As mentioned earlier, iron mold sand coated casting uses the rigidity of iron mold to eliminate the shrinkage cavity and porosity of nodular iron castings and improve the dimensional accuracy of castings. The good thermal conductivity and heat storage capacity of iron mold make the grain of iron mold sand coated casting refined, the mechanical properties improved, and energy conservation has another advantage. The cooling process of iron mold sand coated casting determines its final microstructure, and then affects the mechanical properties of iron mold sand coated casting. When the iron mold sand coated casting changes from austenite state, the faster the cooling speed, the more pearlite, the finer the pearlite and the denser the structure. Iron mold sand coated casting process can create better heat dissipation conditions than sand mold by adjusting the thickness of iron mold and sand coated layer. At present, the heat treatment of most cast iron castings can not be achieved stably. Therefore, this chapter will discuss the research on the cooling process of iron mold sand coated casting.
The working condition of crankshaft casting is very complex and plays an important role in engine work. Crankshaft casting materials should have the characteristics of external hardness and internal toughness, such as high tensile strength, surface strength, fatigue strength, hardness and wear resistance. At the same time, the core of crankshaft casting should have enough toughness. In addition, in order to comply with the development trend of automotive materials in the world and reduce the self weight of vehicles, lightweight materials should be used to reduce fuel consumption, reduce environmental pollution and save oil resources; Try to select or develop alternative materials, simplify the production process and reduce the production cost. This part mainly studies the process optimization of as cast nodular cast iron crankshaft.
At present, the commonly used crankshaft casting materials in China mainly include forged steel and nodular cast iron. Ordinary forged steel materials usually need heat treatment (quenching and tempering or normalizing) to improve mechanical properties and processability. The overall processing technology is complex, with long processing time and high energy consumption. By adding trace alloying elements such as vanadium (V), niobium (NB) and titanium (TI), the grain structure of the matrix can be refined, the comprehensive mechanical properties of the steel can be improved, the heat treatment step can be omitted and the energy consumption can be saved.
Nodular cast iron crankshaft castings can be divided into cast heat treated crankshaft castings, austenitic austempered ductile iron (ADI) crankshaft castings and as cast pearlite ductile iron crankshaft. Its brands include qt600.3, qt700.2, qt800.2, qt900.2 and qt900-5. Compared with forged steel, ductile iron has the advantages of low density, good process performance and low cost.
Nodular cast iron is more and more widely used in crankshaft castings of various engines. Generally speaking, the development trend of materials for crankshaft castings is to replace forged steel crankshaft castings with nodular iron crankshaft castings; Ductile iron crankshaft castings have developed high-performance ADI materials and low-cost as cast ductile iron materials in high-power engines and light engines respectively.