Microstructure Formation in Hot-Wire TIG Welding Inconel 625 Overlay Layer

Literature Overview and Research Background

This study note presents a focused examination of a technical paper on the microstructure formation mechanisms in Inconel 625 overlay layers produced by hot-wire TIG welding on carbon steel substrates. While similar in subject matter to the previous study on stainless steel substrates, this paper specifically addresses the carbon steel substrate case, which presents distinct metallurgical challenges including higher dilution rates, increased risk of martensite formation in the heat-affected zone, and greater susceptibility to hydrogen-induced cracking. The paper provides detailed metallographic analysis, phase identification, and mechanical property characterization of the overlay layer, with particular emphasis on the fusion line region where the most critical microstructural transitions occur.

Core Technical Findings

Substrate Influence on Overlay Microstructure

The choice of carbon steel substrate (Q345R, equivalent to ASTM A516 Gr. 70) significantly influences the overlay microstructure compared to stainless steel substrates. The lower thermal conductivity and higher carbon content of carbon steel result in higher dilution rates (15-30% for the first pass) and more pronounced microstructural gradients across the overlay thickness. The fusion line region exhibits a complex microstructure consisting of a mixture of austenite, martensite, and carbide phases, with the exact proportions depending on the cooling rate and dilution level.

Fusion Line Microstructure Analysis

The fusion line region is the most critical area for microstructural integrity and corrosion resistance. In hot-wire TIG Inconel 625 overlays on carbon steel, the fusion line exhibits a gradient microstructure transitioning from the carbon steel substrate through a dilution zone to the overlay material. The dilution zone, typically 0.2-0.5 mm thick, contains a mixture of austenite and martensite due to the rapid cooling and high carbon equivalent of the dilution zone. This martensitic region can be brittle and susceptible to cracking, particularly under thermal cycling conditions.

The study recommends that the first pass be designed to minimize the thickness of the dilution zone through careful control of penetration depth. A shallow penetration strategy, achieved by using lower arc current and higher wire feed speed, produces a thinner dilution zone with reduced martensite content. The recommended first-pass parameters for carbon steel substrates are: arc current 180-220 A, wire feed speed 4.0-5.0 m/min, travel speed 500-700 mm/min, and wire preheat temperature 300-350°C.

Phase Evolution and Precipitation Behavior

The phase evolution in hot-wire TIG Inconel 625 overlays on carbon steel follows a distinct pattern from the stainless steel substrate case. In the upper layers of the overlay (beyond the CET position), the microstructure is predominantly single-phase FCC γ austenite, consistent with wrought Inconel 625. However, near the fusion line, the dilution from the carbon steel substrate introduces carbon, manganese, and silicon, which promote the formation of carbide phases (NbC, TiC, and Cr7C3) and, in some cases, Laves phase (Mo-rich intermetallic).

The Laves phase is of particular concern because it is brittle and can serve as a crack initiation site under thermal cycling or mechanical loading. The study identifies a critical dilution threshold of approximately 20% above which Laves phase formation becomes significant. This threshold is lower than for stainless steel substrates due to the higher carbon content of carbon steel, which promotes carbide and intermetallic phase formation.

Grain Orientation and Solidification Pattern

The solidification pattern in hot-wire TIG Inconel 625 overlays on carbon steel is characterized by a clear columnar-to-equiaxed transition. Near the fusion line, columnar grains grow epitaxially from the substrate, with grain orientation perpendicular to the fusion line. The CET occurs at approximately 2.0-3.0 mm from the fusion line, which is deeper than for stainless steel substrates due to the higher thermal conductivity contrast between the overlay and the carbon steel substrate. The equiaxed grain region extends from the CET position to the overlay surface, with grain sizes of 35-55 μm.

The grain orientation in the columnar region is influenced by the thermal gradient, which is steeper for carbon steel substrates due to their lower thermal conductivity. This steeper thermal gradient promotes more pronounced columnar grain growth and delays the CET, resulting in a larger columnar grain fraction in the overlay. The engineering implication is that the columnar grain region is more susceptible to transverse cracking, particularly under thermal cycling conditions, because the grain boundaries are aligned perpendicular to the primary stress direction.

Process-Structure-Property Relationships

Effect of Thermal Input on Microstructure

The thermal input, defined as the heat input per unit length of weld (Q = V × I × η / v, where V is arc voltage, I is arc current, η is arc efficiency, and v is travel speed), is a primary factor governing microstructure in hot-wire TIG welding. The study investigates thermal inputs in the range of 10-25 kJ/mm, with the following observations:

1. Low thermal input (10-14 kJ/mm): Produces fine columnar grains (25-35 μm) near the fusion line with a shallow CET position (1.5-2.0 mm). Higher cooling rates promote equiaxed grain formation but increase dilution.
2. Moderate thermal input (14-19 kJ/mm): Produces medium-sized grains (35-45 μm) with a CET position of 2.0-2.5 mm. This is the recommended range for most applications.
3. High thermal input (19-25 kJ/mm): Produces coarse grains (45-60 μm) with a deep CET position (2.5-3.0 mm). Excessive thermal input increases the risk of Laves phase formation and reduces mechanical properties.

