Zhuojin Pipe Fitting Co., LtdMIG Melting Injection WC-Reinforced Wear-Resistant Layer - Literature Study Note
Literature Overview
This paper, published in the Journal of Shenyang Ligong University in 2011 by Liu Aiguo from the School of Materials Science and Engineering, investigates the application of MIG melting injection technology to produce tungsten carbide (WC) particle-reinforced wear-resistant coatings on low-carbon steel substrates. The study combines process development with comprehensive microstructural characterization and tribological testing, providing a thorough understanding of the relationship between processing parameters, microstructure, and wear resistance performance.
Process Description and Working Principle
The MIG melting injection process is a variant of gas metal arc welding in which solid particles are injected into the weld pool through a side nozzle or a coaxial delivery system. In this application, WC particles are injected into the molten weld pool created by the MIG arc on the low-carbon steel substrate. The particles melt partially or completely and become incorporated into the solidifying weld metal, forming a composite wear-resistant layer.
The process parameters used in this study include:
- Shielding gas: CO2 or argon-CO2 mixture
- Wire: Low-carbon steel or alloy steel MIG wire
- WC particles: Commercially available WC powder with controlled particle size distribution
- Substrate: Low-carbon steel plate
- Multi-pass welding: Multiple passes to build up the wear-resistant layer thickness
| Process Parameter | Typical Range | Effect on Coating |
|---|---|---|
| Welding current | 150-250 A | Higher current increases dilution and WC melting |
| Welding voltage | 20-28 V | Affects arc power and pool size |
| Travel speed | 100-300 mm/min | Affects heat input and dilution ratio |
| Wire feed speed | 3-8 m/min | Controls metal deposition rate |
| WC particle size | 10-100 μm | Affects melting behavior and distribution |
| Shielding gas flow rate | 10-20 L/min | Affects arc stability and particle delivery |
Microstructural Analysis
The scanning electron microscopy analysis reveals several important microstructural features of the WC-reinforced wear-resistant layer.
The WC particles are distributed relatively uniformly throughout the coating layer, with no significant settling or segregation observed. This uniform distribution is critical for achieving consistent wear resistance across the coating surface. The absence of particle settling indicates that the process parameters were well-controlled to maintain adequate turbulence in the weld pool.
Adjacent to the WC particles, the microstructure shows the formation of iron tungsten carbide phases, specifically Fe3W3C and/or Co3W3C. These phases form as a result of the interaction between the molten steel matrix and the WC particles. The presence of these intermediate carbide phases is important because they provide a metallurgical bond between the hard WC particles and the steel matrix, improving the overall cohesion of the composite coating.
Further from the WC particles, a fishbone-shaped eutectic microstructure is observed. This eutectic structure forms during the solidification of the weld metal and is characteristic of high-carbon, high-alloy weld deposits. The fishbone morphology indicates a specific solidification pattern that can influence the mechanical properties of the coating.
The substrate metal adjacent to the coating shows a martensitic microstructure, indicating that the heat input was sufficient to transform the low-carbon steel substrate into martensite in the heat-affected zone. This is a significant finding because martensite in the HAZ can be brittle and susceptible to cracking. However, the paper does not report any cracking in the HAZ, suggesting that the heat input was controlled to minimize this risk.
WC-type carbides are observed to precipitate along the grain boundaries of the substrate metal in a network pattern. This grain boundary carbide precipitation is a result of the carbon enrichment in the HAZ due to the interaction between the substrate and the WC-containing weld metal.
| Microstructural Feature | Location | Phase Composition | Implication |
|---|---|---|---|
| WC particles | Throughout coating | WC | Primary wear-resistant phase |
| Fe3W3C/Co3W3C | Adjacent to WC particles | Iron/cobalt tungsten carbide | Metallurgical bonding phase |
| Fishbone eutectic | Away from WC particles | Eutectic mixture | Affects coating toughness |
| Martensite | Substrate HAZ | Martensite | Risk of cracking, needs tempering |
| WC-type carbides | Grain boundaries of substrate | WC | Carbon enrichment effect |
Wear Resistance Performance
The most striking finding of this paper is the exceptional wear resistance of the WC-reinforced coating. The wear resistance of the coating is approximately 850 times that of the base low-carbon steel substrate. This extraordinary improvement is attributed to the combined effect of the hard WC particles and the hardened matrix.
