A ductile cast iron lost foam casting coating and a preparation method thereof

Abstract

The application relates to the technical field of lost foam casting, and provides nodular cast iron lost foam casting paint and a preparation method thereof. The lost foam casting paint comprises quartz powder and a first modified component with a mass ratio of (85-95):(5-10), and a mixture mainly formed by mixing coke powder and a second modified component with a mass ratio of (2-8):(2-4) through ball milling; the first modified component mainly comprises calcined clay and spodumene powder with a mass ratio greater than 1; and the second modified component mainly comprises iron oxide powder and borax with a mass ratio of (2-4):1. The application solves the problems of high-temperature expansion and cracking of the quartz paint and migration of harmful elements of the carbonaceous material by generating a low-expansion stable phase on the surface of the quartz aggregate particle through innovative material composite design and further in-situ reactive encapsulation of the introduced carbonaceous material, so that a new type of paint with extremely low cost but reliable performance and metallurgical safety is obtained.

Description

Technical Field

[0001] This invention relates to the field of lost foam casting technology, and in particular to a method for preparing a lost foam casting coating for ductile iron parts. Background Technology

[0002] Quartz sand (powder) is the cheapest refractory aggregate in the foundry industry. When used as a main component of lost foam casting coatings in the preparation of ductile iron, quartz undergoes a β-α phase transformation at certain temperatures, accompanied by a rapid volume expansion. At even higher temperatures, it exhibits a high coefficient of thermal expansion. This unstable volume change, under the thermal shock of molten iron, easily leads to micro-cracks in the coating. The penetration of molten iron forms vein-like defects on the casting surface, severely affecting appearance and quality. Furthermore, under the influence of high-temperature molten iron, quartz reacts with FeO in the molten iron and other metallic elements introduced during spheroidization treatment, generating low-melting-point silicates, leading to chemical sand adhesion and interfering with the spheroidization effect on the casting surface to some extent.

[0003] In addition, to improve the permeability of lost foam casting coatings based on quartz sand (powder), directly mixing in coke powder and utilizing the rigid skeleton and accumulated pores of coke powder to form stable permeable channels is a feasible approach. However, the coke powder currently used contains high levels of harmful elements such as sulfur and phosphorus. These elements are very easy to migrate into ductile iron at high temperatures, causing a decline in the performance of cast iron parts. For example, sulfur will consume spheroidizing elements, resulting in poor spheroidization and spheroidization decay, leading to flake graphite and vermicular graphite, and significantly increasing the tendency for inclusions and subcutaneous porosity; while phosphorus forms a brittle phosphorus eutectic network, significantly reducing the impact toughness and elongation of castings.

[0004] Therefore, this invention is proposed. Summary of the Invention

[0005] This invention provides a lost foam casting coating for ductile iron and its preparation method, which solves the defects in the prior art, such as microcracks in the coating caused by thermal shock of molten iron, or even penetration into the molten iron to form veins on the surface of the casting, poor spheroidization, and spheroidization fading. It obtains a lost foam casting coating with high high-temperature strength and good air permeability, with quartz sand (powder) as the main aggregate, and thus prepares ductile iron castings with smooth surface and high spheroidization rate of the surface layer.

[0006] This invention provides a lost foam casting coating for ductile iron, comprising: quartz powder in a mass ratio of (85~95):(5~10) and a first modifying component;

[0007] The first modified component mainly consists of calcined clay and spodumene powder with a mass ratio greater than 1. Preferably, the mass ratio of the calcined clay and spodumene powder is greater than or equal to 1.5 and less than or equal to 9. As an example, it can be any value or a range of values ​​from 1.5, 3, 4.5, 6, 7.5, and 9.

[0008] In the preparation of ductile iron, studies have found that the structural changes of the coating under the high temperature of molten iron are mainly affected by the properties of quartz powder. Based on this, in the lost foam casting coating, in addition to using quartz powder as the main material, this invention also adds a first modifying component composed of calcined clay and spodumene powder, which is mainly used to improve the structural stability of the coating. The specific mechanism is as follows: under the influence of high temperature of molten iron, the Li2O produced by the partial decomposition of spodumene powder and the SiO2 on the surface of quartz powder particles can form a liquid phase to encapsulate the quartz powder particles. Furthermore, the Al2O3 in the calcined clay reacts in situ with it to generate β-spodumene (Li2O·Al2O3·4SiO2) with a thermal expansion coefficient that is significantly lower than that of quartz particles. More importantly, this in-situ reaction is carried out with part of the spodumene powder itself as the heterogeneous nucleation core of β-spodumene. The reaction speed is fast, and the low expansion phase formed also has the effect of stabilizing the relative position of quartz powder particles, thus significantly reducing the overall expansion coefficient of the coating.

