CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure Formation of Inconel 625 Overlay by Hot Wire TIG Cladding
Literature Overview and Process Context
Hot-wire TIG (HWT) cladding represents an advanced variant of gas tungsten arc welding that integrates a separate wire feed mechanism with the TIG arc, enabling significantly higher deposition rates while maintaining the arc stability and microstructural control characteristic of TIG welding. This process is particularly valuable for depositing nickel-based alloy overlay layers such as Inconel 625 on carbon steel and low-alloy steel substrates in applications requiring exceptional corrosion resistance, including chemical processing equipment, offshore platforms, and nuclear components.
The study investigates the microstructure formation mechanisms in Inconel 625 overlay layers deposited by HWT cladding, with particular attention to the effects of process parameters on grain morphology, phase distribution, and solidification behavior. Inconel 625 (UNS N06625) is a precipitation-hardenable nickel-iron-chromium alloy containing approximately 21-23% Cr, 8-10% Mo, and 3-4.5% Nb, offering outstanding resistance to pitting, crevice, and stress corrosion cracking in aggressive environments.
Process Parameters and Their Influence on Microstructure
The HWT process parameters are systematically varied to establish their influence on the overlay microstructure. The key parameters and their effects are summarized below:
| Parameter | Range Studied | Effect on Microstructure |
|---|---|---|
| Arc Current | 80-180 A | Higher current → larger grain size, deeper penetration |
| Travel Speed | 80-250 mm/min | Slower speed → thicker beads, coarser grains |
| Wire Feed Speed | 1.5-4.0 m/min | Higher feed → more deposited material, wider beads |
| Shielding Gas | Ar or Ar+5%He | He addition → deeper penetration, finer grains |
| Wire Diameter | 1.0-2.4 mm | Larger wire → higher deposition rate, coarser structure |
| Torch Angle | 5-15° from vertical | Forward angle → shallower penetration, finer grains |
| Interpass Temperature | ≤150°C | Higher interpass temp → reduced thermal gradient |
The thermal input (heat input) is the primary parameter governing solidification behavior and grain morphology. The study demonstrates that low thermal inputs (below 0.8 kJ/mm) produce columnar dendritic structures with fine inter-dendritic spacing, while higher thermal inputs (above 1.5 kJ/mm) promote equiaxed grain formation through increased constitutional undercooling and reduced thermal gradient.
The solidification microstructure of Inconel 625 HWT deposits is characterized by the sequential precipitation of gamma (γ) austenite, gamma prime (γ') precipitates, and delta (δ) phase during cooling. The relative amounts and morphologies of these phases are directly influenced by the cooling rate, which is in turn controlled by the process parameters.
Microstructural Analysis and Phase Evolution
The as-deposited microstructure of Inconel 625 HWT overlays typically consists of a two-phase matrix of austenite (γ) and ferrite (δ), with the relative proportions varying with local composition and solidification conditions. The δ phase forms as a result of the Mo and Nb content, which stabilize ferrite in the Ni-Cr-Mo-Nb system. The δ phase content is particularly important for controlling the susceptibility to cracking during subsequent welding operations.
| Microstructural Feature | Low Heat Input (<0.8 kJ/mm) | High Heat Input (>1.5 kJ/mm) |
|---|---|---|
| Grain Morphology | Columnar dendrites | Mixed columnar and equiaxed |
| Grain Size (avg) | 50-100 μm | 150-300 μm |
| δ Phase Content | 5-15% | 15-30% |
| Carbide Precipitation | Fine MC carbides (NbC, TiC) | Coarser MC carbides |
| Hardness (HV) | 250-300 | 280-350 |
| Solidification Cracking Susceptibility | Higher | Lower |
The formation of MC-type carbides (primarily NbC and TiC) is a characteristic feature of Inconel 625 solidification. These carbides form at dendrite boundaries and inter-dendritic regions, and their size, distribution, and volume fraction are governed by the cooling rate. At low cooling rates (high heat input), the MC carbides have sufficient time to grow and coarsen, resulting in larger particles that can act as stress concentrators and crack initiation sites.
The δ phase content is particularly significant for subsequent welding operations because it influences the susceptibility to hot cracking in the weld overlay. Excessive δ phase (above 25-30%) can promote solidification cracking due to the formation of low-melting-point eutectics at the δ/γ interface. The study recommends controlling the δ phase content below 20% through appropriate selection of heat input and interpass temperature.
Effect of Multi-Pass Deposition on Microstructure
Multi-pass HWT cladding introduces additional metallurgical complexity due to the thermal cycling experienced by previously deposited layers. Each subsequent pass subjects the underlying layers to a tempering or annealing cycle, which modifies the microstructure through dissolution and re-precipitation of phases.
The study demonstrates that the microstructure of the first pass undergoes the most significant transformation during multi-pass deposition. The initial columnar dendritic structure is partially recrystallized during the thermal cycles of subsequent passes, resulting in a mixed columnar-equiaxed structure with refined grain size. This beneficial effect of thermal cycling is analogous to the effect of interpass reheating in conventional multi-pass welding.
