Production technology of high-quality gray cast iron

High-quality gray cast iron only has high strength hardness, but also has good cutting performance. The organization should be as uniform as possible, that is, evenly distributed, medium-sized A-type graphite; evenly distributed, medium medium-fine pearlite matrix; as few inclusion particles as possible; as little as possible free distribution of cementite phosphorus Eutectic; pure material.

In recent years, many car joint ventures have encountered a very difficult problem in the localized production of cylinder blocks, that is, all localized materials have poor cutting performance. The reasons can be divided into two major aspects. One is congenital deficiencies, that is, there are gaps in basic parts hardware, such as smelting technology, raw material quality, etc.; the other is the gap in technological level, such as incubation technology alloying technology. The following will elaborate on these aspects.

1 smelting technology

Discussing smelting technology must first understand how the carbon in the molten iron is obtained, because the different ways of obtaining carbon in the molten iron will have a fundamental impact on the shape distribution of graphite.

Obtaining high-quality graphite is the key technology for producing high-quality gray cast iron. High-quality graphite refers to uniformly distributed, medium medium-fine A type graphite. All technological measures should focus on this goal.

The main gap between the domestically produced gray cast iron material foreign countries is the form of graphite. There are many deep-seated reasons for this gap. The fundamental reason is the different ways of obtaining carbon in the molten iron. There are two ways to obtain carbon, one is to obtain more pig iron; the other is to obtain hot metal carburization, these two methods have a huge impact on the performance of gray cast iron, that is, the same molten iron chemical composition, performance will be The difference is 1~2 grades. If the carbon in the molten iron is obtained by carburizing, the graphite has a good shape, uniform distribution, moderate size. If the carbon in the molten iron is obtained by adding more pig iron, the shape of the graphite is poor, some coarse graphite is often produced, which is the so-called pig iron heredity.

Different hot metal smelting methods carburizing technologies are also different. Electric furnace smelting requires adding carburizing agent to the molten iron. For cupola smelting, the key is to increase the melting temperature of molten iron. Foreign cupolas basically use hot air to supply air, some add oxygen. The quality of coke is good, the molten iron melting temperature is high, which can reach 1550 degrees Celsius. Therefore, the carburization rate of coke in the molten iron is high, the carbon in the molten iron is basically The above are all obtained by carburizing, pig iron is basically added added in a small amount. Compared with foreign countries, domestic cupola smelting technology is relatively backward. Most of them use cold air to supply air. The quality of coke is paid attention to. The molten iron smelting temperature is low, generally about 1450 degrees Celsius. The carburizing rate of coke is low, the carbon in the molten iron is only Can be obtained by adding more pig iron. The more pig iron is added, the worse the performance of the material, which is a fundamental innate deficiency.

For electric furnace smelting, there is also the problem of how carbon is obtained. Many people often adopt a simple method of only using pig iron scrap steel to produce cast iron. This method of adding pig iron in large quantities seriously affects the quality of castings. Therefore, it is necessary to add carburizing agent to increase carbon, reduce the amount of pig iron added, fundamentally improve the shape of graphite, improve the performance of the material.

2 Quality of raw materials

The quality of raw materials depends on the content of trace elements in the raw materials.

The trace elements in cast iron mainly come pig iron scrap steel. High-quality pig iron must be used for pig iron, carbon steel must be used for scrap steel, alloy steel is strictly prohibited.

Due to the large amount of pig iron added, the quality of pig iron has an important influence on the quality of castings. The good pig iron in our country is produced the Benxi vein. However, due to limited resources, some other ores are added to the pig iron produced, the content of certain trace elements is much higher than before.

Because the accumulation of trace elements to a certain extent will cause the cast iron to produce bad graphite structure affect the matrix structure, sometimes the effect is fatal, so it is necessary to understand the characteristics of various trace elements.

