Study On The Surface Aluminization And Oxidation Treatment Of8407Die Steel And Its Melting-loss And Thermal Fatigue Properties

The surface treatment to improve the associated performance of casting dies is one of the most interesting subjects at present. As a common material of casting die for aluminum alloy,8407die steel is suffering from the surface melting-loss and thermal fatigue failures due to the repeated action of alternating thermal stresses and the eroding of molten aluminum alloy. Consequently, it’s significant to find a new surface treatment method to improve both melting-loss and thermal fatigue resistance.The commenly used surface treatment technologies such as boriding, oxidation, nitridation and so on, can form a compound layer on the surface of casting die to impove its melting-loss resistance by preventing die from direct contact with molten aluminum. However, the difference between these compound layers and die substrate in terms of thermal properties leads stress concentration, thus deteriorating the more important thermal fatigue resistance of casting die. Considering the defects of the above-mentioned methods, aluminization and subsequent oxidation treatment of8407steel surface is proposed and studied, aiming at improving both the melting-loss and thermal fatigue resistance by forming a Fe-Al-O complex oxide layer in this work.First of all, hot-dip aluminization experiment was carried out based on the theoretical analysis. The influences of hot-dip temperature and time on aluminization layer thickness were investigated and the sectional microstructure and composition of aluminization layer were analyzed, arriving at the following appropriate aluminization condition:hot dipping for about10min at750C. Cold spray of aluminum and subsequent diffusion process was conducted on the basis of theoretical analysis. The influences of diffusion temperature and time on aluminization layer thickness were investigated and sectional microstructure and composition of aluminization layer were analyzed, arriving at the following appropriate aluminization condition:aluminum coat thickness of about100p,m, aluminization layer thickness in the range of40to60μm, diffusion temperature of550C and diffusion time of about4h. Comparing the aluminization layer quality obtained with these two aluminization methods, it’s more reasonable to conduct pre-aluminization treatment on8407die steel by means of spray and subsequent diffusion process.Secondly, the oxidation behaviors of aluminized specimens were studied under different atmospheres in the high temperature oxidation experiment based on the theoretical calculation. In addition, the influences of oxidation temperature and time on oxide film thickness were discussed and microstructure and mineral phases of the films were examined in the experiment, arriving at the following appropriate oxidation condition:oxidation for above3h at570℃under pure oxygen. The potential-pH diagrams of Fe-H2O and Al-H2O systems are applied to determine the conditions where both Fe3O4and Al2O3could form on the Fe-Al alloy surface as follows:solution pH in the range of4.6to8.3. The electrolyte solution was prepared according to this, and room temperature hard anodization experiment was carried out. The influences of sulfosalicylic acid concentration, oxidation time and current density on oxide film thickness and hardness were investigated and the microstructure, composition and mineral phases of the films were examined. The orthogonal tests were carried out to acquire the optimum condition at25℃as below:sulfosalicylic acid concentration of50g/L, oxidation time of60min and current density of2.5A/dm2. Comparing the oxide film quality formed with these two oxidation methods, it’s more reasonable to conduct oxidation process on aluminized8407steel by means of room temperature hard anodization.Surface untreated specimens and surface aluminization and subsequent oxidation treated specimens were immersed into molten ADC12alloy under static or dynamic conditions respectively to compare their melting-loss behaviors. In melting-loss test, surface condition and sectional microstructure of specimen were observed, and compositions of transverse section of untreated specimen were analyzed. Weight loss and melting-loss rate were adopted to evaluate melting-loss property. The results show that the surface aluminization and subsequent oxidation treatment can dramatically improve the melting-loss resistance under both static and dynamic conditions. The melting-loss mechanism of8407steel was analyzed with experiment results, and the anti-melting-loss mechanism of oxide film was discussed from three aspects, namely:property of oxide film, the combination status between oxide film and substrate, and the wettability between oxide film and molten aluminum.In the last part of this work, thermal fatigue test was performed with cycle heating method to compare thermal fatigue behaviors of samples with and without aluminization and subsequent oxidation treatment. In the test, thermal fatigue crack morphology formed on the surface was observed, and relationship between main crack lengths and cycles was discussed. The thermal fatigue property can be judged according to thermal fatigue main crack length and its reciprocal. The results show that the surface aluminization and subsequent oxidation treatment could improve both the initiating resistance and propagating resistance of thermal fatigue crack. The thermal fatigue mechanism of8407die steel was analyzed with experiment results, and the anti-thermal-fatigue mechanism of oxide film was clarified from two stages of thermal fatigue crack, one being initiating stage, the other being propagating stage.