A method for preparing green sand and its casting application method

Abstract

The invention provides a preparation method of green sand and a casting application method thereof. The green sand comprises high-silica sand, sodium bentonite, industrial water and pregelatinized starch. The green sand is prepared by weight to comprise 87 to 88.7 parts of high-silica sand, 8 to 9 parts of sodium bentonite, 2.5 to 3 parts of industrial water and 0.8 to 1 part of pregelatinized starch. The green sand can improve the adhesion and water retention between sand particles in green sand steel casting, thereby reducing sand sticking on the surface of steel castings, realizing the green sand process production of carbon steel valves, and ensuring that the production of carbon steel valves has the characteristics of high mechanization, high automation, high product stability and high pass rate.

Description

Technical Field

[0001] The invention belongs to the field of casting, and in particular relates to a method for preparing green sand and a casting application method thereof. Background Art

[0002] Cast carbon steel, due to its excellent plasticity, yield strength, tensile strength, and high weldability, is widely used in pressure-resistant valve components in industries such as petrochemicals, mining, water conservancy, and natural gas. Since cast carbon steel valves are key components of pressure vessels, strict requirements are imposed on their performance, composition, and surface quality to ensure the reliability and safety of engineering applications. Currently, the production process for carbon steel valve castings generally utilizes methods with low water content, such as coated sand molding, water glass sand, and resin sand. However, these production methods, due to their multiple steps, require extensive manual processing and cannot achieve mechanized, batch, or continuous production. This results in high manufacturing costs and severely restricts the development of national infrastructure.

[0003] Currently, the green sand process in the cast iron industry is characterized by high mechanization and automation, high product stability, and a high pass rate. The green sand process uses a certain proportion of water, sand, bentonite, and a small amount of pulverized coal to mix and create a loose sand. This is then squeezed through a mold to form a concave sand mold. Molten, quenched and tempered iron is poured into the prepared sand mold. After cooling, the product blank is formed. The product is then separated and surface treated through pouring and riser separation to form the product. The green sand process is widely used in the production of cast iron products, but this process cannot be applied to the casting of carbon steel valves for the following reasons: First, the green sand process contains a certain amount of water, which causes the water to vaporize instantly after the high-temperature molten steel is cast, resulting in a large number of surface and internal pores in the product. Second, the casting temperature of cast steel is much higher than that of cast iron, generally reaching above 1580°C, significantly higher than the casting temperature of cast iron of around 1400°C. Green sand does not have a surface resin layer like coated sand, and its refractoriness is insufficient. This leads to severe chemical sand adhesion on the surface of the cast product after casting, which is extremely difficult to clean. Therefore, the green sand process cannot be used in the production of cast carbon steel valves at present. Summary of the Invention

[0004] In order to overcome the shortcomings and problems of the prior art, the present invention provides a green sand for carbon steel valve casting and its preparation method and casting application method, so as to solve the problem of using green sand process for the production of cast carbon steel valves and improve the production efficiency of cast carbon steel valves.

[0005] The present invention is achieved through the following technical solutions:

[0006] A method for preparing green sand, comprising the steps of:

[0007] S101. Prepare materials, including high silica sand, sodium bentonite, industrial water, and pregelatinized starch. Prepare by weight: 87-88.7 parts of high silica sand, 8-9 parts of sodium bentonite, 2.5-3 parts of industrial water, and 0.8-1 part of pregelatinized starch.

[0008] S102, start the sand mixer, add 95% of the prepared amount of high silica sand into the sand mixer, wait until the high silica sand is dispersed, and then add 95% of the prepared amount of sodium bentonite and 95% of the prepared amount of pregelatinized starch into the sand mixer;

[0009] S103, dry-mixing the high silica sand, sodium bentonite, and pregelatinized starch in a sand mixer until the sodium bentonite and pregelatinized starch are coated on the surface of the high silica sand;

[0010] S104, adding 70% of the total amount of industrial water into the sand mixer at a rate of 0.2 parts per second, and continuing mixing for 300 to 320 seconds;

[0011] S105. Take the mixed sand sample and test its wet compressive strength, moisture content, compaction rate, and air permeability. The moisture content is maintained within the range of 2.4-3.0% and the compaction rate is maintained within the range of 33%-38%. If the moisture content is below the lower limit, industrial water is added according to the difference. If the moisture content is above the upper limit, the sand is discarded. The wet compressive strength is maintained within the range of 180-230 kPa. If the moisture content is below the lower limit, sodium bentonite is added according to the difference. If the moisture content is above the upper limit, the sand is discarded.

[0012] S106. After the above requirements are met, the green sand is prepared by continuing to mix for 120 to 150 seconds.

