2, 2 melting process control (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 decays. Irregular, wormlike or flake 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. Previous Next Copper Conductor Power Cable,Aluminium Conductor Power Cable,Power Cable,Pvc Power Cable BEIJING FINECAST INTERNATIONAL CORPORATION , https://www.cfic-finecast.com
First, the order of feeding should be correct. Be careful not to allow direct contact between magnesium and scrap steel. To melt the magnesium with a low melting point, first react with silicon to form a Mg-Si phase to reduce the loss of magnesium.
Second, the composition of the melt should be uniform. In addition to utilizing the self-inductive agitation of the intermediate frequency furnace, it is also necessary to manually and properly stir the alloy to homogenize the alloy composition during the smelting process. In the smelting process, it is necessary to prevent the phenomenon of “running magnesiumâ€, “shelving materials†and “touching the furnaceâ€.
Third, the thickness of the alloy ingot should be appropriate. If the alloy ingot is cooled, the thickness of the ingot is too thin, and its surface area is large. It is easy to cause more magnesium combustion oxidation during the cooling process of the alloy. If it is too thick, the composition of the alloy element is not the same, which is likely to cause segregation of components during solidification. A suitable thickness is generally 10-15 MM.
Fourth, the sieve size should be graded. The solidified alloy ingot is subjected to surface cleaning of oxides and picking up inclusions before crushing and screening. According to the size of the user's ladle, the particle size is packaged in stages, but there is no alloy powder.
2, 3 chemical composition test
A qualified spheroidizing agent, in addition to the appearance of dense, no inclusions, etc., more important is the content and uniformity of its chemical composition.
In the spheroidizing agent, in addition to the analysis of conventional elements such as Re, Mg, Si, Ca, etc., the analysis of MgO in the alloy is often neglected by the manufacturer and the manufacturer. This is also related to the unconformed MgO national analysis standard. The same alloy uses different analytical methods in different manufacturers, and the conclusions of the components are different. This requires alloy manufacturers and foundry users to achieve a uniform acceptance analysis standard to comply with.
3. Evaluation of the quality of spheroidizing agent
What kind of spheroidizer is the best? This is often the constant topic of many foundries. In fact, it can be said that the spheroidizing agent is not the best, only suitable, the best for yourself.
So, how do you evaluate the quality of the spheroidizer?
The data from the reaction thermodynamics of spheroidizing elements in molten iron, the reaction kinetics and the factors affecting the quality of the spheroidizing agent production process, and the factors affecting the process of the foundry process are presented. The criteria for determining the quality of the spheroidizing agent are as follows:
(1) Affinity of spheroidizing elements with elements such as sulfur and oxygen in molten iron. It has strong affinity and forms compounds with it as a foreign core in the solidification process of iron liquid, such as spheroidized elements such as rare earth, magnesium and calcium.
(2) The spheroidizing element changes the ability of the graphite form to change from a sheet to a sphere.
(3) The density and boiling point of the spheroidizing agent are lower than that of the molten iron. The small density can be automatically floated in the molten iron, the boiling point is lower than the temperature of the molten iron treatment, and the magnesium can be converted into a gaseous state at the treatment temperature, and has a self-stirring effect, thereby improving the spheroidizing effect.
(4) The magnesium oxide content in the spheroidizing agent is equal to about 10% of the magnesium content.
(5) The spheroidizing agent is dense, non-segregated, non-shrinking and shrinking, and not powdered.
(6) The spheroidizing agent has a uniform particle size distribution, no powdery alloy, and an obtuse angle multi-deformed particle is preferred.
(7) The above conditions basically cover the quality requirements during the production and use of the spheroidizing agent, and therefore can be used as a unified standard for determining the quality of both the supply and demand sides.
4, the choice of spheroidizing agent
The quality evaluation standard of the above spheroidizing agent can also be said to be the basis for our selection of spheroidizing agent, but it is explained according to the general principle and common phenomenon. The specific selection of spheroidizing agent is now in accordance with the habit of our foundry. Analyze.
The most common problems encountered by foundries in the use of spheroidizers are:
(1) The spheroidizing agent is inaccurate and fluctuates.
(2) The particle size of the spheroidizing agent powdered alloy is not satisfactory.
(3) The spheroidizing agent is not dense, floats quickly, and burns severely.
(4) The MgO content is too high, the reaction is too intense, the spheroidization treatment is poor, and the amount of spheroidizing agent added is too large.
(5) The decline is fast after spheroidization.
(6) The white mouth tends to be large after spheroidization.
So, what kind of spheroidizing agent can you choose to avoid the above problems?
This of course depends on the factory's smelting conditions, casting structure, organization and performance requirements and other specific conditions. For the convenience of description, this article is divided into the following types.
