In order to proceed to the consideration of pig iron, it is necessary to understand the general composition of this product and its qualities. So, cast iron is an alloy that consists of a material such as iron, carbon and several other impurities.
General description of cast iron
Depending on the impurities used for smelting cast iron, its properties also change. However, there are those features that should be supported in any case. One of them is the mass fraction of carbon in the composition. This parameter must be at least 2.14%. If the carbon content is lower, then it is no longer cast iron, but steel. It is important to understand that, as such, ordinary cast iron is not produced. In the process of obtaining this material at the end of the operation, two types of additives are always added, according to which there is a separation into foundry or pig iron. One of the features of this raw material also lies in the fact that the temperature required for its melting is 250-300 degrees higher than for steel. To melt this substance, a temperature of 1200 ° C is required.
How to get cast iron?
It is immediately worth noting here that the production of pig iron or ordinary is almost identical processes, and therefore it makes no sense to describe both. Consider only the general technology of melting.
So, to get this substance, you need to spend a large amount of resources. The main working raw materials are coke and water. In order to manage to melt a ton of pig iron, you need to take about 550 kg of coke or about 900 liters of water. The amount of ore that will be spent on processing cannot be determined exactly for each batch, since its consumption depends entirely on the percentage of iron. However, it is unprofitable to use absolutely any ore, if you look from the point of view of the economy. For this reason, raw materials are used that contain more than 70% of iron in their composition. It is also important to note that before smelting the ore is enriched, and only after it enters the blast furnace, it is in them that the process of producing pig iron occurs. Electric furnaces smelted only 2% of the total amount of material.
First stage
The entire melting process is divided into several stages that are interconnected.
The procedure begins with the fact that ore is loaded into the furnace of the furnace, which contains magnetic iron ore. In addition, you can use ore, which contains aqueous iron oxide or its salt. Together with the loading of the working mineral, coking coal is loaded into the furnace . Their main task is to maintain high temperatures. In order to quickly melt the ore and gain access to iron, a flux is sent to the furnace. The substance, which is a catalyst, contributes to a more rapid decomposition of ore.
It is important to note here that before loading into the furnace, the ore usually undergoes a process of crushing, washing, and drying. All these stages contribute to the removal of excess impurities, as well as an increase in the melting speed.
Second phase
The second stage of smelting pig iron smelting is started when all the necessary materials have been loaded into the blast furnace. Burners are started that heat the coke, and it heats the ore. It is important to know that during heating, coke begins to emit carbon into the air, which passes through it, reacts with oxygen and forms oxide. This volatile substance is actively involved in recovery processes. However, this process proceeds only as long as air remains in the furnace. The more gas inside the blast furnace, the weaker this effect, and over time it completely stops. When this moment comes, all the gas available inside the furnace leaves to maintain a high temperature inside the unit.
All excess carbon is mixed with the molten substance, absorbed by iron, which forms cast iron. All elements that are not melted during the melting process float to the surface, from where they are removed. After completion of this cleaning process, there comes a time when various additives are added to the molten feed. What kind of cast iron will eventually turn out depends on what type of additives will be used.
What are cast iron?
If we consider in more detail exactly the conversion substance, then we can note several distinctive qualities. Firstly, the content of manganese and silicon in the composition is much lower, and secondly, it is used to produce steel by the oxygen-converter method. If we talk about foundry iron, then it is used to produce a wide variety of products. It is also important to note here that all material belonging to this group is divided into several types.
Next, you should know that, depending on its composition, pig iron is divided into classes:
- P1 and P2 - this is the marking of a common remaking substance;
- PF1, PF2 and PF3 are phosphorous raw materials;
- PVK1, PVK2 and PVK3 are a group of high-quality cast iron;
- pig iron cast iron PL 1 and PL2 is a category of materials related to foundry.
For example, you can consider the content of these substances in raw materials having an average quality indicator. The Si content is from 0.2 to 0.9%, Mn is from 0.5 to 1.5%, P is not more than 0.3%, S is not more than 0.06%.
Chemical Features
If we consider the chemical composition required by the technical conditions, an important feature should be noted. The main purpose of pig iron is remelting into steel, and therefore the requirements for its quality and composition are determined by steelmaking processes.
One of the weaknesses of such a technological process was that it is not able to cope with such an impurity as sulfur. And since the main difference between cast iron and steel in the carbon content, it becomes clear that the main task that must be performed is the removal of carbon from the composition. In order to achieve this goal, it is necessary that the chemical composition allows the oxidation process to be carried out. It is through the oxidation of carbon that it is removed from pig iron.
