Plasma Cleaning and Nitriding of Cast Irons Under Industrial Contamination

Abstract Protective oils and oxide films are frequently present on cast-iron tooling and can compromise early-stage surface activation during plasma nitriding. Here, gray and ductile cast irons were intentionally soiled with two industrial protective oils and pre-oxidized either by controlled furnace exposure or by water exposure to reproduce mixed organic and inorganic contamination states. TGA/DTG of the oils informed a staged heating profile to limit abrupt outgassing and avoid uncontrolled cracking with carbonaceous redeposition. Three in-chamber plasma-cleaning routes were integrated with a fixed nitriding cycle without breaking vacuum: CN1 (80% N₂/20% H₂), CN2 (80% N₂/20% O₂ → 80% N₂/20% H₂), and CN3 (80% N₂/20% O₂ → 100% H₂ → 80% N₂/20% H₂). SEM/EDS shows that tool protective oil is generally reset to substrate-dominated Fe–N–O–Si chemistries, whereas sheet protective oil more frequently yields localized Ca–P–(Ba, S)-bearing deposits, particularly on ductile iron and water-oxidized surfaces. Distribution-aware statistics of compound-layer thickness and top-surface microhardness confirm that cleaning effectiveness is expressed as spatial uniformity: conditions lacking residue-marker signals exhibit tighter thickness and microhardness distributions, while residue-prone scenarios show broader scatter and occasional discontinuities. Across all conditions, the mean compound-layer thickness ranged from 2.65 to 10.22 μm, while the mean top-surface microhardness ranged from 686 to 1065 HV 0.025 . The highest-ranked conditions combined compound-layer thicknesses of 5.71–6.76 μm with top hardness values of 1021–1065 HV 0.025 and scores between 90.6 and 100.0. CN3 provided the most robust integrated route by combining a synergistic oxidative oil fragmentation with the strongest hydrogen reduction stage prior to nitriding, thereby minimizing residue and oxide microdomains and improving the reproducibility of compound-layer formation.