Microstructure Formation in Hot-Wire TIG Welding Inconel 625 Overlay

Literature Overview and Research Background

This study note examines a technical paper investigating the microstructure formation in Inconel 625 (UNS N06625) overlay layers produced by hot-wire TIG welding on austenitic stainless steel substrates. Inconel 625 is a nickel-chromium-molybdenum superalloy containing 62% nickel, 22% chromium, 9% molybdenum, and 3% niobium, widely used for corrosion-resistant cladding in chemical processing, marine, and nuclear applications. The hot-wire TIG process introduces unique thermal cycling conditions that influence grain morphology, phase precipitation, and solidification behavior differently from conventional TIG or PTA cladding. Understanding these microstructural evolution mechanisms is essential for predicting overlay performance in aggressive environments.

Core Technical Findings

Thermal Cycle Characteristics

The hot-wire TIG process produces thermal cycles that differ significantly from conventional TIG welding. Peak temperatures in the weld pool reach approximately 1400-1500°C, with cooling rates in the range of 10-50°C/s depending on process parameters and substrate thermal mass. The preheated wire (typically 300-400°C) contributes additional heat to the weld pool, resulting in peak temperatures approximately 50-100°C higher than conventional TIG for equivalent arc current. The cooling rate is also influenced by the wire feed rate, with higher wire feed rates producing slower cooling due to increased heat input per unit length.

Grain Morphology and Orientation

Metallographic examination of hot-wire TIG Inconel 625 overlays reveals a distinct grain morphology evolution from the fusion line to the overlay surface. Near the fusion line, columnar grains grow epitaxially from the substrate, with grain orientation influenced by the thermal gradient direction. As the overlay thickness increases, the columnar-to-equiaxed transition (CET) occurs at approximately 1.5-2.5 mm from the fusion line, resulting in equiaxed grains in the upper layers. The grain size in the equiaxed region ranges from 30-50 μm, which is coarser than the 20-30 μm typically observed in conventional TIG overlays.

The CET is influenced by several factors, including cooling rate, solidification velocity, and grain refiner content. In the hot-wire TIG process, the reduced cooling rate and increased solidification velocity promote earlier CET compared to conventional TIG. The niobium content in Inconel 625, which forms NbC and Nb(N,C) particles, acts as a grain refiner and contributes to the equiaxed grain formation in the upper layers.

Phase Constitution and Precipitation

X-ray diffraction (XRD) analysis of hot-wire TIG Inconel 625 overlays confirms a single-phase face-centered cubic (FCC) γ matrix structure in the bulk of the overlay. However, near the fusion line, minor amounts of Laves phase (Mo-rich intermetallic) may be detected in regions with dilution rates exceeding 15%. The Laves phase forms due to the interaction between the high molybdenum content of Inconel 625 and the dilution from the stainless steel substrate, which introduces additional carbon and chromium.

In the heat-affected zone (HAZ) of the substrate, sensitization and carbide precipitation are concerns. For 304L stainless steel substrates, chromium carbide (Cr23C6) precipitation at grain boundaries is observed in the region experiencing temperatures between 800-1100°C. For 316L substrates, the lower carbon content reduces sensitization risk but does not eliminate it entirely. The hot-wire TIG process, with its higher thermal input, produces a wider sensitization zone compared to conventional TIG, necessitating careful consideration of post-weld heat treatment.

Dilution Effects on Microstructure

The dilution rate in hot-wire TIG Inconel 625 overlays ranges from 5-25% depending on the pass sequence and process parameters. The first pass on the base metal typically exhibits the highest dilution (15-25%), while subsequent passes show progressively lower dilution (5-12%). The dilution rate directly affects the microstructure through its influence on the solidification temperature range, cooling rate, and phase stability. Higher dilution rates promote the formation of columnar grains and increase the risk of Laves phase precipitation, while lower dilution rates favor equiaxed grains and a single-phase γ matrix.

Process-Structure-Property Relationships

Effect of Process Parameters on Microstructure

The study systematically varies arc current, wire feed speed, and travel speed to establish process-structure relationships. The key findings include:

1. Increasing arc current from 200 A to 300 A increases grain size from 30 μm to 50 μm and shifts the CET position from 1.5 mm to 2.5 mm from the fusion line.
2. Increasing wire feed speed from 3.0 m/min to 5.0 m/min reduces dilution from 20% to 10% and promotes equiaxed grain formation throughout the overlay.
3. Increasing travel speed from 400 mm/min to 800 mm/min increases cooling rate from 15°C/s to 45°C/s, resulting in finer grains but higher dilution due to reduced weld pool volume.

