Effect of inoculation treatment on Machinability of gray cast iron

Another important link in obtaining high-performance gray cast iron is inoculation treatment. Before the solidification of molten iron, a certain amount of alloy material (called inoculant) is added to form a large number of submicroscopic cores in the molten iron, which promotes the activation of carbon atom micro region and the formation of eutectic group in the liquid phase, changes the solidification mode of molten iron, improves the structure of gray cast iron, increases the number of graphite nuclei, and can promote the smooth growth of graphite. Inoculation treatment is the way to improve the properties of gray cast iron through foreign substances. Modification treatment will be used in aluminum alloys. In fact, inoculation is different from modification. Inoculation mainly affects the nucleation process and promotes grain dissociation to refine grains. Inoculation only affects the nucleation process, only improves but cannot change the specific phase shape. Modification mainly affects the growth process and morphology of crystals. In gray cast iron, they are often called inoculation treatment or modification treatment, and in aluminum alloy, they are used to be called modification treatment. Both are found in cast steel.

(1) Inoculation treatment

Gray cast iron is not inoculated, the microstructure is unstable, and white mouth is easy to appear at the thin wall of the casting, while coarse graphite flakes are easy to appear at the thick wall, resulting in low mechanical properties of the casting. In order to obtain castings with consistent quality, inoculation treatment must be carried out.

Gray cast iron material for cylinder block requires not only good mechanical properties, but also good machinability and casting process properties. Reducing carbon content and alloying can not only improve the strength of gray cast iron, but also deteriorate the casting process properties and processability. This problem can be effectively solved by inoculation treatment. Inoculation treatment can promote the formation of graphite core, so as to promote graphitization and improve the morphology and distribution of graphite. Under the condition of basically unchanged carbon equivalent, it can reduce the white tendency and section sensitivity, and make gray cast iron obtain better mechanical and processing properties. In this way, low carbon equivalent molten iron can be used to produce high-strength gray cast iron.

The addition of inoculant is very small, which can not basically affect the chemical composition of gray cast iron, but can affect its microstructure, so it can improve its mechanical properties and processability. The effects of proper inoculation treatment on gray cast iron are as follows: eliminate or reduce the tendency of white cast iron while promoting graphitization; Control graphite morphology, avoid supercooling structure, and obtain fine and evenly distributed flake A-type graphite; Reduce the casting stress, reduce the sensitivity of wall thickness, and reduce the difference of microstructure and hardness at different wall thickness; Eutectic clusters are easy to nucleate, increase and refine eutectic clusters, promote the formation of fine flake pearlite structure, and improve the mechanical properties, cutting properties, compactness and wear resistance of gray iron castings.

(2) Inoculation treatment method

When producing gray iron castings, once the inoculant is determined, the selection of inoculation treatment method is very important, and the quality of products is determined by its inoculation effect. Different inoculation treatment methods have advantages and disadvantages. So far, no inoculation treatment method can adapt to various production conditions. Different casting enterprises choose the most suitable method from their own actual conditions. Most manufacturers adopt the inoculation methods of tapping inoculation, pouring inoculation and pretreatment before inoculation. Ladle inoculation is the inoculation of molten iron when pouring from the furnace to the ladle. This is a simple and convenient inoculation treatment method, which is the most widely used at present.

Generally, the amount of inoculant added to gray cast iron is 0.2% − 0.4% of the amount of molten iron. If too much inoculant is added, gray cast iron parts may have slag inclusion, shrinkage porosity and some negative effects such as ladle slag accumulation, slag inclusion, filter piece blockage and shrinkage porosity. In order to ensure the quality of castings, the pouring is usually completed within 10 minutes after inoculation. It is best to carry out post inoculation when molten iron is injected into the mold.

Pouring inoculation is post inoculation (instantaneous inoculation), which is carried out when molten iron is poured into the mold from the pouring ladle. There is no incubation decay, and 75 ferrosilicon is generally used for inoculation. At present, there are many casting inoculation methods, but flow inoculation, in mold inoculation and inoculation wire inoculation are most suitable for flow production line.

(3) Main conditions for producing inoculated gray cast iron

① Reasonable selection of chemical composition

The composition selection of inoculated gray cast iron is the same as that of ordinary gray cast iron, which should be closely combined with the wall thickness of the casting. The metallurgical characteristics of molten iron are also affected by the burden ratio. By improving the metallurgical properties of molten iron, inoculation treatment can play a better role. It is also considered that the increase of pig iron in the charge can significantly reduce the hardness and shrinkage of gray cast iron, avoid shrinkage porosity and shrinkage defects, and improve the machinability.

② Molten iron must have a certain overheating temperature

Temperature, chemical composition and purity are the three Metallurgical indexes of molten iron. The higher the temperature of molten iron, the higher the purity of molten iron, which is more conducive to improve the casting performance. More importantly, if the temperature of molten iron is increased within a certain range, the graphite can be refined, the matrix structure can be fine, and the tensile strength can be improved. For inoculated gray cast iron, the requirement of superheated molten iron is to purify molten iron and improve undercooling, so as to add a large number of artificial cores in the case of inoculation and force the gray cast iron to eutectic solidification under controlled conditions, so as to achieve the purpose of inoculation. In order to inoculate gray cast iron well and change its solidification characteristics to the greatest extent, it is necessary to have a fairly high overheating temperature (1450 − 1470 ℃).

③ Add a certain amount of inoculant

At present, inoculants have been commercialized, and 75 ferrosilicon alloy is used most. In recent years, special inoculants for strengthening inoculation elements such as strontium, cerium, barium, titanium, pickaxe and rare earth have been added, which has promoted the development of inoculated gray cast iron. The amount of inoculant is related to the composition and temperature of molten iron, the degree of oxidation and cooling rate, the wall thickness of castings, the type of inoculant and the inoculation method. Generally, the higher the grade of molten iron, the greater the amount of inoculant to be added. The general addition amount of inoculant is 0.2% − 0.7%. If strong inoculant is selected, the dosage can be appropriately reduced.

④ Inoculation method

The inoculation method has developed greatly since 1980. It is the most commonly used method to add inoculant evenly above the tapping tank, which is called one-time inoculation. This method is convenient and easy to operate. However, inoculant consumption is large and easy to decay. Therefore, the time from inoculation to solidification can be shortened as far as possible to prevent inoculation recession and obtain good inoculation effect.