Experimental material design of heat resistant cast iron for automobile exhaust manifold

The structure of the material determines the performance of the material, and the matrix structure of the material is often determined by its chemical composition. Therefore, the chemical composition of the material requires strict control, and there are upper and lower limits for the element composition of the material in each major standard. The choice of material composition suitable for mass production of automobile exhaust manifold must have good molding performance and relatively reasonable price cost on the basis of meeting the material structure of standard requirements. The chemical composition of materials should be determined according to the influence of alloy elements on properties.

(1) Carbon (c):

Carbon is one of the important elements in nodular cast iron. The content of carbon not only determines the amount and distribution of graphite and the formation of carbides, but also has a great influence on the fluidity and shrinkage of nodular cast iron, and even on the strength of nodular cast iron. When the carbon content is less than 0.2wt%, the strength of the material is difficult to meet the use requirements. At the same time, when casting, it will lead to excessive dendrite and large shrinkage. When the content is more than 0.6wt%, the carbide will coarsen and the thermoplasticity will be very poor.

Therefore, controlling carbon content is very important to obtain high quality ductile iron. In order to improve the casting process, the upper limit of the allowable range of the technical acceptance standard should be selected for the carbon content, and the best choice of the quality fraction of C is 2.3% – 3.5%.

(2) Silicon (SI):

Si is the most common element to promote graphitization, improve transformation temperature and oxidation resistance, and is one of the important elements in cast iron. Si element can form one of the existing elements of SiO2 with good protective effect on the surface of cast iron. Si element can form SiO2 passivation film with good protective effect on the surface of cast iron, which has excellent oxidation resistance and self-healing ability in various atmospheres; Si is also an element of strengthened ferrite phase; for alloys containing niobium, molybdenum can promote the generation of excess phase and solid solution of strengthened alloy.

According to the report, only when the content of Si element reaches 4% or more, can it have the effect of oxidation resistance and thermal fatigue. With the addition of Si element, the strength increases, the plasticity decreases, the expansion coefficient increases, and the thermal conductivity decreases. When the content exceeds 6%, it basically does not meet the requirements of engineering materials.

In view of the requirements of heat resistance, referring to the silicon content of ggg-simo51 and ggg-nisicr3552, considering the limitation of the saturation number of silicon content, the Si content can be taken as the lower limit of the technical standard, namely 4.0% – 5.0%.

(3) Mo:

The melting point of molybdenum is more than 2600 ℃, which is beneficial to creep properties and fracture stress of materials. Especially in ferrite, its role is more prominent.. It often forms intermetallic compound MoSi2 with silicon. MoSi2 can improve the wear resistance, high temperature corrosion resistance and oxidation resistance of materials, but the processability at room temperature will be reduced. 0.2% molybdenum and 4% silicon have good mechanical properties at high temperature. Considering the effect of molybdenum and silicon, the amount of molybdenum is generally less than 1%.

(4) Nickel (Ni):

Nickel is the main element to promote the formation of austenite phase, and nickel does not form carbide with carbon, which can significantly reduce the tendency of white cast iron, but it is easy to segregate in the casting process, and it is easy to form intermetallic compounds with chromium and niobium and other trace elements. The high temperature dimensional stability of nickel rich cast iron is good, and it can work in high temperature condition for a long time. For a period of time, nickel used to be the limiting element, and its price was expensive. Considering the cost, its content was often not high. Generally, its content in Austenitic Ductile iron was controlled at 18-36 mass%.

(5) Chromium (CR):

Chromium is also an element to increase the embrittlement tendency. When the content of chromium exceeds 17%, σ phase is easy to precipitate, which makes aging brittleness insensitive. Chromium can reduce the critical transformation temperature of austenite, and its oxide can slow down the oxidation rate. Chromium is often used in combination with nickel to further improve the heat resistance and corrosion resistance of materials. However, when the content of chromium is too much, it is easy to form network Cr7C3 compound with carbon, which can improve the fracture strength of the material and cut the matrix structure. Therefore, the content of chromium should also be controlled, generally in the range of 1.6-1.8%.

(6) Manganese (Mn):

Manganese is a stable austenite phase element, its price is only about 20% of nickel, to some extent, it can replace nickel; at the same time, manganese oxide also has self-protection, and its properties are relatively stable with the change of temperature. However, the content of manganese should not be too high. It is easy to form network carbide with carbon and precipitate on the grain boundary, reducing the toughness and plasticity of the material. So the content of manganese is not more than 0.95%.

(7) Other elements (S, P, etc.)

P can inhibit the spheroidization of graphite, but its content should not be too high. Although sulfur can promote the graphitization, as a metallurgical impurity, its content should not be too high. Therefore, s, P and other elements are generally controlled within 0.03%.

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