[0013] Furthermore, the SiO2 content of the high-silica sand is greater than 99.2%, the Fe2O3 content is less than 0.05%, the Al2O3 content is 0.2-0.5%, and the remaining impurities are less than 0.25%.

[0014] Furthermore, the AFS particle size of the high silica sand is 58-62, the three-sieve concentration rate of 70-140 mesh is greater than 92%, the moisture content is less than 0.1%, the mud content is less than 0.2%, the loss on ignition is less than 0.3%, and the pH value is 7-7.5.

[0015] Furthermore, the sodium bentonite has a montmorillonite content greater than 85%, a blue absorption capacity of 34 g / 100 g (methylene blue method), a moisture content of 8% to 12%, a wet compressive strength greater than 120 kPa, a hot wet tensile strength greater than 4 kPa, a swelling value ml / 3 g greater than 98%, an expansion multiple ml / 2 g greater than 35%, and a particle size of 200 meshes accounting for greater than 90%.

[0016] Furthermore, the moisture content of the pregelatinized starch is ≤12%, the pregelatinization degree is ≥85%, the 100-mesh sieve pass rate is ≥93%, the viscosity is >500 mPa·s, the pH value is 7-7.5, the volatile matter is ≥78%, and the loss on ignition is ≥99.5%.

[0017] Furthermore, when the allocation is performed without meeting the relevant parameters in step S104, the total time taken shall not exceed 500 seconds, otherwise it shall be scrapped.

[0018] Furthermore, the green sand prepared in step S105 needs to be stored in a closed space when waiting to be used.

[0019] Furthermore, the sand mixer is a double-roller sealed sand mixer.

[0020] The present invention is also achieved through the following technical solutions:

[0021] A green sand casting application method comprises the following casting steps:

[0022] S201, injecting the green sand into the molding machine cavity, compacting it and then unpacking it to form a sand mold, and adjusting the compaction force of the equipment to ensure that the sand mold index is 85 to 90 degrees (type B hardness tester for molding sand);

[0023] S202, deoxidizing the molten steel after smelting and tempering and pouring it into a sand mold, with a starting pouring temperature of 1600°C to 1620°C, a minimum casting temperature of >1580°C, and a casting speed of 8 to 10 kg per second;

[0024] S203. When the sand mold after casting cools down to a surface temperature of less than 80 degrees, the casting is subjected to a vibration sand removal method.

[0025] Furthermore, the sand mold is cast within 15 minutes after compaction.

[0026] The sodium bentonite in the present invention is a hydrophilic mineral with an exchangeable cation content of more than 600mL / kg, and is mainly composed of Na+, with a content greater than 50%. The sodium bentonite has a large water absorption capacity of more than 400%, and the water absorption and dehydration process is long. Therefore, the crystal structure of this type of bentonite is stacked in sheets, so that after it absorbs water and expands, it will not lose water quickly when encountering high-temperature molten steel. The gas generated by the water loss speed after encountering high temperature will not accumulate and can be discharged through the mold exhaust design, avoiding the instantaneous gasification of water after high-temperature molten steel casting, which causes a large amount of surface and internal gas in the product. Porosity, and the high silica sand used has high temperature resistance and high impact resistance. The pregelatinized starch used has a porous, hydrogen bond rupture structure, and has the characteristics of cold water solubility, good cold water stability, strong water retention, thickening, adhesion, etc., especially after mixing, its effect can be further exerted. In green sand casting, the adhesion and water retention between sand particles can be improved, thereby reducing sand sticking to the surface of steel castings. Therefore, the present invention can realize the green sand process production of carbon steel valves, and can make the production of carbon steel valves have the characteristics of high mechanization, high automation, high product stability and high pass rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 These are appearance diagrams of castings according to Examples 1 to 6 of the present invention. DETAILED DESCRIPTION

[0028] To facilitate understanding by those skilled in the art, the present invention is further described in detail below with reference to specific embodiments.

[0029] A method for preparing green sand comprises the following steps:

[0030] Select high silica sand, sodium bentonite and pregelatinized starch that meet the requirements. The parameters of each material are as follows:

[0031] High-silica sand has a SiO2 content of 99.5%, a Fe2O3 content of 0.03%, an Al2O3 content of 0.3%, and a remaining impurity content of 0.17%. The AFS particle size of high-silica sand is 61, the concentration of the 70-140 mesh three-sieve is 95%, the moisture content is 0.08%, the mud content is 0.1%, the loss on ignition is 0.2%, and the pH value is 7.5.

[0032] The montmorillonite content of sodium bentonite is 92%, the blue absorption is 34g / 100g (methylene blue method), the moisture content is 11%, the wet compressive strength is 120kPa, the hot wet tensile strength is 4.2kPa, the swelling value ml / 3g is 99%, the expansion times ml / 2g is 38%, and the proportion of particles with a particle size of 200 mesh is 92%.