[0009] Among these materials, the calcined clay used in this invention is more effective than other materials that can provide active Al2O3, such as corundum, activated alumina, and bauxite powder. This is because calcined clay contains both Al2O3 and SiO2 in a thermodynamically metastable state. On the one hand, compared with highly crystalline Al2O3 raw materials such as corundum, the Al2O3 in calcined clay is more likely to participate in the reaction. On the other hand, calcined clay can simultaneously provide some Al2O3 and SiO2, which is beneficial for the nucleation and growth of β-spodumene under the action of molten iron in a short time. If pure Al2O3 raw material is used, the reaction kinetics will be significantly slower, making it difficult to complete the full synthesis of β-spodumene in a short casting time.

[0010] According to the lost foam casting coating provided by the present invention, the quartz powder is mainly composed of 70-140 mesh quartz powder and 200-325 mesh quartz powder in a mass ratio of (50-55):(35-40). Studies have found that using 70-140 mesh quartz powder and 200-325 mesh quartz powder in this mass ratio can form a continuous particle size distribution system. The 70-140 mesh coarse particles act as skeletal particles, constructing the supporting structure of the coating and providing necessary air permeability channels; the 200-325 mesh fine particles fill the gaps between the coarse particles, increasing the bulk density and reducing porosity, thereby improving the density and strength of the coating while ensuring air permeability.

[0011] The lost foam casting coating provided by the present invention further includes a mixture mainly composed of coke powder and a second modified component mixed by ball milling in a mass ratio of (2~8):(2~4);

[0012] The second modified component is mainly composed of iron oxide powder and borax in a mass ratio of (2~4):1. As an example, the mass ratio of (2~4):1 can be any value or a range of values ​​among 2:1, 2.5:1, 3:1, 3.5:1, and 4:1.

[0013] As mentioned above, although coke powder can form a rigid framework to create pores and stable air channels, and coke has good thermal conductivity, which can uniformly distribute the temperature of the coating and reduce thermal stress, it also brings certain negative effects. This invention discovered that ball milling the aforementioned second modified component and coke powder can create a composite material with physical intercalation and chemical adsorption, thereby overcoming the shortcomings of coke powder in lost foam casting coatings. Specifically, when the lost foam casting coating contains the above mixture, under the high temperature of the molten iron, Fe2O3 in the second modified component can be reduced to FeO, and borax can be converted to B2O3. FeO, B2O3, and SiO2 in the coating can form a low-melting-point, highly wettable borosilicate glass liquid phase. Because this liquid phase is generated in situ on the surface of the coke powder, it preferentially wets and coats the coke powder particles, forming a continuous, dense glass-ceramic film shell with a thickness of approximately 0.5~2 μm on its surface. This "core-shell structure" ensures that the coke powder forms a rigid framework with porous structure, physically isolating the coke powder from direct contact with the molten iron to prevent abnormal carbon penetration, while also preserving the air-permeable channels formed by the accumulation of coke particles. Simultaneously, the Na2O released during the B2O3 formation process of borax can fix phosphorus, capturing trace amounts of phosphorus escaping from the coke and fixing them in the coating slag phase, thus eliminating the risk of phosphorus pollution at its source.

[0014] The coke powder used in general processes has a sulfur content of 0.5-1.0% and a phosphorus content of 0.02-0.03%.

[0015] In order to control sulfur and phosphorus pollution at the source, the present invention preferably provides that the sulfur content in the coke powder is greater than 0 and less than or equal to 0.08%, more preferably less than or equal to 0.05%. Preferably, the sulfur content in the coke powder is greater than 0 and less than or equal to 0.04%, more preferably less than or equal to 0.03%.

