Gray cast iron has good casting process and mechanical properties, excellent wear resistance, vibration damping and thermal conductivity, and low price. Therefore, it is still the preferred material for manufacturing engines. Due to the rapid development of automobile industry, the requirements for cast iron material and cost reduction are higher and higher. Low cost casting of high-strength thin-walled gray iron has become the main research content of automobile engine. At present, the material of cylinder block at home and abroad has basically formed standardization and standardization, with little change in chemical composition, material brand, body hardness and metallographic structure. Foreign cylinder blocks generally use low alloy cast iron equivalent to HT250 or higher in China, with elastic modulus of 105gpa ~ 110GPa and more than 98% pearlite structure in the matrix.
At present, the material of cast iron cylinder block is mainly gray cast iron, However, there is little difference in the content range of conventional five elements in gray cast iron used in domestic factories (mass fraction) C: 3.0% – 3.5%, Si: 1.8% – 2.5%, Mn: 0.6-0.9%, s < 0.15%, P < 0.15%. With the improvement of truck diesel engine power, the requirements for material properties are also increasing, and the grade of gray iron used is increased from HT250 to HT300, or even ht350. In addition to the conventional five elements, a small amount of alloy elements such as Cr, Cu, Mo and V are usually added to the cast iron to promote the pearlescent luster of gray iron It can improve the strength and thermal fatigue performance of castings and reduce the sensitivity of wall thickness.
Yin Qinghua analyzed the reasons for the unstable properties of high strength gray cast iron cylinder block materials. Pretreatment agent was used to improve the nucleation, decay resistance and inoculation ability of molten iron; Select high-quality c-increasing agent to increase the number of graphite cores; Change the liquid inlet position of molten iron filling mold; Properly reduce the content of C, Si and Mn in raw molten iron and increase the content of S; Reduce the addition amount of gf300 alloy, change gf300 alloy into flow modification agent, and add it into molten iron together with flow inoculant during pouring. The composition of molten iron treated above is shown in table 1-8 and table 1-9. The tensile strength of the cylinder body has been steadily improved and met the technical requirements. It is used in the production of 6DL series cylinder body and DEUTZ series cylinder body.
The effect of C and Si elements on mechanical properties. With the increase of C and Si elements, the hardness and mechanical properties of the casting decrease, but the hardness inside the cylinder block decreases more.
Selecting the right cast iron material for an automobile engine block involves considering various properties that are crucial for the engine’s performance, durability, and manufacturability. Traditionally, grey cast iron has been the material of choice for engine blocks due to its advantageous properties, but advancements in materials engineering have also made ductile cast iron a viable option for certain applications. Here’s a comparison and analysis of why these materials are favored:
Grey Cast Iron for Engine Blocks
Advantages:
- Thermal Conductivity: Grey cast iron has good thermal conductivity, which is beneficial for dissipating the heat generated by the engine during operation, thus helping in maintaining optimal operating temperatures.
- Machinability: The presence of graphite flakes in grey cast iron acts as a lubricant, making it easier to machine than many other materials. This property allows for more straightforward manufacturing of complex engine block geometries.
- Damping Properties: Grey cast iron exhibits excellent vibration damping capabilities, which reduces noise and wear in the engine, contributing to a smoother operation and longer engine life.
- Cost: Generally, grey cast iron is less expensive to produce than ductile cast iron or other alternative materials like aluminum alloys, making it a cost-effective choice for engine block manufacturing.
Disadvantages:
- Weight: Grey cast iron is heavier than aluminum, which can contribute to increased fuel consumption.
- Tensile Strength and Toughness: While sufficient for many applications, the tensile strength and toughness of grey cast iron are lower than those of ductile cast iron. This can be a limiting factor in high-performance or heavy-duty engines.
Ductile Cast Iron for Engine Blocks
Advantages:
- Strength and Toughness: Ductile cast iron provides higher tensile strength and toughness compared to grey cast iron, which makes it suitable for more demanding engine applications, including high-performance and heavy-duty engines.
- Weight: While still heavier than aluminum, certain compositions of ductile cast iron can offer a better strength-to-weight ratio compared to grey cast iron, contributing to slightly improved fuel efficiency without sacrificing durability.
- Thermal Conductivity: Although generally lower than that of grey cast iron, the thermal conductivity of ductile cast iron can still meet the requirements for effective heat dissipation in engine blocks.
Disadvantages:
- Cost: The production of ductile cast iron involves additional steps (such as the addition of magnesium or cerium) to spheroidize the graphite, which can make it more expensive than grey cast iron.
- Machinability: While still machinable, ductile cast iron does not machine as easily as grey cast iron due to the difference in the type of graphite present (spherical nodules vs. flakes).
Conclusion
The choice between grey and ductile cast iron for automobile engine blocks largely depends on the specific requirements of the engine, including performance, weight, cost, and manufacturing considerations. Grey cast iron remains a popular choice for standard passenger vehicles due to its excellent machinability, thermal conductivity, and cost-effectiveness. On the other hand, ductile cast iron is favored for more demanding applications where higher strength and toughness are required, despite its higher cost and slightly more challenging machinability.