Product Introduction

17 major causes and control of poor nodulation of ductile iron

In the 1960s and 1970s, the production of ductile iron was mainly the use of cupola, due to the poor quality of coke (large lumpiness, low density, low fixed carbon content, sulfur content); The liquid iron temperature is low; The preparation of spheroidizing agent used is not perfect; The sulfur and phosphorus content of pig iron is high, so the quality of nodular cast iron produced is poor, and the spheroidizing quality is unstable. At present, the production of ductile iron is mostly smelted by electric furnace, and the level of furnace temperature is easy to control; The quality of raw materials such as pig iron is good; There are many types of spheroidizing agents and good quality, so the quality of ductile iron is relatively easy to control. However, poor spheroidization is still one of the main defects in ductile iron production.

The poor nodulation is manifested in the fracture of the casting (generally observe the fracture of the pouring gate), with large black spots or obvious small black spots; The sound produced by knocking the casting is not clear; There are more thick flake graphite on the metallographic microstructure, and a small amount of spherical, clustered graphite, or dendritic graphite (sometimes poor spheroidization has a feature on the metallographic, that is, in the thick flake graphite cluster, individual spherical graphite is also very round).

The causes of poor spheroidization are generally affected by the following three categories of factors: the amount of residual magnesium or rare earth is too low (but when the rare earth content is too high, the graphite roundness becomes worse, and the casting is easy to produce white holes and shrinkage); Weak or declining inoculation; Interference elements are too high.

However, in the actual production process, there are many factors that cause poor spheroidization, including technical problems, operational problems, and management problems.

1. Poor quality of spheroidizing agent

Although the contents of Mg and RE in the nodulating agent meet the quality requirements by testing, due to poor melting technology, the MgO content is high (the nodulating agent contains MgO > 1%, which may have an impact on the nodulating quality), MgO has little effect on improving the nodulating quality, but it makes ductile iron easy to produce slag inclusion defects; Spheroidizing agent contains less elements such as Ca, the reaction is intense during spheroidizing treatment, and Mg is burned more.

Preventive measures: do not use poor quality nodulator (to investigate suppliers, manufacturers, first buy a small amount, try and then buy in bulk). The nodulating agent is easy to be oxidized by moisture when placed for too long.

2. Improper nodularization treatment in front of the furnace

After the nodulating agent was poured into the hole of the dam, the nodulating agent was not evenly patted. The surface covering is small, or the covering layer is thin, or the gap of the nodule block is not filled, after inserting the iron liquid, not only the exposed nodule agent melts immediately, but the iron liquid enters the gap of the nodule block in large quantities, and directly melts the nodule agent or rushes the nodule agent up the floating iron liquid surface, the reaction is too fast and too early, and the Mg burns more.

Preventive measures: Pour the nodulating agent into the pit at the bottom of the bag flat, properly rammed, and then spread the inoculated ferrosilicon covered above and properly rammed, and cover the surface with an appropriate amount of ductile cast iron chips (ramming) or a certain thickness of ductile cast iron plate. This not only fills the gap of the alloy, but also has a certain thickness of the covering layer.

3. The original iron liquid has high sulfur content

Sulfur is the main despheroidizing element, and high sulfur content will seriously affect the nodulating quality. When wS > 0.06% in the original iron liquid, it is difficult to obtain qualified ductile iron quality even if more nodulating agents are added. In the process of nodularization, the Mg in the nodulating agent first reacts with the S in the liquid iron to form the slag of MgS, and the remaining Mg plays the nodularization role, and the same is true for RE. Due to the lack of spheroidizing elements, the spheroidizing quality is affected. The sulfur content of liquid iron is high, even if a large amount of nodulating agent is added, if the pouring time is too long, the slag is not clean, and the "sulfur return" phenomenon will occur, affecting the quality of the casting in the late stage of pouring. The main source of sulfur in the original iron liquid is: the use of coke or new pig iron with high sulfur content.

