Zhuojin Pipe Fitting Co., LtdIron-Based Amorphous Coating on Q235 Steel by TIG Overlay Welding
Literature Overview
Published in 2020 in Materials Reports, this study by Li Yue and colleagues from Nanchang University investigates the microstructure, mechanical properties, and thermal stability of iron-based amorphous alloy coatings deposited on Q235 carbon steel substrates via TIG overlay welding. Supported by the Jiangxi Provincial Academic and Technical Leaders Program (Grant 20182BCB22001), the research addresses the challenge of achieving amorphous or nanocrystalline coatings through welding processes, where the inherent high cooling rates of thin overlay deposits can potentially suppress crystallization.
Microstructure and Phase Analysis
The researchers employed X-ray diffraction, optical microscopy, scanning electron microscopy, nanoindentation, and differential thermal analysis to characterize the overlay coatings. The following table summarizes the key findings:
| Characterization Method | Key Finding |
|---|---|
| XRD | Crystallization phases: α-Fe, Cr₁₂Fe₃₆Mo₁₀, CrFe₄ |
| Optical microscopy | Uniform, dense coating with metallurgical bonding to substrate |
| SEM | Crystallization increases with welding heat input |
| Nanoindentation | Maximum hardness: 7.55 GPa (3× substrate) |
| DTA | Maximum onset crystallization temperature: 506.8°C |
The coating exhibited metallurgical bonding with the Q235 substrate without visible defects, confirming the feasibility of amorphous alloy deposition through arc welding processes. However, the degree of crystallization increased with welding heat input, indicating that the cooling rate achievable through TIG overlay welding is sufficient to partially suppress crystallization but not completely prevent it. The crystallization phases identified (α-Fe, Cr₁₂Fe₃₆Mo₁₀, and CrFe₄) suggest that the amorphous matrix undergoes partial devitrification during solidification, with the extent depending on the local cooling rate.
Heat Input Effects on Properties
The relationship between welding heat input and coating properties follows clear trends:
- Hardness: Decreases with increasing heat input, with maximum hardness reaching 7.55 GPa at the lowest heat input conditions. This is approximately three times the hardness of the Q235 substrate, demonstrating significant surface hardening capability.
- Crystallization: Increases with heat input, as higher heat input reduces cooling rates and allows more time for nucleation and growth of crystalline phases.
- Thermal stability: The onset crystallization temperature decreases with increasing heat input, with the maximum value of 506.8°C achieved at the lowest heat input. This indicates that coatings deposited with lower heat input retain better thermal stability, maintaining their amorphous or nanocrystalline character to higher temperatures.
Engineering Practice Considerations
For engineers considering amorphous alloy overlay coatings for wear and corrosion protection applications, the following practical guidelines emerge from this research:
- Heat input minimization: Low heat input welding parameters should be selected to maximize the amorphous content and associated properties. TIG welding with small diameter tungsten electrodes and low current settings are recommended.
- Single-pass thin deposits: Thin, single-pass deposits maximize cooling rates and minimize crystallization. Multi-pass strategies should be avoided unless post-weld rapid quenching is available.
- Temperature limitation: The onset crystallization temperature of approximately 500°C defines the upper service temperature limit for maintaining the beneficial amorphous/nanocrystalline microstructure.
The 7.55 GPa hardness achieved represents exceptional surface hardening, comparable to or exceeding conventional hardfacing alloys, while potentially offering improved toughness and corrosion resistance due to the absence of grain boundaries in the amorphous regions.
Study Insights and Reflections
This research highlights an important intersection between rapid solidification metallurgy and welding technology. The ability to achieve partially amorphous coatings through conventional TIG welding opens new possibilities for surface engineering, particularly for applications requiring extreme hardness combined with corrosion resistance. The finding that heat input is the critical parameter controlling crystallization degree provides a clear process optimization target. However, the relatively low onset crystallization temperature of approximately 500°C limits the application to moderate-temperature service environments. For high-temperature applications, alternative coating technologies such as plasma spraying or laser cladding may be more appropriate, as they offer even higher cooling rates. Nevertheless, for room-temperature to moderate-temperature wear protection applications, TIG-deposited amorphous alloy coatings represent a promising and cost-effective technology that warrants further industrial development.