The Erosion Behavior of Die-Steel Salt-Bath-Nitrocarburized with Lithium-Iron Compound Oxide Layer in Molten Al-Si-Cu Alloy

Erosion property of die-steel in molten metal is an important factor to determine the lifetime of a die-casting mold. Post-oxidation (that is oxidation after nitriding or nitrocarburizing) very effectively improves the resistance to erosion in molten metals. However, effects of the oxide layer on the surface have not yet been completely clarified, including nitrided layers, especially a compound nitride layer composed of ε-Fe_2-3N and γ’-Fe₄N phases and a nitrogen-diffusion layer. In this study, we have investigated the effect of surface microstructures on erosion behavior in molten Al-Si-Cu alloy JIS-ADC12, using nitrocarburized JIS-SKD61 steel, which is frequently used for die-casting or forging. Nitrocarburizing was conducted by a molten-salt process using cyanate with lithium ion at 853K and 873K, which was designed for thickly and densely forming a lithium-iron compound oxide layer on the surface. The surface microstructures obtained by the process include lithium-iron compound oxide layer, the compound nitride (ε-Fe_2-3N and γ’-Fe₄N phases) layer, and nitrogen-diffusion layer. The erosion property of the nitrocarburized die-steel was evaluated by weight loss of cylindrical test specimens with a 16mm diameter after immersing and rotating them at 200rpm in molten Al-Si-Cu alloy at 953 and 1023K for various times. The erosion behavior at the interface reaction layer between the specimen and the molten metal was investigated using an optical microscope, SEM, XRD, and EPMA. The results are as follows. 1) The lithium-iron compound oxide layer improves the resistance to erosion much more than the ferrous oxide layer composed largely of magnetite (generally formed by the post-oxidation). The erosion resistance of the steel is found to be increased as the compound oxide layer thickness increases, which can inhibit the reaction with the molten metal. 2) The compound nitride layer has limited effectiveness against the erosion resistance because it can be decomposed at a temperature over 923K. 3) The nitrogen-diffusion layer is not thought to effectively improve the erosion resistance.