Quality and selection of the hottest rare earth no

The quality and selection of rare earth nodulizer (4)

5. Common defects caused by improper application of nodulizer

casting defects such as inclusions, holes, cracks (refer to pores, lock holes, cracks, cold shuts, etc.) often affect the mechanical, physical and chemical properties, processing properties of castings, and determine the quality of castings. Ductile iron castings may produce almost all casting defects, but due to its production mode, crystallization law, casting properties and other casting alloys, ductile iron often has some unique defects

what are the defects of nodular iron castings related to nodular agents, or the defects of nodular iron castings caused by nodular agents

PP developed in this paper has the advantages of low price, low density, easy processing and reuse; However, PP has the disadvantages of large molding shrinkage, high low-temperature brittleness and easy aging. The study shows that almost all the defects of ductile iron parts are related to spheroidizing agent. This mainly includes the following aspects:

(1) graphite ball Alienation: irregular graphite appears in graphite ball alienation, such as lump, tadpole, worm, horn or other non-circular spheres. 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 due range, such as the residual magnesium is too high, which 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, the molten iron with high carbon equivalent is easy to produce broken graphite, and the concentration area of broken graphite is generally called gray spots. The appearance of vermicular graphite is due to the lack of residual spheroidized elements or the presence of excess titanium and aluminum

(2) graphite floating: in thick walled ductile iron parts with hypereutectic composition, there is often a graphite dense area 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 is different from that of molten iron, and the graphite directly precipitated from hypereutectic molten iron is buoyant upward. The floating degree of graphite is related to carbon equivalent, the type and residue of spheroidizing elements, casting solidification time, pouring temperature and other factors. Magnesium can increase the eutectic carbon content of ductile iron. For molten iron with the same carbon equivalent, increasing its residual magnesium can reduce graphite floating. The excessive residual rare earth is conducive to the formation of burst graphite

(3) anti white cast iron: the white cast iron structure of general iron castings is easy to appear in the surface layer, sharp corners, cracks and so on, while the anti white cast iron defects are on the contrary, and the carbide phase appears in the center of the medium section, hot spots and other parts of the casting. When the residual amount of nodular elements is too much, it can promote the formation of anti white defects. Rare earth elements are stronger than magnesium. They generally achieve unprecedented progress in reducing the weight of bulletproof plate, reducing the number of trauma and enhancing threat protection, which can increase the undercooling of nodular iron during the formation of microstructure

(4) subcutaneous pinhole: the subcutaneous pinhole mainly contains hydrogen, but also a small amount of carbon monoxide and nitrogen. When the residual magnesium content is too high, it also strengthens the tendency to absorb hydrogen from the wet type, thus increasing the probability of subcutaneous pinholes. In addition, the long residence time of nodular molten iron can also increase the number of pinholes

(5) shrinkage cavity and porosity: shrinkage cavity often occurs in the last solidification part of the casting (hot spot, riser neck and casting connection, inner corner or ingate and casting connection), which is a hole hidden in the interior of the casting or connected with the exterior. Shrinkage porosity occurs macroscopically at the hot spot, and most of the subtle shrinkage holes are interconnected inside. Related to spheroidized elements, residual magnesium and rare earth should not be too high, which has a significant effect on reducing macro and micro shrinkage porosity. The shrinkage porosity tendency is almost proportional to spheroidized elements

(6) black slag: it usually occurs in the upper part of the casting (pouring position), which 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 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, which has an obvious effect in reducing black slag

(7) spheroidization recession: This is because the spheroidized molten iron stays for a long time, the residual magnesium decreases gradually, the slag is not removed in time, and the sulfur will return to the molten iron, so that the graphite in the solidified structure decreases or even disappears, and decays into irregular, vermicular or sheet graphite. This spheroidization recession is related to the low content of rare earth in the spheroidizing agent or the low addition of spheroidizing agent, but it is also undesirable to increase its addition immediately, because the high residual magnesium will increase the amount of slag and cementite, and the graphite ball will degenerate into tadpole graphite in thick and large sections. The production practice shows that the low sulfur content of molten iron is the most effective way to prevent spheroidization recession

including the defects of nodular iron castings, almost all of them have something to do with the composition of nodularizing agent and the amount of SEBS foaming material. However, we can't expect nodularizing agent 2 to be used as an agent for metallographic microscope to solve many problems, let alone all problems, because the role of nodularizing elements and the amount of nodularizing agent are both advantages and disadvantages. Nodularizing agent is only a very important factor in the stable production control system of nodular cast iron, Only when combined with other supporting measures can spheroidization be carried out stably