Analysis of Several Common Defects in Ductile Iron Parts
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The influencing factors of defects such as shrinkage, shrinkage, slag inclusion, subcutaneous porosity, nodularity of spheroidization, and poor nodularity often encountered in the production of ductile iron parts are analyzed in detail, and some effective preventive measures are proposed according to the actual situation of production.
(1) Graphite ball alienation
Graphite sphere alienation occurs irregular graphite, such as lumps, maggots, 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 spherical graphite grows in the radiation direction. When the amount of residual spheroidizing elements in the casting exceeds the expected range, such as when 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 easily produce strong graphite. When there are more residual rare earths, high-carbon-equivalent molten iron is liable to produce fragmented graphite, and the concentrated area of fragmented graphite is generally referred to as "grey spots". The appearance of worm-like graphite is due to insufficient residual nodular elements or excessive titanium and aluminum.
(2) Graphite floating
In thick-walled nodular iron with hypereutectic composition, at the top of the pouring position, a graphite-intensive area often appears, that is, the phenomenon of "floating from beginning to end". This is because graphite and molten iron have different densities, and the graphite directly precipitated from the hypereutectic molten iron is subject to buoyancy. Caused by upwards. The degree of graphite floating is related to factors such as carbon equivalent, nodular element type and residual amount, casting solidification time, pouring temperature and other factors. Magnesium can increase the eutectic carbon content of ductile iron, and the molten iron with the same carbon equivalent can reduce the graphite floating by increasing its residual magnesium content. The residual rare earth content is too high, which helps the burst graphite to grow.
(3) Anti-white mouth
Generally, the white mouth structure of cast iron parts is easy to appear in the surface layer, sharp corners, and crevices, etc., where the cooling is faster, and the reverse white mouth defects are the opposite. The carbide phase appears in the middle section of the casting, the heat section and other parts. When the residual amount of nodularizing elements is too much, it can promote the production of anti-whitening defects. Rare earth elements are stronger than magnesium. They can generally increase the degree of undercooling when nodular iron structures are formed.
(4) Subcutaneous pinhole
Subcutaneous pinholes mainly contain hydrogen, but also 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 form is also strengthened at the same time, so the probability of subcutaneous pinholes is increased. In addition, the long residence time of nodular molten iron can also increase the number of pinholes.
(5) Shrinkage and shrinkage
Shrinkage often occurs in the last solidified part of the casting (the joint of the riser, the connection between the neck and the casting, the inner corner or the connection between the gate and the casting), and it is a hole hidden inside the casting or communicating with the exterior. Shrinkage occurs macroscopically at the thermal node, and the small shrinkage holes are mostly interconnected inside the holes. Related to the nodularizing element, it is necessary to control the residual magnesium and rare earths not to be too high, which has a significant effect on reducing macro and micro shrinkage. The tendency to shrink is almost proportional to the nodularizing element.
(6) Black slag
It generally occurs in the upper part (casting position) of the casting, and is mainly divided into block, rope and fine 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, 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 the molten iron), and the residual rare earths have a strong affinity with oxygen and are reducing the amount of black Slag has obvious effects.
(7) Nodularity decline
This is because the nodular molten iron has a longer residence time, the residual magnesium gradually decreases, the slag is not removed in time, and the sulfur will return to the molten iron. Flake graphite. This spheroidizing decay has a certain relationship with the low content of rare earth in the spheroidizing agent or the low amount of spheroidizing agent, but it is also not advisable to increase the amount of slagging agent because the residual amount of magnesium is high and the amount of slag Both cementite and cementite will increase, and graphite spheres will be transformed into corrugated graphite in thick sections. Production practice shows that the low sulfur content of the original molten iron is the most effective to prevent the nodularity from declining.
Including the defects of ductile iron castings, almost all have a relationship with the composition and amount of nodularizers, but we cannot expect nodularizers to solve many problems, let alone all the problems, because of the role of nodular elements and nodularity. The addition amount of the agent is a combination of pros and cons. Nodulizing agent is only a very important factor in the stable production control system of nodular cast iron. Only in combination with other supporting measures can the nodularizing process be stable.
Nanjing Kirin Scientific Instrument Group Co., Ltd.