Zhuojin Pipe Fitting Co., LtdEffect of Powder Spraying Method on CO2 Gas Shielded Weld Overlay Microstructure and Wear Resistance
Literature Overview
This research by Ma Chunli and colleagues from Jiamusi University's Engineering Research Center for Metal Wear-Resistant Materials and Surface Technology, published in the Journal of Jiamusi University (Natural Science Edition, 2015, Vol. 33, No. 4, pp. 606-608), investigates the influence of powder feeding methods on the microstructure and wear resistance of CO2 gas shielded arc welding (GMAW) overlay deposits. The study compares in-gun (internal) powder feeding with out-of-gun (external) powder feeding for a Cr-Ti-Mn-B alloy powder system, providing valuable insights into process optimization for hardfacing applications.
Process Configuration and Materials
The study employs a CO2 gas shielded welding process with an optimized Cr-Ti-Mn-B alloy powder as the overlay material. The key variable is the powder delivery method:
- In-gun (internal) feeding: Powder is fed through the internal channel of the welding gun, typically through a separate powder tube within the gun body.
- Out-of-gun (external) feeding: Powder is delivered to the arc zone from an external nozzle or hopper positioned near the gun tip.
The Cr-Ti-Mn-B composition is selected for its ability to form hard carbide and boride phases that provide excellent wear resistance. Chromium contributes carbide formation and oxidation resistance, titanium promotes hard carbide precipitation, manganese enhances base strength and austenite stability, and boron facilitates the formation of hard boride phases.
Key Findings on Weld Formation and Microstructure
The in-gun powder feeding method produced overlay layers with superior weld bead formation and higher alloy utilization efficiency compared to the out-of-gun method. The powder melted more uniformly in the arc zone when fed internally, resulting in more homogeneous alloy distribution throughout the weld cross-section.
| Parameter | In-Gun Feeding | Out-of-Gun Feeding |
|---|---|---|
| Bead Formation | Excellent | Good |
| Alloy Utilization | High | Moderate |
| Microstructural Uniformity | High | Moderate |
| Surface Hardness (HRC) | 65 | Lower |
| Surface Microhardness (HV) | 730 | Lower |
| Core Microhardness (HV) | 811 | Lower |
| Wear Loss Rate | < 0.2% | Higher |
The microhardness results are particularly noteworthy. The core microhardness of 811 HV exceeded the surface microhardness of 730 HV, indicating that the alloying elements and hard phases are more concentrated in the interior of the overlay layer. This is consistent with the mechanism of in-gun feeding, where powder particles are entrained in the molten weld pool and distributed throughout the cross-section rather than being primarily deposited on the surface.
Microstructural Analysis
Metallographic examination and X-ray diffraction (XRD) analysis revealed that the in-gun fed overlay layers exhibited more uniform microstructural distribution. The hard carbide and boride phases were dispersed more evenly throughout the weld metal, contributing to the high and uniform hardness values. In contrast, the out-of-gun method likely produced a more heterogeneous distribution of hard phases, with potential segregation of alloying elements toward the surface or in specific regions of the weld cross-section.
The formation of hard phases in Cr-Ti-Mn-B systems typically involves:
- Chromium carbides (Cr7C3, Cr23C6) providing base hardness and wear resistance
- Titanium carbides (TiC, TiC0.7) contributing exceptional hardness
- Iron-boron compounds (Fe2B, FeB) forming hard boride phases
- Possible formation of complex carbides involving multiple alloying elements
Wear Testing Results
The wear testing demonstrated that the in-gun fed overlay layers achieved a wear loss rate of less than 0.2%, representing a significant improvement in wear resistance. This excellent performance is attributed to the synergistic effect of multiple hard phases uniformly distributed in a ductile matrix, providing both hardness and toughness. The combination of Cr, Ti, Mn, and B in the alloy system creates a microstructure where hard carbide and boride particles are supported by a relatively tough matrix, resisting both abrasive and adhesive wear mechanisms.
Engineering Practice Implications
For industrial hardfacing applications, the choice of powder feeding method has direct implications for production efficiency and repair quality:
- In-gun feeding is preferred when high alloy utilization, uniform microstructure, and consistent hardness are required. It is particularly suitable for automated or semi-automated welding operations where process repeatability is critical.
- Out-of-gun feeding may be advantageous in situations where the welding gun design does not accommodate internal powder channels, or when a surface-enriched hard layer is desired for specific applications.
- CO2 shielding provides adequate arc protection for most hardfacing applications while being economical and readily available. However, the oxidizing nature of CO2 can lead to some oxygen pickup in the weld metal, which may affect the quality of hard carbide and boride formation.
Critical Reflections
The study's focus on a single alloy system (Cr-Ti-Mn-B) limits the generalizability of the findings. Different alloy compositions may respond differently to the two powder feeding methods, particularly if the powder particle size distribution, melting point range, or alloying element reactivity varies significantly. Future research should examine the interaction between powder characteristics (particle size, shape, composition) and feeding method to establish more comprehensive process guidelines.
Additionally, the study does not address the long-term wear performance under specific service conditions such as high temperature, corrosive environments, or impact loading. The laboratory wear testing, while providing useful comparative data, may not fully capture the complex wear mechanisms encountered in actual industrial applications.
Study Insights and Conclusions
This research provides clear evidence that in-gun powder feeding in CO2 GMAW hardfacing produces superior overlay layers in terms of weld formation, microstructural uniformity, hardness, and wear resistance compared to out-of-gun feeding. The HRC 65 surface hardness and sub-0.2% wear loss rate achieved with the in-gun method demonstrate the effectiveness of this approach for wear-resistant overlay applications. For engineers specifying hardfacing procedures, the powder feeding method should be considered a critical process parameter that directly influences the final performance of the overlay layer. The findings support the use of internal powder feeding systems for applications demanding high and consistent wear resistance.