Preventive measures: use pig iron with low sulfur content and return charge and coke; To master the relationship between the amount of pelletizing agent added and the sulfur content of the original iron solution; Desulphurization measures are taken before the furnace and in the process of nodularization (spraying lime water on the coke, it is easier to desulphurization in the electric furnace, adding soda surface or caustic soda in the nodularization package).

The nodular interference elements inserted into the charge are too high, such As Ti, Sb, AS, Pb, Al, Sn, etc. Although rare earth elements have the ability to weaken or cancel the despherodizing interference elements, too much interference elements in liquid iron will still deteriorate the shape of graphite balls (deformed graphite). Even if spheroidized, the physical properties of ductile iron materials will tend to be very brittle. Therefore, in the production of QT400-18 and low-temperature ductile iron, high purity pig iron should be selected.

4. Improper placement of iron ladle

When the iron is discharged, the iron liquid directly rushes to the nodulizing agent pressed in the pit, which not only washes away the covering, but also makes the alloy block directly affected by the high-temperature iron liquid, or prematurely melts and reacts violently, or quickly floats to the surface of the iron liquid surface, where the melting and burning damage is absorbed by the air, reducing the absorption rate of the iron liquid to Mg.

Preventive measures: Place the position of the iron packet to avoid the direct impact of the iron liquid on the alloy, so that the iron liquid can drown the alloy smoothly and quickly and reach a certain depth instantaneously, extend the floating distance of the alloy, and facilitate the alloy to be fully absorbed by the iron liquid.

5. The iron solution starts slowly

If the liquid iron is too slow at the beginning and the liquid surface rises slowly in the packet, when the liquid iron is submerged in the alloy, the surface part of the alloy will begin to melt and then float. Due to the short distance between the surface of the alloy and the surface of the liquid iron, a large number of the alloy will float on the surface of the liquid iron before it has time to melt, and Mg will melt and burn on the surface of the liquid iron and be absorbed by the air. The absorption rate of Mg in liquid iron was reduced.

Preventive measures: For the cupola, there should be sufficient liquid iron in the front furnace cylinder, the mud around the blocked iron mouth should be cleaned before the iron is removed, the iron mouth should be quickly opened when the iron is removed, so that the liquid iron will soon reach 2/3 of the depth of the liquid iron package capacity (that is, a certain depth), and the spherodizing reaction at this time, due to the large distance between the alloy surface and the liquid iron surface, the alloy will float in the liquid iron through a long distance. The alloy floats, melts and is fully absorbed by the liquid iron. The absorption rate of the nodulating element Mg in the nodulating agent is high, and the quality of ductile iron is good. Electric furnace iron is more convenient, start fast out, slow out or stop out when the reaction is violent, continue to iron out to the required amount when the reaction is stable, if the reaction is stable, as far as possible first fast and then slow (in the middle of the stop).

6. Loading the nodulator too early or the liquid iron in the dike pit is not emptied

After pouring, the bottom of the ladle is red hot, and the temperature is higher than 900℃. If the nodulizing agent is loaded immediately, Mg and RE will lose part of them under high temperature baking (there is smoking phenomenon); If the liquid iron in the dike pit is not emptied, the Mg loss is more. In addition, the superheated preheating temperature will also promote the premature melting of the nodulator.

Preventive measures: Let the ladle cool down for a period of time, fill the nodule before the iron liquid, at the same time, pour the remaining iron liquid in the ladle in time after pouring, and clean the slag in the ladle.

7. The temperature of spheroidized iron liquid is too low

When the temperature of molten iron is lower than 1390℃, the alloy is not easy to melt, the nodularization reaction is not complete, and the nodularization level is difficult to meet the requirements. During the floating process, due to the low temperature of liquid iron, the nodulating agent cannot be quickly melted and absorbed, causing the nodulating agent to float to the liquid surface of liquid iron to melt and burn.

