Zhuojin Pipe Fitting Co., LtdPowder Filling Rate Effect on Composite Powder-Particle and Solid Wire Surfaced High-Chrome Alloy
Literature Overview and Process Innovation
The paper by Gong Jianxun, Yao Huiwen, Cheng Shiyao, Liu Chao, and Huang Hongjiang (2020), published in the Welding Journal, presents a novel open-arc surfacing process for high-chrome alloy deposition that combines pre-placed composite powder particles with an H08A solid welding wire as the arc carrier. This approach, funded by the Hunan Provincial Natural Science Foundation (Project 2015JJ5031), was developed at Xiangtan University and represents an innovative alternative to conventional flux-cored wire surfacing methods for depositing high-chrome wear-resistant alloys.
Process Description and Powder Preparation
The composite powder particles are prepared through a multi-step manufacturing process:
- Dry mixing: The powder components (high-chrome alloy powders) are dry-mixed to achieve initial homogeneity.
- Wet mixing with binder: A binder is added and the mixture is wet-mixed to ensure uniform distribution of all components.
- Rotational granulation: The wet mixture is processed in a rotary granulator to form uniform particles.
- Sintering: The granulated particles are sintered to achieve the required mechanical strength and metallurgical bonding between powder components.
- Screening: The sintered particles are screened to achieve a uniform particle size in the range of 10 to 30 mesh (approximately 0.6 to 2.0 mm).
The prepared composite powder particles are pre-placed on the weld bead path, and the H08A solid welding wire serves as the arc carrier and provides the primary heat input. The arc melts both the pre-placed powder particles and the solid wire, creating a molten pool that solidifies into the high-chrome alloy overlay.
Powder Filling Rate and Microstructural Evolution
The powder filling rate, defined as the ratio of powder particle volume to the total volume of deposited material (powder plus wire), is the key process variable investigated in this study. The powder filling rate was varied from 30 percent to 45 percent, and the resulting microstructural changes were characterized using optical microscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM).
The microstructural evolution with increasing powder filling rate is summarized as follows:
| Powder Filling Rate | Microstructure Type | Primary Matrix Phase | M7C3 Morphology |
|---|---|---|---|
| 30 percent | Hypoeutectic | Gamma-Fe (austenite) | Discontinuous network along grain boundaries or dendritic |
| Intermediate | Transitional | Mixed gamma-Fe and alpha-Fe | Mixed morphology |
| 45 percent | Hypereutectic | Alpha-Fe (ferrite) | Particulate or blocky |
This microstructural transition is of significant metallurgical importance. The hypoeutectic structure at 30 percent powder filling rate is characterized by an austenitic matrix with M7C3 carbides forming along grain boundaries in a discontinuous network or dendritic pattern. This morphology, while providing some wear resistance, is susceptible to intergranular fracture and spalling under severe wear conditions.
At 45 percent powder filling rate, the structure transitions to hypereutectic, with the primary matrix phase changing from austenite (gamma-Fe) to ferrite (alpha-Fe). The M7C3 carbide morphology changes from a continuous network to discrete particles or blocks, which is a more favorable morphology for wear resistance because it eliminates the continuous brittle network that serves as a crack initiation site.
Wear Performance and Mechanism
The wear test results demonstrate that the high-chrome alloy overlay deposited using this method exhibits excellent wear resistance, comparable to overlays deposited using flux-cored wire surfacing. The wear mechanism analysis identifies two primary mechanisms:
- Abrasive micro-cutting: Hard M7C3 carbide particles resist the cutting action of abrasive particles, resulting in shallow grooves and reduced material removal rate. The particulate morphology of M7C3 at higher powder filling rates is particularly effective at resisting micro-cutting because the discrete particles provide uniform resistance across the wear surface.
- Microscopic spalling: Under cyclic loading conditions, material is removed in small fragments through fatigue processes. The microstructure of the overlay, particularly the distribution and morphology of the M7C3 carbides, influences the resistance to spalling. The particulate M7C3 morphology at 45 percent powder filling rate provides better spalling resistance than the network morphology at 30 percent because the discrete particles do not create continuous crack paths.
Process Advantages and Economic Analysis
The open-arc surfacing process using composite powder particles and solid wire offers several advantages over conventional methods:
| Advantage | Description |
|---|---|
| Process simplicity | No flux-cored wire manufacturing required; uses standard solid wire |
| Economic efficiency | Powder particles are less expensive than flux-cored wire |
| Flexibility | Powder composition can be easily adjusted without changing wire manufacturing |
| Equipment compatibility | Compatible with standard GMAW or FCAW equipment |
| Wear performance | Comparable to flux-cored wire deposited overlays |
The economic advantage is particularly significant. Flux-cored wire manufacturing requires specialized equipment and processes, while the composite powder particle method uses standard powder metallurgy techniques that are widely available. The ability to adjust the powder composition independently from the wire material provides additional flexibility for optimizing the overlay properties for specific applications.
Study Insights and Engineering Implications
This research demonstrates that the open-arc surfacing process with pre-placed composite powder particles is a viable and economically attractive alternative to flux-cored wire surfacing for high-chrome alloy deposition. The microstructural control achieved through powder filling rate adjustment is particularly noteworthy, as it provides a straightforward process variable for tailoring the overlay microstructure to specific wear conditions.
The transition from hypoeutectic to hypereutectic structure with increasing powder filling rate is a well-known phenomenon in cast high-chrome alloys, but its demonstration in a welding overlay context is significant. It confirms that the welding solidification process can produce similar microstructural transitions to casting, and that the powder filling rate serves as an effective lever for controlling this transition.
The particulate M7C3 morphology achieved at higher powder filling rates is consistent with the well-established principle that discrete hard particles in a ductile matrix provide superior wear resistance compared to continuous networks of hard phases. This finding reinforces the importance of carbide morphology, not just carbide content, in determining overlay wear performance.
This study opens up new possibilities for the design of high-performance wear-resistant overlays through the strategic combination of powder metallurgy and arc welding, demonstrating that innovative process approaches can achieve wear performance comparable to established methods while offering greater flexibility and lower cost.