The pleat pattern on the surface of the defect is caused by the solidification shrinkage of the molten iron, reflecting the characteristics of the shrinkage cavity. It can be seen that there is an oxide film on the surface of the defect. It can be seen that there are some long-standing dendrites on the surface of the casting defects, and the surface has microscopic pinholes, and the shrinkage is due to the development of the developed dendrites in multiple directions without closure, and the shrinkage of molten iron is not caused. Therefore, it cannot be judged that the casting defect is a shrinkage defect. From the inner surface of the hole pleat pattern 1000 × hole inner surface oxide film 300 × sheet saw undeveloped, small and bare head dendrites, reflecting the presence of gas in the defect.
In order to determine whether slag inclusions were present, several similar substances were analyzed using a PHILIPS EDAXPV-9100 X-ray energy spectrometer. No obvious slag inclusions were found, so the possibility of the defects due to slag inclusions could be excluded. Sex. According to the above analysis, it can be determined that the nature of the defect should be both a pore and a shrinkage defect. But whether it is mainly stomata or shrinkage hole is the main reason for its analysis.
Analysis of the cause of the defect First, the hole around the cylinder head bolt is surrounded by a V-shaped core, a transitional water core, a camshaft cavity and a gas cavity, and the conditions are rather poor. After pouring the molten iron, a large amount of heat is released during the solidification process. Since the molten iron is directly in contact with the sand core, the heat is first transmitted to the periphery of the cavity and the sand core, and then transferred to the surrounding molding sand, and the heat in the sand core can only be After the heart is discharged, the rapid release of heat is relatively difficult <1>. The resin sand has a small heat storage coefficient and poor heat dissipation conditions, which makes the cooling conditions worse. The hole of the cylinder head bolt is originally large (the maximum thermal pitch circle is á104mm), and the transitional water chamber is large and thin, and the size of the heart is relatively small. After pouring, it is surrounded by molten iron and heated, and the heat generated cannot be quickly released. And the large amount of gas generated after the heat of the transitional water chamber cannot be quickly eliminated and the release of heat is delayed, which invisibly increases the "hot section" of the hole of the cylinder head bolt in the direction of the transitional water chamber, which is biased from the defect The direction of the transitional water chamber can be verified.
The structure around the hole of the 400× cylinder head bolt in the hole is second, and the hole of the cylinder head bolt is near the ingate. From the start of pouring to the end of casting, it is subject to the influence of hot iron water and is in a state of overheating, which prolongs the solidification time of this part and increases the tendency of shrinkage cavities. Although the hole of the cylinder head bolt is located at the bottom of the casting position and the filling head of the molten iron seems to be relatively high during the solidification process, the thickness of the vertical plate above it and the wall thickness of the surrounding part are only 15-20 mm, which is solidified earlier than it. So that its feeding channel is blocked, thus forming an "isolated hot section."
In addition, due to the high residual manganese content in the molten iron, the high residual magnesium content and the unsatisfactory inoculation effect, the white mouth of the molten iron tends to be too large, and the tendency of solidification and shrinkage of the part is also increased. Due to the above factors, the liquid cylinder shrinkage and solidification shrinkage during the cooling and solidification process are not supplemented by the molten iron, thereby forming shrinkage cavities. After the shrinkage cavity is formed, it is in a vacuum state, and the gas in the molten iron is "forced" to absorb the negative pressure in the hole <2>. The source of the gas is mainly the gas which precipitates as the solubility of the gas decreases as the temperature decreases during the solidification of the molten iron. For those with large volumes, some of them may be invasive gases. Since the hole of the cylinder head bolt is in an overheated state together with the transition water cavity core, the interface of the contact is not dense, and the gas suction passage is provided for the shrinkage hole at the heat joint, and the iron liquid is discharged due to the poor discharge of the transition water cavity core. The large amount of gas generated after heating has a large instantaneous pressure. When the condition of invading pores <3>: PA>Po+Pm+Pz is met, the gas invades the molten iron and some of the bubbles enter the shrinkage cavity.
The effect has been processed by adopting additional measures such as adding external cold iron, improving the V-shaped core and the exhaust pipe arrangement of the transition water cavity, and controlling the white mouth tendency of the molten iron. More than 40 sets of cylinder head bolt holes have been processed. No defects were found in the parts, which proved that the cause of the casting defects of the cylinder head bolt holes was correct and the measures taken were effective.
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