Casting defects such as inclusions, holes, cracks (referring to pores, keyholes, cracks, cold shuts, etc.) often affect the mechanical properties, physical and chemical properties, and processing properties of castings, and determine the quality of castings. Almost all casting defects may occur in ductile iron parts, but due to its production method, crystallization law, casting performance and other casting alloys, ductile iron often has some unique defects.
So what are the defects of nodular iron parts related to nodulizers, or what are the defects of nodular iron parts caused by nodulizers?
The research in this paper shows that almost all the defects of nodular iron parts are related to the nodularizer. This mainly has the following aspects:
1. Dissimilation of graphite balls: irregular graphite appears in the dissimilation of graphite balls, such as agglomerates, tadpoles, worms, horns or other non-spherical shapes. This is due to the fact that the local crystal growth mode and growth rate deviate from the normal growth law when the spherical graphite grows along the radiation direction. When the amount of residual spheroidizing elements in the casting exceeds the expected range, such as the residual magnesium is too high, exceeding the minimum amount required to maintain graphite spheroidization, it will also affect the crystallization conditions of graphite, and it is easy to produce tadpole graphite. When there are many residual rare earths, high-carbon equivalent molten iron is prone to produce fragmented graphite, and the concentrated area of fragmented graphite is generally called “gray spot”. The appearance of vermicular graphite is due to insufficient residual spheroidizing elements or excessive titanium and aluminum.
2. Graphite floating: In the thick-walled ductile iron parts with hypereutectic composition, there is often a dense area of graphite at the top of the pouring position, that is, the phenomenon of “floating at the beginning and end”. This is because the density of graphite and molten iron is different, and the hypereutectic molten iron directly The precipitated graphite is caused by the upward buoyancy. The degree of graphite floating is related to factors such as carbon equivalent, type and residual amount of spheroidizing elements, casting solidification time, and pouring temperature. Magnesium can increase the eutectic carbon content of ductile iron. For molten iron with the same carbon equivalent, increasing the amount of residual magnesium can reduce graphite floating, and the amount of residual rare earth is too high, which is conducive to the formation of explosive graphite.
3. Anti-whitening: The whitening structure of general iron castings is easy to appear in the surface layer, sharp corners, drape, etc., which are cooled faster. The anti-whitening defect is the opposite, and the carbide phase appears in the center of the middle section of the casting, hot section etc. When the residual amount of spheroidizing elements is too much, it can promote the generation of anti-white defects. The rare earth elements are stronger than magnesium, and they can generally increase the supercooling degree when the nodular iron structure is formed.
4. Subcutaneous pinholes: The subcutaneous pinholes mainly contain hydrogen, but also a small amount of carbon monoxide and nitrogen. When the amount of residual magnesium is too high, it also strengthens the tendency to absorb hydrogen from the wet type at the same time, thus increasing the chance of subcutaneous pinholes. In addition, the long residence time of spheroidized molten iron can also increase the number of pinholes.
5. Shrinkage cavity shrinkage: Shrinkage cavity often appears in the final solidification part of the casting (hot joint, riser neck and casting connection, inner corner or inner gate and casting connection), which is hidden inside the casting or connected to the outside holes. Shrinkage porosity appears macroscopically at hot joints, and fine shrinkage holes are mostly connected to each other inside the holes. What is related to the spheroidizing elements is that the residual magnesium and rare earth should not be too high, which has a significant effect on reducing macroscopic and microscopic shrinkage porosity, and the tendency of shrinkage porosity is almost proportional to the spheroidizing elements.
6. Black slag: It generally occurs in the upper part of the casting (pouring position), and is mainly divided into massive, rope-like and finely crushed black slag. The main component of black slag is magnesium silicate, which is formed by the reaction of MgO and SiO2 in molten iron, and is affected by its relative content. Therefore, one of the measures to control black slag is to reduce the residual amount of magnesium (when adding 0.15% magnesium, the total amount of slag accounts for about 0.1% of the weight of molten iron), and the residual rare earth has a strong affinity with oxygen. There is an obvious effect on slag.
7. Nodularization decline: This is due to the long residence time of nodularized molten iron, the residual magnesium gradually decreases, the slag is not removed in time, and the sulfur will return to the molten iron, reducing or even disappearing the graphite in the solidified structure, and decaying to Irregular, worm-like or flake graphite. This kind of spheroidization decline has a certain relationship with the low content of rare earth in the spheroidizer or the low addition amount of the nodulizer, but it is not advisable to increase the amount of the nodulizer immediately, because the residual magnesium is high and the amount of slag is high. and cementite will increase, and in the thick and large sections, the graphite balls will be transformed into tadpole-shaped graphite. Production practice shows that low sulfur content in the original molten iron is the most effective in preventing nodularization decline.
8. Including the defects of nodular iron parts, almost all of them are related to the composition and dosage of nodulizers, but we cannot expect nodulizers to solve many problems, let alone solve all problems, because the role of nodulizers and The amount of nodulizer added has both advantages and disadvantages. The nodulizer is only a very important factor in the stable production control system of nodular cast iron. Only when it is combined with other supporting measures can the nodularization process be carried out stably.