Iron is the quality of cast iron

In the family of foundry alloys, the development of various types of cast alloys depends on the development of their advantages and the suppression of disadvantages. Cast iron has better casting performance than cast steel and non-ferrous alloys, and it is easy to cast castings with complicated shapes and uneven wall thickness and less casting defects. Therefore, the utilization rate of the molten iron is high, and the product has a wide application range. In addition, graphite cast iron has some innate properties such as friction reduction, cushioning, machinability, fatigue fracture and the like.

In the material competition, it is very important to improve the specific strength of cast iron, increase the toughness, and eliminate the inherent defects. It is very important to reduce the weight of castings and prolong the service period. In order to ensure the reliability and stability of cast iron performance, it is necessary to have good quality of molten iron first.

The connotation of a molten iron quality

The quality of molten iron includes: temperature, composition and composition accuracy, harmful element content, quantity of non-metallic inclusions, gas solubility and workability outside the furnace.

The control of molten iron quality is divided into three parts: smelting, pre-furnace treatment and filtration. The following options are briefly described.

Second smelting

1. The cupola is going to a new level

1 Obtain high temperature molten iron

Although the casting temperature of cast iron parts is not high, most cast irons are between 1260 ° C and 1400 ° C. However, from the viewpoint of obtaining high-quality molten iron, there should be a higher melting temperature. High-temperature melting ensures high iron output temperature, Fe, Si, Mn less burning, stable furnace conditions, and small fluctuations in chemical composition. Taking developed countries as an example, the fluctuation of C in the cupola can be controlled at ±0.05%, and the fluctuation of Si can be controlled at ±0.1%. High temperature melting, S, O, H, N and inclusions in the molten iron will be reduced, and the poor heritability of the charge will be weakened.

There are three ways to obtain high-temperature molten iron from cupola: one is good furnace type, such as two rows of large spacing, cold air + high-quality coke with fixed carbon above 85%, and the tapping temperature can be above 1480 °C. The second is oxygen-enriched air supply. This method is simple, and the temperature adjustment is rapid and timely, but it is limited by the oxygen supply conditions. The third is hot air supply. The air temperature of different hot air devices ranges from 150 °C to 600 °C, and is suitable for factories with long furnace opening time.

Most of China's foundries are small in scale, so "Method One" will be the main measure in a considerable period of time. Cold air operation, the heat utilization rate of the cupola is low. Considering the strategy of sustainable development, the hot air cupola meets the national policy of energy conservation, and can be combined to solve environmental pollution and meet social and ecological requirements. With the development of the world economic integration process, the foundry industry will restructure and merge, expand the scale, and implement intensive production. The hot air operation will gradually expand its application range. The two questions that must be considered when using hot air are whether there is sufficient continuous operation time, as well as the amount of one-time investment and the payback period of the investment.

The hot air of the furnace using the physical heat of the furnace gas, the air temperature is between 150 ° C and 250 ° C, and the temperature is stable above 300 ° C. This kind of furnace is matched with medium-quality (fixed carbon >80%) coke to obtain iron liquid of 1450 ° C ~ 1500 ° C. At the same time, the cupola is made of physical heat and chemical heat. The air temperature is between 400 ° C and 600 ° C, and the tapping temperature can exceed 1500 ° C. The wind temperature is above 350 °C, the cupola has obvious metallurgical effect, the oxidizing atmosphere in the furnace is greatly improved, the slag is active, FeO<3% in the slag, the iron liquid S is low, the burning loss of the elements in the furnace is reduced, and the wet bottom operation is [ Si] has increased. Considering the economic strength of China's factories, we should focus on developing efficient, practical and economical hot air cupolas.

In addition to high-quality coke and hot air, cast calcium carbide containing CaO and special silicon carbide pressed iron are added to the furnace to assist in warming. Casting calcium carbide is also a strong desulfurizer; special silicon carbide briquettes as a charge have both the function of increasing silicon and increasing carbon and promoting slag desulfurization.

2 Stable furnace size

Low-quality lining materials will bring a series of adverse effects: lining erosives contaminate the molten iron; erosive materials change the physicochemical properties of the slag, thereby affecting the forward and desulfurization effects of the furnace; furnace expansion, destroying the stability of smelting, opening When the furnace time is long, the temperature of the molten iron will decrease, the burning loss will increase, and the molten iron will oxidize.

