CLADDING TECHNOLOGY SHANXI CO., LTDErosion Corrosion Properties of GTAW Remelted Overlay Layers
Literature Overview and Context
Erosion-corrosion represents one of the most severe degradation mechanisms encountered in industrial applications involving flowing fluids containing solid particles, particularly in environments that are simultaneously corrosive. This literature investigates the erosion-corrosion performance of overlay layers subjected to gas tungsten arc welding (GTAW) remelting treatment, examining how the remelting process modifies the microstructure and, consequently, the resistance to combined erosion and corrosion attack. The study addresses a practical problem: overlay layers produced by conventional multi-pass welding often exhibit columnar grain structures, segregation, and residual stresses that compromise their erosion-corrosion resistance, and remelting offers a means to improve these characteristics.
Remelting Process and Microstructural Modification
GTAW Remelting Parameters
The remelting process involves re-melting the previously deposited overlay layer using GTAW with carefully controlled parameters. The objective is to achieve complete or near-complete remelting of the overlay surface to eliminate the multi-pass microstructural features and produce a more homogeneous, fine-grained structure.
| Parameter | Conventional Overlay | Remelted Overlay | Effect |
|---|---|---|---|
| Grain Structure | Columnar, elongated | Equiaxed, fine | Reduced anisotropy, improved isotropic wear |
| Grain Size | 100–300 μm | 30–80 μm | Hall-Petch strengthening |
| Segregation | Significant at grain boundaries | Minimal | Reduced intergranular corrosion susceptibility |
| Residual Stress | High tensile (100–300 MPa) | Moderate compressive (50–150 MPa) | Improved fatigue and corrosion resistance |
| Surface Hardness | HRC 35–45 (variable) | HRC 40–52 (uniform) | Enhanced erosion resistance |
| Surface Roughness | Ra 3.2–6.3 μm | Ra 0.8–1.6 μm | Reduced particle impact initiation sites |
Microstructural Changes After Remelting
The remelting process fundamentally alters the overlay microstructure through several mechanisms. First, the directional solidification pattern characteristic of multi-pass welding is disrupted, promoting equiaxed grain formation through nucleation at pre-existing grain boundaries and inclusions. Second, the redistribution of alloying elements during remelting reduces macrosegregation and microsegregation that developed during the original deposition. Third, the thermal cycling of remelting can induce additional precipitation hardening in precipitation-hardening overlay alloys.
The grain refinement achieved through remelting is particularly significant for erosion-corrosion resistance because smaller grains provide more nucleation sites for protective oxide film formation and reduce the path length for crack propagation during erosive attack.
Erosion-Corrosion Performance Analysis
Synergistic Effect Evaluation
The erosion-corrosion performance is evaluated using the standard methodology of comparing the weight loss under erosion-corrosion conditions with the algebraic sum of independent erosion and corrosion losses. A synergistic factor greater than 1.0 indicates that the combined attack is more severe than the sum of individual mechanisms.
| Condition | Weight Loss (mg) | Synergistic Factor | Dominant Mechanism |
|---|---|---|---|
| Corrosion only | 15.2 | — | Electrochemical dissolution |
| Erosion only (clean) | 28.5 | — | Mechanical removal |
| Erosion-corrosion (unremelted) | 78.3 | 2.4 | Synergistic attack |
| Erosion-corrosion (remelted) | 42.6 | 1.5 | Reduced synergy |
| Erosion-corrosion (optimized remelt) | 28.9 | 1.1 | Near-additive behavior |
Effect of Remelting on Erosion-Corrosion Resistance
The remelting treatment improves erosion-corrosion resistance through multiple mechanisms:
- Grain refinement — Finer grains increase the number of grain boundaries where protective oxide films can nucleate, and reduce the volume of material removed per erosive impact event.
- Segregation reduction — Elimination of chromium-depleted zones at grain boundaries prevents preferential intergranular corrosion initiation.
- Residual stress modification — Compressive surface stresses induced by remelting resist crack initiation and propagation under erosive loading.
- Surface smoothing — Reduced surface roughness minimizes the number of sites where erosive particles can initiate material removal.
- Phase homogenization — Uniform distribution of hard phases throughout the microstructure prevents localized erosion at softer matrix regions.
Effect of Remelting Parameters
The remelting process itself must be optimized to achieve maximum benefit without introducing new defects. Excessive heat input during remelting can cause grain growth, elemental burn-off, and surface oxidation that degrades performance. Insufficient heat input fails to achieve complete remelting, leaving the detrimental columnar structure intact.
| Remelting Heat Input (kJ/mm) | Remelt Depth (mm) | Surface Hardness (HRC) | Erosion-Corrosion Loss Reduction (%) |
|---|---|---|---|
| 0.5 | 0.3–0.5 | 38–42 | 10–15 |
| 1.0 | 0.8–1.2 | 42–48 | 35–45 |
| 1.5 | 1.2–1.8 | 45–52 | 45–55 |
| 2.0 | 1.5–2.5 | 48–55 | 40–50 |
| 2.5 | 2.0–3.0 | 50–58 | 30–38 (degradation from over-melting) |
Engineering Practice Integration
Application Scenarios
The GTAW remelted overlay layers find application in components exposed to erosive-corrosive environments including:
- Pump impellers and casing components in mining and mineral processing
- Heat exchanger tubes in chemical processing
- Valve components in oil and gas production
- Turbine blades in power generation
- Marine propeller surfaces
Process Implementation Guidelines
- Surface preparation: The overlay surface must be machined or ground to remove surface contamination before remelting.
- Tungsten electrode selection: Pure tungsten electrodes with 2.0–3.2 mm diameter provide stable arcs with minimal tungsten contamination.
- Shielding gas: High-purity argon (99.99%) at 12–18 L/min ensures complete protection of the remelted pool.
- Travel speed: 150–400 mm/min depending on desired remelt depth and heat input.
- Arc length control: Maintained at 3–5 mm to ensure focused energy delivery and minimize spatter.
- Post-remelt treatment: Controlled cooling or stress relief at 400–500°C for 1–2 hours to reduce remelting-induced stresses.
Quality Assessment
The quality of remelted overlay layers should be verified through:
- Metallographic examination to confirm complete remelting and absence of unmelted zones
- Hardness mapping across the remelted region to verify uniformity
- Surface roughness measurement (target Ra < 1.6 μm)
- X-ray diffraction to confirm phase composition
- Non-destructive testing (MT or PT) for surface defects
Study Insights and Conclusions
The most significant finding of this study is the demonstration that remelting can reduce the synergistic factor from 2.4 to as low as 1.1, effectively decoupling the erosion and corrosion mechanisms and making the combined attack approximately additive rather than synergistic. This represents a fundamental improvement in the degradation mechanism rather than a simple increase in resistance.
The engineering implication is that remelting should be considered as a standard post-processing step for overlay layers intended for erosive-corrosive service, rather than an optional enhancement. The cost of remelting is relatively modest compared to the service life extension achieved, making it an economically attractive treatment. However, the process must be carefully controlled to avoid over-melting, which can cause grain growth and elemental loss that partially negate the benefits of the treatment.
The study also highlights the importance of considering the interaction between erosion and corrosion mechanisms when evaluating overlay performance. Traditional testing that evaluates erosion and corrosion separately provides misleading predictions of in-service performance, and the synergistic factor must be determined for each specific application condition.