Casting production process of ductile iron 500-7

Based on the structural and dimensional characteristics of castings, appropriate chemical compositions are selected, and a special ladle for ductile iron is used to strengthen the control of the spheroidization process and stably produce as cast QT500-7 thin-walled small ductile iron parts that meet technical requirements. The production of as cast high-strength ductile iron can be achieved through measures such as alloying to strengthen the matrix structure. However, the production of QT500-7 ductile iron with good as cast comprehensive performance is difficult due to the presence of many trace alloying elements in raw materials such as pig iron, scrap steel, and alloys that strengthen the matrix structure, resulting in a decrease in the performance of ductile iron. Here is a summary of our company's years of production practice for small (weighing 1-10Kg) and thin-walled ductile iron parts (wall thickness 5-20mm).

Requirements for Performance and Metallographic Structure: Tensile Strength ≥ 500N/mm

Yield strength ≥ 320N/mm2, elongation ≥ 7, Brinell hardness 170-230, spheroidization grade ≤ 3, pearlite 15% -45%, cementite ≤ 1, graphite size ≥ 6. The selection of chemical composition plays an important role in the microstructure and mechanical properties of cast pig iron. According to the structural characteristics, size, and requirements of the casting, the chemical composition selection is as follows: 3.6% to 3.9% C, the original iron liquid Si content is selected as 1.2% to 1.6%, the final Si content is controlled at 2.6% to 2.9%, the carbon equivalent (CE) is 4.5% to 4.7%, 0.2% to 0.3% Mn,<0.05% P,<0.03% S,<0.03% RE residue, 0.035% to 0.6% Mg, and the RE residue/Mg residue should be ensured to be ≤ 2/3.

The enterprise where the author works uses a 0.5T medium frequency furnace to melt molten iron, with a tapping temperature above 1520 ℃. Adopting the 0.5T ductile iron special pit pouring ladle punching method for spheroidization treatment. The spheroidizing agent is FeSiMg8RE5 with GB4138-88, and the inoculant is a silicon barium composite inoculant. After passing the pre furnace inspection, it can be quickly poured. Control the pouring time within 15 minutes to prevent spheroidization and deterioration.

3.1 The amount of molten iron treated with spheroidizing agent is 0.5 tons, and the particle size of the spheroidizing agent should be 10-20mm. The mass fraction of spheroidizing agent particles smaller than 10mm should not exceed 10%. The spheroidizing agent should be dried and preheated, preferably at 150-200 ℃. If conditions permit, a special hopper can be made for baking at the furnace mouth. We hope to produce ductile iron with high liquid iron temperature, low gas content, no oxidation, and stable chemical composition. Nowadays, various types of blast furnaces are commonly used in China to melt molten iron. Due to the unique characteristics of each furnace type, it is necessary to choose a furnace type that is more conducive to the production of ductile iron based on practical considerations. Our company uses a medium frequency furnace, which is environmentally friendly. The temperature of the molten iron is easy to control, the chemical composition is stable, and there is little fluctuation.

3.2 Pouring ladle

3.2.1 Size requirements: Use specialized ductile iron pouring ladle. The ratio of height to diameter of a general ladle is 1.1-1.2, and the ratio of height to diameter of a ductile iron ladle is 1.5 (diameter refers to the diameter of the ladle mouth). The area inside the pit accounts for 50% to 60% of the bottom area of the ladle, and the depth can accommodate the added spheroidizing agent, inoculant, and covering agent. The diameter of the ductile iron ladle mouth is smaller than the diameter of the general ladle mouth of the same specification. Considerations for ladle design:

A、 The height of the package ensures sufficient height above the spheroidizing agent, increasing the pressure of the molten iron column. The spheroidizing agent floats for a long time, which is conducive to the absorption of molten iron. At the same time, the magnesium bubbles float up and take a long time to escape from the surface of the molten iron;

B、 If the diameter of the package is small, the amount of magnesium bubbles escaping at the same time decreases, the oxidation burn is small, and the temperature loss slows down. Size and structure of pouring ladle

3.2.2 Drying degree requirements: The ladle must be dried thoroughly before use.

3.3 Processing methods

3.3.1 Preheating of the ladle: The ladle used for the first time must be thoroughly baked with chopped wood. When used again, it can be baked with chopped wood or ironed with molten iron.

