Quality Control Technology of Gray Cast Iron Cylinder Block Head Casting

Cylinder block cylinder head are the parts with large size, shape, high technical requirements in automobiles. The quality of cylinder head cylinder head castings comprehensively reflects the technological level, management level personnel quality of the foundry. At present, automobile design strives to reduce unit power weight, reduce fuel consumption, improve economic efficiency. Therefore, castings such as cylinder blocks cylinder heads strive to reduce wall thickness weight under the premise of ensuring strength rigidity. At the same time, a series of requirements are put forward for this type of castings in terms of dimensional accuracy, surface roughness, cavity cleanliness, material properties, internal quality, material uniformity dimensional stability.

    Compared with the technical conditions of cylinder blocks cylinder heads of various foreign manufacturers, the content range of the conventional five elements in the chemical composition has little difference. the perspective of product technical conditions, the range of chemical composition is relatively wide, but in actual production, each factory often selects a narrower composition range according to its own production conditions user requirements to ensure the quality performance of castings. The components specified in the technical standards can produce qualified castings stably.

Before the 1960s, the equivalent grade HT200 was generally used to produce cylinder blocks cylinder heads, but now the equivalent HT250 grade is widely used. Improving the grade first requires strict control of the chemical composition, especially the carbon silicon content. When British foundries produce cylinder blocks cylinder heads, the deviation of carbon content of cast iron is controlled within ±0.06%, the deviation of silicon content is controlled within ±0.08%. The second automobile produces 6100 cylinder block. In order to make the material grade stably reach HT250, it is required to control carbon 3.20%-3.35%, silicon 1.9%-2.2%, carbon equivalent 3.85%-4.05%. If the carbon equivalent is lower than 3.85%, the shrinkage tends to increase the cylinder leakage rate increases. If the carbon equivalent is higher than 4.05%, the mechanical properties are difficult to meet the grade requirements stably. The metallographic structure of the cylinder block cylinder head are all required to be based on type A graphite without exception. The pearlite content of the matrix is required to be more than 95%, the cylinder barrel part must meet the specified requirements. In order to ensure that the amount of pearlite in the matrix is as high as possible, the silicon content should be too high, the cylinders produced at home abroad are generally controlled between 0.6% 0.7%. Phosphorus is very sensitive to the impact of shrinkage. Although the technical conditions of cylinder blocks cylinder heads at home abroad generally stipulate that the phosphorus content is allowed to reach 0.12% to 0.15%, in fact, many manufacturers have strict limits on the phosphorus content in production. . Statistics accumulated over a long period of time show that the lack of compactness of castings is always associated with high phosphorus content. In recent years, with the large-scale use of scrap steel as furnace material abroad, the phosphorus content of cylinder blocks cylinder heads has been greatly reduced, generally controlled below 0.10%, but even within such a range, the phosphorus content of castings is at the upper lower limit. The difference in compactness is still obvious. Low phosphorus content also has its disadvantages. When phosphorus is less than 0.05%, fins sticky sand will be produced on the surface of the casting. This defect is gradually reduced with the increase of phosphorus content. But for castings such as cylinder blocks cylinder heads, the key issue is to improve the compactness reduce the rate of leakage waste. Therefore, phosphorus should be strictly controlled. The British foundry stipulates phosphorus at 0.05%~0.06%. The West German Volkswagen Foundry my country Second Automobile Foundry No. 1 both require phosphorus content to be less than 0.08% when producing cylinder blocks cylinder heads. SI, C used to think that sulfur is a harmful element in steel, hope that the lower the content, the better, but in recent years, studies in various countries have reached a consistent conclusion that when the sulfur content in the molten iron is less than 0.04%, when ordinary ferrosilicon is inoculated The effect is poor, the gestation declines faster.

Countries attach great importance to the control of trace harmful elements in cast iron. The gray cast iron can deteriorate the graphite structure greatly reduce its mechanical properties. The study pointed out that even if the gray cast iron contains only harmful effects on the mechanical properties, it will show the demand of the West German public cylinder block. It must also be controlled. Excessive amounts will cause pinhole defects in castings, excessive ferrite in the metallographic structure, unqualified mechanical properties. It is believed that there is a danger of pinholes when the content of molten iron exceeds. When the nitrogen content of cast iron is one, nitrogen can passivate the tip of the flake graphite promote the pearlite matrix to improve the mechanical properties, but when the nitrogen content exceeds, it promotes the formation of crack-like defects. In order to limit the nitrogen content in cast iron, it is required to control the nitrogen content of the resin sand used to produce the cylinder block cylinder head core, the nitrogen content of the core sand is required to be lower than that in the core sand.

