CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Powder Spraying Method on CO2 Gas Shielded Weld Overlay Microstructure and Wear Resistance
Literature Overview
Published in 2015 by Ma Chunli and colleagues from the Ministry of Education Engineering Research Center for Metal Wear-Resistant Materials and Surface Technology at Jiamusi University, this paper investigates the influence of powder feeding methods on the microstructure and wear resistance of CO2 gas shielded arc weld overlay deposits. The work was supported by a national science and technology program project and a university-level research grant, indicating its significance in the field of wear-resistant surface engineering. The study addresses a practical problem in industrial welding: the variability in powder delivery systems used in wire-plus-powder and wire-only powder overlay processes, and how this variability affects the final performance of the overlay.
Experimental Setup and Powder Feeding Methods
The authors compare two distinct powder feeding configurations in a CO2 gas shielded arc welding setup: a side-suction (lateral) powder feeder and a center-powder (axial) powder feeder. In the side-suction method, the powder is introduced laterally into the arc zone through a nozzle positioned at an angle to the welding axis. In the center-powder method, the powder is fed axially through the center of the torch, directly into the arc.
| Parameter | Side-Suction Feeder | Center-Powder Feeder |
|---|---|---|
| Powder trajectory | Angled entry into arc | Axial entry into arc |
| Arc disturbance | Moderate | Minimal |
| Powder utilization | Lower (more scatter) | Higher (more directed) |
| Typical powder composition | Hardfacing alloy (Cr-C, Cr-B, Cr-Cr2C) | Same as side-suction |
| Wire diameter | 1.2 mm to 1.6 mm | 1.2 mm to 1.6 mm |
| Shielding gas | CO2 (99.5% purity) | CO2 (99.5% purity) |
The overlay deposits were produced on low-carbon steel substrates, and the microstructure was examined using optical microscopy and scanning electron microscopy (SEM). Wear resistance was evaluated using a pin-on-disk tribometer under dry sliding conditions.
Microstructural Analysis
The microstructure of the overlay deposits showed clear differences between the two powder feeding methods. In the side-suction deposits, the microstructure exhibited a coarser grain structure with more pronounced dendritic segregation. This is attributed to the asymmetric heat input caused by the lateral powder introduction, which creates a temperature gradient across the weld bead. The dendrites grew preferentially in the direction of the heat gradient, resulting in compositional segregation of carbon and alloying elements.
In contrast, the center-powder deposits displayed a more uniform microstructure with finer dendrite spacing and reduced segregation. The axial powder feeding produced a more symmetric thermal profile, promoting more uniform solidification. SEM analysis revealed that the center-powder deposits contained a higher proportion of hard carbide phases (Cr7C3, Cr23C6) distributed more uniformly throughout the matrix, compared to the side-suction deposits where carbides tended to cluster at the interdendritic regions.
The fusion zone microstructure also differed. In both cases, a dilution zone was observed where the base metal alloyed with the overlay material. However, the dilution zone in the side-suction deposits was wider and more irregular, while in the center-powder deposits it was narrower and more consistent. This has implications for the overall wear resistance of the overlay, as the dilution zone typically has lower hardness and is more susceptible to wear initiation.
Wear Resistance Results
The wear resistance tests confirmed the microstructural observations. The center-powder deposits exhibited approximately 15% to 25% higher wear resistance compared to the side-suction deposits under identical testing conditions. The improved wear resistance is attributed to three factors:
- More uniform distribution of hard carbide phases, which provide more consistent resistance to abrasive wear.
- Finer dendrite spacing, which reduces the likelihood of intergranular fracture during wear.
- Narrower dilution zone, which means a larger fraction of the overlay thickness is composed of the wear-resistant material rather than the diluted transition zone.
The authors also noted that the CO2 shielding gas contributed to a certain degree of carbon pickup in the overlay, which promoted carbide formation. However, the CO2 also introduced some porosity, particularly in the side-suction deposits where the gas flow pattern was more turbulent. This porosity further degraded the wear resistance of the side-suction deposits.
Engineering Implications
From a practical standpoint, this research has direct implications for the selection of powder feeding systems in industrial hardfacing and wear-resistant overlay applications. For applications where wear resistance is critical and the overlay thickness is limited, the center-powder feeding method is clearly superior. However, center-powder feeders are generally more expensive and require more precise maintenance than side-suction feeders. The choice of feeding method should therefore be made based on a cost-benefit analysis that considers the severity of wear conditions, the required overlay thickness, and the availability of qualified equipment and personnel.
The study also highlights the importance of process parameter optimization beyond just the powder feeding method. Factors such as welding current, voltage, travel speed, and powder flow rate all interact with the powder feeding configuration to determine the final overlay quality. A systematic approach, such as the Taguchi method or response surface methodology, would be recommended for optimizing these parameters in conjunction with the selected powder feeding method.
Summary
This paper provides clear evidence that the powder feeding method in CO2 gas shielded weld overlay significantly influences both the microstructure and the wear resistance of the resulting deposits. The center-powder feeding method produces more uniform microstructures with finer carbide distribution and higher wear resistance, while the side-suction method, though simpler and less expensive, results in coarser microstructures and lower wear performance. For engineers selecting overlay processes for wear-critical applications, this research supports the investment in center-powder feeding technology when the performance requirements justify the additional cost.