the defects of ductile iron related to spheroidizing agent

Casting defects such as inclusions, holes, cracks (referring to pores, keyholes, cracks, cold partitions, etc.) often affect the mechanical properties, physical and chemical properties, and processing properties of the castings, and determine the quality of the 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. Research shows that almost all defects of ductile iron are related to spheroidizing agent.

the-defects-of-ductile-iron-related-to-spheroidizing-agent

 

Mainly have the following aspects:

1.Graphite Ball Alienation

Irregular graphite appears in the dissimilation of graphite balls, such as clumps, tadpoles, worms, horns, or other non-spherical shapes. This is because 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, if the residual magnesium is too high, exceeding 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 more residual rare earths, hot metal with high carbon equivalent is easy to produce fragments of graphite, and the concentrated area of fragments of graphite is generally called "grey spots". The emergence of vermicular graphite is due to insufficient residual spheroidizing elements or excessive titanium and aluminum content.

2.Graphite floating

In 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 from beginning to end". This is because the density of graphite and molten iron are different, and the graphite directly precipitated by hypereutectic molten iron is subjected to buoyancy. Caused by upwards. The degree of graphite floating is related to the carbon equivalent, the type and residual amount of spheroidizing elements, the solidification time of the casting, the pouring temperature and other factors. 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 helps to promote the explosion of graphite.

3.Inverted

In general, the white structure of iron castings tends to appear in the surface, sharp corners, and drape seams that cool faster. On the contrary, the white mouth defects are reversed. The carbide phase appears in the middle section center and hot joints of the casting. When the residual amount of spheroidizing elements is too much, it can promote the generation of anti-white 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 pinhole

The hypodermic 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, thereby increasing the probability of subcutaneous pinholes. In addition, the long residence time of spheroidized molten iron can also increase the number of pinholes.

5.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), it is a hole hidden inside the casting or connected to the outside. Shrinkage porosity appears macroscopically at the hot joints, and most of the tiny shrinkage holes are interconnected inside the holes. Related to the spheroidizing element is that the residual magnesium and rare earth must not be too high, which has a significant effect on reducing the macroscopic and microscopic shrinkage. The shrinkage tendency is almost proportional to the spheroidizing element.

6.Black slag

It generally occurs on the upper part of the casting (pouring position), and is mainly divided into block, rope and fine black slag. Magnesium silicate, the main component of black slag, 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 the black slag is to reduce the residual amount of magnesium (when magnesium is added 0.15%, the total slag accounts for about 0.1% of the weight of the molten iron), and the residual rare earth has a strong affinity with oxygen, which reduces the amount of black The slag has obvious effect.

7.Spheroidization

This is because the spheroidized molten iron stays for a long time, the residual magnesium is gradually reduced, 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 decays into irregular, worm-like or Flake graphite. This kind of spheroidization decline is related to the low rare earth content in the spheroidizing agent or the low amount of spheroidizing agent added. However, it is not advisable to increase the amount immediately because the residual magnesium content is high and the amount of slag is high. Both cementite and cementite will increase, and the graphite ball will be transformed into tadpole-like graphite in the thick section. Production practice shows that the low sulfur content of raw molten iron is the most effective to prevent spheroidization and decline.

The defects of nodular iron parts are almost all related to the composition and amount of nodulizer, but we cannot expect nodulizer to solve many problems, let alone solve all problems, because the effect of nodular element and the amount of nodulizer are both The pros and cons coexist, the spheroidizing agent is only a very important factor in the stable production control system of spheroidal graphite cast iron. Only when combined with other supporting measures can the spheroidizing treatment be carried out stably.