High-density high-pressure-resistant nodular cast iron gate valve casting process

Abstract

The invention relates to the field of gate valve casting, in particular to a high-density high-pressure-resistant nodular cast iron gate valve casting technology. The raw materials are selected from 40-60 parts of Q10 high-purity pig iron, 30-40 parts of low-carbon waste steel, 25-35 parts of ductile iron foundry returns, 1.5-2.5 parts of a calcium carbide desulfurizer, 0.5-1 part of cryolite powder, 2-3 parts of magnesium-rare earth-calcium alloy wires, 0.5-1 part of a barium-containing ferrosilicon alloy, 0.3-0.5 part of a strontium-containing ferrosilicon alloy and 0.2-0.4 part of a zirconium-containing ferrosilicon alloy. According to the method, the nano particles are added to enable the graphite nodules to be finer and denser, liquid nitrogen is used for cooling and eliminating internal holes, then pouring is conducted in a vacuum environment to avoid bubbles, then subzero treatment is conducted to enhance the hardness of the material, and finally laser is used for cladding the superhard alloy layer on the sealing face, so that the strength, compactness and temperature resistance limit of the nodular cast iron can be greatly enhanced; and the use function is improved.

Description

TECHNICAL FIELD

[0001] The application relates to the casting technology field of gate valves, in particular to a high-density high-pressure-resistant nodular cast iron gate valve casting process. BACKGROUND

[0002] The nodular cast iron gate valve is a gate valve with nodular cast iron as the main pressure-bearing component material, and the core feature is that the graphite form in the cast iron is changed from flaky to spherical through spheroidizing treatment process, so that the strength, toughness and pressure resistance of the material are significantly improved, and the nodular cast iron gate valve is suitable for fluid cutting control under medium and high pressure working conditions.

[0003] Under the background of the traditional nodular cast iron gate valve casting process, a high-density high-pressure-resistant nodular cast iron gate valve cannot be cast. In the process of using the traditional nodular cast iron gate valve, the graphite form is out of control, the micro shrinkage is difficult to eliminate, and the stability of the pearlite is insufficient, which affects the use function. Therefore, a high-density high-pressure-resistant nodular cast iron gate valve casting process is needed. SUMMARY

[0004] The purpose of the present application is to solve the problems existing in the prior art, and a high-density high-pressure-resistant nodular cast iron gate valve casting process is provided.

[0005] In order to achieve the above purpose, the application adopts the following technical scheme:

[0006] The high-density high-pressure-resistant nodular cast iron gate valve casting process comprises the following steps:

[0007] S1: raw material preparation;

[0008] A: 40-60 parts of Q10 high-purity pig iron, 30-40 parts of low-carbon scrap steel, 25-35 parts of nodular cast iron return material, 1.5-2.5 parts of calcium carbide desulfurizer, 0.5-1 part of cryolite powder, 2-3 parts of magnesium-rare earth-calcium alloy wire, 0.5-1 part of barium-containing silicon iron alloy, 0.3-0.5 part of strontium-containing silicon iron alloy, and 0.2-0.4 part of zirconium-containing silicon iron alloy are selected;

[0009] B: The Q10 high-purity pig iron, low-carbon scrap steel and nodular cast iron return material are subjected to shot blasting treatment to remove surface oxides;

[0010] C: The Q10 high-purity pig iron, low-carbon scrap steel and nodular cast iron return material are placed in a 200 DEG C oven for drying for 2h to remove moisture;

[0011] S2: melting control;

[0012] A: The Q10 high-purity pig iron, low-carbon scrap steel and nodular cast iron return material are added into a medium-frequency induction furnace for melting;

[0013] B: When the molten iron reaches 1450-1550℃, inject calcium carbide desulfurizer for deep desulfurization;

[0014] C: Use spectrometer to detect and ensure that the sulfur content is ≤0.010%;

[0015] S3: Spheroidizing treatment;

[0016] A: Transfer the desulfurized molten iron into a special treatment package;