4, 1 melting conditions
The cupola melting of ductile iron is used in about 80% of enterprises in China. Because of the low temperature of molten iron, high sulfur content and other impurities, it requires strong spheroidizing agent for desulfurization and slag removal. Therefore, high grade should be used. A spheroidizing agent, such as FeSiMg10Re7, FeSiMg8Re7, FeSiMg8Re5; and for an electric furnace or "double" molten iron ductile iron, a spheroidizing agent having a low rare earth and a low magnesium content, such as a spheroidizing agent containing Mg1-6 and Re4-8, is more commonly used.
4, 2 casting thickness
Castings of different wall thicknesses and weights are not the same because of their inconsistent cooling and cooling conditions. For thin-walled small parts, the solidification is fast and the degree of supercooling is large, which is suitable for the growth of spheroidal graphite. At the same time, it is easy to appear carbides and increase the tendency of white mouth. When the residual magnesium exceeds 0.07%, it is more likely to produce carbides. Therefore, low rare earth should be selected. Low-magnesium spheroidizing agent; for thick-section ductile iron parts (wall thickness above 100mm), due to the small solidification rate at the central part, there is a spheroidization decline phenomenon, it is easy to use high-grade spheroidizing agent or increase the amount of spheroidizing agent added. (0.01-0.02% higher than the residual magnesium content of ordinary ductile iron). However, if the residual rare earth is too high, it will cause the explosion of graphite and anti-white mouth phenomenon. Therefore, it has been studied to add a small amount of anti-spheroidizing elements (such as 0.005% antimony, or antimony, tin) to the molten iron to neutralize the excess rare earth element. There is also the choice of bismuth-based heavy rare earth spheroidizing agent, which has stronger anti-recession ability than lanthanide spheroidizing agent, and the tendency of white mouth is also small.
4, 3 pearlite and ferritic castings
The main factors affecting the pearlite content in ductile iron structure are the characteristics of solidification structure, cooling rate through eutectoid zone, carbon and silicon content, alloying element type and content. The solidified structure has few graphite spheres and large size, which is not conducive to the full diffusion of carbon, which is beneficial to increase pearlite and reduce ferrite. The high austenite carbon content and the high cooling rate of castings have a tendency to increase pearlite. The pearlite structure can be stabilized by using a spheroidizing agent containing copper, bismuth or nickel or by adding elements such as copper, bismuth, nickel and tin to the molten iron. For ferritic ductile iron, it is necessary to control the content of these elements. In addition, since rare earth elements increase the tendency of supercooling of molten iron, when producing ferrite castings, it is suitable to use a spheroidizing agent with a low rare earth content (Re content should not be higher than 5). %).
5, common defects caused by improper application of spheroidizing agent
Casting defects such as inclusions, holes, cracks (referring to air holes, 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 spheroidizing agent, or the defects of the ductile iron parts due to the factors of the spheroidizing agent?
Studies in this paper have shown that almost all of the ductile iron defects are related to the spheroidizing agent. This mainly has the following aspects:
(1) Alienation of graphite spheres: irregularities appear in graphite spheres, 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 spheroidal 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 reinforced graphite. When there are many residual rare earths, high carbon equivalent iron water is easy to produce broken graphite, and the concentrated area of ​​broken graphite is generally called "grey spot". The appearance of worm-like graphite is due to insufficient residual spheroidal elements or excessive titanium and aluminum.
(2) Graphite floating: In thick-walled ductile iron parts with hypereutectic composition, at the top of the casting position, a graphite-dense area often appears, that is, the phenomenon of “floating at the beginning and endingâ€, which is due to the difference in density between graphite and molten iron, and the hypereutectic molten iron directly The precipitated graphite is caused by buoyancy. The degree of graphite floating is related to factors such as carbon equivalent, type and residual amount of spheroidizing 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 residual magnesium content and reduce the floating of graphite. The residual rare earth content 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 on 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 section of the casting, which is hot. Sections and other parts. When the residual amount of the spheroidal element is too large, there is an effect of promoting the occurrence of the anti-white mouth defect. The rare earth element is stronger than the magnesium, and they generally increase the degree of subcooling in the formation of ductile iron.
(4) Subcutaneous needle hole: The subcutaneous needle hole mainly contains hydrogen, and also has 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 needle holes 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 solidification part of the casting (hot joint, joint between the riser neck and the casting, internal corner or joint between the gate and the casting), hidden in the interior of the casting or connected to the exterior The hole. Shrinking, macroscopic appearance in the hot section, fine shrinking holes, mostly inside the holes. Related to the spheroidal element is to control the residual magnesium and rare earth can 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 the molten iron when 0.15% of magnesium is added), and the residual rare earth has a strong affinity with oxygen, and the black is reduced. The slag has obvious effects.
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 agent The amount of addition is a 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.