spheroidizing

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

Then what are the defects of the ductile iron parts related to the spheroidizer, or the defects of the ductile iron parts due to the factors of the spheroidizing agent?

Studies have shown that almost all of the ductile iron defects are related to the spheroidizing agent.

This mainly has the following aspects:

(1) Graphite ball dissimilation

Graphite ball dissimilation occurs in irregular graphite, such as agglomerates, braids, 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 spheroidal graphite grows in the radiation direction. When the amount of residual spheroidizing elements in the casting exceeds the expected range, if the residual magnesium is too high, which exceeds the minimum amount required to maintain the spheroidization of the graphite, it will also affect the crystallization condition of the graphite, and it is easy to produce sturdy graphite. When the residual rare earth is large, the high carbon equivalent molten iron is liable to produce crushed graphite, and the concentrated area of ​​the broken graphite is generally referred to as "grey spot". The appearance of vermicular graphite is due to insufficient residual spheroidal elements or excessive titanium and aluminum.

(2) Graphite floating

In the thick-walled ductile iron with hypereutectic composition, at the top of the casting position, there is often a dense graphite region, that is, the phenomenon of “floating at the beginning and ending”. This is because the density of graphite and molten iron is different, and the graphite directly precipitated by hypereutectic molten iron is buoyant. Caused by upwards. The degree of graphite floating is related to factors such as carbon equivalent, type and residual amount of spheroidized elements, solidification time of castings, and pouring temperature. Magnesium can increase the eutectic carbon content of ductile iron. The molten iron with the same carbon equivalent can increase the amount of residual magnesium to reduce the floating of graphite. The amount of residual rare earth is too high, which is helpful for the rise of burst graphite.

(3) Anti-white mouth

Generally, the white mouth structure of cast iron parts tends to appear in the surface layer, sharp corners, creases, etc. where the cooling is faster, and the reverse white mouth defects are opposite. The carbide phase appears in the middle part of the casting, the hot section and the like. When the residual amount of spheroidizing elements is too large, there is an effect of promoting the occurrence of anti-white mouth defects. The rare earth elements are stronger than magnesium, and they generally increase the degree of subcooling in the formation of ductile iron.

(4) Subcutaneous needle hole

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

(5) Shrinkage shrinkage

Shrinkage holes often appear in the final solidified part of the casting (hot joint, the neck of the riser neck and the joint of the casting, the inner corner or the joint between the inner gate and the casting), and are holes hidden in the interior of the casting or communicating with the exterior. Shrinking, macroscopic appearance in the hot section, fine shrinking holes, mostly inside the holes. Related to the spheroidal element is that the control of residual magnesium and rare earth should not be too high, which has a significant effect on reducing macroscopic and microscopic shrinkage, and the tendency to shrink is almost proportional to the spheroidized element.

(6) Black slag

It generally occurs in the upper part of the casting (casting position) and is mainly divided into blocks, ropes and finely divided black slag. The main component of black slag, magnesium silicate, is formed by the reaction of MgO and SiO2 in molten iron and is affected by its relative content. Therefore, as one of the measures to control the black slag is to reduce the residual amount of magnesium (the total amount of slag is about 0.1% of the weight of molten iron when 0.15% of magnesium is added), and the residual rare earth has a strong affinity with oxygen, reducing black The slag has obvious effects.

(7) Spheroidization decline

This is because the spheroidized molten iron has a longer residence time, the residual magnesium is gradually reduced, the slag is not removed in time, and the sulfur is returned to the molten iron, so that the graphite in the solidified structure is reduced or even disappeared, and the decay becomes irregular, worm-like or Flaky graphite. This spheroidization decline has a certain relationship with the low content of rare earth in the spheroidizing agent or the low amount of spheroidizing agent added, but it is also undesirable to increase the amount of spheroidizing agent, because the residual amount of magnesium is high, and the amount of slag is high. Both cementite and cementite will increase, and in the thick section, the graphite spheres will become stellite graphite. Production practice shows that the low sulfur content of the original molten iron is the most effective to prevent spheroidization decline.

Including some ductile iron defects, almost all related to the composition and amount of spheroidizing agent, but we can not expect the spheroidizing agent to solve many problems, but can not solve all problems, because the role of spheroidizing elements and spheroidizing agents The addition amount is the coexistence of advantages and disadvantages. The spheroidizing agent is only a very important factor in the stable production control system of ductile iron. Only when combined with other supporting measures, can it be stably spheroidized.