1 ductile iron solidification shrinkage process theory
What is the process to solve and prevent shrinkage of the Nodular Graphite Iron Castings, the current consensus is that the mold must have sufficient rigidity and strength, chemical composition close to the eutectic composition, strengthen the spheroidization and fertilization treatment to produce enough Graphite expansion. However, 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 casting the use of 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, So that part of the graphitization ahead of time; but also have the literature expressed dissent, that the ductile iron shrinkage is greater than the expansion, there must be external fill, riser can not be later than the casting solidification, the role of cold iron is the first liquid Ahead of time and faster, more conducive to the earlier timely replenishment, the expansion and contraction has no effect. The focus is on whether the graphitization expands from self-refraction or emphasizes external shrinkage.
From the analysis of some typical castings and the summary of the solution, simply emphasize the external fill or emphasize the graphitization of the expansion of self-complement can not be stable control of the casting shrinkage. How to effectively increase the use of graphite and expansion of the use, mainly depends on the casting structure, molten iron smelting treatment level and casting conditions. One of the views expressed in suggests that pressure changes occur inside the molten iron during the cooling and solidification process, and this part is prone to shrinkage when the internal pressure is less than a certain level or to a negative pressure.
Principle of process design
(1) small parts or thin parts of the casting should be considered external fill, solidification shrinkage process requires external pressure support
The equilibrium solidification theory also suggests that small pieces require external strong shrinkage, which can actually be expanded to say that the so-called small pieces are small pieces of small weight or large thin-walled parts, including thin-walled parts. Such small pieces because of its solidification speed, due to the ductile iron ductile expansion and solidification shrinkage time difference can not use the pressure superposition to achieve self-complement, the need for external external hydraulic pressure support and liquid supplement.
In the thin pieces of cast process design, should first consider the fill. (2) thick parts or large parts of the thick parts do not need to fill or only need to shrink the thick part of the casting thick parts or thick pieces, because of its large amount of graphitization, and easy to achieve the expansion of graphite and coagulation shrinkage Superimposed on each other, so self-complement can be achieved. The outside only need to provide a large amount of shrinkage in the initial solidification of the pressure support, the latter can be completely self-expansion of the pressure generated by self-refraction.
Thick or thick wall in the process design, can be achieved by the expansion of self-refraction of the way to deal with graphite, and less consider the use of external fill.
(3) the filling of the casting is not limited to the external shrinkage casting there are structural differences, the thin wall of the filling does not have to rely on the external set of riser to achieve the adjacent parts of the thick and thick parts of the same can be Thinning at the thin wall, with the traditional concept is to establish and achieve the appropriate temperature gradient (solidification order), so that the liquid static pressure can be maintained at a certain level, to meet the thinner coagulation shrinkage needs.
In the process of casting with poor thickness, it is possible to consider the establishment of a certain order of solidification, taking into account the above two principles, that is, the thin part of the thick part of the fill, and thick parts are considered only to achieve limited shrinkage.
(4) cold iron can adjust the thick heat section of the solidification rate and casting temperature gradient, the casting of the role of the filling is conditional.
The role of cold iron in addition to the casting can speed up the rate of solidification to improve the solidification gradient, the ductile iron also has a prominent improvement in casting graphite size and shape, improve the level of spheroidization and to prevent the ball and the decline of graphite and other effects, but cold The use of iron also has many side effects on the castings. In the past people understand the shortcomings of cold iron is easy to produce white mouth, cold iron stoma and so on. In fact, the use of cold iron also has a bad effect on the shrinkage of the castings. If the cold iron changes the solidification sequence and the temperature gradient favorable to the above principles, the casting will produce shrinkage defects; in addition, When there are more than one cold iron, the cold iron gap is too large and the gap between the intercoolers (or cold iron) may form new hot sections and shrinkage defects. Therefore, when the cold iron is set, Cooling conditions, but also to prevent the cold iron to change the inherent balance of the state, or cause a new hot section, or block the liquid static pressure transmission channel (fill channel), resulting in casting shrinkage defects; multiple cold iron placed in parallel To avoid excessive cold iron gap, you can increase the effective chill area by increasing the cold chill effect, rather than blindly increase the cold iron thickness.