[0033] The moisture content of the pregelatinized starch is 11%, the pregelatinization degree is 89%, the 100-mesh sieve pass rate is 95%, the viscosity is 550 mPa·s, the pH value is 7.2, the volatile matter is 81%, and the loss on ignition is 99.6%.

[0034] Industrial water is filtered through 200 mesh before use.

[0035] Prepare each material according to the mass ratio, one mass part is 30 kg, and the amount of materials prepared for Examples 1 to 5 is shown in Table 1:

[0036] Table 1 Preparation ratio of each ingredient

[0037]

[0038] Use a double-roller sealed sand mixer, start the sand mixer, and put 95% of the prepared high-silica sand into the sand mixer. After the high-silica sand is dispersed, put 95% of the prepared sodium bentonite and 95% of the prepared pregelatinized starch into the sand mixer.

[0039] In the sand mixer, high silica sand is dry-mixed with sodium bentonite and pregelatinized starch until the sodium bentonite and pregelatinized starch are wrapped on the surface of the high silica sand. The dry-mixing time is generally controlled within 120 seconds.

[0040] Add 70% of the prepared amount of industrial water into the sand mixer at a rate of 0.2 parts per second and continue mixing for 300 to 320 seconds.

[0041] Take the mixed sand samples to test the wet compressive strength, moisture, compaction rate and air permeability.

[0042] The moisture content is maintained at 2.4-3.0% and the compaction rate is maintained at 33%-38% (Note: if the moisture content is controlled to meet the requirements, the compaction rate can be controlled to meet the requirements). If it is lower than the lower limit, industrial water is added according to the difference. If it is higher than the upper limit, it is scrapped until the relevant parameters are adjusted to meet the requirements, as shown in Table 2:

[0043] Table 2 Ingredients for moisture content adjustment (using a 3-ton sand mixer)

[0044]

[0045] The wet compressive strength is kept in the range of 180-230kPa. If it is lower than the lower limit, sodium bentonite is added according to the difference.

[0046] If the upper limit is exceeded, the product will be scrapped until the relevant parameters meet the requirements, as shown in Table 3:

[0047] Table 3 Ingredients for adjusting wet compressive strength (using a 3-ton sand mixer)

[0048]

[0049] The total time for adjusting the above moisture content and wet compressive strength shall not exceed 500 seconds, otherwise it shall be scrapped.

[0050] After adjusting Examples 1-5 to meet the relevant parameter requirements, the measured results are shown in Table 4:

[0051] Table 4 Measured parameters of sand samples in various examples

[0052]

[0053] After the adjustment is completed, continue mixing for 120 to 150 seconds to complete the preparation of green sand. After preparation, the green sand is placed in a closed space for storage.

[0054] Casting of carbon steel valves:

[0055] Example 6 uses currently commonly used green sand. The green sand used in this example is specifically ZGS 98-70 / 140 (61A). The green sand prepared in Examples 1 to 5 and the green sand in Example 6 are used to cast carbon steel valves. 30 pieces are cast in each example. The casting steps are as follows:

[0056] Green sand is injected into the molding machine cavity. The molding machine adopts a horizontal static pressure molding machine (Qingdao Xindong ACE-5 model). After compaction, the sand mold is formed. The surface hardness of the sand mold is adjusted to 85-90 degrees (special hardness tester for molding sand, type B) by adjusting the compaction force of the equipment.

[0057] The molten steel after smelting and tempering is deoxidized and poured into the sand mold. The starting pouring temperature is 1600℃~1620℃, the minimum casting temperature is >1580℃, and the casting speed is 8~10kg per second. The casting is completed within 15 minutes after the sand mold is compacted and formed.

[0058] When the sand mold after casting cools to a surface temperature of less than 80 degrees, the casting is sanded by vibration.

[0059] like Figure 1 The following are appearance diagrams of the castings of Examples 1 to 6. The castings produced in Examples 1 to 6 were tested, and the test results are shown in Table 5:

[0060] Table 5 Casting test results

[0061]

[0062] Average number of pores: Calculate the total number of defective pores (one pore is considered as one if the pore diameter is 0.2mm or larger) on the casting surface and machined surface of 30 castings, and then average it for each casting.

[0063] Average chemical bonding area ratio: Calculate the ratio of the total chemical bonding area to the total component surface area.

[0064] Explanation of terms:

[0065] Porosity: These smooth, pear-shaped, round, oval, or needle-shaped holes of varying sizes form on the surface of steel castings due to the presence of gas in the molten metal. Their primary hazard: Porosity can reduce the airtightness of valve seals, leading to leakage and affecting product operation.