The hardness of the WC particles in the coating has not been reduced compared to the original WC powder, indicating that the particles have not undergone significant melting or decomposition during the welding process. This is a critical finding because excessive WC melting would reduce the hardness and wear resistance of the coating.
The matrix hardness is approximately Hv700, which is significantly higher than the base steel (typically Hv150-200 for low-carbon steel). This high matrix hardness is attributed to the martensitic microstructure and the high carbon content resulting from the interaction with WC particles.
Importantly, the paper reports that multi-pass welding does not significantly increase the degree of WC melting and decomposition. This is a crucial finding for practical applications because most wear-resistant coatings require multiple passes to achieve adequate thickness. The ability to maintain WC integrity through multiple thermal cycles is essential for producing thick, reliable coatings.
Engineering Practice Considerations
The MIG melting injection process offers several advantages for producing WC-reinforced wear-resistant coatings. First, it is a relatively simple process that can be performed with standard MIG welding equipment with the addition of a particle injection system. Second, the process is amenable to automation and can be integrated into existing production lines. Third, the multi-pass capability allows for the production of thick coatings without significant degradation of WC integrity.
However, there are several practical challenges that must be addressed. The presence of martensite in the substrate HAZ is a concern, as martensite can be brittle and susceptible to cracking, especially in thick sections or under residual stress. Post-weld heat treatment (tempering) may be necessary to relieve residual stresses and improve the toughness of the HAZ.
The grain boundary carbide precipitation in the substrate HAZ is another concern. While these carbides can improve wear resistance, they can also reduce the toughness of the substrate. In applications where the substrate must withstand impact or cyclic loading, this could be a limiting factor.
| Application Consideration | Concern | Mitigation Strategy |
|---|---|---|
| Substrate HAZ toughness | Martensite formation | Post-weld tempering treatment |
| Grain boundary carbides | Reduced substrate toughness | Control heat input, use low-carbon wire |
| WC melting | Reduced coating hardness | Optimize process parameters, control heat input |
| Coating adhesion | Thermal mismatch cracking | Control cooling rate, use interlayer |
| Multi-pass welding | Cumulative thermal effects | Control interpass temperature |
Key Insights and Reflections
This paper demonstrates the remarkable potential of MIG melting injection technology for producing high-performance wear-resistant coatings. The 850-fold improvement in wear resistance compared to the base steel is extraordinary and highlights the effectiveness of WC particle reinforcement.
The microstructural analysis provides valuable insights into the metallurgical interactions between the WC particles and the steel matrix. The formation of Fe3W3C and Co3W3C phases adjacent to the WC particles is particularly interesting, as these phases provide a metallurgical bond that enhances the cohesion of the composite coating. The fishbone eutectic microstructure further from the particles indicates a complex solidification process that can influence the coating's mechanical properties.
The finding that multi-pass welding does not significantly increase WC melting is practically important and suggests that the process is robust for producing thick coatings. However, further investigation into the effects of interpass temperature and welding sequence on WC integrity would be valuable.
One limitation of the paper is the lack of detailed discussion of the coating's resistance to specific wear mechanisms (abrasion, adhesion, erosion, etc.). The wear test results are presented in terms of relative wear resistance, but more detailed tribological characterization would provide a more complete understanding of the coating's performance.
Summary
This 2011 paper by Liu Aiguo presents a comprehensive study of MIG melting injection technology for producing WC-reinforced wear-resistant coatings on low-carbon steel. The research demonstrates exceptional wear resistance (850 times the base steel), uniform WC particle distribution, and good metallurgical bonding through the formation of intermediate carbide phases. The microstructural analysis reveals a complex interaction between the WC particles and the steel matrix, including martensite formation in the HAZ and grain boundary carbide precipitation. The finding that multi-pass welding does not significantly degrade WC integrity is practically important for producing thick coatings. The work highlights the potential of MIG melting injection as a versatile and effective technology for surface engineering applications, while also identifying important considerations for practical implementation such as HAZ toughness and post-weld treatment requirements.