Ductile iron is an important casting metal material developed in China in the past 40 years. Because of the small stress concentration caused by spheroidal graphite, the splitting effect on the matrix is also small, so the tensile strength, ductility and toughness of ductile iron are higher than other cast iron. Compared with the steel of the corresponding organization, the plasticity is lower than that of steel, and the fatigue strength is close to that of ordinary medium carbon steel. The yield ratio can reach 0 7 to 0 8, which is almost twice that of ordinary carbon steel, and the cost is lower than that of steel, so its application It is becoming more and more widespread.
Of course, ductile iron is not perfect, in addition to the general casting defects, it will also produce some unique defects, such as shrinkage, slag inclusion, subcutaneous pores, spheroidization and decline. These defects affect the performance of the casting and increase the casting rejection rate. In order to prevent the occurrence of these defects, it is necessary to analyze and precision casting, summarize various influencing factors, and propose preventive measures to effectively reduce the occurrence of defects and improve the mechanical properties and production efficiency of castings. This article will discuss the main common defects of ductile iron parts: shrinkage, shrinkage, slag inclusion, subcutaneous pores, graphite floating, poor spheroidization and spheroidization.
1 shrinkage shrinkage
1.1 Influencing factors
(1) Carbon equivalent: increase the amount of carbon, increase the expansion of graphitization, and reduce shrinkage and shrinkage. In addition, increasing the carbon equivalent can also improve the fluidity of the ductile iron and facilitate the feeding. The empirical formula for producing high quality castings is C% + 1/7Si% > 3 9%. However, when the carbon equivalent is increased, the casting should not cause other defects such as graphite floating.
(2) Phosphorus: The phosphorus content in the molten iron is too high, so that the solidification range is enlarged, and the low-melting-point phosphorus eutectic is not replenished at the final solidification, and the casting shell is weakened, so that the shrinkage cavity and the shrinkage are increased. The tendency to produce. The general plant control phosphorus content is less than 0 08%.
(3) Rare earth and magnesium: If the residual amount of rare earth is too high, the graphite shape will be deteriorated and the spheroidization rate will be lowered, so the rare earth content should not be too high. Magnesium, on the other hand, is a strongly stable carbide element that hinders graphitization. It can be seen that the amount of residual magnesium and the amount of residual rare earth increase the tendency of the white iron of the ductile iron to decrease the expansion of the graphite, so when their content is high, the tendency of shrinkage and shrinkage is also increased.
(4) Wall thickness: When the surface of the casting is formed into a hard shell, the higher the temperature of the internal molten metal, the larger the liquid shrinkage, and the volume of shrinkage and shrinkage not only increases in absolute value, but also increases the relative value. In addition, if the wall thickness changes too suddenly, the isolated thick section cannot be replenished, and the tendency to cause shrinkage shrinkage increases.
(5) Temperature: high pouring temperature is conducive to feeding, but too high will increase the amount of liquid shrinkage, which is unfavorable for eliminating shrinkage and shrinkage. Therefore, the pouring temperature should be reasonably selected according to the specific conditions, generally 1300~1350 °C is suitable. .
(6) The compactness of the sand type: If the compactness of the sand type is too low or uneven, the phenomenon of cavity expansion occurs under the action of metal static pressure or expansion force after casting, so that the original metal is not enough to be fed. Lead to shrinkage and shrinkage of the casting.
(7) pouring riser and cold iron: if the pouring system, riser and cold iron are improperly set, the molten metal cannot be solidified in sequence; in addition, the number and size of the riser and the connection with the casting will not affect the riser Shrink effect.
1.2 Preventive measures
(1) Control the composition of molten iron: maintain a high carbon equivalent (>3 9%); reduce the phosphorus content as much as possible (<0 08%); reduce the amount of residual magnesium (<0 07%); use rare earth magnesium alloy to treat, The residual amount of rare earth oxide is controlled at 0 02% to 0 04%.
(2) The process design should ensure that the casting can continuously replenish the high-temperature molten metal from the riser during solidification, and the size and quantity of the riser should be appropriate, and strive to achieve sequential solidification.
(3) If necessary, use cold iron and subsidies to change the temperature distribution of the casting to facilitate sequential solidification.
(4) The pouring temperature should be 1300 ~ 1350 ° C, the pouring time of a pack of molten iron should not exceed 25 min, so as to avoid spheroidization decline.
(5) Improve the compactness of the sand type, generally not less than 90; uniform sand impact, moisture content should not be too high, to ensure that the mold has sufficient rigidity.
