Effect of gas nitriding on microhardness and microstructure of laser cladding iron-based alloy coating

The demanding service environment of feed screws in injection molding machines, characterized by metal friction, extrusion, and polymer corrosion, necessitates enhanced hardness, wear resistance, and corrosion resistance. Traditional strengthening methods involve nickel-based coating followed by nitriding, yet iron-based coatings, which are more cost-effective. This study compares laser-clad iron-based alloy coatings with those postgas nitriding, utilizing various characterization techniques to assess changes in microhardness, metallography, phase composition, and elemental distribution. The nitriding process significantly alters the microstructure and microhardness of iron-based coatings, forming a distinct nitride layer and a transition layer. Nitrogen ions penetrate the iron-based coating surface, forming γ′-Fe4N and ɛ-Fe2-3N phases upon saturation, resulting in a 200 μm-thick nitride layer with a 5 μm compound layer at the surface and a diffusion layer primarily of α-Fe(N). The γ′ phase achieves a maximum microhardness of 1214.1 HV near the surface, which decreases with depth and nitrogen content reduction. At 180 μm depth, microhardness reverts to prenitriding levels of 850 HV. Below the nitride layer, the absence of nitrogen leads to ferrite decomposition and a further reduction in hardness, with an average drop from 811.4 to 480.9 HV. Furthermore, the disproportionate phase ratio within the ɛ/γ′ dual-phase system results in a marked deterioration of wear resistance and corrosion resistance in the nitrided specimens. This phase imbalance induces microstructural incompatibilities, compromising both tribological performance and electrochemical stability under operational conditions.