3.3.2 The amount of spheroidizing agent added is 1.0% to 1.2%, the amount of inoculant added is 0.8% to 1.0%, and the amount of covering agent added is 0.5% to 1.0%

3.3.3 Loading method: First, place the spheroidizing agent into the concave pit at the bottom of the package and compact it. Then, cover it with inoculant and compact it again. Finally, add the covering agent and iron cover plate in sequence and compact the iron cover plate (if conditions permit, it is best to use dry, clean and oil-free ductile iron chips of the same brand instead of the iron cover plate).

3.3.4 Specific treatment process: Pour the molten iron into the ladle from the furnace to ensure that the first stream of molten iron falls into the center of the iron cover plate, avoiding the molten iron from impacting the edge of the iron cover plate and causing premature contact and reaction with the spheroidizing agent. It is best to control the molten iron to start the spheroidizing reaction when it overflows 300mm of the spheroidizing agent. After the molten iron is discharged, quickly cover the spheroidizing treatment ladle with an iron sheet cover. After the reaction is stable, take the iron sheet cover to remove the slag and sprinkle the covering agent.

Summary of Production Results

4.1 Composition of castings and performance of specimens:

4.2 Sample Graphite and Matrix Metallographic Images 5 Quality Control of Production Process The author summarizes years of practical experience in the production of thin-walled (wall thickness 5-20mm) small (weight 1-10 Kg) as cast ductile iron castings and realizes that in order to meet the technical requirements for the metallographic and performance of QT500-7, it is very important to control the content of pearlite in the matrix structure while ensuring good spheroidization. The author believes that the following points are the most critical for stable production of as cast QT500-7 ductile iron:

5.1 Iron liquid out of the furnace temperature: If the iron liquid out of the furnace temperature is too high, it will cause the spheroidization reaction time to advance, that is, the iron liquid will start spheroidizing before reaching a sufficient height. At the same time, the iron liquid will churn too violently, causing severe Mg burning and a decrease in absorption rate. Moreover, the growth and decay will be rapid, resulting in poor spheroidization or decay; If the temperature of the molten iron is too low, it can cause slow or even unresponsive spheroidization and initiation of the molten iron. The temperature of the 0.5T ladle decreases by approximately 80-100 ℃ during the spheroidization reaction process, so the temperature of the molten iron treatment after being discharged from the furnace should be 100-120 ℃ higher than the pouring temperature. Therefore, the temperature of the molten iron treatment after being discharged from the furnace should be controlled between 1500-1520 ℃

5.2 Control the carbon and silicon content in the molten iron, strictly following the principle of "high carbon and low silicon".

5.3 The content of RE residue and Mg residue should be strictly controlled, and the ratio of RE residue to Mg residue should be ensured to be ≤ 2/3. 5.4 Use high-quality raw materials, stabilize material sources, and strictly control the content of sulfur and trace harmful elements. To this end, it is necessary to strengthen the management of furnace materials, which should be classified and stacked with clear markings, and weighed accurately when adding materials.

5.4.1 Pig iron: It is best to choose iron containing less than 2% silicon and less than 0.1% phosphorus. Its tonnage shall not be less than 50 tons to maintain a relatively stable period of ductile iron quality.

5.4.2 Recycled materials: Scrap ductile iron and sprues of this grade must be removed of sand and oxides, sorted and stacked, and labeled with chemical composition.

5.4.3 Scrap steel: High quality carbon scrap steel should be used as much as possible. At the same time, those with severe corrosion and a thickness of less than 2mm should not be used as nodular iron furnace materials to reduce oxides and facilitate desulfurization. The block size of the above furnace materials should be less than 1/3 of the furnace inner diameter to prevent the influence of shed materials on the temperature of the molten iron.

5.5 Spheroidizing agents must be sealed and stored to prevent oxidation and moisture.