Effect of Feldspar on Microstructure and Wear Resistance of Arc Hardfacing Alloys

Literature Overview

The paper by Liu Chao and Gong Jianxun, published in the Journal of Ordnance Materials and Science (Vol. 47, No. 6, 2024), investigates the effect of feldspar content on the microstructure and wear resistance of arc hardfacing alloys produced using composite powder particles and solid wire electrodes. The study employs a lateral flux addition method, where feldspar-containing flux blocks are placed adjacent to the welding arc to modify the arc morphology and the composition of the molten pool. The research was supported by the Hunan Provincial Natural Science Foundation (Grant No. 2021JJ30669).

Experimental Methodology

The experimental approach is innovative in several respects. The use of a high-speed camera to capture arc morphology provides direct visual evidence of how the feldspar flux affects the arc shape and coverage area. This is critical for understanding the mechanism by which the flux influences the welding process and the resulting deposit composition.

The characterization methodology includes X-ray diffraction (XRD) for phase identification, scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS) for microstructural analysis and elemental mapping, and pin-on-disc wear testing for wear resistance evaluation. This comprehensive characterization provides a thorough understanding of the structure-property relationships in the hardfacing deposits.

The composite powder particles used in the study are designed to provide a high volume fraction of hard phases in the deposit. The solid wire serves as the base material, while the powder particles act as reinforcement. The feldspar flux modifies the arc environment to improve the melting and incorporation of the powder particles into the weld deposit.

Arc Morphology and Process Mechanism

The high-speed camera observations reveal a significant change in arc morphology with the addition of feldspar-containing flux blocks. Without the flux, the arc assumes a typical bell-shaped (campanula) configuration, which is characteristic of standard GMAW welding. With the addition of feldspar, the arc expands outward and transforms into a hemispherical shape. This change in arc geometry increases the arc coverage area, which has two important consequences.

First, the increased arc coverage area provides more thermal energy to the composite powder particles, promoting their melting and incorporation into the weld pool. This results in a higher volume fraction of reinforcement particles in the deposit. Second, the hemispherical arc shape provides more uniform heat distribution across the weld pool, which can improve the homogeneity of the deposit microstructure.

The feldspar acts as a flux that modifies the arc plasma composition. Feldspar is a silicate mineral (typically KAlSi3O8 or NaAlSi3O8) that decomposes at welding temperatures to release potassium or sodium oxides and silicon oxides into the arc plasma. These species modify the arc conductivity and shape, causing the expansion observed in the high-speed camera images.

Microstructural Evolution and Wear Resistance

The microstructural analysis reveals that the primary carbide phase in the hardfacing alloy is M7C3, which is a common carbide in medium-alloy hardfacing alloys. The morphology of the M7C3 carbides is significantly affected by the feldspar content. At low to moderate feldspar levels, the primary M7C3 carbides transform from a fine strip-like morphology to a plate-like (lamellar) morphology. This morphological change is associated with an increase in wear resistance.

The plate-like M7C3 carbides are more effective in resisting wear than the fine strip-like carbides because they provide better load-bearing capacity and crack deflection. The plate-like carbides are oriented preferentially in the direction of the applied load, creating a more effective barrier against abrasive particles. The increased volume fraction of M7C3 carbides, resulting from improved powder particle incorporation, further enhances wear resistance.

However, the study identifies an optimal feldspar content. Beyond this optimum, excessive feldspar causes the arc to expand too much, which increases the preheating of the solid wire and enhances its melting rate. This leads to a slight decrease in powder particle incorporation because the increased base metal dilution reduces the relative contribution of the reinforcement particles. Consequently, the M7C3 carbide volume fraction decreases, and the wear resistance declines.

Engineering Practice and Optimization

The findings have direct implications for the design and optimization of composite powder hardfacing processes. The lateral flux addition method offers a practical approach to modifying arc behavior without changing the base electrode or wire composition. This is advantageous because it allows for process optimization through flux selection and placement rather than requiring changes to the electrode design.

For engineering applications, the key is to determine the optimal feldspar content for a specific application. This requires systematic testing with varying feldspar levels, monitoring arc morphology, and evaluating the resulting deposit microstructure and wear resistance. The optimal feldspar content will depend on the specific electrode composition, welding parameters, and the desired wear resistance level.

The study also highlights the importance of arc morphology control in composite powder welding. The arc shape directly influences the thermal input distribution, which affects both the melting behavior of the reinforcement particles and the solidification microstructure of the deposit. Controlling the arc through flux addition is a practical and effective method for optimizing the welding process.

Key Technical Insights

The most significant finding is the non-monotonic relationship between feldspar content and wear resistance. This indicates that there is an optimal flux level that maximizes the beneficial effects of arc expansion while avoiding the detrimental effects of over-expansion. This type of optimization is common in welding process development, where multiple competing factors must be balanced to achieve the desired outcome.

The morphological transformation of M7C3 carbides from strip-like to plate-like is particularly noteworthy. This suggests that the solidification conditions in the weld pool, influenced by the arc morphology and heat input distribution, play a critical role in determining the carbide morphology. Understanding and controlling this relationship is essential for producing high-performance hardfacing deposits.

Summary and Reflections

This study provides valuable insights into the role of flux addition in modifying arc behavior and deposit microstructure in composite powder hardfacing. The use of feldspar as a flux to expand the arc and improve powder incorporation is a practical and cost-effective approach. The identification of an optimal feldspar content, beyond which performance degrades, underscores the importance of systematic optimization in welding process development. For engineers working on wear-resistant component repair, this study offers a clear pathway for improving hardfacing performance through flux engineering. The key lesson is that arc morphology is not merely a process variable but a critical design parameter that directly influences deposit quality, and that simple flux additions can have profound effects on welding outcomes.