Mechanical Property Implications

The microstructural differences between hot-wire TIG and conventional TIG Inconel 625 overlays translate into measurable differences in mechanical properties. Hardness values in hot-wire TIG overlays range from 230-280 HV, compared to 250-300 HV for conventional TIG overlays. The slightly lower hardness is attributed to the coarser grain structure and potential minor Laves phase formation near the fusion line. Tensile strength of the overlay material is approximately 700-800 MPa, consistent with wrought Inconel 625 specifications.

Corrosion Resistance Assessment

Electrochemical polarization testing in 3.5% NaCl solution at room temperature shows that hot-wire TIG Inconel 625 overlays with dilution rates below 10% exhibit pitting corrosion potentials (Epit) of 0.3-0.5 V vs. SCE, comparable to wrought Inconel 625. Overlays with dilution rates exceeding 15% show reduced Epit values of 0.1-0.3 V, indicating compromised pitting resistance due to chromium and molybdenum depletion. Intergranular corrosion testing per ASTM G48 Practice A confirms that overlays with dilution below 12% pass the 5% sulfuric acid-copper sulfate test, while higher dilution levels may result in intergranular attack.

Quality Control and Process Optimization

Welding Procedure Qualification

The hot-wire TIG welding procedure for Inconel 625 overlay must be qualified per NB/T 47014 or ASME IX, Section IX, Qualification Standards. The qualification requirements include:

Multi-Pass Welding Strategy for Microstructure Control

A three-pass welding strategy is recommended for achieving optimal microstructure in hot-wire TIG Inconel 625 overlays:

1. First pass (bonding pass): Applied on the base metal with parameters optimized for maximum penetration and bond strength. Expected dilution: 15-25%. Grain structure: predominantly columnar.
2. Second pass (filler pass): Applied on the first pass with parameters optimized for low dilution and uniform composition. Expected dilution: 8-15%. Grain structure: columnar-to-equiaxed transition.
3. Third pass (cap pass): Applied on the second pass with parameters optimized for surface quality and minimum dilution. Expected dilution: 3-8%. Grain structure: predominantly equiaxed.

The interpass temperature should be maintained at 100-200°C to prevent excessive grain growth while ensuring adequate weldability. Post-weld heat treatment at 1050°C for 1 hour followed by air cooling is recommended for overlays exceeding 3 mm thickness to relieve residual stresses and dissolve any Laves phase that may have formed near the fusion line.

Integration with Engineering Practice

Application in Chemical Processing Equipment

Hot-wire TIG Inconel 625 overlays are widely specified for chemical processing equipment, including heat exchangers, reactors, and piping systems exposed to aggressive media such as hydrochloric acid, sulfuric acid, and chloride-containing environments. The microstructural understanding gained from this study directly informs the selection of welding parameters to ensure that the overlay microstructure is optimized for corrosion resistance in the specific service environment. For example, in chloride-containing environments, the absence of Laves phase is critical to prevent localized corrosion, which requires dilution control below 10% in the upper layers.

Comparison with PTA Cladding

Compared to plasma transferred arc (PTA) cladding, hot-wire TIG produces coarser grains and slightly higher dilution rates. However, hot-wire TIG offers significant advantages in terms of equipment cost, process flexibility, and suitability for field applications. For applications requiring extremely low dilution (below 5%) and fine-grained microstructure, PTA remains the preferred method. For applications where dilution of 5-15% is acceptable and equipment cost is a consideration, hot-wire TIG provides an economically viable alternative.

Key Questions and Reflections

The study highlights several areas requiring further investigation. The long-term stability of the microstructure under thermal cycling conditions, particularly in high-temperature service, is not fully characterized. The effect of hot-wire TIG residual stress on creep fatigue behavior in high-temperature applications warrants further study. Additionally, the influence of substrate material (304L vs. 316L vs. 321) on the microstructure evolution of the overlay layer is an area that merits more systematic investigation.

Study Insights and Implications

The hot-wire TIG welding of Inconel 625 overlays produces microstructures that are generally suitable for corrosion-resistant applications, provided that dilution is controlled below 15% and the multi-pass welding strategy is properly executed. The coarser grain structure compared to conventional TIG is a trade-off for the significantly improved deposition rate, and this trade-off is generally acceptable for most engineering applications. The key engineering implication is that process parameter optimization, particularly the wire feed speed to travel speed ratio, is the primary lever for controlling dilution and, consequently, microstructure and corrosion performance. Engineers should prioritize dilution control in WPS development and qualification testing, and should consider post-weld heat treatment for overlays exceeding 3 mm thickness to ensure long-term microstructural stability.