Mechanical Property Characterization

The mechanical properties of hot-wire TIG Inconel 625 overlays on carbon steel show significant variation across the overlay thickness, reflecting the microstructural gradients. Hardness measurements reveal values of 280-320 HV near the fusion line (due to martensite and carbide formation) decreasing to 230-260 HV in the upper layers (single-phase austenite). Tensile strength of the upper layers is approximately 700-800 MPa, consistent with wrought Inconel 625 specifications, while the fusion line region shows reduced tensile strength of 550-650 MPa due to the brittle martensitic microstructure.

The bond strength between the overlay and the substrate is a critical performance parameter. The study reports bond strength values of 85-95% of the base metal tensile strength, measured per ASTM G123 using a bond tensile test. The bond strength is primarily governed by the quality of the fusion line, with clean, well-bonded fusion lines achieving bond strengths above 90% of the base metal strength.

Corrosion Resistance and Microstructure Correlation

Corrosion resistance testing reveals a strong correlation between dilution rate and pitting corrosion resistance. Overlays with dilution rates below 12% exhibit pitting corrosion potentials of 0.3-0.5 V vs. SCE in 3.5% NaCl solution, comparable to wrought Inconel 625. Overlays with dilution rates of 15-25% show reduced pitting potentials of 0.0-0.2 V, indicating significant degradation of pitting resistance. The presence of Laves phase near the fusion line further reduces corrosion resistance by creating galvanic couples with the surrounding austenite matrix.

Quality Control and Process Optimization

Critical Process Parameters for Microstructure Control

The study identifies the following critical process parameters for controlling overlay microstructure in hot-wire TIG welding of Inconel 625 on carbon steel:

Post-Weld Heat Treatment Considerations

Post-weld heat treatment is strongly recommended for hot-wire TIG Inconel 625 overlays on carbon steel, particularly when the overlay thickness exceeds 2 mm or when the dilution rate exceeds 15%. The recommended heat treatment is solution annealing at 1050°C for 1 hour followed by air cooling. This treatment dissolves any Laves phase that may have formed near the fusion line, relieves residual stresses, and homogenizes the composition across the overlay thickness. The heat treatment does not significantly affect the microstructure of the upper layers, which are already in a single-phase austenitic condition.

However, heat treatment of the entire assembly (overlay + substrate) must be carefully evaluated for carbon steel substrates, as the high-temperature exposure may affect the substrate properties. For thick-walled vessels, the heat treatment temperature and time must be optimized to avoid excessive grain growth in the substrate HAZ while ensuring complete dissolution of Laves phase in the overlay.

Integration with Engineering Practice

Application in Carbon Steel Vessel Cladding

Hot-wire TIG Inconel 625 overlays on carbon steel substrates are widely used in the fabrication of pressure vessels, heat exchangers, and piping systems where the substrate provides structural strength and the overlay provides corrosion resistance. The microstructural understanding gained from this study is directly applicable to the development of welding procedure specifications (WPS) for these applications. The key engineering requirements are: (1) dilution control below 15% in the upper layers, (2) absence of Laves phase in the overlay, (3) bond strength exceeding 90% of the base metal strength, and (4) intergranular corrosion resistance meeting ASTM G48 requirements.

Comparison with Alternative Processes

When compared with submerged arc welding (SAW) overlay, hot-wire TIG produces cleaner welds with less slag inclusion and better control of dilution. However, SAW overlay achieves higher deposition rates (8-12 kg/h vs. 3-5 kg/h for hot-wire TIG), making it more suitable for thick overlay applications. When compared with plasma transferred arc (PTA) cladding, hot-wire TIG produces coarser grains and higher dilution rates, but offers lower equipment cost and greater process flexibility. The selection of the optimal process depends on the specific application requirements, including overlay thickness, dilution tolerance, and production volume.

Key Questions and Reflections

The study raises important questions regarding the long-term performance of hot-wire TIG Inconel 625 overlays on carbon steel substrates under thermal cycling conditions. The martensitic region near the fusion line, while providing adequate bond strength, may be susceptible to cracking under cyclic thermal loading, particularly in high-temperature service. Additionally, the influence of hydrogen pickup from the carbon steel substrate on the hydrogen embrittlement susceptibility of the overlay layer warrants further investigation, particularly for applications in hydrogen-containing environments.

Study Insights and Implications

The hot-wire TIG welding of Inconel 625 overlays on carbon steel substrates produces microstructures that are generally suitable for corrosion-resistant applications, provided that dilution is controlled below 15% and appropriate post-weld heat treatment is applied. The key engineering implication is that the carbon steel substrate presents more challenging metallurgical conditions than stainless steel substrates, requiring more careful process parameter optimization and more rigorous qualification testing. Engineers should prioritize dilution control, fusion line quality, and Laves phase avoidance in WPS development, and should consider post-weld heat treatment as a standard practice for overlays exceeding 2 mm thickness. The hot-wire TIG process, when properly qualified and controlled, offers a practical and economical solution for Inconel 625 cladding of carbon steel pressure vessels and equipment.