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

Literature Overview

This study investigates the influence of plasma transferred arc (PTA) cladding deposition speed on the microstructural evolution and hardness distribution of Ni60 (Stellite 6) overlay coatings applied to carbon steel substrates. The research examines a range of deposition speeds from 40 to 120 mm/min, with other process parameters held constant, including arc current of 200 A, arc voltage of 22 V, gas flow rate of 15 L/min (argon), and powder feed rate of 180 g/min. The Ni60 powder composition includes approximately 58-62% Ni, 28-32% Cr, 5-7% Co, 1-2% Mo, 0.8-1.2% Fe, and 0.8-1.0% C. The substrate material used was Q235 carbon steel with a base hardness of approximately 120 HBW.

Core Viewpoints and Technical Analysis

The primary finding of this research is that deposition speed exerts a dominant influence on the cooling rate, which in turn governs the phase transformation behavior within the overlay layer. At lower deposition speeds (40-60 mm/min), the heat input per unit length is relatively high, resulting in slower cooling rates. This promotes the formation of larger cellular dendrites and facilitates the precipitation of M23C6 carbides along dendrite boundaries. At higher deposition speeds (100-120 mm/min), the increased cooling rate suppresses the growth of cellular structures, leading to finer microstructural features and a higher volume fraction of M7C3 carbides.

The hardness results demonstrate a clear trend: as deposition speed increases from 40 to 120 mm/min, the average microhardness of the overlay layer increases from approximately 420 HV to 620 HV. This improvement is attributed to three mechanisms: (1) finer dendrite spacing reducing the lever arm for dislocation motion, (2) increased carbide density providing greater obstacle density for dislocation glide, and (3) enhanced solid solution strengthening due to the higher supersaturation of alloying elements in the faster-cooled matrix.

Microstructural Evolution Mechanism

The microstructural transition from M23C6-dominated to M7C3-dominated carbide phases can be understood through the lens of thermodynamic and kinetic considerations. At low cooling rates associated with slow deposition speeds, the system has sufficient time for carbon diffusion, allowing the formation of thermodynamically stable M23C6 carbides which require higher chromium content. As the cooling rate increases with higher deposition speeds, the kinetic barrier to M23C6 nucleation becomes prohibitive, and the system preferentially forms M7C3 carbides which have lower nucleation energy barriers and require less carbon diffusion distance.

The dilution rate decreases with increasing deposition speed because the shorter interaction time between the arc and substrate reduces the amount of base material melted and incorporated into the melt pool. This lower dilution rate means a higher concentration of alloying elements (Cr, Co, Mo) from the Ni60 powder is retained in the final microstructure, further contributing to the hardness improvement through solid solution strengthening and carbide formation.

Interface Bonding and Defect Analysis

At deposition speeds exceeding 100 mm/min, the reduced heat input can lead to incomplete melting of the previous layer, potentially causing interpass lack of fusion. The study reports that at 120 mm/min, minor interpass porosity was observed near the interface between the first and second overlay layers. This defect formation is attributed to insufficient thermal accumulation to ensure complete remelting of the prior layer surface. The recommended upper limit for practical application is 100 mm/min, where the balance between hardness and defect susceptibility is optimal.

Engineering Practice Implications

For industrial applications requiring high wear resistance with acceptable productivity, a deposition speed of 80-100 mm/min represents the optimal window for Ni60 PTA cladding on carbon steel substrates. This speed range provides microhardness values exceeding 540 HV while maintaining low defect rates. In applications such as pump shafts, valve seats, and hydraulic cylinder liners where Ni60 cladding is commonly specified, selecting deposition speed within this range ensures both performance and quality consistency.

The study also highlights the importance of multi-layer deposition strategies. For thick overlay requirements (exceeding 3 mm), a graded speed approach is recommended: slower speeds (60-80 mm/min) for the first layer to ensure adequate bond strength and minimize cracking, followed by faster speeds (80-100 mm/min) for subsequent layers to achieve the target hardness profile.

Study Insights and Reflections

This research provides valuable quantitative data for process parameter optimization in PTA cladding operations. The clear correlation between deposition speed and microstructural refinement offers a practical lever for adjusting overlay performance without changing the base alloy composition. From a production planning perspective, the ability to achieve 620 HV at 120 mm/min (compared to 420 HV at 40 mm/min) represents a potential productivity gain of 200% with a 48% improvement in hardness, which has significant economic implications for high-volume cladding operations.

However, the study's findings must be interpreted with caution regarding substrate compatibility. The results are specific to Q235 carbon steel substrates, and different base materials (such as low-alloy steels or stainless steels) may exhibit different dilution behaviors and interface reactions. Engineers should validate these speed-hardness relationships for their specific material combinations through trial cladding and characterization before implementing process changes in production environments.

This study underscores the fundamental principle that cladding performance is not solely determined by the overlay alloy composition but is equally dependent on the processing conditions that govern microstructural development. A comprehensive understanding of the speed-microstructure-property relationship is essential for achieving consistent, high-performance Ni60 overlay coatings in industrial applications.