The composition of the overlay layer also evolves with multi-pass deposition due to dilution from the base metal and intermixing between passes. The study quantifies the dilution rate as a function of pass number and process parameters, finding that the first pass typically experiences 15-25% dilution from the base metal, while subsequent passes show 5-10% dilution from the previously deposited layer. This dilution effect influences the final composition and, consequently, the phase balance and mechanical properties of the overlay.
Mechanical Properties and Performance Evaluation
The mechanical properties of Inconel 625 HWT overlays are evaluated through hardness testing, tensile testing, and impact testing, with particular attention to the properties of the base metal-to-overlay transition zone. The results demonstrate that the overlay hardness is significantly higher than the base metal, with a gradual transition through the dilution zone.
| Property | Base Metal (AISI 304) | Transition Zone | Overlay Layer |
|---|---|---|---|
| Hardness (HV) | 180-220 | 220-280 | 280-350 |
| Tensile Strength (MPa) | 500-600 | 550-650 | 600-750 |
| Elongation (%) | 40-50 | 30-40 | 35-45 |
| Impact Energy (J at 25°C) | 150-200 | 80-120 | 100-150 |
The transition zone hardness of 220-280 HV is within acceptable limits for most applications, but the study notes that if the base metal is a high-strength low-alloy steel with carbon equivalent above 0.40%, the transition zone may require post-weld heat treatment to reduce hardness and prevent cracking.
The corrosion resistance of the HWT overlay is evaluated through potentiodynamic polarization testing and intergranular corrosion testing per ASTM A263. The overlay demonstrates excellent resistance to pitting and crevice corrosion in chloride-containing environments, with pitting resistance equivalent number (PREN) values exceeding 35. The intergranular corrosion resistance is satisfactory when the δ phase content is controlled below 20%, as excessive δ phase can promote chromium-depleted zones at grain boundaries.
Engineering Practice and Process Optimization
For practical HWT cladding operations, the study recommends a systematic approach to process parameter selection based on the FMEA (Failure Mode and Effects Analysis) methodology. The primary failure modes identified include:
| Failure Mode | Root Cause | Prevention Measure |
|---|---|---|
| Solidification cracking | Excessive δ phase, high thermal input | Control heat input, limit δ phase to <20% |
| Porosity | Inadequate shielding, wire feed instability | Optimize gas flow, maintain stable wire feed |
| Lack of fusion | Insufficient heat input, poor joint preparation | Increase current, clean base metal surface |
| Excessive dilution | High travel speed, thin base metal | Reduce current, increase wire feed speed |
| Undercut | Torch angle too high, insufficient overlap | Optimize torch angle, maintain 50-70% overlap |
The study also addresses the issue of residual stress management in HWT overlays. The thermal stresses generated during deposition can reach values approaching the yield strength of the overlay material, particularly in thick multi-pass deposits. The recommended approach involves controlling the interpass temperature below 150°C and applying a post-weld stress relief treatment at 425-450°C for 1 hour per 25 mm of overlay thickness.
Study Insights and Independent Reflection
This research provides valuable insights into the microstructure formation mechanisms of Inconel 625 HWT overlays, with particular emphasis on the interplay between process parameters, solidification behavior, and final microstructure. The systematic approach to parameter optimization, combined with detailed microstructural characterization, offers a solid technical foundation for engineers seeking to implement HWT cladding in production environments.
One of the most significant findings is the demonstration that HWT can produce overlay microstructures comparable to or better than those achieved by conventional TIG overlay, while offering deposition rates that are 2-3 times higher. This combination of microstructural quality and productivity makes HWT an attractive option for high-volume cladding applications where both performance and cost are critical considerations.
The study also raises important questions about the long-term stability of the overlay microstructure under service conditions. The δ phase and MC carbides that form during solidification may undergo coarsening or phase transformation during prolonged exposure to elevated temperatures, potentially affecting the corrosion resistance and mechanical properties of the overlay. Engineers should consider these long-term stability issues when selecting HWT parameters for applications involving sustained high-temperature service.
The practical implications of this research extend to the qualification of HWT welding procedures under NB/T 47014 and ASME IX. The study provides the technical data necessary for establishing acceptance criteria for HWT overlay procedures, including requirements for microstructural evaluation, hardness mapping, and corrosion testing of the overlay layer.
In conclusion, this study offers comprehensive technical guidance for the HWT cladding of Inconel 625 overlay layers, covering process parameter optimization, microstructure control, and mechanical property evaluation. The findings support the adoption of HWT as a viable and efficient alternative to conventional TIG overlay for depositing high-performance nickel-based alloy cladding layers on steel substrates. Engineers involved in bimetal product manufacturing and pressure vessel fabrication should consider HWT as a preferred cladding method for applications requiring the exceptional corrosion resistance and mechanical properties of Inconel 625.