When the furnace is opened at 8–12h, the quality of the lining material should be emphasized. The moisture content of the lining material should be small and the density of the blasting should be large. When the furnace is continuously operated, the furnace body and the tuyere should be water-cooled, and the furnace body and the hearth are made of high-aluminum brick, ASC brick and ASC. The continuous working time of this type of furnace varies from 1 to 6 weeks depending on the system design, lining quality and management level. At present, there are manufacturers that produce high-quality lining materials and companies that sell foreign products.

2. Use medium frequency induction furnace

In recent years, the author visited the Yangtze River Delta, the Pearl River Delta and the Jiaodong area, and found that there are many factories that use induction furnaces or twins. The quality of the induction furnace molten iron is often the supply condition specified by foreign buyers and domestic OEMs.

In the 1990s, the third-generation transistor module IF furnace took the second generation of traditional thyristor intermediate frequency furnace and replaced it. The furnace is characterized by power saving, fast melting speed, rapid temperature rise, simple operation, reliable operation, no interference to the power grid, and high power factor. Some can also achieve one melting, another one holding or preheating one-to-two operation.

The variable frequency intermediate frequency furnace can meet different process requirements of different melting stages. When melting, fast melting is achieved with higher frequency; when carbon is added, lower frequency is used, the stirring force of molten iron is increased, and the carbon increasing process is accelerated, thereby shortening the time and saving electricity. Some furnaces can also automatically adjust the power according to the height of the liquid level. Recently, the wide furnace induction furnace introduced abroad can realize the direct loading of large materials.

3. Do not ignore trace elements in pig iron

The pig iron factory generally only specifies Si, Mn, P, S, and has an unlimited amount of C. The trace elements are natural with the source and are not detected. The bottom code is unclear. The trace elements in pig iron in China include Cu, Cr, Mo, V, Ni, Ti, Sn, Al, Pb, Bi, Zn, As, Sb and the like. Among them, Cu, Cr, Mo, V, Ni are harmless, Al is considered neutral, As, Zn, and Pb are harmful, and Ti, Sb, Bi, and Sn are sensitive elements, which are favorable below a certain value, and there are disadvantages in exceeding.

In general, the trace elements in pig iron are not important for ordinary cast iron parts, but they are not to be considered for ductile iron parts and important gray iron parts. Germany stipulates that the total amount of trace elements in pig iron is ≤T≤0.0745%, and in Japan ΣT≤0.089%, Ti is required to be no more than 50% of the total. Domestic sources have suggested that pig iron for ductile iron T≤0.1%, of which Ti<0.045%.

Some foreign companies have explicit provisions on Ti in cast iron. The foundry should calculate the corresponding requirements for the amount of Ti in pig iron according to the ratio of charge.

In the past, some factories used some pig iron when there were some abnormalities in metallographic structure and casting defects, which was not related to the heritability of pig iron.

4. Induction furnace carbonation

If the induction furnace is mainly made of scrap steel, carbon is produced in the furnace to produce synthetic cast iron, and the steel is generally pure iron. Because there is no genetic and fertility of pig iron, the graphite in the cast iron structure has the characteristics of fineness and uniform distribution. The spheroidization rate of ductile iron is high. The low carbon gray iron is easy to obtain the type A graphite and pearlite matrix, and the wall thickness Sensitivity is reduced. Because the phosphorus is very low, the defects of shrinkage and leakage of the engine parts are greatly reduced.

Commonly used carbon reductants are graphite recarburizers (electrode blocks or high-quality natural graphite, graphite-like) and carbonaceous recarburizers (such as metallurgical coke). The electrode is pure in texture, the ash content is <0.5%, and the S and N contents are only 0.1%. When using natural graphite and carbonaceous recarburizers, special attention should be paid to the amount of ash, S and N, and to prevent moisture. The absorption rate of the recarburizer is 65 to 70% when the furnace bottom is added (charged), and 75 to 80% when the mirror is added. To some extent, the recovery rate increases with the increase of the treatment temperature, but after the equilibrium temperature, some of the carbon is consumed in the reaction (SiO2) + 2C = [Si] + 2CO, which in turn reduces the recovery.

The melting point of carbon is 3727 ° C. The carbon atoms enter the molten iron by dissolution and diffusion. Therefore, the particle size of the recarburizer and the agitation of the molten pool are very important for the carbonation process.

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