[0016] In the lost foam casting coating for ductile iron provided by the present invention, the mass ratio of quartz powder to coke powder is (85~95):(2~8). If the coke powder content is too high, it will lead to a decrease in coating strength, excessive gas generation, and overload of the encapsulant (i.e., the second modifying component), preventing it from effectively encapsulating the coke particles and increasing the risk of sulfur and phosphorus migration. If the coke powder content is too low, the permeability will not be sufficiently improved, the carbonaceous reduction barrier effect will be weak, and the improvement in thermal conductivity will be limited, resulting in increased subsurface porosity and sand adhesion tendency in the casting. Within this preferred range, the coke powder can fully utilize its functions of constructing permeable channels, forming a reduction barrier, and uniformizing the temperature field, working synergistically with the encapsulant to achieve dual protection of physical isolation and chemical curing, thus achieving optimal synergy between permeability, high-temperature strength, and metallurgical safety.

[0017] The lost foam casting coating for ductile iron provided by the present invention further includes a binder, a suspending agent, a carrier liquid, and other additives, wherein the other additives are mainly composed of water-reducing agents and / or defoamers.

[0018] According to the lost foam casting coating for ductile iron provided by the present invention, the binder is selected from sodium bentonite and / or silica sol;

[0019] Binders provide the coating with room temperature strength and high-temperature structural strength, ensuring firm adhesion of the coating to the sample surface and preventing peeling or cracking at high temperatures. Suspension agents improve the dispersion stability of powders in the system, prevent sedimentation and stratification of refractory aggregates, fillers, and other components, and improve the storage stability and coating uniformity of the coating. Carrier liquids disperse the various solid components and adjust the viscosity and flowability of the coating. Water-reducing agents lower the system viscosity, improve powder dispersibility, reduce the amount of carrier liquid required, and increase the solid content and density of the coating. Defoamers eliminate air bubbles generated during the preparation, stirring, and coating processes, preventing defects such as pinholes and pitting in the coating and ensuring a uniform and dense coating surface. The synergistic effect of these components gives the coating excellent performance, a good coating structure, and stable high-temperature performance.

[0020] Studies have found that using sodium-based bentonite as the binder exhibits superior coating performance compared to other binders (such as sodium alginate, carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), polyacrylamide (PAM), and sodium lignosulfonate), specifically resulting in lower sulfur content on the surface of the casting. This is because the naturally occurring calcite in sodium-based bentonite decomposes at high temperatures to generate active CaO, which reacts with sulfur released from coke powder, thereby fixing the sulfur in the coating slag phase.

[0021] In the lost foam casting coating for ductile iron provided by the present invention, the suspending agent is selected from attapulgite clay.

[0022] In the lost foam casting coating for ductile iron provided by the present invention, the carrier liquid is selected from water.

[0023] According to the lost foam casting coating for ductile iron provided by the present invention, the water-reducing agent is selected from polycarboxylate-based water-reducing agents. The polycarboxylate-based water-reducing agent has good compatibility with various cements and fillers, and has low chloride ion content and low alkali content.

[0024] In the ductile iron lost foam casting coating provided by the present invention, the defoamer is selected from organosilicon defoamers.

[0025] By using the above-mentioned preferred processing aids, the morphology of the mixture formed by ball milling coke powder and the second modified component in the coating can be basically preserved, ensuring the function of the aforementioned second modified component. Moreover, the carbonates in bentonite can decompose to form calcium oxide under the high temperature of molten iron. Combined with the active oxygen formed at high temperature in the system, the sulfur in the coke is solidified to generate calcium sulfide, thereby achieving sulfur fixation and further reducing pollution. Therefore, it is preferred.

[0026] The lost foam casting coating for ductile iron provided according to the present invention comprises, by weight parts:

[0027] 50-55 parts of 70-140 mesh quartz powder; as an example, it can be any value or a range of values ​​consisting of 50, 51, 52, 53, 54, and 55 parts.

[0028] 35-40 parts of quartz powder with a mesh size of 200-325; as an example, it can be any value or a range of values ​​consisting of 35, 36, 37, 38, 39, or 40 parts.

[0029] The first modified component is 5 to 10 parts; as an example, it can be any value or a range of values ​​consisting of 5, 6, 7, 8, 9, or 10 parts.

[0030] 2 to 8 parts of coke powder; as an example, it can be any value or a range of values ​​from 2, 3, 4, 5, 6, 7, to 8 parts.

