Zhuojin Pipe Fitting Co., LtdEffect of Plasma Surfacing Speed on Microstructure and Hardness of Ni60 Coatings
Literature Overview
The paper by Luo Shengyang and Yuan Zhentao (2018), published in Hot Working Technology (Vol. 47, No. 5, pp. 227–230), investigates the influence of plasma surfacing travel speed on the microstructure, phase composition, and microhardness of Ni60 single-layer, single-pass surfacing coatings deposited on 304 stainless steel substrates. The work was supported by the Yunnan Provincial Department of Education Grant (2017ZZX136) and represents a focused parametric study within the broader field of plasma arc surfacing technology.
Core Technical Content
Plasma arc surfacing (PAS) is a widely used technique for applying hardfacing overlays on critical components in oil and gas pipelines, mining equipment, and power generation systems. Ni60 is an austenitic nickel-based hardfacing alloy renowned for its exceptional work-hardening capacity, making it ideal for components subjected to severe abrasion and impact loading. The study systematically varied the travel speed and employed optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), and microhardness testing to characterize the resulting coatings.
Key Findings on Travel Speed Effects
The optimal travel speed was identified at 200 mm/min, where the coating exhibited a wetting angle of 55°, a dilution rate of 4.5%, and the best macroscopic deposition quality. At this speed, the microstructure in the mid-region of the Ni60 coating consisted of columnar dendrites with precipitates growing laterally on small crystal facets. As the travel speed increased beyond this optimum, the crystallinity of Fe-Ni phases and boride phases first increased and then decreased, while the coating hardness followed a similar non-monotonic trend—rising initially and then falling.
| Parameter | Optimal Value (200 mm/min) | Trend with Increasing Speed |
|---|---|---|
| Wetting angle | 55° | Degrades beyond optimum |
| Dilution rate | 4.5% | Increases with speed |
| Grain morphology | Fine columnar dendrites | Coarsens at extreme speeds |
| Fe-Ni phase crystallinity | Good | First increases, then decreases |
| Boride phase crystallinity | Good | First increases, then decreases |
| Average hardness | Maximum | First increases, then decreases |
Interpretation of Technical Points
The non-monotonic behavior of hardness and phase crystallinity with travel speed can be attributed to competing thermal effects. At low travel speeds, the heat input is excessive, leading to grain coarsening and potential over-melting of the substrate, which increases dilution and degrades coating properties. At high travel speeds, insufficient heat input causes incomplete melting of the surfacing wire, resulting in poor fusion, reduced crystallinity of hard phases, and lower hardness. The 200 mm/min speed represents the thermal balance point where sufficient melting occurs without excessive substrate dilution.
The columnar dendritic structure observed at the optimal speed is characteristic of rapid solidification under the high cooling rates typical of plasma arc surfacing. The presence of Fe-Ni solid solution phases and boride compounds (NiB, Ni₂B) is critical for the wear resistance of Ni60 coatings. Borides contribute hardness, while the austenitic matrix provides the work-hardening response under impact loading.
Integration with Engineering Practice
In pipeline and fitting repair operations, Ni60 plasma surfacing is commonly applied to valve seats, pump impellers, and wear rings in oilfield service. The findings of this study have direct implications for field repair procedures:
- Process parameter selection: Operators must calibrate travel speed to approximately 200 mm/min for typical Ni60 wire diameters (1.6–2.0 mm) on stainless steel substrates, adjusting for wire diameter, current, and arc voltage.
- Dilution control: A dilution rate of 4.5% is acceptable for wear applications; however, for corrosion-critical applications, dilution must be kept below 3% to maintain Ni60's austenitic integrity.
- Quality verification: Post-surfacing hardness testing should target HV 500–600 in the as-deposited condition, with expectation of further hardening to HV 800–1000 under operational loading.
Key Questions and Reflections
The study raises several questions worth further investigation. First, the single-pass, single-layer configuration limits the applicability of findings to multi-pass repairs commonly encountered in field conditions. Second, the absence of wear testing data (e.g., pin-on-disk, taber abrasion) leaves the practical wear performance correlation incomplete. Third, the effect of interpass temperature in multi-layer builds on the phase evolution is not addressed.
From a metallurgical perspective, the observation that boride crystallinity peaks at intermediate speeds suggests that boride precipitation is thermally activated but kinetically limited at extreme cooling rates. This insight is valuable for optimizing post-weld heat treatment schedules where additional boride precipitation is desired.
Study Insights and Implications
This work provides a clear, experimentally validated process window for plasma surfacing of Ni60 on austenitic stainless steel substrates. The identification of 200 mm/min as the optimal travel speed, coupled with the understanding of the underlying microstructural mechanisms, offers practical guidance for welding engineers specifying surfacing procedures in maintenance and repair operations. The systematic approach—varying one parameter while holding others constant—demonstrates good experimental design and yields actionable results. However, the study would benefit from incorporating multi-pass configurations and direct wear testing to bridge the gap between microstructural characterization and field performance prediction.