Casting Process Optimization, Microstructures And Properties Of Grey Cast Iron Engine Cylinder Blocks

Grey cast iron is the commonly-used cylinder block materials. In this paper, ingredient design of the cast iron melt, microstructures and mechanical properties of the cast iron, causes and elimination methods of main defects in the castings were studied systemically in order to optimize the production processes, promote the microstructures, mechanical properties and qualities and lower the rejection rate of the grey cast iron engine block castings. The study therefore lays the foundation for preparing the thin-wall and high-strength engine block castings.Morphology, size and distribution of the TiN particles in an automobile engine cylinder cast (HT250) were studied in terms of microstructure observation and thermodynamic calculation. Nucleation and growth mechanism of TiN was also explored. The size of the TiN particles from foundry returns is the largest (>20 μm), due to the hereditary of TiN, while that of the TiN particles formed from the cast iron melt is in the middle (4-8 μm) and that of the TiN particles precipitated from the under-cooled austenite is the smallest (1-2 μm). Crystallization of TiN in the cast iron melt depends on the solubility product of [%Ti][%N]. As the [Ti] content in cast iron melt is around 0.045 wt%([%Ti][%N]=2.47×10-4 at the time), TiN begins to crystallize as the iron melt is under-cooled to 1368 ℃. Owing to the same crystal structures and nearly equal lattice constants of TiN as those of MnS, TiN particles crystallize preferentially on the MnS substrates in a nearly regular quadrangle shape, following a non-heterogeneous nucleation mechanism.Effects of the steel scrap content on microstructures, mechanical properties and fracture mechanism of the grey iron were studied. As the scrap steel content increased, microstructures of the grey iron are improved, and hardness and strength of the grey iron are therefore increased. The grey iron by adding 100 wt%scrap steel has the highest mechanical properties, casting properties and machining properties, due to the optimal microstructures of small and curved graphite flakes, refined pearlite-colonies and MnS and TiN inclusions. The refined, curved graphite and the refined pearlite change the crack propagation type in the grey iron cast, the crack propagation stress then increases.Causes on the common casting defects in the grey iron casts of the cooperative company were also investigated. Chilling tendency mainly results from the quiescence at a high temperature for a long time. It leads to rapid increase/decrease of the hardness/ machining property of the grey iron. Blow holes are formed due to the high sand moisture and the gas entrapment during casting. They usually appear in the key positions of the grey cast iron, which are regular in shape with oxide skins and a small amount of inclusions on their inner surfaces. Slag holes (sand holes) are formed by the residual dross and the sand mold burr in the grey iron cast. They often appear on the surface of the cylinder, which are irregular and have a composition of Al2O3, SiO2 and MnO2. Based on the results mentioned above, several solutions were proposed concerning the smelting, pouring and solidification cooling processes. As a result, the foundry defects and reject rate of the grey iron casts are effectively decreased to maintain the steady fabrication of the grey iron casts of the cooperative company.