Zhuojin Pipe Fitting Co., LtdMicrostructure and Properties of Plasma Arc Overlay on Q235 Electrolytic Breaking Hammer Head
Literature Overview
Published in Surface Technology of China (2012, Vol. 25, No. 3, pp. 47-51), this study by Cao Hongmei and colleagues from Wuhan University investigates the application of plasma arc overlay (PAO) technology to restore and enhance the performance of Q235 steel electrolytic aluminum breaking hammer heads. The F40 alloy powder cladding layer is deposited onto the Q235 base material, and the resulting microstructure, hardness distribution, wear resistance, and corrosion resistance are comprehensively characterized. The research was supported by the China Power Investment Corporation Science and Technology Project (2011074NXLKJX) and represents a direct industrial application of surface engineering in the aluminum electrolysis industry.
Core Technical Content
Plasma Arc Overlay Process Characteristics
Plasma arc overlay is a highly efficient surface engineering technique that utilizes a high-temperature, high-velocity plasma jet to simultaneously melt and project powder onto the substrate surface. Compared to conventional arc welding overlay methods, PAO offers several distinct advantages: precise control of dilution rate (typically 5-15%), excellent layer-to-substrate metallurgical bonding, low heat input to the base material, and the ability to deposit layers with compositions closely matching the feedstock powder. These characteristics make PAO particularly suitable for the repair and upgrade of critical industrial components where dimensional accuracy and low residual stress are essential.
Microstructural Analysis
The overlay layer exhibits a typical columnar crystal structure, which is characteristic of directional solidification under the high cooling rates associated with plasma arc processing. The columnar grains grow perpendicular to the substrate surface, reflecting the strong thermal gradient established during the single-pass or multi-pass deposition. The interface between the overlay layer and the Q235 base metal shows excellent metallurgical bonding, indicating proper wetting and diffusion at the fusion boundary. The absence of cracks, voids, or delamination at the interface confirms that the process parameters were well-optimized for this substrate-overlay system.
Hardness and Wear Performance
The average microhardness of the plasma overlay layer is measured at 444 HV₀.₁, which is approximately twice that of the Q235 base material. This significant improvement in hardness is attributed to the alloying elements in the F40 powder (typically containing Cr, Mo, and other strengthening elements) and the refined microstructure resulting from the rapid solidification in the plasma arc process. The wear resistance test using a friction-wear tester demonstrates that the overlay layer exhibits 1.6 times the wear resistance of the base material. While the hardness improvement is a factor of 2, the wear resistance improvement is somewhat lower (1.6×), suggesting that the wear mechanism is not purely hardness-dependent and that microstructural factors such as grain orientation and phase distribution also play significant roles.
| Property | Base Material (Q235) | Plasma Overlay Layer (F40) | Improvement Factor |
|---|---|---|---|
| Microhardness | ~220 HV₀.₁ (estimated) | 444 HV₀.₁ | ~2× |
| Wear Resistance | Baseline | 1.6× baseline | 1.6× |
| Corrosion Rate (Rcorr) | ~14.8 mm/a (estimated) | 3.524 × 10⁻⁴ mm/a | ~42,000× reduction |
Corrosion Resistance
Perhaps the most striking finding in this study is the dramatic improvement in corrosion resistance. The corrosion rate of the overlay layer is measured at 3.524 × 10⁻⁴ mm/a, which represents a reduction of approximately 42,000 times compared to the base material. This extraordinary improvement is attributed to the formation of a passive chromium-rich oxide film on the overlay surface, combined with the alloying elements in the F40 powder that enhance passivation. In the context of aluminum electrolysis, where hammer heads are exposed to molten cryolite and aluminum fluoride salts at elevated temperatures, this level of corrosion protection is of immense practical value.
Engineering Practice Integration
The electrolytic aluminum breaking hammer head is a critical tool in the aluminum smelting industry, used to break the carbon anode crust that forms on the surface of the molten electrolyte. These hammer heads are subjected to severe combined degradation mechanisms: high-temperature thermal cycling, mechanical impact loading, abrasive wear from the carbon crust, and chemical corrosion from the molten salt environment. The Q235 base material, while inexpensive and readily available, offers limited resistance to any of these degradation mechanisms.
The application of plasma arc overlay with F40 alloy powder addresses all three degradation modes simultaneously. The improved hardness and microstructure provide enhanced wear resistance against the abrasive carbon crust, the alloying elements provide corrosion protection against the molten salt environment, and the low dilution rate of PAO ensures that the beneficial alloy composition is maintained throughout the overlay thickness. From an economic perspective, the extension of hammer head service life through overlay repair is far more cost-effective than periodic replacement, particularly when considering downtime costs in continuous aluminum production.
Key Questions and Reflections
The study raises several important questions for further investigation. First, the long-term durability of the overlay under actual production conditions—particularly under thermal cycling between ambient temperature and the operating temperature of the electrolytic cell (typically 950-970°C)—remains to be confirmed. Laboratory corrosion tests conducted at room temperature may not fully represent the aggressive conditions encountered in service.
Second, the impact resistance of the overlay layer is not evaluated in this study, yet the hammer head is subjected to significant impact loading during operation. The columnar crystal structure, while beneficial for hardness, may be susceptible to intergranular cracking under impact conditions. Post-weld heat treatment or multi-layer deposition with varying layer orientations could potentially address this concern.
Third, the study does not report on the residual stress state of the overlay or the base material. In plasma arc overlay, residual stresses can be significant and may affect the fatigue life and dimensional stability of the component. Stress relief treatments or process parameter optimization to minimize residual stresses should be considered in production applications.
Study Insights and Outlook
This work demonstrates the transformative potential of plasma arc overlay technology in extending the service life of industrial components subjected to combined degradation mechanisms. The findings provide a compelling case for the adoption of surface engineering solutions in the aluminum electrolysis industry, where component downtime and replacement costs are substantial. The dramatic improvement in corrosion resistance—reducing the corrosion rate by over four orders of magnitude—highlights the synergistic benefits of alloying and microstructural refinement achieved through plasma arc processing. Future research should focus on accelerated life testing under simulated production conditions, optimization of multi-layer deposition strategies for enhanced impact resistance, and cost-benefit analysis comparing overlay repair to full component replacement.