Ductile iron is nearly 40 years to develop China's important casting metal materials. As the spherical graphite caused by the small concentration of stress on the matrix of the split is also smaller, so the ductile iron tensile strength, plasticity and toughness are higher than other cast iron. Compared with the corresponding steel, the plasticity is lower than that of steel. The fatigue strength is close to that of medium carbon steel, and the yield ratio is up to 0 7 ~ 0 8, which is almost twice that of ordinary carbon steel, and the cost is lower than steel. Increasingly.
Of course, ductile iron is not perfect,Nodular Graphite Iron Castings it will produce in addition to the general casting defects, but also produce some unique defects, such as shrink loose, slag, subcutaneous stomata, spheroidization and recession. These defects affect the casting performance, so that the casting scrap rate increased. In order to prevent these defects, it is necessary to analyze and precision casting, summed up the various influencing factors, put forward preventive measures, can effectively reduce the defects, improve the mechanical properties of the casting and production efficiency. This article will discuss the main common defects of ductile iron: shrinkage, shrinkage, slag, subcutaneous stomata, graphite floating, spheroidization and spheroidization.
1.1 influencing factors
(1) carbon equivalent: increase the amount of carbon, increasing the graphitization of expansion, can reduce shrinkage shrinkage. In addition, to improve the carbon equivalent can also improve the mobility of ductile iron, is conducive to filling. The empirical formula for producing high quality castings is C% + 1 / 7Si%> 3 9%. However, when the carbon equivalent is increased, other defects such as graphite floatation should not be caused.
(2) phosphorus: iron in the high phosphorus content,Nodular Graphite Iron Castings so that the scope of expansion of solidification, while low melting point of phosphorus eutectic in the final solidification can not get the supply, and the casting shell weakened, so there is increased shrinkage, The tendency to produce. General plant control of phosphorus content of less than 0 08%.
(3) rare earth and magnesium: rare earth residual capacity will deteriorate the shape of graphite, reduce the rate of spheroidization, so rare earth content should not be too high. And magnesium is a strong stabilization of carbide elements, hinder the graphitization. It can be seen that the residual magnesium and the amount of residual rare earth will increase the dolomite tendency of the ductile iron, so that the expansion of graphite decreases, so when their content is high, will increase the shrinkage, shrinkage tendencies.
(4) wall thickness: When the surface of the casting after the formation of hard shell, the internal metal liquid temperature is higher, the greater the liquid shrinkage, the shrinkage,Nodular Graphite Iron Castings shrink the volume of not only the absolute increase in the relative value also increased. In addition, if the wall thickness changes too suddenly, the isolated thick section can not be filled, so that shrinkage shrinkage tend to increase.
(5) temperature: pouring temperature is high, is conducive to fill, but too high will increase the amount of liquid contraction, to eliminate shrinkage, shrink loose, so should be based on the specific circumstances of reasonable choice pouring temperature, generally 1300 ~ 1350 ℃ is appropriate The
(6) sand compactness: If the sand tightness is too low or uneven, so that after pouring in the metal static pressure or expansion force under the action of the cavity to produce the phenomenon of expansion, resulting in the original metal is not enough to fill Resulting in shrinkage and shrinkage of the casting.
(7) pouring riser and cold iron: If the pouring system,Nodular Graphite Iron Castings riser and cold iron set improperly, can not guarantee the order of solidification of metal liquid; In addition, the number of risers, and the size of the connection with the casting when it will affect the riser Shrink effect.
1.2 Preventive measures
(<0 0%); reduce the amount of residual magnesium (<0 07%); the use of rare earth magnesium alloy to deal with, Rare earth oxide residue is controlled at 0 02% ~ 0 04%.
(2) process design to ensure that the casting in the solidification can be constantly from the riser to add high-temperature metal liquid, riser size and quantity to be appropriate, and strive to achieve the order of solidification.
(3) the use of cold iron and subsidies to change the temperature distribution of the casting in order to facilitate the order of solidification.
(4) pouring temperature should be 1300 ~ 1350 ℃, a pack of molten iron pouring time should not exceed 25min, in order to avoid spheroidization decline.
(5) to improve the sand compactness, generally not less than 90; hit sand uniform, moisture content should not be too high to ensure that the mold has enough rigidity.