8. The temperature of spheroidized iron solution is too high

The temperature of the nodulized iron is too high, the covering agent and the nodulizing agent are melting too fast, because the density of pure Mg is 1.74g/cm 3, the melting point is 651℃, the boiling point is 1105℃, even because Mg and Si combine to improve the melting point of the alloy, but it is lower than 1400℃, not to mention the nodulizing temperature is often 1490~1520℃, and some may be higher. According to the size of the casting and the thickness of the casting wall, when it is really necessary to increase the nodularization temperature, it is also necessary to take relatively "low temperature treatment and high temperature casting" measures. In addition, the temperature of liquid iron is too high, liquid iron is often seriously oxidized, because Mg and RE are easy to produce a combination reaction with oxides, high temperature makes a large amount of Mg, RE loss and evaporation, reducing the absorption rate.

9. Spheroidizing agent is small in lumpiness and more pulverized

When the nodularity of the agent is small and pulverized for a long time, although the nodularity treatment method is the same, because there is no gap between the alloy blocks, the melting reaction can only be peeled slowly layer by layer, if the same steps are poured, there may be bad nodularity in the first few boxes, and good nodularity in the last few boxes.

Prevention measures: According to the size of the liquid iron package, that is, the amount of the liquid iron spheroidizing treatment, the size of the lump size of the nodulator is selected. If there is too much crushed powder, it needs to be sifted. If the nodularization reaction is too slow, the steel can be used to pound the alloy through the iron liquid several times, so that the iron liquid is drilled into the alloy in order to speed up the nodularization reaction.

10. Spheroidizing agent block is too large

The nodulated agent block is too large, and in the process of floating and melting on the edge, it is not absorbed by the liquid iron in time, but floats to the surface of the liquid iron to melt and burn, and is emitted into the air and wasted.

The choice of the lumpness of the nodulator is determined by the size of the liquid iron package, that is, the amount of the liquid iron nodulator.

11. The amount of spheroidizing agent added is small

The amount of nodulating agent added is related to the material requirements, the sulfur content of the iron liquid, the quality of the iron liquid, the nodulating temperature, the size of the casting and other factors. There are two reasons for the amount of nodulating agent added: first, the design requirements for the amount of addition itself is small; Second, the amount of iron liquid is not well controlled, and the amount of iron liquid exceeds the requirements.

12. Liquid oxygen of iron

The oxygen content of liquid iron after oxygenation is high, due to the strong affinity of O and Mg, the effective nodulating element Mg in the nodulating agent first combines with O to produce MgO slag, and the remaining Mg plays the nodulating role of graphite, due to the oxygen loss of a large number of Mg, the remaining Mg is not enough to ensure the amount of graphite spheroidality, so the nodulating grade is low and the nodulating quality is poor.

Preventive measures: pay attention to the low coke (carbon) height of the cupola to prevent liquid iron oxygenation; Electric furnace melting, do not use too oxidized charge, to prevent the liquid iron temperature is too high or high temperature for a long time heat preservation, especially 10t furnace molten iron, each time the nodulization treatment of 1t, when the nodulization treatment after a few packages, not only the liquid iron in the furnace due to the long residence time, not only the lack of "crystal core", and easy oxidation. After a few packages of nodularization treatment, the first "pretreatment" is carried out in the furnace, adding an appropriate amount of silicon carbide, deoxidizer, carburizer, ferrosilicon, etc., for deoxidation treatment, and appropriately add some nodulator.

13. Bag depth ratio and bag pit

(1) The ratio of the depth H of the spheroidization package to the direct D is: H/D = 1.5~2. If the spheroidized bag is used to treat half of the bag, it will go against the original intention of high warp ratio.

(2) The depth of the pit of the spheroidizing bag should remain 20~30mm after loading the spheroidizing agent and covering agent, and the liquid iron will enter the pit and melt into a semi-solid substance with the covering agent, delaying the premature outbreak of the spheroidizing agent, and can improve the yield of Mg.

