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Casting defects such as inclusions, holes, cracks (referring to pores, keyholes, cracks, cold insulation, etc.) often affect the mechanical properties, physical and chemical properties, and processing properties of castings, and determine the quality of castings. Ductile iron parts may have almost all casting defects, but due to their different production methods, crystallization laws, casting properties and other casting alloys, ductile iron often has some unique defects.

Xinyuanzhu casting research shows that almost all ductile iron defects are related to nodularizers. This mainly includes the following aspects:

1. Graphite ball dissimilation
Graphite ball dissimilation appears irregular graphite, such as agglomerates, tadpoles, worms, horns or other non-spherical shapes. This is due to the deviation of the local crystal growth pattern and growth rate from the normal growth law when the spheroidal graphite grows along the radiation direction. When the amount of residual spheroidizing elements in the casting exceeds the proper range, such as the residual magnesium is too high, and exceeds the minimum amount required to maintain graphite spheroidization, it will also affect the graphite crystallization conditions, and it is easy to produce tadpole strong 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 "grey spot". The appearance of vermicular graphite is due to insufficient residual spheroidizing elements or excessive titanium and aluminum content.

2. Graphite float
In the thick-walled ductile iron parts with hypereutectic composition, there is often a graphite-intensive area at the top of the pouring position, that is, the phenomenon of "floating from beginning to end". This is because the density of graphite and molten iron is different, and the graphite directly precipitated from the hypereutectic molten iron is affected by buoyancy. caused upwards. 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 residual magnesium content can reduce graphite floating, and the residual rare earth content is too high, which is helpful for the rise of cracked graphite.

3. Anti-white mouth
Generally, the white mouth structure of iron castings is easy to appear in the surface layer, sharp corners, drape seams, etc. that cool faster, while the reverse white mouth defect is the opposite. When the residual amount of spheroidizing elements is too large, it has the effect of promoting the occurrence of anti-whitening defects. Rare earth elements are stronger than magnesium, and they can generally increase the degree of supercooling when the ductile iron structure is formed.

4. Hypodermic needle hole
The subcutaneous pinhole mainly contains hydrogen, but also a small amount of carbon monoxide and nitrogen. When the residual magnesium content is too high, the tendency to absorb hydrogen from the wet form is also enhanced, thereby increasing the probability of hypodermic pinholes. In addition, the long residence time of spheroidized molten iron can also increase the number of pinholes.

5. Shrinkage cavity shrinkage
Shrinkage cavities often appear at the last solidified part of the casting (hot joint, the connection between the riser neck and the casting, the inner corner or the connection between the inner gate and the casting), and it is a hole hidden inside the casting or connected to the outer surface. Shrinkage porosity occurs macroscopically at the thermal joint, and the tiny shrinkage holes are mostly interconnected inside the holes. Related to the spheroidizing element, the residual magnesium and rare earth should not be controlled too high, which has obvious effect on reducing macroscopic and microscopic shrinkage, and the shrinkage tendency is almost proportional to the spheroidizing element.

6. Black Slag
It generally occurs in the upper part of the casting (pouring position), and is mainly divided into block, rope and fine 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 0.15% of magnesium is added, the total amount of slag accounts for about 0.1% of the weight of molten iron), and residual rare earth has a strong affinity with oxygen. Slag has obvious effect.

7. Globalization decline
This is because the spheroidized molten iron stays for a long time, the residual magnesium gradually decreases, the slag is not removed in time, and the sulfur returns to the molten iron, which reduces or even disappears the graphite in the solidified structure, and decays into 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 spheroidizer, but it is not advisable to increase its addition amount immediately, because the residual magnesium content is high, and the amount of slag is reduced. and cementite will increase, and in thick and large sections, the graphite ball will be transformed into tadpole-like graphite. Xinyuanzhu casting production practice shows that low sulfur content in raw molten iron is the most effective to prevent spheroidization decline. Including the defects of ductile iron parts, almost all are related to the composition and addition amount of the nodularizing agent, but we cannot expect the nodulizing agent to solve many problems, let alone solve all problems, because of the effect of nodularizing elements and the nodularizing agent. The added amount of ductile iron has both advantages and disadvantages. Nodularizing agent is only a very important factor in the stable production control system of ductile iron. Only when combined with other supporting measures, can the spheroidizing treatment be performed stably.

Xinyuanzhu Group specializes in the production of Ductile iron castings, Pig iron gray iron casting, Steel casting, Brass Casting, Malleable Iron Casting, and undertakes batch casting processing of castings. Welcome guests to cooperate with pictures or samples.