Zhuojin Pipe Fitting Co., LtdEffect of Powder Filling Rate on Microstructure and Wear Resistance of Composite Powder-Granules Overlay Alloy
Literature Overview
This paper published in Trans. Weld. Join. Inst. China (Vol. 41, No. 7, 2020, pp. 53–58) by Gong Jianxun and colleagues from Xiangtan University investigates a novel open-arc self-shielded overlay welding method that uses pre-placed composite powder-granules (10–30 mesh) as the alloying source, with H08A solid wire serving merely as the arc carrier. The work is funded by the Hunan Provincial Natural Science Foundation (Grant 2015JJ5031). The authors systematically examine how the powder filling rate (30%–45%) influences the microstructure and tribological performance of the resulting high-chromium alloy overlay deposit.
Core Technical Approach
The methodology is elegant in its simplicity. Powder components are dry-mixed, then wet-mixed with a binder, granulated through rotary granulation, sintered, and sieved to produce composite powder-granules in the 10–30 mesh range. These granules are pre-placed on the weld path before welding begins. The H08A solid wire functions solely as an arc-stabilizing medium, transferring heat to melt the pre-placed powder. This approach eliminates the need for flux-cored wire, reducing material cost and process complexity significantly.
| Parameter | Value / Range |
|---|---|
| Powder mesh size | 10–30 mesh |
| Powder filling rate studied | 30%–45% |
| Arc carrier wire | H08A solid wire |
| Alloy type | High-chromium alloy |
| Welding method | Open-arc self-shielded overlay |
| Characterization | OM, XRD, SEM, wear testing |
Microstructural Evolution with Filling Rate
The key finding is that increasing the powder filling rate from 30% to 45% causes a fundamental metallurgical transition:
- At 30% filling rate: The deposit exhibits a hypoeutectic microstructure with a γ-Fe (austenite) primary matrix. M7C3 carbide phases appear as discontinuous intergranular networks or dendritic structures.
- At 45% filling rate: The deposit transitions to a hypereutectic structure with an α-Fe (ferrite) primary matrix. M7C3 carbide phases evolve into granular or blocky morphologies, which are far more beneficial for wear resistance.
This transition is critical from an engineering standpoint. The granular M7C3 carbides at higher filling rates provide superior load-bearing capacity and resistance to micro-cutting during abrasive wear, whereas the network-type carbides at lower filling rates are prone to cracking under contact stress.
Wear Mechanism Analysis
The wear testing reveals two dominant mechanisms:
- Micro-cutting — Hard M7C3 carbide particles act as cutting tools against the counterface, but their granular morphology at high filling rates prevents catastrophic material removal.
- Micro-spalling — Thin layers of material are removed through cyclic contact stress, but the high volume fraction of dispersed carbides at 45% filling rate impedes crack propagation through the matrix.
The wear resistance achieved is comparable to that of conventional flux-cored wire overlay deposits of high-chromium alloy, which is a significant practical achievement given the substantially simpler process.
Engineering Practice Insights
From a production standpoint, this method offers several advantages worth considering for field applications:
- Cost reduction: Eliminating flux-cored wire and using pre-placed powder significantly lowers consumable costs.
- Process flexibility: The powder composition can be adjusted independently of the arc carrier wire, allowing rapid alloy design iteration.
- Coating thickness control: The powder filling rate directly governs the dilution ratio and thus the final alloy composition of the overlay layer.
However, several practical challenges must be addressed before widespread adoption:
- Pre-placing powder on vertical or overhead surfaces requires additional fixturing or adhesive bonding.
- Powder loss during welding (blow-off) must be monitored to maintain consistent filling rate.
- The open-arc nature of the process may be limited in outdoor or windy conditions without shielding.
Reflections and Implications
This research demonstrates a philosophy that resonates with lean manufacturing principles: achieving equivalent performance through process innovation rather than material complexity. The transition from hypoeutectic to hypereutectic microstructure with increasing powder content is well-documented in high-chromium alloy metallurgy, but the practical implementation via pre-placed granules with a plain carbon steel wire is novel and commercially attractive. Engineers working on wear-resistant overlays for mining equipment, cement mill liners, and pipeline components should consider this approach as a cost-effective alternative, particularly where large coating areas require economical deposition rates.