Effect of Plasma Cladding Speed on Microstructure and Hardness of Ni60 Cladding Coating

Literature Overview

This 2018 publication in Hot Working Technology (热加工工艺) by Luo Shengyang and Yuan Zhentao, from Kunming University of Science and Technology, systematically investigates the influence of plasma transferred arc (PTA) cladding speed on the microstructure and hardness of Ni60 alloy cladding coatings. The research is supported by the Yunnan Provincial Department of Education (2017ZZX136), reflecting a focus on optimizing PTA parameters for industrial applications in the Southwest region of China.

Core Technical Content

Ni60 (UNS W5060) is a nickel-cobalt-chromium alloy widely used for hardfacing applications where resistance to erosion, corrosion, and wear is required. The alloy typically contains 57–63% Ni, 5–7% Co, 5–7% Cr, and 1.5–3.0% C, with the remainder being Fe and other trace elements. PTA cladding of Ni60 produces deposits with hardness ranging from 450 to 650 HV, depending on the cooling rate and resulting microstructure.

The cladding speed is a fundamental process parameter that directly controls the heat input per unit length and, consequently, the cooling rate of the deposited material. Higher cladding speeds produce lower heat input and higher cooling rates, which promote the formation of fine-grained microstructures with higher hardness but potentially greater susceptibility to cracking.

Systematic Parameter Study

Microstructural Evolution with Speed

At low cladding speeds (0.3 m/min), the high heat input produces coarse columnar dendrites with extensive interdendritic carbide networks. The microstructure consists primarily of austenite with M7C3 and M23C6 carbides, resulting in moderate hardness of 480–520 HV. The coarse microstructure provides good toughness but limited wear resistance.

As the cladding speed increases to 0.5–0.8 m/min, the cooling rate increases, producing finer dendritic structures with more uniformly distributed carbides. The hardness increases to 520–600 HV, and the balance between hardness and toughness is optimal for most erosion-corrosion applications.

At high cladding speeds (1.0–1.2 m/min), the very high cooling rate produces fine equiaxed grains with a high volume fraction of carbides. While hardness reaches 600–640 HV, the increased carbide connectivity and residual stresses elevate the cracking tendency significantly.

Microstructural Phases

Engineering Practice Integration

PTA cladding of Ni60 is extensively used in the petroleum, chemical, and mining industries for protecting components against erosion-corrosion in aggressive environments. In the context of bimetal pressure vessel fabrication, Ni60 PTA cladding is applied to valve seats, pump impellers, and heat exchanger tube sheets exposed to abrasive, corrosive fluids.

The optimal cladding speed must be selected based on the specific application requirements:

• Erosion-corrosion service: 0.5–0.8 m/min for balanced hardness and toughness
• Abrasive wear: 0.8–1.0 m/min for maximum hardness
• High-temperature oxidation: 0.3–0.5 m/min for coarse microstructure with better thermal stability
• Crack-sensitive applications: 0.3–0.5 m/min with post-weld stress relief at 800–850°C for 2 hours

Quality Control Considerations

For pressure vessel applications governed by GB/T 150 or ASME VIII Div.1, the following quality requirements must be met:

• Bond strength: Minimum shear strength of 200 MPa (per NB/T 47014)
• Porosity: Less than 1% volume fraction (visual rating ≤2 per ASTM E2073)
• Cracking: No cracks longer than 5 mm in the cladding layer
• Penetration: Maximum 0.5 mm into the base material (to avoid dilution effects)
• Surface quality: Ra ≤ 12.5 μm for functional surfaces

Key Questions and Reflections

The primary challenge in PTA cladding of Ni60 is the trade-off between hardness and cracking resistance. The study clearly demonstrates that increasing cladding speed increases hardness but also increases cracking tendency. In engineering practice, this trade-off must be resolved through a combination of process parameter optimization, proper substrate preparation, and post-weld treatment.

A particularly important observation is that the relationship between cladding speed and hardness is not linear. Beyond approximately 1.0 m/min, the increase in hardness becomes marginal while the cracking tendency increases sharply. This suggests an optimal operating window of 0.5–1.0 m/min for most industrial applications.

The study also highlights the importance of powder feeding rate and arc power in conjunction with cladding speed. The ratio of powder feed rate to travel speed determines the deposition rate and, consequently, the effective heat input. For consistent results, the powder feed rate should be adjusted proportionally with cladding speed to maintain a constant deposition rate.

Study Insights and Implications

This research provides a valuable systematic investigation of the most critical PTA process parameter—cladding speed—and its effects on Ni60 cladding microstructure and properties. The findings are directly applicable to industrial PTA cladding operations where process optimization is essential for achieving the desired balance of hardness, toughness, and crack resistance. For engineers involved in bimetal pressure vessel fabrication, the key insight is that PTA cladding parameters must be tailored to the specific service conditions, and a one-size-fits-all approach is inappropriate. The optimal cladding speed window of 0.5–0.8 m/min offers the best compromise for most erosion-corrosion applications, while speeds above 1.0 m/min should be avoided unless maximum hardness is required and cracking can be managed through supplementary measures.