However, it must be understood here that during the oxidation of carbon other impurities will also be affected - silicon, manganese, and to a lesser extent - iron. The resulting substances during this process are called oxides, after which they are transferred to the discharge of slag. The end product of such an industry is ferrous slag - this is waste with a high iron content, which significantly complicates the removal of sulfur from the composition. For this reason, the mass fraction of the element S should be minimal in the composition of pig iron.
Recycling in other devices
Depending on what method the cast iron was processed into steel, different technical conditions for the composition will be presented.
Using an oxygen-converter device, you can get rid of such impurities as phosphorus. The higher the mass fraction of this element, the higher the cold brittleness of the raw material (cracking at low temperatures).
If you take, for example, open-hearth furnaces, then you can melt cast iron into steel of almost any kind. However, it is important to monitor the quantitative content of phosphorus and silicon. The higher the mass fraction of these elements, the more expensive the remodeling process will be. In addition, the time required to complete the work is greatly increased. For this reason, in the composition of the material, their content should not exceed the average values according to technical documentation. It should be noted that the manganese content in pig iron is not limited. This is because it contributes to processes associated with the removal of sulfur.
Converted cast iron is characterized in that its silicon content is higher - up to 1.2%.
State standard
As is the case with other industrial materials, cast iron must be manufactured according to the strict rules described in the state standard. For pig iron - GOST 805-95 establishes all the technical conditions by which it should be created. The quantitative content of all chemical elements in each group is regulated.
Technical requirements according to GOST
The documentation indicates the points that must be observed in any case, but there are those that are set by the consumer in an agreement with the manufacturer.
The first category includes the following rules:
- Cast iron grades related to PL1 and PL2 should be delivered to the processing sites with the obligatory indication of the mass fraction of carbon in the composition.
- If pig iron is smelted from ores belonging to copper-bearing ores, then the mass fraction of this element should ultimately not exceed 0.3%.
- The production of this material is carried out in ingots, without pinching, with one pinch or two pinch max. In places of pinch, the thickness of ingots (ingot) shall not exceed 50 mm.
- Mass ingots should not exceed such values as: 18, 30, 45, 55 kilograms.
- There should not be any residual slag on the surface of these aggregates.
Customer requirements
GOST 805 for pig iron also regulates several technical requirements that the consumer is entitled to establish when ordering from the manufacturer. These include the following items:
- Grades of pig iron related to PL1 and PL2 should be made with a mass fraction of carbon in the composition from 4 to 4.5% inclusive.
- If we consider the same grades PL1 and PL2, which will subsequently be used for the manufacture of nodular iron castings, the mass fraction of chromium in such a substance should not exceed 0.04%. Also, in the manufacture of high-quality pig iron in accordance with GOST, for the further production of piston rings, the manganese content should be limited to 0.3%, and chromium to 0.2%.
- If there are no special applications, then ordinary conversion and high-quality material should be manufactured with a manganese content of more than 1.5%. If pig iron of the phosphorous group is produced, the phosphorus content is more than 2%.
- The mass fraction of silicon in such brands as PL1, PF1 and PVK1 should be more than 1.2%.
- A very important point is the sulfur content, which is allowed no more than 0.06% in types of cast iron P1, P2 and PL1, PL2.
Acceptance and quality control
The document also establishes the rules for the acceptance of goods and quality control operations.
This material may only be accepted in batches. A batch is considered to be cast iron belonging to the same brand, group, type and type, as well as having a document that confirms the quality of the products. Most often, such papers indicate: the trademark of the enterprise that manufactured the product; name of the enterprise acting as a consumer; brand, group, class and category of cast iron, stamp control and a few more points.
If we talk about control methods, then it is necessary to check the quality of the flakes. To do this, use of magnifying devices is optional. In order to carry out quality control regarding flakes, the method used was agreed upon between the consumer of the product and the manufacturer. If the lot weight is up to 20 tons, then 10 samples of flakes are taken from different places. If the mass exceeds 20 tons, then 20 samples must be taken from the surface of cast iron.
Structural quality
It is worth adding that there is a special division of cast iron into such types as: white, gray, malleable, high-strength. The division into types is carried out depending on the structure of the material.
For example, the category of white cast iron includes that batch of material in which all carbon is in a chemically bound state and also has the appearance of cementite. Due to the presence of this substance, the color of cast iron becomes white, hence the name.
If we talk about gray cast iron, here the main distinguishing quality is carbon, which is presented in the form of graphite with the shape of curved plates or flakes. Due to the large number of these elements, a break in cast iron is gray. An alloy of iron with carbon is produced in large quantities in China, Japan, Russia, India, South Korea, and Ukraine.