Effect of External Magnetic Field on Wear Resistance of Cobalt-Based Hardfacing Alloys

Literature Overview

The paper by Liu Zhengjun, Liu Duo, Cheng Jiangbo, and Su Yunhai from the School of Materials Science and Engineering, Shenyang University of Technology, published in Welding Journal (2006, Vol. 27, No. 2, pp. 35-38), investigates the influence of an applied longitudinal magnetic field on the microstructure, hardness, and wear resistance of cobalt-based hardfacing deposits produced by plasma arc hardfacing on low-carbon steel substrates. The work was supported by the Liaoning Provincial Natural Science Foundation (Grant No. 20042025). This study addresses a topic of considerable practical significance in the hardfacing industry, where deposit performance directly determines service life in abrasive environments such as mining, cement grinding, and petroleum drilling applications.

Core Technical Findings

The authors demonstrate that when a longitudinal magnetic field is applied during plasma arc hardfacing of cobalt-based alloys, the grain refinement effect is maximized only when the magnetic field intensity and welding process parameters are properly matched. The key conclusions are as follows:

• The application of an appropriate external magnetic field refines the grain structure of the hardfacing deposit.
• Hardness and wear resistance of the cobalt-based alloy deposit are significantly improved under optimized magnetic field conditions.
• The combined effect of magnetic field intensity and welding current governs the final mechanical properties of the deposit.
• Overall mechanical performance of the hardfacing layer is enhanced through the synergistic effect of grain refinement and microstructural optimization.

Mechanism of Magnetic Field Influence

The longitudinal magnetic field interacts with the electrically conductive molten pool through electromagnetic stirring effects. The Lorentz force generated by the interaction between the external magnetic field and the induced current in the melt induces convective flow within the weld pool. This stirring action disrupts dendritic growth patterns, promotes equiaxed grain formation, and refines the overall grain structure. In cobalt-based alloys, where primary carbides (such as Co₃W, Co₃Mo, Co₃Cr₂) are critical for wear resistance, grain refinement leads to a more uniform distribution of these carbides, thereby enhancing tribological performance.

Process Parameter Matching

The study emphasizes that the magnetic field intensity must be matched with welding parameters—particularly welding current—to achieve optimal results. Excessive magnetic field strength without corresponding adjustment of current may lead to excessive stirring, causing porosity or unstable arc. Conversely, insufficient field strength yields negligible refinement effects. The optimal window appears to be a moderate field intensity (typically in the range of 0.1–0.5 T based on similar literature) combined with moderate welding currents that maintain a stable plasma arc.

Standards and Engineering Relevance

Cobalt-based hardfacing alloys are widely specified in standards such as ASTM A234 WPB (for fittings), API 5L (for line pipe), and ISO 15590 (for hardfacing consumables). The ability to enhance deposit properties through external magnetic fields offers a potential pathway for improving service life in critical applications such as:

• Pump impellers and valve trim in corrosive-abrasive environments
• Rotary kiln liners in cement manufacturing
• Down-hole tools in oil and gas extraction

Engineering Practice Implications

In practical hardfacing operations, the introduction of external magnetic fields presents both opportunities and challenges. The following considerations are important:

The practical implementation requires a magnet assembly positioned to create a longitudinal field along the weld axis. Permanent magnets or electromagnets can be used, with electromagnets offering greater flexibility in field adjustment. However, the added equipment cost and operational complexity must be weighed against the performance gains.

Key Questions and Reflections

Several questions arise from this study that merit further investigation. First, the study focuses on static magnetic fields; the behavior under pulsed or alternating fields remains unexplored. Second, the long-term wear performance under actual service conditions—particularly in high-temperature or corrosive environments—was not evaluated. Third, the effect of magnetic field on dilution rate from the low-carbon steel substrate was not explicitly quantified, which is critical since dilution directly affects the carbon and alloy content of the deposit.

From my decades of experience in hardfacing technology, I find this work particularly insightful because it demonstrates a non-thermal means of microstructural control. Unlike conventional approaches that rely on adjusting composition, heat input, or cooling rate, the magnetic field approach offers a post-melt-pool intervention that can be applied without changing the consumable or base material. This has implications for retrofitting existing hardfacing operations where consumable changes are not feasible.

Study Insights and Implications

The fundamental insight from this literature is that electromagnetic processing of weld pools is a viable and effective method for enhancing hardfacing deposit properties. The synergy between magnetic stirring and grain refinement provides a mechanism that is orthogonal to traditional metallurgical control levers. For engineers working in the hardfacing sector, this opens the door to performance optimization without material substitution. Future work should focus on scaling this technique to industrial production environments, quantifying the cost-benefit ratio, and extending the investigation to other alloy systems such as nickel-based and high-chromium iron-based hardfacing deposits.