at the interface. This is acceptable for most applications but would be unacceptable for applications requiring full Inconel 625 corrosion resistance throughout the overlay thickness.

Key Reflections and Study Insights

The most significant contribution of this study is the demonstration that high-quality Inconel 625 overlay welding is achievable in the vertical-down position with proper parameter selection and technique. This has important practical implications for the repair of large vertical components such as heat exchanger tubes, reactor internals, and pressure vessel shells, where repositioning for horizontal or flat welding is impractical or impossible.

The finding that heat input must be kept below 0.9 kJ/mm is a critical process window that must be communicated to welders and welding engineers. The temptation to use higher current and slower travel speed to improve penetration must be resisted, as it leads to excessive sagging and porosity.

The importance of shielding gas selection cannot be overstated. The addition of just 2% CO2 to the argon shielding gas makes a dramatic difference in weld quality, reducing porosity and improving bead profile. This is a simple and cost-effective improvement that should be standard practice for vertical-down nickel alloy welding.

From a standards perspective, the study highlights the need for qualification procedures that specifically address vertical-down welding. Standard welding procedure specifications (WPS) developed for horizontal or flat position welding may not be directly applicable to vertical-down welding, and separate qualification may be required per ASME IX or ISO 15614.

The study also underscores the importance of welder skill and technique in achieving consistent results. Even with optimal parameters, a welder who does not understand the unique challenges of vertical-down welding will produce inferior results. Training and certification programs should include specific modules on vertical-down welding of nickel-based alloys.