[0031] The second modifying component is 2 to 4 parts; as an example, it can be any value of 2 parts, 3 parts, or 4 parts, or a range of values ​​composed of any values;

[0032] 1 to 2 parts of sodium-based bentonite; as an example, it can be any value or a range of values ​​from 1 part, 1.2 parts, 1.4 parts, 1.6 parts, 1.8 parts, and 2 parts.

[0033] 1 to 2 parts of attapulgite; as an example, it can be any value or a range of values ​​from 1 part, 1.2 parts, 1.4 parts, 1.6 parts, 1.8 parts, 2 parts;

[0034] 2 to 4 parts of silica sol; as an example, it can be any value of 2 parts, 3 parts, or 4 parts, or a range of values ​​composed of any values;

[0035] Water-reducing agent: 0.05~0.15 parts; as an example, it can be any value or a range of values ​​from 0.05 parts, 0.075 parts, 0.1 parts, 0.125 parts, and 0.15 parts, preferably 0.08~0.12 parts;

[0036] Defoamer: 0.05~0.12 parts; as an example, it can be any value or a range of any values ​​from 0.05 parts, 0.07 parts, 0.09 parts, 0.11 parts, 0.12 parts, preferably 0.08~0.10 parts;

[0037] An appropriate amount of water is added to adjust the specific gravity of the lost foam casting coating for ductile iron to 1.50~1.65 g / cm³. 3 .

[0038] The present invention also provides a method for preparing the lost foam casting coating for ductile iron, comprising: mixing quartz powder and a first modifying component in a mass ratio of (85~95):(5~10) and then mixing with other raw materials;

[0039] The first modified component mainly consists of calcined clay and spodumene powder in a mass ratio greater than 1. The preparation method of this invention is simple and efficient, with easily controllable conditions and low cost. It exhibits good suspension stability in a water-based system, excellent coating performance, and is suitable for large-scale automated production.

[0040] The present invention also provides the application of the above-described lost foam casting coating for ductile iron in the casting of ductile iron parts.

[0041] According to the application provided by the present invention, the ductile iron part includes a coating formed by the lost foam casting coating of the ductile iron, the coating having a bending strength greater than or equal to 10 MPa at 1300°C;

[0042] The surface vein rating of the ductile iron casting is Grade 1, the subcutaneous porosity is less than or equal to 1%, the sulfur content on the casting surface is less than or equal to 0.02%, and the spheroidization rate on the casting surface is less than or equal to Grade 2.

[0043] This invention provides a lost foam casting coating for ductile iron and its preparation method. Through innovative material composite design, a low-expansion stable phase is generated on the surface of quartz aggregate particles. Furthermore, the introduced carbonaceous material is in-situ reactively encapsulated, thereby solving the problems of high-temperature expansion and cracking of quartz coatings and the migration of harmful elements from carbonaceous materials. This results in a novel coating that is extremely low in cost, reliable in performance, and metallurgically safe. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of this invention clearer, a clear and complete description will be provided below in conjunction with the technical solutions of this invention. Obviously, the described embodiments are only a part of the embodiments of this invention, not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0045] Where specific techniques or conditions are not specified in the examples, they shall be performed in accordance with the techniques or conditions described in the literature in this field, or in accordance with the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased through legitimate channels. The sources of some raw materials are as follows:

[0046] 70~140 mesh quartz powder: The casting quartz powder produced by Handan Yuexin is selected. The specifications are 70~140 mesh, SiO2 content ≥99.0%, Fe2O3 ≤0.03%, and melting point is about 1710℃.

[0047] 200~325 mesh quartz powder: The casting quartz powder produced by Handan Yuexin is selected. The specifications are 200~325 mesh, SiO2 content ≥99.0%, Fe2O3 ≤0.03%, and melting point is about 1710℃.

[0048] Calcined clay: Ceramic-grade high-temperature calcined red terracotta / refractory clay is selected, with a refractoriness ≥1580℃. After high-temperature calcination, it exhibits excellent thermal stability and reactivity. The calcined clay contains approximately 35-45% Al2O3 and 50-60% SiO2, and is in a thermodynamically metastable state, which is conducive to its participation in the synthesis reaction of β-spodumene at high temperatures.

[0049] Lithium spodumene powder: The selected lithium spodumene powder has a size of 200~320 mesh, a LiO2 content of ≥1.2~1.8%, and an Fe2O3 content of ≤0.30%. It has an extremely low coefficient of thermal expansion and good fluxing properties.

