Common defects in the production of ductile iron by lost foam casting process
Part 1
The common defects in the production of ductile iron by lost foam casting process are as follows:
1. It is easy to cause low spheroidization grade
The main reasons for the low spheroidization grade of ductile iron produced by lost foam casting process are as follows:
Spheroidizer and inoculant
Poor quality of spheroidizer: Although the content of RE and Mg in the spheroidizer meets the standard, due to poor smelting technology, the MgO content is high, which will affect the spheroidization quality, and the spheroidizer contains less elements such as Ca, and the reaction is fierce during the spheroidization treatment, resulting in more Mg burning. If the spheroidizer is left for too long and oxidized by moisture, the spheroidization effect will also be reduced.
Inoculant problem: The amount of inoculant added is small, or the inoculation process is imperfect, the inoculation effect is not strong or decays, which affects graphitization and is not conducive to obtaining a good spheroidization grade.
Original iron liquid
High sulfur content: Sulfur is the main anti-spheroidizing element. When wS>0.06% in the original iron liquid, it is difficult to obtain qualified quality even if more spheroidizer is added. If the pouring time is long and the slag is not clean, it will also "re-sulfurize", affecting the quality of the casting.
Interference elements are too high: the spheroidizing interference elements such as Ti and Pb brought in with the charge are too high, which will deteriorate the shape of the graphite ball. Even if spheroidized, the physical properties of the material will become brittle.
In terms of molten iron temperature
The temperature is too low: when the spheroidizing molten iron temperature is lower than 1390℃, the alloy is not easy to melt, the spheroidizing reaction is incomplete, and the spheroidizing level is difficult to meet the requirements.
The temperature is too high: the molten iron temperature is too high, the spheroidizing agent melts too fast, Mg is lost and evaporated in large quantities, and the molten iron is severely oxidized, which reduces the absorption rate of the spheroidizing agent.
In terms of process operation
Improper operation of the spheroidizing treatment before the furnace: if the amount of spheroidizing agent added is insufficient, or the amount of molten iron is not well controlled, the amount of iron discharged is too much, and the spheroidizing agent is relatively small. When using the flushing method for spheroidization, if the operation is not standardized, it will also lead to low magnesium absorption rate.
Pouring process problems: The pouring temperature is not suitable. If the pouring temperature is too high, carbon and silicon will be burned seriously. If the pouring temperature is too low, it will not be conducive to spheroidization, and the spheroidization rate and spheroidization level will be reduced. In lost foam casting, the white mold decomposes a large amount of hydrogen. If it cannot be extracted from the mold in time by negative pressure, the hydrogen will enter the molten iron, forming a white mouth tendency, which is not conducive to the precipitation of graphite balls.
Other aspects
The white mold materials, coatings and vacuum used in lost foam casting will also affect the spheroidization of ductile iron. For example, the density of the white mold material is large, and more hydrogen is decomposed; the coating has poor air permeability or the vacuum degree is low after pouring, which will increase the contact time between hydrogen and molten iron and affect the precipitation of graphite balls.
2. Porosity defects
The main reasons for the porosity defects in the production of ductile iron by lost foam casting process are as follows:
Pattern material aspect
The pattern material has a large gas emission and decomposes at high temperature to produce a large amount of gas. If the gas cannot be discharged in time, pores will be formed in the casting.
The density of the pattern material is uneven, resulting in different local gas emission, which is also easy to cause pores.
Coating
The uneven thickness of the coating layer or partial detachment will cause the molten metal to directly contact the molding sand during the filling process, and the gas in the molding sand will enter the molten metal to form pores.
The permeability of the coating is poor, and the gas generated by the decomposition of the pattern and the gas in the molding sand are difficult to be discharged through the coating layer, so they gather in the casting to form pores.
Casting system
The design of the casting system is unreasonable, such as the small number of inner runners, small cross-sectional area, and too fast filling speed of the molten metal, which will cause the gas decomposed by the pattern to be discharged too late and be drawn into the molten metal to form pores.
The design of the pouring cup is unreasonable, which cannot effectively prevent slag and gas from entering the mold cavity, and will also increase the probability of pores.
Molding sand
The permeability of the molding sand is poor, and the gas is difficult to be discharged in the molding sand, which will form pores in the casting.
The moisture content in the molding sand is too high. During the pouring process, the moisture evaporates to produce a large amount of water vapor, which easily invades the molten metal to form pores.
