Preparation method and casting application method of green sand

Abstract

The invention provides a preparation method and a casting application method of green sand, the green sand comprises 87-88.7 parts by mass of high silica sand, 8-9 parts by mass of sodium bentonite, 2.5-3 parts by mass of industrial water and 0.8-1 part by mass of pre-gelatinized starch, the adhesion and the water-retaining property among sand grains can be improved in green sand cast steel, and the green sand can be used for casting the cast steel with the high silica sand, the sodium bentonite, the industrial water and the pre-gelatinized starch. And therefore, sand adhering to the surface of the steel casting is reduced, green sand process production of the carbon steel valve can be achieved, and the production of the carbon steel valve has the characteristics of high mechanization, high automation, high product stability, high percent of pass and the like.

Description

Technical Field

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

[0002] Cast carbon steel is widely used in pressure-resistant valve parts in petrochemical, mining, water conservancy, natural gas and other industries due to its good plastic toughness, yield strength, tensile strength and high weldability. Since cast carbon steel valves are key components of pressure vessels, there are strict requirements on their performance, composition and surface quality to ensure the reliability and safety of engineering applications. At present, the production process of carbon steel valve castings generally adopts coated sand molding, water glass sand, resin sand and other methods with low water content for production. However, these production methods require a lot of manual processing due to their multiple processes, and cannot achieve mechanized, batch and continuous production, resulting in high manufacturing costs, which seriously restricts the development of national infrastructure.

[0003] At present, the green sand process in the cast iron industry has the characteristics of high mechanization, high automation, high product stability and high qualified rate. The green sand process uses a certain proportion of water, sand, bentonite and a small amount of coal powder to mix and make loose sand, and then extrude and mold it into a concave sand mold through a mold. The melted and tempered molten iron is cast in the prepared sand mold, and the product blank is formed after cooling, and then the product is formed through pouring and riser separation and surface treatment. The green sand process is widely used in the production of cast iron products, but this process cannot be used in the production of valves made of cast carbon steel. The reasons are as follows: First, the green sand process contains a certain amount of water, which causes the water to instantly gasify after the high-temperature molten steel is cast, resulting in a large number of surface and internal pores in the product; second, because the casting temperature of cast steel is much higher than that of cast iron, the general casting temperature reaches more than 1580°, which is significantly higher than that of cast iron, about 1400°, and the green sand has no surface resin layer like coated sand, and the refractoriness is insufficient, which leads to serious chemical sand sticking on the surface of the 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 a preparation method and a casting application method thereof, so as to solve the problem of using green sand process for producing 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, 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, put 95% of the prepared amount of high silica sand into the sand mixer, and after the high silica sand is dispersed, put 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 the 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 to mix for 300 to 320 seconds;

[0011] S105, take the mixed sand sample to test the wet compressive strength, moisture, compaction rate, and air permeability; the moisture content is maintained in the range of 2.4-3.0% and the compaction rate is maintained in the range of 33%-38%. If it is lower than the lower limit, industrial water is added according to the difference, and if it is higher than the upper limit, it is scrapped; the wet compressive strength is maintained in the range of 180-230 kPa. If it is lower than the lower limit, sodium bentonite is added according to the difference, and if it is higher than the upper limit, it is scrapped;

[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 >99.2%, the Fe2O3 content is <0.05%, the AlO2 content is 0.2-0.5%, and the remaining impurities are <0.25%.

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

[0015] Furthermore, the sodium bentonite has a montmorillonite content of >85%, a blue absorption amount of 34g / 100g (methylene blue method), 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>98%, an expansion multiple ml / 2g>35%, and a particle size of 200 mesh accounts for >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 mPa·s>500, 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 casting application method of green sand comprises the following casting steps:

[0022] 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 guaranteed to be 85 to 90 degrees on the surface (type B hardness tester for molding sand);

[0023] S202, deoxidize the molten steel after smelting and tempering and pour it 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;

[0024] S203, when the sand mold after casting cools to a surface temperature of less than 80 degrees, the casting is subjected to sand removal treatment by vibrating sand removal.

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

[0026] The sodium-based bentonite in the present invention belongs to a hydrophilic mineral, and the exchangeable cation content is as high as 600mL / kg or more, and is mainly Na+, with a content greater than 50%. The sodium-based bentonite has a large water absorption capacity, which can reach more than 400%, and the water absorption and dehydration process is long, so 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 dehydration speed after encountering high temperature will not gather and can be discharged through the mold exhaust design, avoiding the instantaneous gasification of water after high-temperature molten steel casting, resulting in a large amount of surface and internal products. 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, the effect can be further exerted, and the adhesion and water retention between sand particles can be improved in green sand casting steel, thereby reducing sand sticking on 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 qualified rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is the appearance diagram of the casting of Examples 1 to 6 of the present invention. DETAILED DESCRIPTION

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

[0029] A method for preparing green sand is described, which specifically 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] The SiO2 content of high silica sand is 99.5%, the Fe2O3 content is 0.03%, the AlO2 content is 0.3%, and the content of other impurities is 0.17%. The particle size AFS of high silica sand is 61, the concentration rate of 70-140 mesh three sieves is 95%, the water content is 0.08%, the mud content is 0.1%, the ignition loss 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 particle size of 200 mesh accounts for 92%.