(3) The width of the pit at the bottom of the bag is better than 1/4 to 1/3 of the diameter of the bottom of the bag, and the pit with a small projection area increases the depth, which is conducive to delaying the explosion.

(4) After the completion of pouring, clean the slag in the package in time, so that each package of nodulating agent into the pit is the same.

14. Spheroidization decline caused by too long pouring time and other reasons

The characteristics of nodularization decline are: good nodularization before the furnace, bad nodularization on the casting; Or the same package of iron liquid, the casting cast first is good at spheroidization, and the casting cast after the casting is not good at spheroidization. The nodularity decline caused by too long pouring time often coexists with inoculation decline. The spheroidization quality of liquid iron is determined by the amount of Mg residue to ensure spheroidization of graphite. Mg has a strong affinity with O and S, Mg and O combine to form MgO and burn off. In particular, S, when S combines with Mg to form MgS slag, floats to the liquid surface; after floating to the liquid surface, Mg in MgS slag combines with O in the air to form MgO and burns off; and the separated S returns to the liquid iron and combines with Mg again. Like a boat, S in the liquid iron keeps taking Mg in the liquid iron to the air and burns off. This is called the "sulfur return phenomenon". With the extension of pouring time, the residual amount of Mg in liquid iron becomes less and less. It is introduced that with the extension of pouring time, the burning loss of Mg in liquid iron is 0.004% for every 1min extension.

Solution: If the pouring time is extended for some reason, the appropriate thickness of the insulation agent can be covered to reduce the contact between the liquid iron and the air, and reduce the amount of Mg burning in the liquid iron. In addition, appropriate flow inoculation measures should be taken to decompose or truncate the graphite that has grown up and is deformed (flake graphite after long growth), so that its shape tends to be spheroidal.

15. Breeds recession

Through metallographic analysis, it can be seen that in the metallographic photos of the gestate recession, the number of graphite balls is small, the diameter of the ball is large, the density is thin, the nodulization level is low, the ferrite content is usually small, the pearlite content is increased, and the presence of carbide. The cause of inoculation decline is that the amount of inoculant added is small, or the inoculation process is not perfect. Since the presence of magnesium is a necessary condition for spheroidization, and the elements in gestation are sufficient conditions for participating in graphitization, it is impossible to make high-quality ductile iron only by talking about spheroidization treatment without paying attention to gestation treatment.

Preventive measures: increase the amount of inoculant added; Use long-acting inoculants containing barium and calcium; The compound inoculation measures of secondary inoculation, floating silicon inoculation and floating current inoculation were adopted.

16. Spheroidizing or pouring ladle wet

When the nodulizing treatment is injected into the iron liquid, the water is decomposed by gasification to produce hydrogen and oxygen, and O will be in the middle and drop some Mg in the nodulizing agent, and become MgO slag, which not only reduces the magnesium content in the iron liquid, but also easily causes the casting to produce slag holes and porosity defects.

17. Site management

The management and stacking of spheroidizing agent is not standardized, which may be mixed with ferrosilicon. The weight of the spheroidizing agent is not allowed, or the skin is not removed, or the weight is wrong. For example, the production quality of ductile iron cast iron in a factory has been very stable, suddenly one day before the night shift, two furnace nodularization, laboratory analysis of silicon content exceeded the standard, after research and analysis, it may be during the day cleaning, the scattered iron silicon on the ground, sorted into the nodularizing agent tank. In addition, the storage time of the nodulating agent is too long and the storage is not good, the oxidation of the nodulating agent will weaken the nodulating effect and affect the quality of the nodulating agent.

The above is about nodular cast iron nodularization treatment methods and poor reasons for the summary of knowledge, nodular cast iron nodularization treatment methods have their own advantages and disadvantages, hope that each nodular cast iron production enterprises can according to their own production conditions and conditions, comprehensive evaluation of each nodular treatment method, develop strengths and avoid weaknesses, choose their own nodular treatment methods

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