Nodular Graphite Iron Castings 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 Nodular Graphite Iron Castings 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, Nodular Graphite Iron Castings is not perfect, 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 Nodular Graphite Iron Castings: 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 Nodular Graphite Iron Castings, 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, 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 Nodular Graphite Iron Castings, 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, 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,Nodular Graphite Iron Castings 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, 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.