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Casting stable and qualified chilled cast iron camshaft blanks

Update:16-06-2022
Summary:

Chilled cast iron camshafts have good wear resistance, […]

Chilled cast iron camshafts have good wear resistance, but poor technological applicability, which affects its application in the automotive market to a certain extent. By reasonably controlling its chemical composition and controlling the molding sand, stable and qualified chilled cast iron camshaft blanks can be obtained.
The material of camshaft can be mainly divided into chilled cast iron, steel, alloy cast iron (hardenable and sub-arc remelting), ductile iron, combined type, etc. The craftsmanship, weight, durability and market application of camshafts of V6 camshaft various materials are different (see the table for details).
Chilled cast iron camshafts have good wear resistance, and are gradually used in the car market, but their technological applicability is poor, which affects its further development to a certain extent. In actual production, factors such as chemical composition of materials, control of molding sand, process temperature control, control of molten iron inoculation and spheroidization in front of the furnace, mold and process design will all affect the technological usability of chilled cast iron camshafts. Let's talk about the impact on the casting of chilled cast iron camshafts from the aspects of chemical composition and sand control.
Table Performance comparison of camshafts with different materials
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chemical composition
1. Appropriate carbon equivalent
The main factors that determine the properties of chilled cast iron are: the quantity and morphology of chilled layer carbides, the amount of graphite and the properties of the metal matrix. When the carbon equivalent (CE=C+1/3Si) is high, it is easy to form a chilled layer, and the amount of carbides in the chilled layer increases, which is very beneficial for obtaining higher hardness of the chilled layer; but it also brings graphite The increase in the number, the deterioration of the graphite morphology, and the coarsening of the matrix structure have adverse effects. Especially when the incubation conditions are not good or there are trace harmful elements, the coarse graphite reduces the effective area of ​​the metal matrix that can bear the load, so that the strength of the metal matrix cannot be exerted normally, thereby reducing the strength of the cast iron.
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Among cast iron materials, pearlite has better strength and hardness, while ferrite is softer and has lower strength. With the increase of the number of C and Si, the number of pearlite will decrease (see Figure 1), the morphology will become more coarse, and the number of ferrite will increase (see Figure 2), which will reduce the tensile strength of the material. Therefore, after ensuring that sufficient cold shock layer is obtained, the carbon equivalent should be controlled at a lower level to obtain higher mechanical properties of the product. However, when the carbon equivalent is too low, in addition to being unfavorable to obtain a good cold shock layer, it will also increase the tendency of shrinkage and cause the product to be scrapped (see Figure 3). Therefore, in the smelting process, controlling the carbon equivalent is a very important factor to solve the material properties of chilled cast iron camshafts. After practice, the author believes that it is usually more appropriate to control the CE value at 3.9~4.2.
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2. The effect of alloying elements on material properties
In chilled cast iron camshafts, the commonly used alloying elements are Cr, Cu, Ni, Mo, etc., and these elements have different effects in molten iron. The main function of Cr is to promote the formation of white mouth and form composite carbides with C and Fe to improve the hardness and wear resistance of the cold shock layer. However, the addition of Cr will increase the amount of carbides in the non-cooling zone, increase the brittleness of the material, and reduce or even disappear the amount of ferrite. At the same time, due to the addition of alloying elements, the graphite is also refined to a certain extent, and the ferrite in it is solid solution strengthened due to a certain amount of alloying elements, so that the cast iron has high strength properties.
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3. The effect of charge ratio on materials
In the past, we have always believed that as long as the chemical composition meets the requirements of the specification, it should be possible to obtain materials that meet the standard mechanical properties. In fact, this view only sees the conventional chemical composition, ignoring the role of some trace elements and harmful elements in it. effect. For example, pig iron is the main source of Ti, so the amount of pig iron used will directly affect the content of Ti in the material and have a great impact on the mechanical properties of the material; The performance impact is very immediate. Therefore, the charge ratio has a direct impact on the mechanical properties of chilled cast iron materials, and is the focus of smelting control. In addition, in order to reduce the influence of charge heredity on the material, it is recommended that the charge should be managed in batches to achieve first-in, first-out.

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