In recent years, the development of incubation treatment technology has been rapid, and there are various effective incubation methods. The selection of appropriate inoculation treatment methods is crucial for the production of gray cast iron parts and is often a key factor in determining product quality. Each fertility treatment method has its advantages, but also certain limitations, and so far there is no treatment method that can adapt to various production conditions. Casting enterprises should select the most suitable method for their specific conditions through experiments based on their characteristics. Once the processing method is determined, the process should be strictly controlled to ensure stable casting quality.
The widely used inoculation methods mainly include: inoculation during casting, inoculation during pouring, in mold inoculation, and pre-treatment before inoculation
1. Pregnancy during iron production
Ladle Inoculation, also known as inoculation during iron casting, is a inoculation process carried out when molten iron flows from a furnace or insulation furnace to the ladle. This processing method is both simple and convenient, and is currently the most widely used process, but when using it, attention must be paid to following the key points of the operation.
Do not add inoculant to the bottom of the empty ladle before pouring. By doing so, some of the inoculant will be oxidized before interacting with the molten iron, and the inoculant is prone to wrap around the slag, resulting in a decrease in its utilization rate. It is best to start the inoculation treatment from the moment when the amount of molten iron in the ladle is about 1/4 of the amount of molten iron after tapping. The granular inoculant should be evenly and dispersed into the liquid flow through a quantitative funnel, and the treatment should be completed when the amount of molten iron in the ladle is about 4/5 of the amount of molten iron. This method of operation can utilize the stirring effect of the liquid flow to completely dissolve the inoculant in high-temperature molten iron, thereby improving its utilization rate. The use of wire feeding method for inoculation treatment has a more stable effect, but currently there are relatively few methods used for inoculation during iron production.
The particle size of the inoculant varies slightly depending on the amount of molten iron being processed: when the amount of molten iron is less than 500kg, the particle size should be 2-5mm; When the amount of molten iron is 500kg, the particle size should be 5-8mm. The amount added is generally 0.2-0.4% of the amount of molten iron processed, which can be adjusted according to the type of inoculant and the thickness of the casting wall, and determined through experiments. Adding too much is not only useless, but also has some negative effects due to incomplete dissolution in the molten iron, such as slag accumulation in the ladle, slag inclusion defects in the casting, clogging of filter components in the pouring system, and casting defects caused by excessive inoculation of the molten iron. If the particle size of the inoculant is too large, it can also affect the inoculation effect and cause casting defects due to incomplete dissolution in the molten iron.
When using 75% silicon iron for inoculation treatment during iron production, the inoculation effect will quickly decline over time. After 5-7 minutes of inoculation, the effect can decline by more than 50%. After about 15 minutes, most or all of the inoculation effect will disappear. To ensure the quality of castings, they are usually poured within 10 minutes after inoculation, and it is best to perform another inoculation when the molten iron is poured into the mold from the ladle.
2. Incubation during pouring
During casting, inoculation, also known as post inoculation or instantaneous inoculation, is a inoculation treatment carried out when the molten iron is poured into the mold. Fine particle inoculants can be added to the liquid flow, or the wire feeding method can be used for incubation.
(1) Fine grained inoculant for incubation
There is no problem of fertility decline during casting. Generally, 75 silicon iron is used as a fertility agent, and fertility agents containing other alloy elements can also be used when there are special requirements. In order to enable the inoculant to quickly dissolve in the iron liquid and be evenly distributed in the iron liquid, fine particles should be used, generally 0.3-0.7mm. The amount added is approximately 0.15-0.2% of the molten iron.
When using a tilting pouring ladle for manual pouring, a relatively simple method is to use a quantitative funnel hung on the ladle nozzle. During pouring, the discharge port is opened, and the inoculant falls into the liquid flow by gravity. For the fixed position of the tilting machine on the production line, it is better to use a micro spiral feeder with a hopper.