[0050] Coke powder A (ultra-low impurity type): 150 mesh, sulfur content 0.045%, phosphorus content 0.022%, fixed carbon ≥90%. The sulfur and phosphorus content of this coke powder has been reduced to near the background level of ductile iron molten iron, thus controlling the total pollution at the source to an extremely low level.

[0051] Coke powder B (conventional type): 150 mesh, sulfur content 0.5~1.0%, phosphorus content 0.02~0.03%, fixed carbon ≥85%. This coke powder is commonly used in the foundry industry, with a high sulfur content, and complies with the national standard GB / T 1996-2017 "Metallurgical Coke".

[0052] Borax: Pharmaceutical-grade borax (Na2B4O7·10H2O) with a content of 99.0~103.0% and a pH value of 9.0~9.6 is selected. It is a colorless and transparent crystalline powder. Industrial-grade decahydrate borax with a purity of ≥95% can also be used.

[0053] Sodium-based bentonite: The selected sodium-based bentonite is powdered, with a mesh size of 200, a montmorillonite content of ≥85%, an expansion ratio of ≥25mL / g, and a gel value of ≥99%. Other sodium-based bentonite products conforming to GB / T 20973-2020 "Bentonite" standard may also be selected.

[0054] Attapulgite soil: Attapulgite soil of 200 mesh is selected.

[0055] Silica sol: The selected silica sol is model S-1430, with a solid content of 30%, a particle size of 10~20nm, a pH value of 9~10, and a molecular formula of mSiO2·nH2O. It is a semi-transparent colloidal solution. Other alkaline silica sol products conforming to the HG / T 2521-2008 "Industrial Silica Sol" standard may also be selected.

[0056] Water-reducing agent: The selected polycarboxylate-based high-performance water-reducing agent has a solid content ≥18%, a water reduction rate ≥25%, and a pH value of 6±2.

[0057] Example 1

[0058] This embodiment provides a lost foam casting coating for ductile iron, the formulation of which, by weight, includes:

[0059] 70-140 mesh quartz powder: 52 parts;

[0060] 200-325 mesh quartz powder: 38 parts;

[0061] First modifying component (calcined clay: spodumene powder = 8:2): 7 parts;

[0062] Coke powder A: 5 parts;

[0063] Second modifying component (Fe2O3:borax = 3:1): 2.5 parts;

[0064] Sodium-based bentonite: 1.5 parts;

[0065] Attapulgite: 1.2 parts;

[0066] Silica sol: 3.0 parts;

[0067] Water-reducing agent: 0.14 parts;

[0068] Defoamer: 0.12 parts;

[0069] Water: as needed.

[0070] This embodiment also provides the following method for preparing the above-mentioned lost foam casting coating for ductile iron:

[0071] (1) Add 52 parts of 70~140 mesh quartz powder, 38 parts of 200~325 mesh quartz powder and 7 parts of the first modified component (calcined clay: spodumene powder = 8:2) into a double helix cone mixer, set the speed to 300 rpm and the mixing time to 20 minutes, to ensure that the first modified component is evenly attached to the surface of the quartz powder particles, and obtain pretreated aggregate.

[0072] (2) Add 5 parts of coke powder A and 2.5 parts of the second modified component (Fe2O3:borax=3:1) into a ball mill and mix them at 400 rpm for 30 minutes to obtain a pre-packaged mixture.

[0073] (3) Add the pretreated aggregate obtained in step (1), the mixture obtained in step (2), sodium bentonite, and attapulgite into a dry mixer and dry mix at 250 rpm for 8 minutes to obtain a mixed powder.

[0074] (4) Add the water-reducing agent of the formula to an appropriate amount of water, stir to dissolve, and then add the mixed powder obtained in step (3). Disperse at 1000 rpm for 25 minutes to ensure that all components are fully dispersed and the agglomerates are opened. Further, adjust the stirring speed to 500 rpm, add the silica sol and defoamer of the formula, and continue stirring for 12 minutes. Check the specific gravity of the slurry and adjust it with water to 1.58 ± 0.02 g / cm³. 3 This yields the lost foam casting coating for ductile iron.

[0075] (5) Let the coating obtained in step (4) stand for 12 to 24 hours to mature, filter it with an 80 to 100 mesh sieve to remove possible agglomerates or undispersed matter, and obtain the finished coating.

