1 ductile iron solidification shrinkage process theory
What kind of process measures are used to solve and prevent shrinkage of the nodular cast iron, the current consensus is that the mold must have sufficient rigidity and strength, chemical composition close to the eutectic composition, strengthen the spheroidization and inoculation to produce sufficient Graphite expansion. But the specific process design in principle there is still controversy: equilibrium solidification theory that ductile iron ductile expansion can offset the coagulation shrinkage, the process should take measures to make unit time contraction and expansion, contraction and contraction in proportion, the use of expansion and Dynamic shrinkage superposition, to achieve the purpose of filling the casting, the idea of using the riser is limited, the riser does not have to be later than the casting solidification, the role of cold iron is to balance the wall thickness difference, eliminate hot section, and can make part of the graphite But also have literature on this objection that the bulky divergence is greater than the expansion, there must be external fill, the riser can not be later than the casting solidification,Chilled Iron Castings the role of cold iron is the first to make the liquid crystal shrink and advance, more Conducive to the earlier timely replenishment, the expansion and contraction has no effect. The focus is on whether the graphitization expands itself or emphasizes external shrinkage.
From the analysis of some typical casting shrinkage defects and the summary of the measures, simply emphasize the external fill or emphasize the expansion of the graphitization from the expansion can not be stable control of the casting shrinkage shrinkage. How to effectively use the graphitization of expansion and make up, mainly depends on the casting structure, iron smelting treatment level and casting conditions. One point of view in the literature suggests that pressure changes occur inside the molten iron during the cooling and solidification process, and this part tends to create shrinkage defects when the internal pressure is less than a certain level or to a negative pressure.
1) Set the riser and raise the height of the riser and delay the solidification time of the riser, which can be considered as a measure to improve the static pressure of the molten iron. It is beneficial to increase the internal pressure and can prevent the shrinkage and shrinkage.
(2) to strengthen the melting of spheroidization treatment, increase the strength and stiffness of the mold, can increase the expansion of graphite pressure and improve its utilization.
(3) control the level of smelting and iron liquid treatment process, control the pouring temperature and speed, can be considered to reduce the condensation caused by negative pressure.
Therefore, it can be considered that all the measures that are conducive to reducing the shrinkage and shrinkage are for the purpose of controlling the pressure in the above formula in favorable trend. But the key factor is that the internal pressure of molten iron is the pressure of a variety of pressure at the same time the superposition of the cast iron crystal solidification characteristics,Chilled Iron Castings a variety of pressure effects may not always play a role. Many of the literature shows that ductile iron in the eutectic temperature above the iron liquid in a large number of graphite core or graphite growth, graphite in the eutectic temperature above the precipitation rate of the fastest, because the flow of liquid iron is still present, a large number of The pressure generated by the graphitization expansion will be transferred through the liquid. The internal pressure of the molten iron is dominated by the hydrostatic pressure, and the eutectic solidification is dominated at the beginning of the eutectic solidification. Before the eutectic solidification, the graphitized expansion may act on Iron liquid internal, but because of the formation of the solidification layer strength is not high, will consume part of the expansion pressure; to the late, casting molten iron fluid has been lost, iron liquid static pressure has been ineffective, need to rely on the growth of graphite expansion Pressure to counteract the negative pressure generated by coagulation shrinkage. Therefore,Chilled Iron Castings the literature that the overall ductile iron shrinkage and ahead of the expansion, expansion and contraction can not be superimposed, there must be external fill, in practice proved this point. At the same time, however, many processes based on equilibrium solidification theory, by graphitization expansion and contraction, have also been successful, and it has been proved that the pressure of graphitization can offset the shrinkage negative pressure.
Pressure and molten iron flow (fill) on the iron liquid for a moment is interlinked, there is pressure there is iron liquid transmission (fill) the power; but stressed that the concept of pressure to better reflect the dynamic of the supplement, Timeliness and directionality.
Therefore, in order to prevent the ductile castings shrinkage defects, not only to improve the graphitization capacity and the expansion of the use of graphitization, to improve the static pressure of molten iron to reduce the cooling liquid coagulation to take measures,Chilled Iron Castings while the casting process as much as possible To ensure that these measures to increase the pressure with the shrinkage generated by the superposition of negative pressure in order to achieve the internal solidification of molten iron at any time at a certain level to meet the casting does not produce shrinkage needs.