Under the condition of pouring through a siphon nozzle using an insulation furnace on the production line, dry air blowing is commonly used abroad. At the beginning of pouring, the sensor controls the opening of the gas valve to add the inoculant into the liquid flow,
The disadvantage of using this feeding method is that some powdered inoculants may be blown away by air and cannot enter the liquid flow. Using mechanical feeding method also has the problem of easy blockage and affecting accurate quantification.
(2) Feeding and nurturing
Feeding and nurturing is also a common method, and the diameter of the core wire used is generally between 5-10mm, with a core material of 75 silicon iron. The amount of inoculant added is approximately between 0.05% and 0.1%.
Under the condition of pouring through a siphon nozzle using an insulation furnace, the core wire is fed near the stopper rod of the nozzle. The feeding device is controlled by a computer, and the core wire is fed while lifting the stopper rod for pouring. The feeding stops when the stopper rod is lowered after pouring.
3. Intratype pregnancy
In mold inoculation is the process of placing the inoculant directly into the pouring system, allowing it to interact with the molten iron during the pouring process. The rate at which the inoculant dissolves into the molten iron is a key consideration in the design of the process plan.
The inoculant used can be fine-grained material, block material made of powder and binder, or prefabricated block.
According to the process characteristics of the casting, the inoculant can be placed in the sprue basin, pouring system, or specially designed filter components.
(1) Fertility in the sprue basin
There are two ways to breed in a sprue basin,
One method is to fix the block shaped inoculant at the bottom of the sprue basin and gradually dissolve it into the molten iron during pouring. When the amount of molten iron poured is large, a stopper can be used, which is more conducive to the dissolution of the inoculant into the molten iron and its uniform distribution in the molten iron.
Another method is to place the block shaped inoculant in the sprue basin, which floats on the molten iron during pouring, also known as floating silicon inoculant, but not in the ladle. When using this method, it should be ensured that the size of the incubation block is still larger than the diameter of the sprue after being dissolved in the molten iron.
Regardless of the method used, there should be a slag blocking plate inside the sprue basin.
(2) Incubation within the pouring system
The block shaped inoculant can be fixed in the sprue cavity under the sprue. When using this method, it is necessary to use prefabricated block shaped inoculants, which gradually dissolve into the molten iron during the pouring process to achieve effective inoculation.
Alternatively, the inoculant can be placed in a specially designed inoculation tank in the transverse runner. When using this method, inoculants with a particle size of 0.3-0.7mm can be used, and the dosage is generally 0.05-0.1% of the amount of molten iron in the mold. During pouring, the molten iron flows over the inoculant. To prevent inoculant particles from entering the casting, the molten iron must pass through an effective filter after passing through the inoculation tank.
(3) Pregnancy through filter
This is a relatively new method of in mold inoculation, in which the inoculant is placed in a specially designed filter, and during pouring, the molten iron flows through the filter for inoculation treatment, while also preventing residue from entering the casting cavity.
When using foam ceramic filters, a fine-grained inoculant is placed in the cavity between the two filters,
If through-hole ceramic filters are used, place the pre fabricated pellet in the cavity between the two filters
4. Pre treatment before conception
The demand for some important thin-walled gray cast iron parts, such as cylinder blocks and heads of internal combustion engines, is constantly increasing, and metallurgical requirements are also increasing. Therefore, the pre-treatment of gray cast iron before inoculation has gradually received widespread attention.
When producing thin-walled gray cast iron parts, pre-treatment before inoculation treatment can not only avoid the occurrence of carbides in the structure, but also minimize the undercooled graphite (B-type, D-type, and E-type).
The research results indicate that the most effective pretreatment agent is crystalline carbonaceous material. For gray cast iron, 85-90% metallurgical silicon carbide is suitable, and crystalline graphite can also be used. The typical dosage is 0.75-1.0%, and the optimal amount needs to be determined through experimentation during application. After adding the pretreatment agent to the molten iron, it takes a certain amount of time to dissolve it and requires stirring. Therefore, it is best to add it to the induction furnace for treatment before tapping. There are also reports that adding it to the ladle can achieve good results as long as the operation is done properly.