Studies On The Improvement Of Molten Aluminum Corrosion And Wear Resistance Of H13 Die Steel By Low-Temperature Liquid Oxy-Nitriding

The demand and application of aluminum alloys in many fields are increasing,for example,aerospace,transportation,electronic devices,building materials,etc.The forming of aluminum alloys can choose to use die casting,hot forging,or hot extrusion,according to the required properties.Contact with the molten aluminum alloys directly causes severe dissolution corrosion on the surface of casting dies.The forging and the extrusion dies usually failed due to suffering severe wear in working.Low-temperature liquid oxy-nitriding surface treatment can produce the oxygen-nitrogen composite modified layers.Besides,low-temperature liquid oxy-nitriding has the advantages of the ability to modify workpieces of complex shapes,low distortion,and easy operation.Therefore,we studied and applied the low-temperature oxy-nitriding on AISI H13 steel,expected to improve the molten aluminum corrosion and wear resistance.Results show that the microstructure of the oxygen-nitrogen composite modified layers can be adjusted by the chosen temperatures（430/450/470℃）and time（4/8/12 h）of the low-temperature liquid oxy-nitriding treatment.The oxide layers formed by low-temperature liquid oxy-nitriding were mainly composed of Fe₂O₃ and/or Fe₃O₄.Temperatures at 430℃ and 450℃ mainly formed Fe₂O₃,while at 470℃ mainly formed Fe₃O₄.The nitriding layer of AISI H13 steel can be divided into the compound layer(>1100 HV_0.1),diffusion layer(1000-1100 HV_0.1),and transition layer(1000-550 HV_0.1),according to the cross-sectional microhardness.The compound layer is mainly composed of ζ-Fe₂N,ε-Fe₃N,and a little γ’-Fe₄N.The diffusion layer is mainly composed of γ’-Fe₄N with a little ε-Fe₃N,and the transition layer is mainly composed of α’N-Fe with a small amount of γ’-Fe₄N.The phase transformation order of nitrides during low-temperature liquid oxy-nitriding of AISI H13 steel is α’-Fe+N→α’N-Fe+N→γ’-Fe₄N+N→ε-Fe₃N+N→ζ-Fe₂N.During low-temperature liquid oxynitriding,the diffusion coefficients of[N]atoms are respectively 1.03×10^-13 m²/s,2.74×10^-13 m²/s,and 6.23×10^-13 m²/s when the treated temperature is 430℃,450℃,and 470℃.The diffusion activation energy of low-temperature liquid oxy-nitriding treatment for AISI H13 steel is 195.4 KJ·mol^-1.Low-temperature liquid oxy-nitriding treatment significantly improves the molten aluminum corrosion resistance of AISI H13 steel.The oxide layer in the oxygen-nitrogen composite modified layer has better molten aluminum corrosion resistance than the nitriding layer.The oxide layer can prevent the molten aluminum from contacting the matrix directly.Meanwhile,the oxide layer has better chemical and thermal stability in molten aluminum.The oxide layer can also reduce the wettability of the molten aluminum to the sample surface.Thus,the diffusion and reaction of molten aluminum with matrix were avoided.The nitriding layer can reduce the formation rate of intermetallic compounds in the molten aluminum corrosion.According to the T-L-K model,the diffusion rate of molten aluminum in the nitriding layer is about 6.1～8.1 times slower than that in the matrix.The oxygen-nitrogen composite modified layers improved the fracture toughness of the intermetallic compounds and reduced defects such as holes in the intermetallic compounds.The oxygen-nitrogen composite modified layer also has a protective effect on the long-term molten aluminum corrosion.The oxygen-nitrogen composite modified layer was rich in oxygen so that formed the bilayer of Fe₃O₄ and Fe-Cr spinel by oxidizing the surrounding nitrides in molten aluminum corrosion.Thus,low-temperature liquid oxy-nitriding can transform the molten aluminum corrosion from dissolution into oxidation.During the thermal fatigue in molten aluminum,the untreated AISI H13 steel suffered severe dissolution corrosion and formed the intermetallic compounds.The intermetallic compounds were easy to initial cracks at the interface of the matrix by thermal fatigue,which promoted the corrosion of molten aluminum.However,the low-temperature liquid oxy-nitriding treatments mainly suffered oxidation corrosion and thermal fatigue.Based on low-temperature liquid oxy-nitriding treatment,the effects of air oxidation and post-oxidation treatment on the corrosion resistance of AISI H13 steel in molten aluminum were comparatively studied.Results showed that the lowtemperature liquid oxy-nitriding treatment had better corrosion resistance to aluminum liquid than the air oxidation treatment.In addition,the post-oxidation treatment can further improve the molten aluminum corrosion resistance of the low-temperature liquid oxy-nitriding treatment.The corrosion resistance of Fe₃O₄ was better than that of Fe₂O₃,due to Fe₃O₄ having higher thermal stability and a lower reaction rate in molten aluminum.The air oxidation and low-temperature liquid oxy-nitriding were more in line with the reaction-controlled wetting.Besides,the low-temperature liquid oxy-nitriding and the post-oxidation samples needed to overcome the capillary pressure during the wetting process,so it was more difficult to be wetted by molten aluminum.Low-temperature liquid oxy-nitriding can improve the wear resistance of AISI H13 steel.The oxide layer had a lubricating effect and thereby can reduce the coefficient of friction.The nitriding layer had higher hardness and better resistance to plastic deformation and ploughing,which can reduce abrasive wear and adhesive wear.At high temperatures,the softening of the matrix and the decomposition of the nitriding layer led to a decrease in hardness,but the hardness of the nitriding layer was still more than twice that of the matrix.The nitriding layer mainly underwent elastic deformation during the high-temperature friction process,while the untreated sample underwent severe plastic deformation and abrasive wear.Oxygen-nitrogen composite modified layer promoted to form thick oxide layer during high-temperature wear.Therefore,the main wear mechanism of the low-temperature liquid oxy-nitriding samples was oxidation wear.The interaction of oxygen and nitrogen composite structure can simultaneously reduce the wear coefficient and wear rate for AISI H13 steel.