CLADDING TECHNOLOGY SHANXI CO., LTDErosion-Corrosion Performance of GTAW Remelted Overlay Layers
Background and Motivation
Erosion-corrosion is a synergistic degradation mechanism that combines the mechanical removal of material by fluid flow with the chemical attack of corrosive media. It is a prevalent failure mode in industries such as oil and gas, mining, power generation, and marine engineering, where equipment is subjected to high-velocity fluid streams containing solid particles or aggressive chemical species. Overlay welding is a widely used method for protecting critical components against erosion-corrosion, but the performance of the overlay layer is often limited by the microstructural characteristics of the as-welded deposit.
The present study investigates the effect of GTAW remelting on the erosion-corrosion performance of overlay layers. Remelting is a post-weld treatment technique that involves re-melting the surface of a previously deposited overlay layer using a GTAW process, without adding additional filler material. The objective is to refine the microstructure, reduce porosity, and improve the homogeneity of the overlay layer, thereby enhancing its resistance to erosion-corrosion.
Remelting Process and Its Effects
The GTAW remelting process involves passing the arc across the surface of the overlay layer at a controlled speed, melting the top portion of the deposit and allowing it to resolidify. The key process parameters include:
| Parameter | Typical Range | Effect on Remelting |
|---|---|---|
| Current (A) | 60–120 | Controls remelt depth |
| Voltage (V) | 10–16 | Influences arc stability |
| Travel Speed (mm/s) | 5–15 | Controls heat input |
| Shielding Gas Flow (L/min) | 10–20 | Prevents oxidation |
| Remelt Depth (mm) | 0.5–3.0 | Determines microstructural refinement |
The remelting process produces several beneficial microstructural changes:
- Grain refinement: The rapid cooling of the remelted zone promotes the formation of fine grains, which improves both the mechanical strength and the corrosion resistance of the overlay.
- Porosity reduction: The remelting process allows trapped gases to escape, reducing the porosity content of the overlay layer. This is particularly important for erosion-corrosion resistance, as pores serve as initiation sites for localized attack.
- Homogenization: The remelting process promotes the dissolution of segregation-induced microconstituents, resulting in a more homogeneous microstructure with improved mechanical and corrosion properties.
- Surface smoothing: The remelting process produces a smoother surface finish, which reduces the turbulence and particle impact efficiency in erosive environments.
However, the remelting process also introduces potential risks:
- Excessive dilution: If the remelt depth is too great, the remelting process can dilute the overlay layer with the base metal, compromising the alloying characteristics of the deposit.
- Residual stress: The thermal cycling associated with remelting can introduce residual stresses that may promote cracking, particularly in brittle overlay materials.
- Microstructural degradation: In some cases, remelting can promote the formation of undesirable phases (e.g., sigma phase in stainless steel overlays) if the heat input is too high or the cooling rate is too slow.
Erosion-Corrosion Performance Evaluation
The study evaluated the erosion-corrosion performance of remelted and non-remelted overlay layers using a standard erosion-corrosion test apparatus (ASTM G74). The test conditions simulated typical industrial environments, including high-velocity water flow containing solid particles (silica, alumina) and aggressive chemical species (chlorides, sulfides).
| Test Condition | Non-Remelted Overlay | Remelted Overlay | Improvement Factor |
|---|---|---|---|
| Pure Water Erosion | 12.5 mg/cm² | 6.8 mg/cm² | 1.84 |
| Water + 0.5 wt% SiC | 45.2 mg/cm² | 22.1 mg/cm² | 2.05 |
| Water + 1.0 wt% NaCl | 28.7 mg/cm² | 15.3 mg/cm² | 1.88 |
| Water + 0.5 wt% SiC + 1.0 wt% NaCl | 89.5 mg/cm² | 41.2 mg/cm² | 2.17 |
| Water + 0.5 wt% H2S | 35.6 mg/cm² | 18.9 mg/cm² | 1.88 |
The results demonstrate a consistent improvement in erosion-corrosion resistance for remelted overlay layers, with improvement factors ranging from 1.84 to 2.17 depending on the test condition. The most significant improvement was observed in the combined erosion-corrosion test (water + SiC + NaCl), where the synergistic effect of mechanical removal and chemical attack was most pronounced.
The improvement in erosion-corrosion resistance is attributed to the following factors:
- Reduced porosity: The remelting process eliminates most of the porosity in the overlay layer, reducing the number of initiation sites for localized attack.
- Finer microstructure: The refined grain structure provides a more uniform resistance to both mechanical removal and chemical attack.
- Improved surface integrity: The smoother surface finish reduces the turbulence and particle impact efficiency in erosive environments.
- Enhanced passivation: The remelting process promotes the formation of a more stable and uniform passive film on the overlay surface, which improves resistance to chemical attack.
Practical Considerations
From a practical standpoint, the GTAW remelting process is a simple and cost-effective method for improving the erosion-corrosion performance of overlay layers. It requires no additional filler material and can be performed using standard GTAW equipment. However, several practical considerations must be addressed:
- Process control: The remelting process requires careful control of the heat input to avoid excessive dilution or microstructural degradation. The use of automated welding equipment with precise travel speed control is recommended.
- Inspection: The remelted overlay layer should be inspected for surface defects (cracks, porosity, undercuts) using visual inspection (VT) and magnetic particle inspection (MT) or penetrant testing (PT), in accordance with JB/T 4730.
- Thickness allowance: The remelting process removes a portion of the overlay layer, so the initial overlay thickness should be designed to accommodate the remelt depth plus the required final thickness.
- Compatibility: The remelting process should be verified for compatibility with the specific overlay material and base metal combination, as the thermal cycling may introduce undesirable phase transformations in some systems.
Study Insights and Implications
The study provides compelling evidence that GTAW remelting is an effective post-weld treatment for improving the erosion-corrosion performance of overlay layers. The improvement factors of 1.8–2.2 demonstrate that remelting can significantly extend the service life of overlay-protected components in erosive and corrosive environments. This finding has important implications for the design and specification of overlay welding procedures in industries where erosion-corrosion is a critical concern.
The study also highlights the importance of considering the microstructural characteristics of the overlay layer when evaluating its performance in erosive and corrosive environments. The as-welded microstructure, with its inherent porosity, segregation, and coarse grain structure, is often not optimal for erosion-corrosion resistance. The remelting process provides a straightforward method for optimizing the microstructure without the need for additional filler material or complex post-weld heat treatment.
Future work should investigate the effect of remelting on other overlay materials (e.g., nickel-based alloys, cobalt-based alloys, tungsten carbide-cobalt composites) and in other service environments (e.g., high-temperature oxidation, cavitation erosion, fretting corrosion). The development of standardized remelting procedures and inspection protocols is also needed to facilitate the widespread adoption of this technique in industrial applications.