[0066] Chemical sand bonding: This defect forms a rough, glossy layer on the surface of metal castings. This defect is primarily caused by a chemical reaction between metal oxides and the sand mold material. Its main hazard is that it affects the casting's appearance, processing properties, and performance.

[0067] It can be seen from the above test results that the present invention can significantly reduce the defective pores in castings and avoid the occurrence of chemical sand sticking, thereby making the castings meet the use requirements.

[0068] The above embodiments are preferred implementations of the present invention and are not intended to limit the present invention. Without departing from the inventive concept of the present invention, any obvious replacements are within the scope of protection of the present invention.

Claims

1. A method for preparing green sand, comprising the steps of: S101. Prepare materials, including high silica sand, sodium bentonite, industrial water, and pregelatinized starch. Prepare by weight: 87-88.7 parts of high silica sand, 8-9 parts of sodium bentonite, 2.5-3 parts of industrial water, and 0.8-1 part of pregelatinized starch. S102, start the sand mixer, add 95% of the prepared amount of high silica sand into the sand mixer, wait until the high silica sand is dispersed, and then add 95% of the prepared amount of sodium bentonite and 95% of the prepared amount of pregelatinized starch into the sand mixer; S103, dry-mixing the high silica sand, sodium bentonite, and pregelatinized starch in a sand mixer until the sodium bentonite and pregelatinized starch are coated on the surface of the high silica sand; S104, adding 70% of the total amount of industrial water into the sand mixer at a rate of 0.2 parts per second, and continuing mixing for 300 to 320 seconds; S105. Take the mixed sand sample and test its wet compressive strength, moisture content, compaction rate, and air permeability. The moisture content is maintained within the range of 2.4-3.0% and the compaction rate is maintained within the range of 33%-38%. If the moisture content is below the lower limit, industrial water is added according to the difference. If the moisture content is above the upper limit, the sand is discarded. The wet compressive strength is maintained within the range of 180-230 kPa. If the moisture content is below the lower limit, sodium bentonite is added according to the difference. If the moisture content is above the upper limit, the sand is discarded. S106. After the above requirements are met, the green sand is prepared by continuing to mix for 120 to 150 seconds.

2. The method for preparing green sand according to claim 1, wherein: The high silica sand has a SiO2 content greater than 99.2%, a Fe2O3 content less than 0.05%, an Al2O3 content of 0.2-0.5%, and other impurity components less than 0.25%.

3. The method for preparing green sand according to claim 1, wherein: The high silica sand has an AFS particle size of 58-62, a 70-140 mesh three-sieve concentration rate of >92%, a moisture content of <0.1%, a mud content of <0.2%, a loss on ignition of <0.3%, and a pH value of 7-7.

5.

4. The method for preparing green sand according to claim 1, wherein: The sodium bentonite has a montmorillonite content of >85%, a blue absorption capacity of 34g / 100g, a moisture content of 8% to 12%, a wet compressive strength of >120kPa, a hot wet tensile strength of >4kPa, a swelling value ml / 3g of >98%, an expansion multiple ml / 2g of >35%, and a particle size of 200 meshes accounting for >90%.

5. The method for preparing green sand according to claim 1, wherein: The pregelatinized starch has a moisture content of ≤12%, a pregelatinization degree of ≥85%, a 100-mesh sieve pass rate of ≥93%, a viscosity of >500 mPa·s, a pH value of 7-7.5, a volatile matter of ≥78%, and a loss on ignition of ≥99.5%.

6. The method for preparing green sand according to claim 1, wherein: When the allocation is performed without meeting the relevant parameters in step S104, the total time taken shall not exceed 500 seconds, otherwise it shall be scrapped.

7. The method for preparing green sand according to claim 6, wherein: The green sand prepared in step S105 needs to be stored in a closed space when waiting to be used.

8. The method for preparing green sand according to claim 7, wherein: The sand mixer is a double-roller sealed sand mixer.

9. The green sand casting application method according to any one of claims 1 to 8, comprising the following casting steps: S201, injecting the green sand into the molding machine cavity, compacting it and unpacking it to form a sand mold, and adjusting the compaction force of the equipment to ensure that the sand mold index is 85 to 90 degrees in surface hardness; S202, deoxidizing the molten steel after smelting and tempering and pouring it into a sand mold, with a starting pouring temperature of 1600°C to 1620°C, a minimum casting temperature of >1580°C, and a casting speed of 8 to 10 kg per second; S203. When the sand mold after casting cools down to a surface temperature of less than 80 degrees, the casting is subjected to a vibration sand removal method.

10. The casting application method according to claim 9, characterized in that: The sand mold is cast within 15 minutes after compaction.

Images