The influence of trace elements can be divided into three main aspects. One is the influence on graphite, such as lead, which will seriously deteriorate the shape of graphite; the other is the influence on the matrix structure, such as vanadium will promote the production of large amounts of sorbite deteriorate the cutting performance; It is the effect on inclusions. For example, titanium can form titanium compound particles affect the cutting performance.

3 cooling rate

The cooling rate of molten iron has a great influence on the graphite morphology metallographic structure, the cooling rate is determined by the molding method. The reason why casting methods such as metal mold casting, metal shell mold casting, shell mold iron shots can produce castings of good quality is precisely because of their relatively high cooling rate. Different modeling methods have different cooling speeds, the impact on material properties cannot be compensated by other technological measures.

For shaft castings such as crankshafts camshafts, surface quenching treatment is often required, which requires very high material quality. Therefore, these castings are basically produced by molding methods with a relatively high cooling rate.

4 alloying process

The main function of alloying is to improve the strength hardness of the casting material. But alloying also has an impact on the morphology of graphite, many people ignore this effect.

Copper is an alloying element often added in the production of gray cast iron. The main reason is that copper has a low melting point, is easy to melt, has a good alloying effect, does increase the white mouth tendency of molten iron. Therefore, many people think that adding more copper will cause harm. But this view is wrong. The appropriate amount of copper added is 0.2%~0.4%. Adding a large amount of copper, adding tin chromium at the same time is harmful to the cutting performance, it will promote a large amount of sorbite structure in the matrix structure.

Titanium has a strong affinity with nitrogen carbon in molten iron, it is easy to form titanium nitride titanium carbide particles. A certain amount of titanium nitride titanium carbide particles are dispersed in the matrix, which can improve the wear resistance of the cylinder. The large number of materials is detrimental to the processing performance, especially for fine boring honing, so it must be restricted. To control the titanium content, it is generally to control the titanium content at the lower limit of the process requirements, that is, 0.03% to 0.05%.

Too much titanium content will cause the graphite to be overcooled easily produce a large amount of D E type supercooled graphite. Therefore, the titanium content must be controlled at the lower limit effective inoculation measures should be taken to eliminate the influence of supercooled graphite.

The burning loss of titanium is serious. When ferro-titanium is directly added to the molten iron, the absorption rate of titanium is unstable the composition is difficult to control, so it should be made into an intermediate alloy added to the molten iron.

Rare earth elements can neutralize many impurity elements in gray cast iron, purify molten iron, especially react strongly with sulfur to form the core of graphite nucleation, which significantly affects material properties. Therefore, the role of rare earth elements in gray cast iron is advantageous. Especially when using high carbon equivalent molten iron to produce thicker castings, the addition of rare earth elements will help refine the graphite, will make the graphite too coarse, improve the material properties.

The negative effect of rare earth elements is mainly to increase the tendency of molten iron to be supercooled, so it is easy to add too much, the specific amount to be added should be adjusted according to the molten iron castings. Rare earths also have a tendency to creep the graphite morphology.

The addition of rare earth elements to the molten iron is also technical. When directly added, the absorption effect is very poor, the slag formation is serious. It must be made into an intermediate alloy, a carrier element is used to bring it into the molten iron.

Both tin antimony are alloying elements that strongly stabilize pearlite. When it is desired to increase the hardness of castings, especially castings with poor cooling conditions, adding a small amount of tin antimony can significantly improve the effect.

The alloying effect of antimony is stronger than that of tin. Tin has no effect on the shape of graphite. Antimony shortens the length of graphite, especially for thick large castings. The effect of shortening graphite is more obvious. Although the length of graphite is shortened, the graphite is refined, there is a small amount of massive graphite. For the production of thick large section castings, whether it is ductile iron gray iron, adding a small amount of antimony is beneficial to improve its graphite morphology.

When the content of antimony does exceed 0.02%, it will have a harmful effect on the properties of the material. However, if the content of antimony is too high, it will embrittle the material produce harmful effects.

