Effect of Powder Filling Rate on Microstructure and Wear Resistance of Composite Powder and Solid Wire Surfaced Alloy

Literature Overview

The paper by Gong Jianxun and colleagues from Xiangtan University, published in the Journal of Welding in 2020, investigates the influence of powder filling rate on the microstructure and wear resistance of high-chromium alloy overlay deposits produced by a novel open-arc self-shielded surfacing technique. The composite powder particles, prepared through dry mixing, wet mixing with binder, rotary granulation, sintering, and sieving to achieve 10 to 30 mesh sizes, are pre-placed on the weld bead and melted using H08A solid wire as the arc carrier. This approach represents a cost-effective alternative to conventional flux-cored wire surfacing for high-chromium alloy deposits.

Core Technical Findings

The study systematically varied the powder filling rate from 30% to 45% and characterized the resulting deposits using optical microscopy, X-ray diffraction, and scanning electron microscopy. The key metallurgical transitions observed are summarized below.

The transition from hypoeutectic to hypereutectic structure with increasing powder filling rate is a critical finding. At lower filling rates, the dilution from the H08A base wire is relatively high, resulting in a carbon-deficient composition that favors the austenitic (gamma-Fe) matrix. As the filling rate increases, the effective carbon and chromium content of the molten pool rises, pushing the composition past the eutectic point and stabilizing the ferritic (alpha-Fe) matrix with copious carbide precipitation.

Microstructural Interpretation

The morphological evolution of M7C3 carbides is particularly significant for wear performance. At 30% filling rate, the M7C3 carbides form a discontinuous network along grain boundaries or exhibit dendritic shapes, which, while providing some hardening effect, can act as crack initiation sites under abrasive loading. At 45% filling rate, the M7C3 carbides adopt a granular or blocky morphology distributed within the ferritic matrix. This morphology is generally considered more favorable for abrasion resistance because the discrete carbide particles impede sliding contact without creating interconnected brittle pathways.

The wear mechanism identified in this study includes two primary modes: micro-cutting by abrasive particles and micro-ploughing. The micro-cutting mechanism dominates when hard abrasive particles exceed a critical size relative to the carbide spacing, while micro-ploughing occurs when the abrasive particles are smaller and interact with the matrix phase. The improved wear resistance at higher filling rates can be attributed to the increased volume fraction and refined morphology of the M7C3 carbides, which collectively enhance the resistance to both wear mechanisms.

Engineering Practice Implications

From an engineering perspective, this technique offers several advantages over conventional flux-cored wire surfacing for high-chromium alloy overlays. The preparation of composite powder particles is relatively straightforward and does not require complex wire manufacturing equipment. The open-arc self-shielded process eliminates the need for shielding gas, making it suitable for outdoor and field applications where gas supply is inconvenient. The wear resistance achieved is comparable to that of flux-cored wire deposits, which validates the technical feasibility of this approach.

However, several practical considerations must be addressed before widespread industrial adoption. The powder pre-placement process requires careful control to ensure uniform distribution along the weld bead, as local variations in powder density could lead to compositional inhomogeneity in the deposit. The open-arc process is inherently more susceptible to atmospheric contamination than gas-shielded processes, particularly in humid or windy environments, which could affect the deposit quality. The dilution ratio between the base metal and the surfacing alloy also needs to be monitored, as excessive base metal dilution can degrade the high-chromium alloy properties.

Study Insights and Reflections

This research demonstrates a creative approach to surfacing alloy design by decoupling the powder preparation from the wire manufacturing process. The ability to precisely control the powder composition and particle size independently of the wire allows for greater flexibility in tailoring the deposit properties. The finding that the microstructure transitions from hypoeutectic to hypereutectic with increasing filling rate provides a practical lever for property optimization. Engineers seeking maximum hardness and abrasion resistance should target filling rates that produce a hypereutectic structure with granular M7C3 carbides, while applications requiring better toughness might benefit from hypoeutectic structures with lower carbide volume fractions.

The economic argument presented is compelling for cost-sensitive applications. High-chromium alloy surfacing using flux-cored wires can be expensive due to the complex wire manufacturing and the cost of the alloy powder in the wire core. By preparing the composite powder separately and using inexpensive H08A solid wire as the arc carrier, the overall process cost is significantly reduced. This makes the technique particularly attractive for large-area surfacing applications such as mining equipment, material handling components, and heavy-duty wear parts where the total surfacing area is substantial.

This work opens an important direction for further research into hybrid surfacing techniques that combine the simplicity of solid wire welding with the compositional flexibility of powder metallurgy. The key challenge for future development lies in standardizing the powder preparation process and establishing reliable quality control procedures to ensure consistent deposit properties across different production batches and operating conditions.