Zhuojin Pipe Fitting Co., LtdErosion-Corrosion Properties of TIG Surface Remelted Hardfacing Layers
Literature Overview
This study by Lei Yucheng, Feng Lianghou, and Zhao Xiaojun (2007), published in Corrosion and Protection, investigates the effect of tungsten inert gas (TIG) surface remelting on the cavitation erosion-corrosion performance of hardfacing layers. The research was supported by the Jiangsu Provincial Industrial Research Project (BE2004089) and conducted at the School of Materials Science and Engineering, Jiangsu University. The work addresses a practical challenge in the repair and protection of components subjected to cavitation erosion, such as pump impellers, marine propellers, and hydraulic turbine components.
Background and Problem Statement
Cavitation erosion occurs when vapor bubbles in a liquid collapse in proximity to a solid surface, generating intense micro-jets and shock waves that cause material removal. When combined with a corrosive environment, cavitation erosion-corrosion (CEC) becomes a synergistic degradation mechanism where the mechanical action of cavitation removes protective corrosion films, exposing fresh metal to corrosion attack, while corrosion weakens the material and facilitates cavitation damage. The resulting mass loss rate is often significantly higher than the sum of individual erosion and corrosion rates.
The study focuses on a hardfacing material designed for cavitation corrosion resistance and investigates how TIG surface remelting can modify the surface microstructure to improve performance. The comparison is made between conventionally ground surfaces and TIG remelted surfaces, providing a direct assessment of the remelting process benefits.
TIG Surface Remelting Process
TIG surface remelting is a solid-state surface modification process where the TIG arc is used to selectively melt the surface layer of a hardfacing deposit without adding filler material. The process creates a thin remelted zone (typically 0.5-2 mm deep) with a modified microstructure, while the bulk of the deposit retains its original characteristics. The key parameters include arc current, travel speed, shielding gas flow rate, and number of passes.
The remelting process produces several beneficial effects:
| Effect | Mechanism | Benefit |
|---|---|---|
| Grain refinement | Rapid solidification in remelted zone | Increased strength and resistance to crack initiation |
| Homogenization | Dissolution of inhomogeneities | Uniform microstructure, reduced stress concentration |
| Phase transformation control | Modified cooling rate | Delayed austenite-to-martensite transformation |
| Surface smoothing | Melting and resolidification | Reduced surface roughness, improved fatigue resistance |
Cavitation Erosion-Corrosion Test Results
The cavitation erosion-corrosion testing was conducted over 45 hours using a standard ultrasonic vibration test rig. The results demonstrate a clear advantage of TIG surface remelting:
| Condition | Cumulative Mass Loss (mg) | Relative Performance |
|---|---|---|
| Ground surface | Higher | Baseline (1.57×) |
| TIG remelted surface | Lower | 1.0× (reference) |
The ground sample exhibited 1.57 times the mass loss of the TIG remelted sample after 45 hours of cavitation erosion-corrosion testing. This represents a 36% reduction in mass loss, which is a substantial improvement for service life extension.
Microstructural Analysis of Damage Mechanisms
The study provides valuable insights into the damage mechanisms operating during cavitation erosion-corrosion:
- Martensite spalling as the primary mass loss mechanism: The hardfacing material undergoes an austenite-to-martensite transformation during the welding process. The resulting martensite is hard but brittle, and under cavitation impact, it spalls from the surface as platelets. This spalling exposes fresh material to further erosion and corrosion, creating a self-accelerating degradation cycle.
- TIG remelting delays the austenite-to-martensite transformation: The remelting process creates a thin layer with a modified cooling rate and thermal history. This delays the transformation of retained austenite to martensite, maintaining a more ductile microstructure at the surface. The delayed transformation means that less brittle martensite is present to spall under cavitation impact.
- Crack suppression by TIG remelting: In the ground condition, cavitation-induced cracks develop along martensite plate boundaries and propagate through the microstructure. These cracks facilitate material removal and corrosion ingress. TIG remelting refines the grain structure and eliminates the coarse martensite plate boundaries, thereby suppressing crack initiation and propagation. The absence of large crack networks prevents the formation of large material spalls.
- Crack propagation along martensite plates: The study specifically notes that corrosion-induced cracks develop along martensite plate boundaries in the ground condition. This is because the martensite plates have a different composition and residual stress state compared to the surrounding retained austenite, creating a preferential path for crack growth. TIG remelting eliminates these plate boundaries by creating a finer, more homogeneous microstructure.
Engineering Practice Implications
This study provides a practical solution for improving the cavitation erosion-corrosion resistance of existing hardfacing deposits without the need for re-welding or replacement. The TIG surface remelting process can be applied in the field or in a workshop, making it a cost-effective maintenance strategy. Key considerations for implementation include:
- Process parameters: The TIG remelting parameters must be optimized to achieve the desired remelted depth and microstructure. Excessive heat input can cause unwanted phase transformations or cracking, while insufficient heat input will not produce the desired surface modification.
- Number of passes: Multiple passes may be required to achieve a uniform remelted layer across the entire surface. The number of passes should be optimized to balance coverage and efficiency.
- Post-remelting treatment: A light grinding or polishing step may be necessary to remove any oxide scale formed during remelting and to achieve the desired surface finish.
- Inspection: Non-destructive testing (such as magnetic particle inspection or eddy current testing) should be performed after remelting to ensure that no cracks or other defects have been introduced.
Study Insights and Reflections
This research highlights the importance of understanding the microstructure-dependent damage mechanisms in cavitation erosion-corrosion. The finding that martensite spalling is the primary mass loss mechanism provides a clear target for material and process optimization. The success of TIG surface remelting in delaying the austenite-to-martensite transformation demonstrates that even a thin surface layer modification can have a significant impact on bulk performance. This approach is particularly valuable for in-service components where full re-hardfacing is impractical or impossible. The study also underscores the synergistic nature of cavitation erosion-corrosion, where mechanical and chemical degradation mechanisms interact to accelerate material loss. Engineers working on cavitation-prone components should consider TIG surface remelting as a viable option for extending service life, provided that the underlying hardfacing material has adequate base properties and the remelting process is properly controlled.