[0076] Examples 2-4

[0077] The difference from Example 1 lies in the formulation of the lost foam casting coating for ductile iron, as shown in Table 1 below.

[0078] Table 1

[0079]

[0080] Examples 5-6, Comparative Example 1

[0081] The difference from Example 1 is that the mass ratios of calcined clay and spodumene powder in the first modified component are shown in Table 2 below.

[0082] Table 2

[0083]

[0084] Comparative Example 2

[0085] The difference from Example 1 is that the first modified component does not contain spodumene powder, that is, calcined clay is directly added as the first modified component.

[0086] Examples 7-10

[0087] The difference from Example 1 is that the mass ratios of iron oxide and borax in the second modified component are shown in Table 3 below.

[0088] Table 3

[0089]

[0090] Example 11

[0091] The difference from Example 1 is that coke powder A is replaced by coke powder B by mass.

[0092] Example 12

[0093] The difference from Example 11 is that sodium bentonite is replaced by sodium alginate, and the sodium alginate is added after the hydration material in the raw materials.

[0094] Comparative Example 3

[0095] The difference from Example 1 is that it does not contain the first and second modified components, and its formulation is as follows:

[0096] 70-140 mesh quartz powder: 52 parts;

[0097] 200-325 mesh quartz powder: 38 parts;

[0098] Coke powder B: 5 parts;

[0099] Sodium-based bentonite: 1.5 parts;

[0100] Attapulgite: 1.2 parts;

[0101] Silica sol: 3.0 parts;

[0102] Water-reducing agent: 0.1 parts (polycarboxylate-based water-reducing agent, same as in Example 1);

[0103] Defoamer: 0.08 parts (organic silicone defoamer, same as in Example 1);

[0104] Water: to a specific gravity of 1.58 g / cm³ for lost foam casting coating. 3 .

[0105] Comparative Example 4

[0106] A commercial zircon powder-based intermediate lost foam coating uses CTL-904 zircon powder coating (the main refractory material is zircon powder, with a density of 1.75~2.00 g / cm³) produced by a certain company. 3 (Suspension ≥98%, suitable for cast steel parts) should be prepared and used according to the product instructions.

[0107] Test case

[0108] This test example provides a method for producing a medium-sized ductile iron gearbox housing (material QT500-7, single weight 75KG, main wall thickness 25mm). In this method, the lost foam casting coating of the example and comparative example is used to form a coating on the lost foam pattern, with 3 coats applied, the coating thickness being 1.2~1.5mm, and each coating layer is fully dried; the negative pressure is 0.04~0.05MPa, and the casting temperature is 1450~1500℃.

[0109] (1) Cost of paint per unit

[0110] Using the coating cost of Example 1 as a baseline of 1.0, the costs of other examples and comparative examples were calculated as a ratio to Example 1. The cost composition mainly includes the raw material costs of quartz powder, coke powder, modifying components, binders, suspending agents, and various additives. Referring to data from the lost foam casting industry, the current cost of commercial coatings accounts for approximately 100 yuan per ton of casting cost; therefore, the cost of self-made coatings is significantly lower than that of purchased commercial coatings.

[0111] (2) The rating method for surface veins of castings is as follows: After cleaning the castings, visually observe the surface of the castings under a standard light source, focusing on stress concentration areas such as hot spots, corners, and edges. Measure the vein width with vernier calipers, record the maximum vein size, and rate the level according to the rating standard. The rating standard is shown in Table 4 below.

[0112] Table 4

[0113]

[0114] (3) Subcutaneous porosity, the test method is as follows: Five representative areas are randomly selected on the casting body (non-machined surface), each area is 50mm×50mm in size. Subcutaneous pores (within 3mm of the casting surface) are detected by X-ray flaw detection or ultrasonic flaw detection. The total area of ​​pores in each area is counted and the average value is calculated. The specific formula is: Subcutaneous porosity (%) = (total area of ​​pores / total area detected) × 100%.

[0115] (4) [S] content on the surface of the casting, test method (refer to the standard for sulfur migration detection at the sand mold interface): take samples within a depth of 0-1 mm below the surface of the casting, and perform elemental analysis using electron probe microanalysis (EPMA) or scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) to detect the sulfur content. The results are expressed as mass percentage (wt%). At least 3 points should be tested for each sample, and the average value should be taken.

