Influence of Cladding Current on Microstructure and Properties of Nickel-Based Alloy Plasma Cladding Layers

Overview and Motivation

Plasma transferred arc (PTA) cladding is a widely used method for applying nickel-based alloy overlay layers on pressure vessel internals, heat exchanger tubes, and pump shafts that must resist severe corrosion and erosion. The cladding current is the single most influential parameter governing the thermal cycle, dilution, and solidification behavior of the deposit. This literature review examines how systematic variation of PTA current affects the microstructure, hardness, corrosion resistance, and bonding strength of nickel-based alloy cladding layers, specifically focusing on Inconel 625 and Hastelloy C276 powder systems.

Process Parameter Matrix

Current (A)	Powder feed rate (g/min)	Travel speed (mm/min)	Dilution (%)	Layer thickness (mm)
200	180	250	8–12	1.2–1.5
300	220	300	15–22	1.5–2.0
400	280	350	25–35	2.0–2.8
500	350	400	38–50	2.5–3.5

The study systematically varied the cladding current from 200 A to 500 A while proportionally adjusting powder feed rate and travel speed to maintain a consistent layer thickness per pass. Dilution was measured by energy-dispersive X-ray spectroscopy (EDS) at the interface between the clad layer and the carbon steel substrate (Q345R or 16MnR).

Microstructural Evolution with Current

At low current (200 A), the cooling rate is moderate (approximately 500–800 K/s), producing a fine dendritic microstructure with isolated γ' and γ'' precipitates. The grain size is small (approximately 10–20 μm), and the carbide content is minimal. At medium current (300–400 A), the cooling rate decreases, and the microstructure transitions to a coarser dendritic pattern with increased Nb-rich MC carbides and Laves phase (Fe₂Nb) formation. At high current (500 A), the dilution exceeds 40 %, and the microstructure becomes significantly affected by the substrate carbon and manganese content, leading to extensive carbide precipitation and potential grain boundary segregation of brittle intermetallic phases.

Current (A)	Hardness (HV)	Grain size (μm)	MC carbide volume (%)	Laves phase
200	260–290	10–20	<1	Absent
300	240–270	20–35	1–3	Trace
400	220–250	35–50	3–6	Present
500	200–230	50–80	6–12	Abundant

Corrosion and Bonding Performance

Corrosion testing in 3.5 % NaCl solution and 20 % H₂SO₄ demonstrates that low-current deposits (200–300 A) exhibit superior corrosion resistance, with corrosion current densities of 0.5–2.0 μA/cm² in saline solution. High-current deposits (500 A) show corrosion current densities exceeding 10 μA/cm² due to the presence of galvanic couples between the Ni-rich matrix and Fe-rich dilution zones. Bond strength testing (shear and peel) reveals that the optimal bonding interface occurs at 300 A, where the dilution is sufficient to promote metallurgical bonding without excessive substrate melting. At 200 A, the bond may be weak due to insufficient heat input; at 500 A, the bond is strong but the overall layer performance is degraded by excessive dilution.

Engineering Implications and Recommendations

For pressure vessel applications governed by NB/T 47014 and ASME IX, the cladding current must be selected to ensure that the overlay layer meets the specified corrosion resistance and mechanical property requirements while maintaining a dilution rate below 30 % as recommended by API 934. The study recommends a current range of 250–350 A for Inconel 625 PTA cladding on low-alloy steel substrates, with corresponding powder feed rates of 200–260 g/min and travel speeds of 280–340 mm/min. Post-cladding solution heat treatment at 1050–1100 °C for 1–2 hours is strongly recommended to dissolve brittle intermetallic phases and homogenize the microstructure, particularly for deposits produced at currents above 300 A. This study provides a clear, quantitative basis for optimizing PTA cladding parameters in engineering practice, reducing trial-and-error and improving first-pass quality.