In the production of cylinder block cylinder head, foreign countries attach great importance to the control of pouring temperature. It is believed that for most cylinder blocks cylinder heads, the most suitable pouring temperature is 1390-1420℃. If the pouring temperature is too high, the shrinkage of the molten iron will increase, which will promote shrinkage porosity in the casting. When the pouring temperature is too low, the casting will produce pore defects.

In order to ensure that the required chemical composition molten iron temperature can be obtained stably in production, dual smelting is commonly used in foreign automobile foundries. Cupola furnaces are mostly used to melt cast iron, a few use electric arc furnaces coreless induction furnaces. The molten iron melted by the cupola has a smaller tendency of chilling shrinkage than the molten iron of the electric furnace. It is easier to obtain a sound casting with a metallographic structure that meets the requirements, the structure performance of the cylinder block cylinder head. Hot metal holding furnaces generally use large-scale core trough industrial frequency furnaces.

For the cylinder block cylinder head, inoculation treatment is a very important link. Inoculation is an important technological measure to eliminate white mouth, improve processing performance, refine eutectic clusters, obtain graphite, refine distribute graphite, improve matrix structure, improve mechanical properties, reduce section sensitivity. Inoculation also has side effects. The result of the refinement of the eutectic cluster often leads to an increase in shrinkage. Because the inoculant contains aluminum, too high an inoculum will also cause pinhole defects due to an increase in the aluminum content in the molten iron. Inoculation increases the degree of nucleation of cast iron increases the number of eutectic clusters, which leads to an increase in the expansion force acting on the mold during the solidification process. As a result, the casting may produce shrinkage shrinkage porosity. Therefore, in the production of cylinder blocks cylinder heads Particular attention should be paid to control the fertility.

Improving the dimensional accuracy of the cylinder block cylinder head castings, reducing the thickness, reducing the weight are common trends abroad.

Improving the precision of castings involves all aspects of raw auxiliary materials, mold tooling, design, technology, management personnel quality. To improve the precision of castings, first of all, we must ensure the compactness of the mold. If the mold is compact enough the rigidity is insufficient, it will withstand the static pressure of molten iron the expansion force caused by the increase of the graphite precipitation volume, which will cause the casting to exceed the required size. High-pressure molding is generally used at home abroad to produce cylinder blocks cylinder heads. Usually the mold rigidity can be guaranteed, but the molding sand performance must be adapted to it. Moisture of molding sand needs to be strictly controlled. Because of high moisture content, such as excess, the mold will be tight. If the breaking index of molding sand is too high, for example, if the index is reached, the toughness is too high it is difficult to compact. The performance indicators of the high-pressure molding sand produced by the British factory for cylinder blocks cylinder heads are generally wet compressive strength, crushing index, moisture, clay content, compaction rate. When mixing sand each time, the amount of fresh bentonite is generally exceeded, the amount of coal powder does exceed, the amount of new sand is added, the amount of starch added is flakes.

Improving the design manufacturing accuracy of mold tooling is also crucial. Germany, Japan, Spain, etc. all have specialized factories specializing in creating tooling molds such as cylinder blocks cylinder heads. These companies have accumulated shrinkage deformation data of various sizes of cylinder blocks cylinder heads when using different cores for casting. Guide the design manufacture of molds. In order to improve the wear resistance of the mold improve the roughness, the mold needs to undergo aging treatment, the surface is nickel-plated chrome-plated polished after processing. The precision of foreign mold manufacturing is up to ± the surface roughness is forest. In contrast, the accuracy of domestic mold manufacturing can only reach the soil, which is one piece. In order to achieve high-precision requirements, copy milling, coordinate files, digital display milling machines digital display EDM machine tools are widely used abroad for processing, their equipment is far better than domestic mold workshops.

Sand boxes mold plates require regular inspections. The British factory requires that the positioning pins positioning bushings be replaced when they are worn out.

In the world casting market, the prices of castings with different complexity levels vary greatly. Among automobile castings, the relatively simple brake drum castings cost about one dollar per ton, while the more complex cylinder block cylinder head castings cost more than US dollars per ton. At present, the export of my country's automobile castings is basically limited to relatively simple castings, complex cylinder block cylinder head castings are still difficult to hit the international market. Even some imported automobile engines are manufactured at high prices abroad. Purchasing cylinder blocks cylinder head castings, if our domestically produced cylinder blocks cylinder head castings can replace foreign products sell them back abroad, the economic benefits are very significant