[0033] The moisture content of the pregelatinized starch is 11%, the pregelatinization degree is 89%, the passing rate of 100 mesh sieve 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 portion is 30 kg, and the amount of materials prepared in Examples 1 to 5 is shown in Table 1:

[0036] Table 1 Preparation ratio of each ingredient

[0037] High silica sand (parts) Sodium bentonite (parts) Industrial water (parts) Pregelatinized starch (parts) Example 1 87.6 8.7 2.7 1 Example 2 87.9 8.5 2.8 0.8 Example 3 88.7 8 2.5 0.8 Example 4 87.5 9 2.6 0.9 Example 5 87 9 3 1

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

[0039] The high silica sand is dry-mixed with sodium bentonite and pregelatinized starch in the sand mixer until the sodium bentonite and pregelatinized starch are coated 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 for testing of wet compressive strength, moisture, compactness 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 meets 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. The details are shown in Table 2:

[0043] Table 2 Moisture content adjustment ingredients list (using 3-ton sand mixer)

[0044]

[0045] The wet compressive strength is maintained in the range of 180-230 kPa. If it is lower than the lower limit, sodium bentonite is added according to the difference. If it is higher than the upper limit, it is scrapped until the relevant parameters meet the requirements. The details are shown in Table 3:

[0046] Table 3 Ingredients for adjusting wet compressive strength (using 3-ton sand mixer)

[0047]

[0048]

[0049] The total time for preparing the above moisture content and wet compressive strength shall not exceed 500 seconds, otherwise it will 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 embodiments

[0052] Moisture content (%) Compactness rate (%) Wet compressive strength(kPa) Example 1 2.55％ 24.2 208.8 Example 2 2.63％ 24.8 204.8 Example 3 2.43％ 23.0 192.7 Example 4 2.49％ 23.5 215.4 Example 5 2.88％ 27.6 216.1

[0053] After the adjustment is completed, the green sand is prepared after the mixing is continued for 120 to 150 seconds. After the preparation is completed, 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 ZGS 98-70 / 140 (61A). The green sand prepared in Examples 1 to 5 and the green sand in Example 6 are used for carbon steel valve casting. 30 pieces are cast in each example. The casting steps are as follows:

[0056] The green sand is injected into the cavity of the molding machine. The molding machine adopts a horizontal static pressure molding machine (Qingdao Xindong ACE-5 model). After compaction, the sand mold is formed by unpacking. 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 the 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] Average number of pores Average proportion of chemical sand bonding area (%) Inspection base number (pieces) Example 1 1.16(35 / 30) 0 30 Example 2 0.96(29 / 30) 0 30 Example 3 1.06(32 / 30) 0 30 Example 4 1.03(31 / 30) 0 30 Example 5 1.47(44 / 30) 0 30 Example 6 7.70(231 / 30) 8.4 30

[0062] Average number of pores: Calculate the total number of defective pores (one pore is considered as one if the pore diameter is larger than 0.2 mm) on the casting surface and machined surface of 30 castings, and then calculate the average 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: Pores of varying sizes with relatively smooth walls and in the shape of pears, circles, ellipses or needles are formed on the surface of steel castings due to the presence of gas in the molten metal. Main hazards: Porosity will reduce the air tightness of valve seals, making them prone to leakage and affecting the normal operation of the product.

[0066] Chemical sand bonding: a rough and shiny sand bonding layer is formed on the surface of metal castings. The formation of this defect is mainly due to the chemical reaction between metal oxides and sand materials. Main hazards: affecting the appearance quality, processing performance and use performance of castings.

[0067] It can be seen from the above test results that the present invention can significantly reduce defective pores in castings and avoid the occurrence of chemical sand sticking, so that 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 protection scope 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, 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, put 95% of the prepared amount of high silica sand into the sand mixer, and after the high silica sand is dispersed, put 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 the 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 to mix for 300 to 320 seconds; S105, take the mixed sand sample to test the wet compressive strength, moisture, compaction rate, and air permeability; the moisture content is maintained in the range of 2.4-3.0% and the compaction rate is maintained in the range of 33%-38%. If it is lower than the lower limit, industrial water is added according to the difference, and if it is higher than the upper limit, it is scrapped; the wet compressive strength is maintained in the range of 180-230 kPa. If it is lower than the lower limit, sodium bentonite is added according to the difference, and if it is higher than the upper limit, it is scrapped; 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, characterized in that: The high-silica sand has a SiO2 content of >99.2%, a Fe2O3 content of <0.05%, an AlO2 content of 0.2-0.5%, and other impurity components of <0.25%.

3. The method for preparing green sand according to claim 1, characterized in that: The high silica sand has a particle size AFS of 58-62, a 70-140 mesh three-sieve concentration rate of >92%, a water 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, characterized in that: The sodium bentonite has a montmorillonite content of >85%, a blue absorption of 34g / 100g (methylene blue method), 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 particles with a particle size of 200 meshes account for >90%.

5. The method for preparing green sand according to claim 1, characterized in that: 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 mPa·s>500, 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, characterized in that: 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, characterized in that: In step S105, the green sand is prepared and needs to be stored in a closed space when waiting for use.

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

9. The casting application method of green sand according to claims 1 to 8, comprising the following casting steps: S201, injecting the green sand into the molding machine cavity, unpacking and compacting 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 on the surface (special hardness tester for molding sand type B); S202, deoxidize the molten steel after smelting and tempering and pour it 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; S203, when the sand mold after casting cools to a surface temperature of less than 80 degrees, the casting is subjected to sand removal treatment by vibrating the 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.

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