Castings produce precipitated pores, how to quickly find out the cause and solve it? (1)Category: Company News Date: 2019-03-21 Click: 3
Porosity defects are common problems in casting production and are also stubborn. Many castings produced by foundries have long-term storage of about 10% of vent defects. This caused great losses to the foundry.
How to solve the problem of porosity in castings?
First of all, we must first understand the cause of the pores in the casting.
The casting pores are divided into a precipitated pore, a contact pore and a reaction pore. The root causes of various pores are different and the solutions are different.
Today, Lao Lu will give you a focus on the characteristics and solutions of the precipitated pores.
Causes and solutions for precipitated pores
We know that in the smelting of medium frequency induction furnace, the molten iron surface is in direct contact with the atmosphere, and the molten iron will be filled with a certain amount of gas from the atmosphere. In the charge, hydrogen, nitrogen and oxygen are also produced for various reasons. These gases are present in the molten iron in atomic form in the high temperature molten iron.
After the molten iron is smelted, when the molten steel is poured into the mold, the molten iron is cooled during the cooling and solidification process, and the gas precipitated in the molten iron is too late to be removed, and the casting will have pores. This kind of pore, we call it a precipitated pore.
The precipitated pores are distributed over a large area on a large section, and are close to the riser position, the hot joint portion, and the higher temperature region.
If there are many pores, the shape of the pores will also appear crack. When the gas content is large, the pores are large and the shape is round.
The gas causing the precipitated pores is mainly hydrogen, followed by oxygen.
Where are the sources of hydrogen and oxygen in casting smelting?
At the beginning, Lao Lu also said that the gas in the casting smelting mainly comes from the atmosphere and the charge.
The smelting iron surface of the intermediate frequency furnace is in direct contact with the atmosphere, and the smelting molten iron is entrained with oxygen in the atmosphere under electromagnetic agitation. This is one of the sources of molten iron.
In addition, in the scrap steel, scrap iron, and pig iron of the smelting charge, if the rust is serious, since the rust is iron oxide, the iron oxide is reduced to the molten iron during high-temperature melting of the iron oxide.
Above, this is the source of oxygen in molten iron.
The source of hydrogen is mainly a moist charge.
If you look at the old irons, you will find that in areas with high air temperatures, if the charge is not baked, it will easily produce precipitated pores.
In summer and during the rainy season, the probability of precipitated stomata in castings is higher than in other seasons. This is related to the moisture of the charge.
Further, the inoculant, the spheroidizing agent, and the alloy added to the furnace may also generate hydrogen if they contain moisture.
The source of hydrogen is mainly in the above aspects.
Therefore, to avoid the formation of precipitated pores, we must control at the source. When selecting the material, when selecting the alloy, try to avoid the rust material.
In the summer, or in areas with high air humidity, the necessary charging of the charge and alloy is carried out. This will eliminate the source of oxygen and hydrogen at the source.
Material protection is one aspect.
In addition to the prevention of materials, in the smelting process and casting process, other measures are needed to prevent the formation of precipitated pores.
First, if the iron gas contains a high gas content, in the smelting, it is necessary to extend the high temperature standing time of the molten iron to facilitate the gas removal in the molten iron.
2. Once the castings have precipitated pores, the molten iron is purified without replacing the smelting materials.
Third, to avoid the generation of pores, under the premise of ensuring that the casting does not produce shrinkage, try to increase the pouring temperature, which is also very beneficial for eliminating the precipitated pores.
Properly controlled materials and alloys, and proper smelting measures, can basically prevent the occurrence of precipitated pores.
However, in addition to materials and smelting processes, when a large proportion of scrap steel is used for casting production, since molten iron is obtained by increasing carbon and silicon for scrap steel, the molten iron has a large liquid shrinkage and a large crystallization temperature range, so that molten iron is also prone to precipitated pores. .
Therefore, if a large proportion of scrap steel is used to smelt the casting, if there is a porosity defect. The first thing to judge is whether it is a nitrogen hole or a precipitated pore.
If it is a precipitated pore, check the chemical composition of the casting to see if the five elements and trace elements exceed the allowable content of the casting.
If it is a nitrogen hole, check the nitrogen content of the recarburizer.
Porosity defects, although common, many foundries are often not well resolved due to the lack of proper countermeasures.
In fact, solving such problems and finding the cause is fundamental.
I hope this article can help to cast colleagues. I also hope that other masters have good measures to control stomata and share them with everyone.
Today, Lao Lu wrote here. Next time, Lao Lu will explain the characteristics of the reaction pores and the countermeasures.