What Is an Iron Foundry?
The general term for alloys consisting mainly of iron, carbon and silicon. In these alloys, the carbon content exceeds the amount that can be retained in the austenite solid solution at the eutectic temperature.
- [zhù ti]
- Cast iron is mainly made of
Precautionary measures
- Cold crack
- Cold crack formation diagram
- When the weld is of cast iron type, cold cracks are liable to occur. Cracks are often accompanied by audible loud brittle fracture sounds, which often occur when welds are longer or when defects with larger rigidity are repaired. The reason for this is: due to the uneven heating of the weldment during the welding process, the welding seam is subject to great tensile stress during cooling. Due to the low strength of cast iron, there is basically no plasticity below 400 . In tensile strength, cold cracks in the weld occur. When white cast iron is present in the weld, the shrinkage of white cast iron (2.3%) is greater than that of gray cast iron (1.26%), so cold cracks are more likely to occur in the weld, especially when the weld is stronger than the base metal In the cooling process, the base material cannot restrain the shrinkage of the weld, and as a result, the base material is torn at the joint. This phenomenon is called "peeling".
- When the welding joint is rigid, the number of weld repair layers is large, and the volume of the weld repair metal is large, so that the weld joint is in a high stress state, such as the yield point of the weld metal is high, it is difficult to relax the high stress of the weld joint through its plastic deformation. , Then the welding crack is easy to occur in the white mouth area or the martensite area of the heat affected zone, forming a cold crack in the heat affected zone.
- The most effective way to prevent cold cracking is to perform an overall preheating of the welding patch at 550 ~ 700 , followed by the welding material using heterogeneous welds.
- Hot crack
- Causes of thermal cracks
- The method to prevent thermal cracking is to adjust the chemical composition of the weld, add rare earth elements, enhance the desulfurization and dephosphorization capabilities, reduce the fusion ratio, and reduce welding stress.
Smelting method of cast iron
- Cupola melting
- Cupola structure diagram
- The basic structure of the cupola is shown in Figure 1. The furnace body, bellows and flue are welded with steel plates. The inside of the furnace is usually lined with a layer of refractory bricks to resist the high temperature effects of coke combustion. For the storage of molten iron, most cupolas are equipped with a forehearth.
- (2) Cupola melting principle
- During the smelting process, the lower part of the furnace body is filled with coke, called bottom coke. A batch of iron materials (pig iron, scrap steel, regrind, ferroalloy, etc.), coke and flux (limestone, fluorite, etc.) are alternately installed on the bottom coke. By blowing air, the bottom coke is strongly burned, and the generated high-temperature furnace gas rises in the direction of the height of the furnace body, so that a layer of iron material on the furnace is melted.
- (3) advantages and disadvantages of cupola melting and its application
- Cupola is the most commonly used cast iron melting equipment. It uses coke as fuel, and the heat generated by the combustion of coke is directly used to melt the charge and increase the temperature of the molten iron, which saves energy compared to electric isolated furnaces and other furnaces. In addition, the equipment is relatively simple and can be used in large and small factories. However, cupola also has certain shortcomings, mainly because the molten iron is in direct contact with coke, so the process of increasing the carbon and sulfur of the molten iron occurs during the smelting process.
- The cupola-electric double furnace smelting method or cupola-inductive electric furnace double smelting method is adopted to make full use of the high melting efficiency of the cupola, the electric furnace and the induction furnace have a strong ability to superheat the molten iron and the chemical composition is easy to control The advantages.
- Induction furnace melting
- Induction furnace
- Induction electric furnace is a furnace that uses the current induction to generate heat to heat and melt iron. The structure of the furnace is divided into core type (Figure 2) and coreless type. In the coreless induction electric furnace, the iron material in the crucible generates an induced current under the action of an alternating magnetic field, and thus generates heat. It melts itself and heats the molten iron process. In the cored induction electric furnace, it is necessary to add molten iron melted by other melting furnaces (such as cupola furnaces). The alternating magnetic field generated in the annular iron core makes the molten iron in the groove process, and uses the molten iron in the groove and the above The molten iron in the molten pool circulates to heat all the molten iron. The coreless induction furnace has the ability to melt solid charge, while the cored induction furnace can only overheat the molten molten iron, but in terms of power consumption of the superheated molten iron, the cored induction furnace is more economical.
- (2) Advantages and disadvantages of induction furnace melting and its application
- Compared with cupola smelting, the advantages of induction furnace smelting are that there will be no carbon and sulfur increase during the smelting process, and the smelting process can cover the molten iron with slag, which can prevent silicon, manganese and alloys in the molten iron to a certain extent The oxidation of the elements reduces the absorption of the molten iron from the furnace gas, thereby making the molten iron relatively pure. The disadvantage of this smelting method is the high cost of electricity.
- Induction electric furnace is suitable for smelting high-quality gray cast iron, alloy cast iron, ductile cast iron and vermicular cast iron. The coreless induction furnace can directly melt the solid charge, and it is more convenient to open and stop the furnace, which is suitable for intermittent production conditions. The core induction electric furnace is inconvenient to start and stop, which is suitable for continuous production. This furnace has low thermal efficiency for melting solid charge and high thermal efficiency for superheated molten iron, so it is suitable for use with cupola furnaces. Both types of induction furnaces are now used in cast iron production.
- Arc furnace melting
- Electric arc furnace
- Electric arc furnace smelting uses the heat generated by the arc between the graphite electrode and the iron material (iron liquid) to melt the iron material and superheat the molten iron. A three-phase electric arc furnace is commonly used in production. The structure of the furnace body is shown in FIG. 3. In the arc furnace smelting process, when the iron material is melted away, the smelting operation to further increase the temperature and adjust the chemical composition is performed under the condition that the slag is covered with the molten iron. The electric arc furnace is divided into acidic and alkaline according to the properties of the slag and the lining refractory. The alkaline electric arc furnace has the ability to desulfurize and dephosphorize.
- (2) Advantages and disadvantages of arc furnace melting and its application
- The advantages of electric arc furnace smelting are the strong ability to melt solid charge, and the molten iron is superheated and chemically adjusted under the condition of slag covering, so it can avoid the molten iron intake and element oxidation. This creates good conditions for smelting low carbon cast iron and alloy cast iron. The disadvantage of the electric arc furnace is that it consumes a lot of power and is not as economical as a cupola from the perspective of melting. Therefore, the cast-iron arc furnace double method is often used in the production of cast iron. Because of the poor resistance to rapid cooling and rapid heating of alkaline electric arc furnace linings, under intermittent smelting conditions, the life of the linings is short, resulting in high melting costs, so acidic electric arc furnaces and cupolas are often used in combination.
Repair of cast iron cracks
- Cast iron parts are brittle and prone to pores during the casting process. Under long-term vibration and impact, it is easy to cause stress concentration and cause the shell to crack. Due to the poor weldability of cast iron and the high sealing requirements of hydraulic equipment, the traditional welding repair process cannot be repaired at all. There are generally no spare parts for such equipment on the site, and purchasing and replacing requires a lot of downtime. Such problems are now mostly repaired with polymer composite materials. The excellent mechanical properties and good adhesion and pressure resistance of polymer metal repair materials make this problem effectively solved. Repair process: According to the site conditions, it is recommended that companies first connect the cracks up and down with electric welding, and weld several points to strengthen the structural force of the shell. Find the end point of the crack and punch a 4.2mm crack stop at the end point to prevent the crack from further extending. Use a sander to polish along the crack, and extend 75px to both sides. After cleaning with absolute ethanol, the polymer metal repairing material is combined with the reinforcing tape to repair the cracks.