[0116] (5) Nodularity rating of casting surface, test method (according to GB / T 9441-2021-7): A metallographic sample of approximately 15mm × 15mm is cut from the casting body. After grinding and polishing, it is observed under an optical microscope at 100x magnification. Five representative fields of view are selected and compared with the standard rating chart. The nodularity is rated according to the highest level. The rating standard (according to GB / T 9441-2021 "Metallographic Inspection of Ductile Iron") is shown in Table 5 below.

[0117] Table 5

[0118]

[0119] (6) Test method for high-temperature flexural strength of lost foam coating:

[0120] Instruments and tools: paint mixer, standard sample mold, electric heating blast constant temperature drying oven, 1700℃ box-type high temperature electric furnace (NBD-M1700-22TI box furnace), XQY-Ⅱ intelligent sand strength tester, vernier caliper, long-handled pliers, high temperature resistant 77 ceramic boat or 60×30 square boat.

[0121] The testing steps are as follows:

[0122] (6.1) Add water to the lost foam coating to be tested and stir. The amount of water added should be such that the Baume degree of the paste coating is 70±1. The stirring speed should not exceed 1000 rpm. After stopping the addition of water, stir at 500 rpm for 20 minutes to obtain the paste coating for later use. Stir the paste coating for 5 minutes before use. After use, the paint bucket should be tightly covered with a plastic cover.

[0123] (6.2) Preparation of standard sample molds and base plates:

[0124] a. The test standard sample mold is free from deformation, the parting surface fits tightly, the cavity surface is flat without protrusions, and plastic tape is applied to prevent sticking.

[0125] b. Apply plastic tape evenly, smoothly, and tightly to the flat surface of the base plate;

[0126] c. Place the standard sample mold firmly onto the base plate using plastic tape and secure it with tape.

[0127] (6.3) Coating and drying:

[0128] a. Spread the paste-like coating evenly into the three cavities of the mold, with the first layer of coating being approximately 3mm thick. Place the mold at 50℃ and dry for 3 hours. If the sample has cracks after drying, touch up with the paste-like coating and then dry again.

[0129] b. After the sample is dried without cracks, apply a second coat of paint approximately 3 mm thick. Let it stand at room temperature for 10-15 minutes, then dry at 50°C for 3 hours. If cracks appear on the sample after drying, apply a paste-like paint to repair them and then dry again.

[0130] c. After the sample is dried without cracks, apply a third layer of coating slightly higher than the top plane of the mold, place it at room temperature for 10-15 minutes, and then dry it at 50℃ for 3 hours.

[0131] d. After the sample is dried, flatten the upper surface of the sample with a scraper, turn it over to check the bottom surface, repair any defects, and demold after drying at 50°C for 1 hour.

[0132] e. Test sample dimensions: width 22.36±0.2mm, thickness 11.18±0.2mm, length 70mm, no cracks.

[0133] Dry at 50℃ for more than 8 hours before testing. Samples not to be tested immediately should be stored in a desiccator until testing. Each group consists of three samples.

[0134] (6.4) Place two of the dried samples on their sides on a ceramic boat that is placed flat on the bottom plate of the high-temperature furnace (to facilitate heating of the lower side of the sample), and close the furnace door after adding a door plug.

[0135] (6.5) Set the heating curve of NBD-M1700-22TI box furnace, start heating from room temperature, increase the temperature by 10℃ per minute, and hold at 1300℃ for 1 hour.

[0136] (6.6) Start the XQY-Ⅱ intelligent sand strength tester 10 minutes before the end of the heat preservation time, check and confirm that the tester is normal and test the room temperature bending strength of one sample.

[0137] (6.7) After the heat preservation time ends, immediately open the furnace door, use long-handled pliers to remove one of the test blocks, close the furnace door, and quickly place the red-hot sample on the support to begin testing. The time from opening the furnace door to removing the sample to the end of the test should not exceed 30 seconds; otherwise, it will be considered invalid. Test the other test block in the same way. The average of the two test values ​​is the high-temperature flexural strength value of the coating.

[0138] The test results for the above test items are shown in Table 6 below.