The alloying effect of chromium is very strong, mainly because the addition of chromium increases the tendency of the molten iron to become white, the castings tend to shrink produce waste products. Therefore, the amount of chromium should be controlled. On the one hand, it is hoped that a certain amount of chromium is contained in the molten iron to increase the strength hardness of the casting; on the other hand, the chromium is strictly controlled at the lower limit to prevent the shrinkage of the casting increase the rejection rate. Traditional experience believes that when the chromium content of the original molten iron exceeds 0.35%, it will have a fatal impact on the casting.

When chromium-manganese-silicon alloy is used, when the chromium content is added by the in-bag method, the white mouth tendency shrinking tendency of the molten iron do increase, the chromium content can be relaxed to 0.5%. The comparative test found that the cylinder block produced by adding copper alone will produce a lot of chrysanthemum-shaped B graphite on the cylinder wall; while the cylinder block produced by chromium manganese silicon, the chrysanthemum-shaped B graphite on the cylinder wall is basically eliminated, the cylinder bore The hardness is improved the production process is stable.

5 inoculation technology

The main purpose of inoculation is to promote the precipitation of graphite eliminate white mouth.

The research on inoculation technology is mainly the research on inoculants inoculation methods. The variety of inoculants in my country is relatively single, mainly 75SiFe. The content of aluminum calcium in 75SiFe has a great influence on the inoculation effect, especially the content of calcium. The 75SiFe we use now has low calcium content poor inoculation effect. In order to improve the inoculation effect, inoculant manufacturers should be required to increase the calcium content. Only 75SiFe containing 1.0%~1.5% calcium 0.8%~1.2% aluminum will have a better incubation effect.

In recent years, our country has developed a variety of inoculants based on 75SiFe to meet the requirements of different types of castings, especially thin-walled high-strength gray iron castings.

a Barium inoculant. Barium has a significant effect in improving the wall thickness sensitivity of castings enhancing the ability to resist recession, that is, it has a long-term effect.

b contains rare earth inoculants. A small amount of rare earth can improve the inoculation effect, significantly reduce the tendency of white mouth, slow down the rate of decline. But the addition amount should be too large, otherwise the white mouth will increase. If it is used in combination with calcium barium, its white mouth tendency will be reduced, the supercooled graphite will be reduced. It has a long-term function is a good inoculant for the production of thin-walled high-strength gray cast iron.

c Inoculant containing strontium. It can reduce shrinkage cavities porosity in thick sections, thin sections can significantly eliminate white mouths slow down.

d Containing zirconium inoculant. The addition of zirconium can improve the resistance to recession, which is beneficial to eliminate pores in the casting, reduce the tendency of white mouth, obtain uniformly distributed Type A graphite. Flow inoculation technology is a very critical technology in the production of ductile iron, but it is rarely used in the production of gray iron. This is mainly due to the lack of understanding of its importance. The production of high-quality gray cast iron is inseparable the flow inoculation technology. Follow-up inoculation can effectively improve graphite morphology promote uniform precipitation of graphite. For the production of thin-walled cylinder castings, it can effectively prevent white holes in the thin-walled parts, can reduce eliminate the free distribution of cementite phosphorus eutectic in the matrix structure, so that the pearlite structure layer spacing is even.

For a long time, many people engaged in the research of cast iron materials have put more energy on the research of ductile iron vermicular graphite cast iron, but have neglected the research on the production technology of gray cast iron. In fact, the production technology of high-quality gray cast iron is the same as that of ductile cast iron, which requires careful study attention.

6 conclusion

In summary, among the five major factors that have a significant impact on the quality of cast iron materials, the impact of cast iron smelting technology, raw material quality, mold cooling rate on cast iron materials is fundamental. If the difference in material is caused by these factors, it is sometimes impossible to make up for it through alloying technology inoculation technology. Therefore, the importance of basic hardware must be clearly understood.