Plasma Cladding Process Windows for Ni60 Overlay Layers

Literature Context and Practical Relevance.

Ni60 nickel boron silicon overlays are widely used for valve seats, pump shafts, and pipe fitting wear pads where abrasive wear and moderate corrosion resistance must be combined. The reviewed work examined plasma powder cladding on an austenitic stainless steel base, which is close to many corrosion resistant alloy or stainless fitting applications. The central practical question is how plasma current and powder feed rate change dilution, boride morphology, hardness, and the hardness step at the fusion boundary. In production, those variables determine whether the overlay will resist cracking, spalling, or localized wear. The study is useful because it links measurable process parameters to microstructural zones rather than reporting only average hardness.

Technical Findings and Parameter Interpretation.

The overlay hardness was clearly higher than the base metal, and the fusion zone showed a distinct hardness transition and elemental diffusion band. At 110 A, the reported hardness was 630 HV, indicating a hard boride reinforced structure. Increasing current or reducing powder feed rate lowered both overlay hardness and the fusion zone gradient, which suggests greater dilution and slower effective cooling. The microstructure changed from a fusion zone to dendritic regions and then to equiaxed regions near the surface. Higher current increased the volume fraction of flower-like eutectic near the surface, while lower powder feed made needle-like structures near the fusion line coarser. At 6 g/min, directional long borides and separated blocks appeared in the middle of the overlay.

Parameter	Observed Microstructural Effect	Engineering Meaning
Current near 110 A.	Hardness around 630 HV with strong boride reinforcement.	Good wear resistance if cracking risk is controlled.
Higher current.	Dilution rises and hardness gradient falls.	More base metal influence and possible softening.
Lower powder feed.	Coarser needle-like structures near fusion line.	Local toughness and fatigue resistance may decline.
Powder feed 6 g/min.	Directional long borides and separated blocks.	Anisotropic wear behavior and crack initiation sites possible.

Engineering Implications and Quality Control Reflections.

For pipe fittings, the most important lesson is that hardness alone is not an acceptance criterion. A 630 HV overlay may look excellent, but coarse borides or a steep hardness transition can become crack initiation sites under thermal cycling or impact load. I would use a PDCA cycle for qualification, starting with a narrow current and powder feed matrix, then checking macro cracks, dilution depth, hardness profile, and metallographic boride morphology. If the overlay is used on thin-wall stainless fittings, preheat and interpass control must be tight because stainless steel has high thermal expansion and plasma heat input can cause distortion. The study also suggests that powder feed rate should not be reduced too far, because a thinner, more diluted bead may look smooth but lose the intended boride reinforcement. A robust procedure should specify a minimum powder feed, a maximum current, and a required cooling rate, then verify by sectioning the first production part.