Zhuojin Pipe Fitting Co., LtdMicrostructure and Wear Resistance of Multi-Element Alloy Powder Coatings on Low Carbon Steel by TIG Overlay Welding
Literature Overview
This paper by Ji Yeyi, Lu Baoshan, Sun Shujuan, Li Youzhi, and Li Qiangwei, published in Foundry Technology (Vol. 39, No. 9, 2018, pp. 2129–2132), investigates the microstructure and wear resistance of multi-element alloy powder coatings deposited on 20 steel (low carbon steel) surfaces using tungsten inert gas (TIG) overlay welding. The coating alloy system is AlCrNiMoFeSi, a complex multi-component system designed to provide enhanced wear resistance through multiple strengthening mechanisms. The study employs pin-on-disk line contact wear testing to evaluate the coating performance and provides detailed microstructural analysis to explain the observed wear behavior.
Core Technical Content and Microstructural Analysis
The multi-element alloy powder coating system AlCrNiMoFeSi is designed to combine several strengthening mechanisms:
- Solid solution strengthening: The dissolution of Cr, Ni, Mo, and Si in the iron matrix provides solid solution strengthening.
- Precipitate strengthening: The formation of hard intermetallic phases such as FeMoSi dendrites provides particle strengthening.
- Grain refinement: The rapid solidification in the weld pool promotes fine grain formation, which increases strength and hardness.
- Oxide film formation: During sliding, the coating surface forms a protective oxide film that reduces friction and wear.
The microstructural analysis reveals that the coating microstructure consists primarily of:
- FeMoSi dendrites: These hard, angular phases are distributed throughout the coating and provide resistance to micro-cutting by abrasive particles.
- BCC (body-centered cubic) matrix: The inter-dendritic regions consist of a BCC ferritic structure that provides toughness and resistance to crack propagation.
- Possible intermetallic phases: Depending on the local composition and cooling rate, additional intermetallic phases such as Cr₂B, Ni₃Si, or MoSi₂ may form.
The pin-on-disk line contact wear test results demonstrate that the coating exhibits significantly improved wear resistance compared to the uncoated 20 steel substrate. The wear mechanism transitions from abrasive wear on the uncoated surface to a combination of abrasive and adhesive wear on the coated surface, with the formation of a protective oxide film during high-speed sliding.
Wear Mechanism Analysis
The wear resistance of the coating can be attributed to several synergistic mechanisms:
| Mechanism | Description | Contribution |
|---|---|---|
| FeMoSi dendrite resistance | Hard dendrites resist micro-cutting by abrasive particles | High |
| BCC matrix toughness | BCC structure resists crack propagation | Medium |
| Oxide film formation | Protective oxide film reduces friction coefficient | Medium-High |
| Solid solution strengthening | Dissolved alloying elements increase matrix hardness | Low-Medium |
The formation of the oxide film during sliding is particularly important. The Cr, Al, and Si in the coating promote the formation of a thin, adherent oxide layer that acts as a protective barrier, reducing the friction coefficient and preventing direct metal-to-metal contact. This oxide film is continuously formed and removed during sliding, maintaining a steady-state wear rate that is significantly lower than that of the uncoated substrate.
Process Parameters and Coating Quality
The TIG overlay welding process parameters are critical to achieving a high-quality coating with the desired microstructure and wear resistance. Key parameters include:
| Parameter | Typical Value |
|---|---|
| Wire/powder feed rate | 3–8 g/min |
| Arc current | 100–200 A |
| Arc voltage | 15–25 V |
| Travel speed | 50–150 mm/min |
| Shielding gas | Argon, 10–20 L/min |
| Preheat | 100–200 °C |
| Interpass temperature | ≤ 250 °C |
| Coating thickness | 1–3 mm |
The dilution rate between the base metal and the coating is a critical parameter. If the dilution is too high, the effective composition of the coating is altered, and the desired strengthening phases may not form in sufficient quantity. If the dilution is too low, the metallurgical bonding between the base and coating may be weak, leading to delamination under wear conditions.
Quality Control and Inspection
The quality of the TIG overlay welded coating should be verified through the following methods:
- Hardness testing: Vickers hardness (HV) testing across the coating cross-section to verify hardness uniformity and the presence of hard phases.
- Metallographic examination: Optical microscopy and scanning electron microscopy (SEM) to examine the microstructure, phase distribution, and any defects such as porosity or cracks.
- Energy dispersive spectroscopy (EDS): To map the elemental distribution and verify the presence of the expected phases.
- Wear testing: Pin-on-disk or ball-on-plate wear tests to quantify the wear resistance of the coating.
- Adhesion testing: Scratch testing or tensile adhesion testing to verify the bonding strength between the coating and the base metal.
Reflections and Study Insights
This paper provides valuable insights into the design and application of multi-element alloy coatings for wear protection. The key innovation is the use of a complex multi-component alloy system that combines multiple strengthening mechanisms, rather than relying on a single mechanism such as carbide precipitation or oxide dispersion. This approach provides a more robust wear resistance that is less sensitive to variations in process parameters and operating conditions.
The formation of the protective oxide film during sliding is an important finding. In many wear applications, the formation of a stable oxide film can dramatically reduce the wear rate by reducing the friction coefficient and preventing adhesive wear. The presence of Cr, Al, and Si in the coating promotes the formation of such a film, and this mechanism is particularly effective at elevated temperatures where oxide film formation is thermodynamically favorable.
The broader lesson is that wear-resistant coatings should be designed to address the specific wear mechanism of the application. In this case, the combination of hard phases for abrasive resistance and oxide film formation for friction reduction provides an effective solution for line contact wear conditions. However, for other wear mechanisms such as erosive wear, corrosive wear, or fatigue wear, different coating compositions and structures may be required.
The TIG overlay welding process is well-suited for this application because it provides excellent control over the weld pool composition and cooling rate, which is essential for achieving the desired microstructure. However, the process is relatively slow compared to other overlay welding methods such as plasma arc welding or laser cladding, and the cost of the multi-element alloy powder may be significant. Therefore, the application should be carefully evaluated for cost-effectiveness before implementation.