A method to improve the reliability of engine cylinder head and silicon solid solution strengthened ferritic vermicular graphite cast iron

Abstract

The invention discloses a method for improving the reliability of an engine cylinder head. Using silicon solid-solution strengthened ferrite vermicular graphite cast iron as a casting material, the engine cylinder head is obtained by sequentially smelting molten iron, refining the molten iron, inoculating the molten iron ladle, and casting; The silicon solid-solution strengthened ferritic vermicular graphite cast iron contains the following chemical components in mass fractions: 2.9-3.3% carbon, 4.0-4.8% silicon, 0.008-0.015% sulfur, 0.009-0.020% magnesium, 0.018-0.035% rare earth, and The amount is iron as well as unavoidable trace elements. Wherein the carbon equivalent needs to be kept at 4.2-4.7%, and the carbon equivalent=mass percentage of carbon+mass percentage of silicon / 3. The engine cylinder head obtained by the invention has high tensile strength, moderate and uniform hardness, small shrinkage tendency of the cylinder head and good thermal fatigue performance.

Description

technical field [0001] The invention relates to the casting of an engine cylinder head, and more particularly relates to a method for improving the reliability of the engine cylinder head. The invention also relates to silicon solid-solution strengthened ferrite vermicular graphite cast iron for improving the reliability of the engine cylinder head. Background technique [0002] The cylinder head is the key component of the engine, and it is also the most prone to reliability problems of the engine. It is installed on the top of the engine block, sealing the cylinder from the upper part and forming the combustion chamber. During the working process of the engine, the combustion zone on the bottom surface of the cylinder head is in direct contact with the high-temperature and high-pressure gas, and bears a large thermal load and mechanical load. The cylinder head is subjected to low-cycle thermal fatigue damage, high-cycle thermal fatigue damage and creep damage during work:...

AI Technical Summary

Problems solved by technology

[0005] The grades of gray cast iron that are more suitable for casting cylinder heads are HT250 and HT275. After the grade HT300, the casting difficulty increases greatly, and the highest can only reach HT350. After that, there is an insurmountable bottleneck for gray cast iron. Therefore, the disadvantage of gray cast iron is: it cannot be used for High-power engine high-strength requirements

[0006] In the prior art, vermicular graphite cast iron is considered to be an ideal material for casting high-power engine cylinder heads. The brands that are more suitable for casting cylinder heads are RuT350 and RuT400. The low-grade RuT300 has low strength and hardness and cannot meet the requirements of cylinder heads. RuT450 and RuT500 with high grades have the following disadvantages:

[0007] (1) The casting performance is poor, and it is difficult to cast it on a cylinder head with a complex structure, because the strength of vermicular iron is determined by the creep rate and pearlite content. The lower the creep rate and the higher the pearlite content, the higher the vermicular iron strength. High, due to the complex structure of the cylinder head, in order to reduce the solidification shrinkage and ensure the thermal conductivity of the vermicular iron, the production of the cylinder head must select a high creep rate, generally requiring a creep rate of more than 80%. Pearlite content, so it is necessary to add Cu, Mo, Sn, Ni and other alloying elements to promote the formation of pearlite, which not only directly increases the production cost, but the most fatal thing is that increasing the content of these alloying elements will lead to an increase in the tendency of shrinkage and porosity, so that Significant increase in casting rejects for cylinder heads

[0008] (2) Poor processing performance. RuT450 and RuT500 are added with Cu, Mo, Sn, Ni and other alloying elements that promote the formation of pearlite. As the pearlite content increases, the hardness of the casting also increases, and the cooling speed of each part of the casting is different. As a result, the content of pearlite is different, which leads to the increase of the hardness difference of each part of the casting, resulting in machining tool breakage or accelerated tool wear, increasing machining costs and reducing machining accuracy

[0009] (3) Insufficient thermal fatigue resistance. The matrix structure of RuT450 and RuT500 is dominated by pearlite (generally greater than 85%). Pearlite is a metastable structure. The cylinder head starts cycle during repeated "start-stop" process. Although the overall temperature of the cylinder head will not exceed the eutectoid temperature due to water cooling, the local temperature at the corners in contact with the high-temperature gas may exceed the eutectoid temperature. After exceeding the eutectoid temperature, the pearlite will transform into austenite , and then when cooling down slowly, the austenite partly transforms into ferrite and graphite, which reduces the strength and hardness, and at the same time produces phase transformation and expansion internal stress, thus accelerating the cracking and failure of the cylinder head

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