High-grade gray cast iron generally refers to gray iron above HT250. In the production process, it can be roughly divided into the following forms: one is inoculated cast iron; the other is alloy cast iron (here mainly introduces rare earth gray cast iron).
1. Inoculated gray iron
In gray cast iron, graphite exists in the form of flakes. The existence of flake graphite seriously damages the performance of the iron matrix. In order to improve the mechanical properties, the number of graphite flakes must be reduced. Inoculated cast iron is essentially a method of obtaining higher mechanical properties by reducing the number of graphite flakes and adding an appropriate amount of inoculant to refine the eutectic group.
In gray cast iron, the main elements are C, Si, Mn, S, and P. Among them, C, Si, and P are elements that promote graphitization, while Mn and S are elements that hinder graphitization. In order to reduce the number of graphite sheets, it is necessary to reduce the content of C, Si, and P, and increase the content of Mn and S. Therefore, its carbon equivalent CE=C+1/3 (Si+P) is generally low, and all belong to hypoeutectic cast iron Sc＜1. However, since carbon exists in two forms in cast iron, one is free graphite; the other is combined cementite. If the carbon equivalent is too low, the carbon will all form cementite during the cooling process of the cast iron, making the cast iron white cast iron with poor mechanical properties. Therefore, in the process of producing inoculated cast iron, chemical composition analysis must be carried out first to select the appropriate carbon equivalent, so that the carbon element forms cementite in addition to the solidification process, and a small amount of graphite must exist, so that pearlite appears in the internal structure of the cast iron, and high mechanical properties are obtained.
In the actual production of inoculated cast iron, since silicon-based inoculants are also added to the original molten iron, when selecting the original molten iron composition, it is generally selected that its carbon equivalent CE is near the edge of the white or pitted area, which depends on the grade of the inoculated cast iron produced. Then, a little addition of inoculants will make the gray iron matrix present fine pearlite.
For Mn and S elements, they are both elements that are conducive to the formation of pearlite. Mn and S elements will also react in the molten iron to generate MnS slag, so when producing inoculated cast iron, the content of Mn element needs to be increased. As for S element, it depends on the actual structural condition of the casting. If the casting does not produce cracks, the control range can be relaxed. In some cases, its content can even be artificially increased.
As for the way of adding inoculants, one is to add it when tapping, and there is also inoculation with flow or inoculation in the mold during the pouring process, which depends on the specific situation. Due to different inoculation methods, the amount of inoculant added will also vary.
The inoculation process is a short-term effect on the original molten iron, so the pouring time of the molten iron must be limited. If the pouring time is too long, the inoculation effect will be lost, and the mechanical properties of the casting will be reduced.
2. Rare earth alloy gray iron
When producing inoculated cast iron, a large amount of scrap steel must be added. If it is smelted in a cupola, it may sometimes be difficult. Moreover, due to the development of the foundry industry, the supply of scrap steel is in short supply, which sometimes directly affects production. For this reason, we must consider a new production plan to produce high-grade gray iron without or with less scrap steel. This process is to use rare earth alloys to smelt high-grade gray iron. First, let us understand the role of rare earth alloys:
(1) Rare earth elements are strong desulfurizers.
(2) Rare earth elements remaining in molten iron will significantly change the graphite morphology of gray iron.
(3) Rare earth can increase the supercooling of cast iron crystallization, hinder graphitization during solidification, and increase the tendency of white cast iron.
The carbon equivalent of nodular cast iron is low, and it belongs to hypoeutectic cast iron, while rare earth gray iron is not. It belongs to eutectic or hypereutectic cast iron. Because rare earth elements are added to hypoeutectic cast iron, eutectic graphite must precipitate between austenite dendrites during solidification. Since rare earth has a large supercooling effect on cast iron crystallization, this interdendritic graphite often precipitates as supercooled graphite, so the mechanical properties deteriorate. The situation is different for eutectic or hypereutectic cast iron. The addition of rare earth elements will cause obvious changes in the cast iron structure. When a small amount of rare earth alloy is added, the graphite is still flaky, but the distribution is slightly uniform. When the rare earth alloy is continued to increase to a certain value, the shape of graphite will change dramatically. It becomes short and thick worm-like, and there is a small amount of spherical graphite. If rare earth elements are continued to be added, the proportion of worm-like graphite will decrease, and the spherical graphite will gradually increase. When it reaches a certain level, due to its strong supercooling effect, part of the ledeburite structure will appear in the matrix. This change in structure will inevitably cause changes in mechanical properties. If the amount of rare earth is increased on this basis, the amount of ledeburite in the matrix will increase and the mechanical properties will decrease.
In summary, in order to use rare earth elements to smelt high-grade gray iron, the two necessary conditions are:
First, the original molten iron must be a eutectic or hypereutectic composition, and its carbon equivalent CE=4.3%-4.8%.
Second, a certain amount of rare earth elements must remain in the molten iron after being treated with rare earth alloys. After multiple measurements, the residual Re=0.06%-0.1%.
Because rare earth is a strong desulfurizer, the molten iron treated with rare earth alloys generally has a relatively low S content, about 0.01%-0.02%.
In addition, when smelting rare earth gray iron, in order to increase the content of pearlite in the matrix, a certain amount of Mn is usually required, and Mn is generally taken between 0.5%-1.5%.
In actual production, in order to ensure the quality of rare earth gray iron, it is necessary to sample and inspect the treated molten iron in front of the furnace, usually using a triangular test block. The best test specimen is one with slight shrinkage on the top and both sides, silver-gray fracture, dense structure, slight shrinkage in the center and a certain width of white opening at the tip of the specimen. If there is a large shrinkage on the top and both sides of the triangular test block, obvious shrinkage in the center, silver-gray fracture, and a large width of white opening, it means that the rare earth alloy is added excessively, and the amount of addition should be reduced, and the inoculation should be strengthened; otherwise, it means that the amount of rare earth alloy added is insufficient, and the amount of rare earth alloy added should be appropriately increased so that the width of white opening on the fracture reaches a certain value.
After many tests, it is completely feasible to use rare earth alloy to smelt high-grade gray iron, and its tensile strength is generally ≥350MPa.
The above are two methods for producing high-grade gray iron. Of course, there are other methods for smelting high-grade gray iron, such as adding a certain amount of alloying elements such as Cu, Ni, Cr, etc. to alloy iron. In any case, only by constantly exploring and learning new things in our work can we improve our professional knowledge and better serve production.
