Zhuojin Pipe Fitting Co., LtdEffect of Plasma Surfacing Travel Speed on Ni60 Overlay Microstructure and Hardness
Literature Overview
This study by Luo Shengyang and Yuan Zhentao from Kunming University of Science and Technology, published in Hot Working Technology (2018, Vol. 47, No. 5), investigates the influence of plasma surfacing travel speed on the microstructure, phase composition, and microhardness of single-layer, single-pass Ni60 overlay coatings deposited on 304 stainless steel substrates. The work was supported by the Yunnan Provincial Department of Education Grant (2017ZZX136). The authors employed metallographic microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and microhardness testing to characterize the coatings systematically.
Core Technical Findings
The central conclusion of this paper is that a travel speed of 200 mm/min represents the optimal processing window for Ni60 plasma surfacing on 304 stainless steel. At this speed, the coating achieves a wetting angle of 55 degrees and a dilution rate of only 4.5 percent, indicating excellent macroscopic quality with minimal substrate contamination. The microstructure in the central region of the coating at this optimal speed consists of columnar dendrites with precipitates growing laterally along small crystallographic planes.
As the travel speed increases beyond this optimum, the crystallinity of both the Fe-Ni intermetallic phase (FeNi_x) and the boride phase exhibits a non-monotonic trend—first increasing and then decreasing. Correspondingly, the microhardness of the overlay follows the same pattern, rising to a peak and then falling. At 200 mm/min, the FeNi_x phase and borides achieve good crystallinity, the coating grains are fine, and the average hardness reaches its maximum value.
Process Parameter Analysis
| Parameter | Optimal Value | Effect on Coating |
|---|---|---|
| Travel speed | 200 mm/min | Best wetting angle (55°), lowest dilution (4.5%) |
| Dilution rate | 4.5% | Minimal substrate alloy contamination |
| Wetting angle | 55° | Good spread and bonding quality |
| Microstructure (optimal) | Columnar dendrites + lateral precipitates | Fine grain, high hardness |
| FeNi_x crystallinity | Peak at 200 mm/min | Maximum hardness contribution |
| Boride crystallinity | Peak at 200 mm/min | Enhanced wear resistance |
The dilution rate of 4.5 percent is notably low, which is significant for Ni60 coatings applied on austenitic stainless steel substrates. In engineering practice, dilution rates below 10 percent are generally considered acceptable for Ni60 overlays, but values below 5 percent are desirable for applications where the Ni-Cr-B-Si alloy system properties must be preserved with minimal dilution by the substrate iron. The low dilution achieved here is attributed to the concentrated heat input of the plasma arc, which creates a narrow melt pool that limits substrate melting.
Microstructural Evolution with Travel Speed
The relationship between travel speed and microstructure can be understood through the lens of solidification theory. At lower travel speeds, the heat input per unit length is higher, resulting in slower cooling rates. This allows more time for diffusion-controlled phase formation but may lead to coarser dendrite arm spacing and potentially more substrate dilution due to a wider melt pool. At higher travel speeds, the cooling rate increases, which promotes finer grains but may also lead to incomplete crystallization of the hard phases if the cooling is too rapid.
The non-monotonic behavior of phase crystallinity and hardness with travel speed reflects a classic trade-off in solidification processing. The optimal speed of 200 mm/min represents a balance where the cooling rate is sufficient to promote fine grain formation and good nucleation of FeNi_x and boride phases, without being so fast that it suppresses crystallization entirely. This is consistent with the concept of a "critical cooling rate" in the Ni60 system, below which the hard boride and carbide phases form preferentially, and above which the matrix tends toward a more solid solution-dominated structure.
Engineering Practice Implications
For engineers applying Ni60 plasma surfacing in industrial settings—such as on pump impellers, valve seats, drilling tools, and wear-resistant linings for mining equipment—the following practical guidelines emerge from this study:
- The travel speed of 200 mm/min should serve as the baseline starting point for process development on austenitic stainless steel substrates.
- Dilution rate must be monitored and controlled below 5 percent for applications requiring full Ni60 alloy properties, particularly in corrosive environments.
- The wetting angle should be inspected visually or measured to ensure proper spread; values above 60 degrees may indicate excessive dilution or poor wetting.
- Multi-pass applications require interpass temperature control to maintain the dilution rate within acceptable limits, as subsequent passes will further dilute the coating with substrate material.
Key Questions and Reflections
One question that arises from this study is whether the optimal travel speed would shift if the substrate were changed to carbon steel or low-alloy steel, which have significantly different thermal conductivity and melting characteristics compared to 304 stainless steel. The lower thermal conductivity of stainless steel means that heat is retained in the substrate more readily, which could influence the melt pool geometry and dilution behavior. In my experience with overlay welding on carbon steel substrates, the optimal travel speed tends to be somewhat higher due to the greater heat dissipation, but this should be verified through systematic experimentation.
Another consideration is the effect of travel speed on the residual stress state of the coating. Rapid solidification at higher travel speeds typically produces higher compressive residual stresses in the coating, which can be beneficial for fatigue resistance but may also increase the risk of cracking in thicker coatings or on substrates with high hardness contrast. This study does not address residual stress, which represents a gap in the investigation.
Study Insights and Outlook
This paper provides a clear and practical process window for Ni60 plasma surfacing on 304 stainless steel, with the 200 mm/min travel speed serving as a well-documented reference point. The combination of low dilution, fine microstructure, and maximum hardness at this speed makes it a robust choice for industrial applications. Future work should extend this investigation to multi-pass coatings, different substrate materials, and post-weld heat treatment effects, as well as incorporating residual stress measurements and tribological testing to provide a more complete picture of coating performance under service conditions.