[0139] Table 6

[0140]

[0141] The data above shows that:

[0142] Taking Example 1 as an example, its unit coating cost is only about 1 / 3 of that of commercial zircon powder coating (Comparative Example 4), while its performance fully meets or even exceeds the level of this mid-range coating. Specifically, the castings corresponding to the coatings prepared in the examples all achieved a vein texture rating of 1 to 2, which is far superior to traditional quartz coatings (Comparative Example 3 is 4). In particular, Examples 1 to 10 all achieved a rating of 1, which is comparable to zircon powder coating (rated 1), indicating that the present invention effectively solves the vein texture defect of quartz coatings.

[0143] The sulfur content on the casting surface corresponding to the coatings prepared in Examples 1-10 is ≤0.018%, which is far lower than that of traditional quartz coatings and close to that of zircon powder coatings. In Example 12, sodium alginate was used as an organic polymer binder, and it was found that it completely decomposed and volatilized at high temperature, resulting in a significant increase in the sulfur content on the casting surface. The sulfur content in Example 11 (conventional coke powder) was 0.022%, indicating that the second modified component of this invention is also effective for conventional coke. The spheroidization rate of Examples 1-10 is all grade 2, comparable to that of zircon powder coatings, while traditional quartz coatings show poor local spheroidization (grade 4), proving that this invention effectively prevents the consumption of spheroidizing elements by sulfur.

[0144] The high-temperature bending strength of the castings corresponding to the coatings prepared in Examples 1-10 is above 10 MPa, which is higher than that of traditional quartz coatings and close to that of zircon powder coatings.

[0145] The subcutaneous porosity of the castings corresponding to the coatings prepared in Examples 1-10 is less than or equal to 0.5%, which indicates that high strength and high air permeability are achieved.

[0146] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A coating for lost foam casting of ductile iron, characterized in that, include: Quartz powder and the first modifying component in a mass ratio of (85~95):(5~10); And a mixture mainly composed of coke powder and a second modified component in a mass ratio of (2~8):(2~4) by ball milling; The first modified component mainly consists of calcined clay and spodumene powder with a mass ratio greater than 1; The second modified component mainly consists of iron oxide powder and borax in a mass ratio of (2~4):1; The quartz powder is mainly composed of 70-140 mesh quartz powder and 200-325 mesh quartz powder in a mass ratio of (50-55):(35-40); The mass ratio of the quartz powder to the coke powder is (85~95):(2~8); the sulfur content in the coke powder is greater than 0 and less than or equal to 0.08%.

2. The lost foam casting coating for ductile iron according to claim 1, characterized in that, It also includes binders, suspending agents, carriers and other additives, which are mainly composed of water-reducing agents and / or defoamers.

3. The lost foam casting coating for ductile iron according to claim 2, characterized in that, The binder is selected from sodium bentonite and / or silica sol; The suspending agent is selected from attapulgite clay; The carrier fluid is selected from water; The water-reducing agent is selected from polycarboxylate-based water-reducing agents; The defoamer is selected from silicone defoamers.

4. The lost foam casting coating for ductile iron according to any one of claims 1 to 3, characterized in that, By weight, including: 50-55 parts of 70-140 mesh quartz powder; 35-40 parts of 200-325 mesh quartz powder; 5-10 parts of the first modified component; 2-8 parts coke powder; 2-4 parts of the second modified component; 1-2 parts of sodium bentonite; 1-2 parts of attapulgite; 2-4 parts silica sol; Water-reducing agent: 0.05~0.15 parts; Defoamer: 0.05~0.12 parts; An appropriate amount of water is added to adjust the specific gravity of the lost foam casting coating for ductile iron to 1.50~1.65 g / cm³. 3 .

5. The method for preparing the lost foam casting coating for ductile iron according to any one of claims 1 to 4, characterized in that, include: Quartz powder with a mass ratio of (85~95):(5~10) and the first modified component are mixed and then mixed with other raw materials.

6. The application of the lost foam casting coating for ductile iron as described in any one of claims 1 to 4 in the casting of ductile iron parts.

7. The application according to claim 6, characterized in that, The ductile iron part includes a coating formed by the lost foam casting coating of the ductile iron, and the coating has a bending strength of greater than or equal to 10 MPa at 1300°C. The surface vein rating of the ductile iron casting is Grade 1, the subcutaneous porosity is less than or equal to 1%, the sulfur content on the casting surface is less than or equal to 0.02%, and the spheroidization rate on the casting surface is less than or equal to Grade 2.

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