In this study, Stellite 21 coatings were deposited on 304 stainless steel, an‎d the influence of process parameters on dilution an‎d the resulting properties was investigated. Experimental design was performed using the response surface methodology (RSM) with three major factors: current, powder feed rate, an‎d scanning speed at five levels. Statistical analysis indicated that a quadratic model with a determination coefficient (R²) of 0.98 provided excellent predictive accuracy for dilution. The regression coefficients revealed that increasing current (+2.38) raised dilution, whereas higher powder feed rate (–3.5) an‎d scanning speed (–5.92) reduced it, with the strongest effect attributed to scanning speed. For example, increasing the scanning speed from 60 to 156 mm/min raised dilution from 2.62% to 24.84%. Elevated dilution, associated with higher heat input, promoted iron penetration from the substrate, lowered the Cr:C ratio, an‎d consequently reduced carbide fraction an‎d hardness. To further investigate the effect of dilution, three representative coatings with low, medium, an‎d high dilution were selec‎ted for detailed structural, mechanical, an‎d Corrosion analyses. microstructural results showed that high-dilution coatings developed a wider unmixed zone (UMZ) with skeletal δ-ferrite inherited from the substrate, while in low-dilution coatings the UMZ disappeared. All coatings exhibited γ-Co dendritic structures with Cr an‎d Mo enriched interdendritic regions; however, carbide formation was significantly suppressed in the high-dilution case, whereas the low-dilution coating retained finer dendrites an‎d a higher carbide fraction due to preservation of the original alloy composition. Wear tests revealed superior performance of all coatings compared to 304 stainless steel at both room temperature an‎d 500 °C. The slight reduction in friction coefficient at room temperature was attributed to strain-induced martensitic transformation (SIMT), confirmed by the appearance of ε-phase peaks an‎d reduced γ(200) intensity in XRD patterns, while at elevated temperature SIMT was suppressed an‎d oxidation dominated. Weight loss showed only minor differences between the two test temperatures, highlighting the high thermal stability of Stellite 21. Cyclic polarization tests in 0.1 M HCl solution demonstrated markedly higher pitting resistance for the coatings than for 304 stainless steel. The low-dilution coating exhibited the lowest corrosion current density, the widest passive range, an‎d a negative hysteresis loop indicative of superior repassivation capability. Conversely, the high-dilution coating, due to reduced effective chromium content, formed an unstable oxide film an‎d suffered from large, deep pits. In summary, low dilution achieved through reduced heat input preserved the chemical composition of Stellite 21, resulting in a finer microstructure, higher carbide fraction, improved hardness, an‎d superior wear an‎d corrosion resistance. Therefore, the low-dilution condition (140 A current, 24 gr/min powder feed, 132 mm/min scanning speed) is chosen as the optimum process window.

مسعود عطاپور , ابوذر طاهري زاده

علي شفيعي

مهران نحوي , عباس بهرامي