2 slag inclusion
2 .1 Influencing factors
(1) Silicon: The oxide of silicon is also a major component of slag inclusion, so the silicon content is reduced as much as possible.
(2) Sulfur: Sulfide in molten iron is one of the main causes of slag inclusion defects in ductile iron. The melting point of the sulfide is lower than the melting point of the molten iron. During the solidification of the molten iron, the sulfide will precipitate from the molten iron, increasing the viscosity of the molten iron, making it difficult for the molten iron or metal oxide in the molten iron to float. Therefore, when the sulfur content in the molten iron is too high, the casting is liable to cause slag inclusion. The sulphur content of the ductile iron original iron solution should be controlled below 0 06%. When it is between 0 09% and 0 135%, the cast iron slag defects will increase sharply.
(3) Rare Earth and Magnesium: In recent years, it has been considered that slag inclusion is mainly caused by oxidation of elements such as magnesium and rare earth, so residual magnesium and rare earth should not be too high.
(4) Pouring temperature: When the pouring temperature is too low, the metal oxide in the molten metal is too high in viscosity due to the viscosity of the molten metal, and it is not easy to float to the surface and remains in the molten metal; when the temperature is too high, the molten slag on the surface of the molten metal It becomes too thin to be easily removed from the liquid surface, often with the influx of molten metal. In actual production, the pouring temperature is too low, which is one of the main causes of slag inclusion. In addition, the choice of casting temperature should also consider the relationship between carbon and silicon content.
(5) Gating system: The design of the gating system should be reasonable, and it has the function of slag blocking, so that the molten metal can smoothly fill the casting mold, and strive to avoid splashing and turbulence.
(6) Type sand: If the surface of the molding sand is adhered with excess sand or paint, they can be combined with the oxide in the molten metal to form slag, resulting in slag inclusion; the sand type has uneven compactness and the wall with low compactness The surface is easily eroded by the molten metal and forms a compound having a low melting point, resulting in slag inclusion in the casting.
2.2 Preventive measures
(1) Control the composition of molten iron: Minimize the sulfur content in the molten iron (<0 06%), and add rare earth alloy (0 1% to 0 2%) to purify the molten iron, and reduce the silicon content and residue as much as possible. The amount of magnesium.
(2) Smelting process: It is necessary to increase the temperature of the molten metal as much as possible, and to seduce properly, so as to facilitate the floating and gathering of non-metallic inclusions.扒 Clean the surface of the molten iron, the surface of the molten iron should be covered with a covering agent (perlite, grass ash, etc.) to prevent oxidation of the molten iron. Choose a suitable casting temperature, preferably no lower than 1350 °C.
(3) The pouring system should make the flow of molten iron smooth, and should be provided with slag collecting slag and slag blocking device (such as filter residue net) to avoid sanding of the sprue.
(4) The compactness of the mold should be uniform and the strength is sufficient; the sand in the mold should be blown off when the box is closed.
3 graphite floating
3. 1 Influencing factors
(1) Carbon equivalent: The carbon equivalent is too high, so that a large amount of graphite is precipitated when the molten iron is at a high temperature. Since the density of graphite is smaller than that of molten iron, graphite is floated to the upper part of the casting by the action of magnesium vapor. The higher the carbon equivalent, the more severe the graphite floating phenomenon. It should be pointed out that too high carbon equivalent is the main reason for graphite floating, but it is not the only reason. Casting size and wall thickness are also important factors affecting graphite floating.
(2) Silicon: Under the condition that the carbon equivalent is constant, appropriately reducing the silicon content helps to reduce the tendency of graphite to float.
(3) Rare earth: When the rare earth content is too small, the solubility of carbon in the molten iron will decrease, and the molten iron will precipitate a large amount of graphite, which will increase the graphite floating.
(4) Spheroidization temperature and incubation temperature: In order to improve the absorption rate of magnesium and rare earth elements, domestic experimental studies have shown that the most suitable molten iron temperature during spheroidization is 1380 to 1450 °C. In this temperature range, as the temperature increases, the absorption rates of magnesium and rare earth increase.
(5) Pouring temperature: In general, the higher the pouring temperature, the greater the tendency of graphite to float, because the castings are in a liquid state for a long time to facilitate the precipitation of graphite. A.P.Druschitz and W.W.Chaput found that if the solidification time is shortened, the graphite floatation tends to decrease as the pouring temperature increases.
(6) Residence time: The residence time between the incubation process and the completion of the casting is too long, which provides conditions for the precipitation of graphite. Generally, this time should be controlled within 10 minutes.