yvonne@nydcasting.com 
Vermicular graphite cast iron has excellent performance such as high wear resistance, shock absorption and superior comprehensive heat fatigue resistance, and has been widely used in production. There are many factors that affect the production stability of cast vermicular graphite cast iron. To produce vermicular graphite cast iron stably and reliably, it is necessary to control the chemical composition and temperature of the raw iron liquid in production, as well as appropriate vermicularization and inoculation treatment methods and reliable furnace control.
1. Melting of raw iron liquid
1.1 Determination of melting equipment
The selection of melting equipment is based on the principle of high efficiency and low consumption on the premise of meeting production needs. The medium frequency induction furnace is used to melt molten iron. Its advantages are: flexible furnace opening; fast heating speed, easy to obtain high-temperature molten iron; low gas content in molten iron; due to the existence of electromagnetic stirring, the temperature and composition of the molten iron are uniform; the molten iron is conducive to metallurgical treatment in the furnace, and the composition is easy to control.
1.2 Selection of furnace charge
The chemical composition of vermicular graphite cast iron is high carbon, high silicon, low phosphorus, low sulfur and a certain manganese content. The conditions for adding inoculants to vermicular graphite cast iron are: high carbon, low silicon, low sulfur, and large inoculation amount. Sulfur and phosphorus are harmful elements of vermicular graphite cast iron. Their content is an important factor affecting the creep rate, so they must be strictly controlled.
2. Creeping and inoculation treatment
2.1 Selection of creeping treatment package
In order to maximize the absorption rate of the creeping agent and enhance the creeping effect, it is decided to use a spheroidizing package with a height to diameter ratio of 2:1 to treat the original iron liquid.
2.2 Selection of creeping agent
The production process of vermicular graphite cast iron is difficult to control. If the amount of creeping agent added is slightly too much, too much spherical graphite is likely to appear; if the amount added is insufficient, flake graphite will be produced. Therefore, it is necessary to reasonably select the creeping agent and strictly control the amount of creeping agent added to ensure a good creeping effect.
Rare earth magnesium silicon iron alloy is selected as the creeping agent. Taking the production of RuT380 vermicular iron castings as an example, since it is a high-strength and high-toughness vermicular iron, it is decided to use a high rare earth alloy vermicular agent with the label FeSiMg4Re17.
The characteristics of the vermicular agent treatment are: stable reaction, no boiling of molten iron, weak self-diffusion ability of rare earth magnesium element, and stirring is required. In actual production, a small amount of spheroidizer with the brand FeSiMg8Re7 is added to the vermicular agent as an initiator to play a stirring role, and the vermicular effect is very good.
The amount of vermicular agent added is generally 0.8% to 1.4% according to the rare earth content in the vermicular agent and the sulfur content of the original iron liquid. The particle size of the vermicular agent should not be too large. According to the amount of molten iron each time, it is generally added in small pieces of 5 to 15 mm.
2.3 Selection of inoculant
By inoculating the molten iron after creeping, white cast iron can be suppressed; the number of vermicular graphite can be increased, the eutectic clusters can be refined, and the mechanical properties, especially plasticity and toughness, can be significantly improved; the distribution morphology of phosphorus eutectics can be changed, thereby improving the comprehensive performance of castings; the creep rate can be changed to prevent creep decay. At present, the most widely used inoculant is FeSi75. The amount of inoculant added should be determined based on the requirements for the mechanical properties of the casting, the silicon content in the original molten iron, and the required silicon content of the casting, generally 0.8% to 1.2%. In order to ensure a good inoculation effect, the inoculant should be added in small pieces of 5 to 15 mm, and it should be kept clean and dry.
2.4 Creeping inoculation treatment method
Because the “non-boiling” creeping agent is selected, the flat-bottomed bag flushing method is used to creep the original iron liquid. The creeping agent and inoculant should be added to the ladle before tapping, especially in continuous production. After a furnace of iron is tapped, the ladle is very hot. Adding alloy too early makes it stick to the ladle bottom, weakening creeping and inoculation. The creeping agent is added first. The inoculant is then placed on top. Dry, clean iron filings cover them to delay the reaction. Wood ash is added for crusting and slag absorption. It prevents air contact and magnesium vapor escape.
2.5 Temperature control
Temperature control is the key to creeping treatment. The tapping temperature is generally controlled at 1420~1450℃. In order to ensure the accurate temperature of the molten iron tapped, an inserted thermocouple is used in front of the furnace to measure the temperature and strictly control the tapping temperature to avoid the temperature being too low to melt the creeping agent or the temperature being too high to burn more magnesium, which will lead to poor creeping effect and even become gray cast iron.
2.6 Pre-furnace inspection of creeping effect
In the pre-furnace inspection of inoculation and creeping effect, triangular specimens are generally used. Pour the triangular specimen, cool it to dark red, and quench it. Observe the fracture after breaking it. The fracture is silvery white, with white tip, shrinkage in the center, and concave on both sides. At the same time, there is an odor of calcium carbide when breaking it, and the knocking sound is similar to that of steel parts. If the creeping is good, otherwise the creeping is poor.
3. Control of the pouring process
The pouring temperature should be controlled at 1380-1410℃ to avoid pores and incomplete pouring in the casting. The treated molten iron should not be left for too long to prevent the treated molten iron from creeping and decaying. Each 400kg molten iron is a bag, and each bag of molten iron is best poured within 10 minutes.
Use the last bag of molten iron to cast the sample for mechanical properties and metallographic inspection. The batches of products and samples must correspond one by one, and be tracked to the cleaning process.
4. Production situation
In actual production, 400kg medium frequency induction furnace is used for smelting to obtain raw iron with relatively stable chemical composition. The raw iron liquid is vermified by flat-bottom ladle flushing method. FeSiMg4Re17 vermification agent is used at 1.1%, with 15% as a detonator. FeSi75 inoculant is used at 1% and added sequentially. Iron filings and plant ash cover it. The raw iron liquid is heated to 1420-1450℃ for tapping. Pouring is completed within 10 minutes after the reaction.
The casting structure is detected by metallographic microscope. The graphite creep rate is 62%, and the basic structure is pearlite + ferrite + carbide, of which pearlite accounts for 85% and eutectic carbide accounts for 5%.
The casting mechanical property test sample is cast, and the processing specification is Φ15mm sample for mechanical property test. The mechanical property meets or exceeds the RuT380 standard.