2.1 influencing factors
(1) silicon: silicon oxide is also a major component of slag, so as much as possible to reduce the silicon content.
(2) sulfur: iron sulfide in the molten iron is the formation of slag defects one of the main reasons. Sulfide melting point is lower than the melting point of molten iron, solidification process in the molten iron, the sulfide will precipitate from the molten iron, increasing the viscosity of molten iron, so that molten iron slag or metal oxides and other easy to float. So the iron liquid sulfur content is too high, the casting is easy to produce slag. Ductile iron raw iron liquid sulfur content should be controlled below 0 06%, when it is 0 09% ~ 0 135%, cast iron slag defects will increase dramatically.
(3) rare earth and magnesium: In recent years, research that slag is mainly due to magnesium, rare earth and other elements due to oxidation, so residual magnesium and rare earth should not be too high.
(4) pouring temperature: pouring temperature is too low, the metal metal metal oxide and other metal liquid viscosity is too high, easy to float to the surface and remain in the metal liquid; temperature is too high, the molten metal surface slag Become too thin, easy to remove from the liquid surface, often with the molten metal into the type. The actual production, casting temperature is too low is caused by slag one of the main reasons. In addition, the choice of pouring temperature should also consider the relationship between carbon and silicon content.
(5) pouring system: gating system design should be reasonable, with slagging function, so that the metal liquid can be smooth filling the mold, and strive to avoid splash and turbulence.
(6) sand: If the sand surface adhesion of excess sand or paint,Nodular Graphite Iron Castings they can be combined with the metal oxide oxide slag, resulting in slag generation; sand compactness is not uniform, low degree of compact wall The surface is easily eroded by the metal liquid and the formation of low melting point of the compound, resulting in casting slag.
2.2 Preventive measures
(1) Control of molten iron composition: Minimize the sulfur content in the molten iron (<0 06%), add appropriate amount of rare earth alloy (0 1% ~ 0 2%) to purify the molten iron, as far as possible to reduce the silicon content and residual Magnesium content.
(2) smelting process: to maximize the temperature of the molten metal baked, suitable for calm, in order to facilitate the non-metallic inclusions floating, gathered. Grilled clean iron liquid surface of the slag, iron surface should be placed on the cover agent (perlite, ash, etc.), to prevent iron liquid oxidation. Choose the appropriate pouring temperature, preferably not less than 1350 ℃.
(3) pouring system to make the molten iron flow is smooth, should be set with slag bag and slagging device (such as filter slag network, etc.), to avoid sprinkler sand.
(4) mold tightness should be uniform, the strength is sufficient; when the box should be blowing the sand in the mold.
3. 1 influencing factors
(1) carbon equivalent: carbon equivalent is too high, so that iron in the high temperature precipitation of large amounts of graphite. As the density of graphite than the molten iron is small, driven by magnesium steam, the graphite floating to the top of the casting. The higher the carbon equivalent,Nodular Graphite Iron Castings the more severe the graphite float. It should be noted that the carbon equivalent is too high is the main reason for the occurrence of graphite floating, but not the only reason, casting size, wall thickness is also an important factor affecting the floating graphite:
(2) silicon: the carbon equivalent of the same conditions, the appropriate reduction in silicon content, help to reduce the tendency to produce graphite floating.
(3) rare earth: rare earth content is too small, the solubility of carbon in the molten iron will be reduced, the molten iron will precipitate a lot of graphite, increased graphite floating.
(4) Spheroidizing temperature and incubation temperature: In order to improve the absorption of magnesium and rare earth elements, domestic experimental studies have shown that the most appropriate iron processing temperature of iron liquid is 1380 ~ 1450 ℃. In this temperature range,Nodular Graphite Iron Castings as the temperature increases, the absorption of magnesium and rare earth increases.
(5) pouring temperature: Under normal circumstances, the higher the pouring temperature, the greater the tendency of graphite floating, this is because the casting for a long time in the liquid is conducive to the precipitation of graphite. A.P.Druschitz and W.W. Chaput study found that if the shortening of the solidification time, with the pouring temperature, graphite floating tendency to reduce.
(6) retention time: after incubation to pouring finished between the residence time is too long for the precipitation of graphite provides the conditions, the general time should be controlled within 10min.