[0017] B: Cover the liquid surface with nodular iron chips and sprinkle cryolite powder to prevent oxidation;

[0018] C: Send magnesium-rare earth-calcium alloy wire through the wire feeder, immediately remove the slag twice after the reaction is complete, and detect that the magnesium residual amount is between 0.045-0.055%;

[0019] S4: Four-stage inoculation treatment;

[0020] A: Pre-embed barium-containing ferrosilicon alloy at the bottom of the treatment package, and add strontium-containing ferrosilicon alloy in the runner during pouring;

[0021] B: Place zirconium-containing ferrosilicon alloy in the pouring cup, and finally set up bismuth-containing inoculation block in the sprue;

[0022] S5: Molding;

[0023] A: First use resin sand molding;

[0024] B: Then inlay zirconia ceramic preform at the sealing surface and flange part;

[0025] C: Finally, spray three layers of gradient coating;

[0026] S6: Pouring;

[0027] A: First move the mold into the vacuum chamber and evacuate the vacuum chamber to 10-2P a ;

[0028] B: Start the pouring system, the bottom iron flows through a 10kA electromagnetic field purification, and simultaneously apply 50Hz mechanical vibration, and finally complete pouring at 1200-1300℃;

[0029] S7: Solidification control;

[0030] A: First pre-embed liquid nitrogen micro-channel chill in the hot spot area;

[0031] B: Then start the directional solidification device, and finally pour high-temperature molten iron in the riser and cover with insulating agent;

[0032] S8: Knockout cleaning;

[0033] A: First open the box after the casting cools to the specified temperature;

[0034] B: rough polishing with cast steel shot first, and then finishing the surface with stainless steel shot;

[0035] S9: heat treatment strengthening;

[0036] A: normalizing strengthening is performed first;

[0037] B: three-stage deep cooling cycle is performed again;

[0038] C: laser cladding WC-Co alloy layer is performed on the sealing surface finally;

[0039] S10: packaging after quality detection.

[0040] Preferably, the titanium content in the Q10 high-purity pig iron in the step S1-A is ≤0.03%, and the sulfur content in the low-carbon scrap steel is ≤0.02%.

[0041] Preferably, the melting furnace temperature of the intermediate frequency induction furnace in the step S2-A is 1500-1600℃, and the melting time is 0.5-1h.

[0042] Preferably, the wire feeding speed in the step S3-C is 28m / min, and the reaction time is 70-80s.

[0043] Preferably, the inner layer of the gradient coating in the step S5-C is nano yttrium oxide, the middle layer is boron carbide + silicon nitride, and the outer layer is graphene zirconium powder.

[0044] Preferably, the temperature of the heating zone in the directional solidification device in the step S7-B is 1200℃, the temperature of the liquid nitrogen cold trap is -196℃, and the casting moves at 0.8mm / min.

[0045] Preferably, the specified temperature in the step S8-A is ≤400-450℃.

[0046] Preferably, the cast steel shot in the step S8-B is Φ1.0mm, and the stainless steel shot is Φ0.3mm.

[0047] Preferably, the three-stage deep cooling cycle in the step S9-B includes soaking in -196℃ liquid nitrogen for 2h first, then standing at room temperature for 1h, and repeating 3 times.

[0048] The beneficial effects of the present application are that: by adding nanoparticles, the graphite ball is made finer and denser, then liquid nitrogen is used to cool and eliminate internal pores, then pouring in a vacuum environment to avoid bubbles, then deep cooling treatment is used to enhance the hardness of the material, and finally laser cladding is used on the sealing surface to form a superhard alloy layer, so that the strength, density and temperature resistance limit of the ductile iron can be greatly enhanced, and the use function is improved. DETAILED DESCRIPTION

[0049] Example 1

[0050] The casting process of the high-density high-pressure-resistant nodular cast iron gate valve comprises the following steps:

[0051] S1: raw material preparation;

[0052] A: 40 parts of Q10 high-purity pig iron, 30 parts of low-carbon scrap steel, 25 parts of nodular cast iron return material, 1.5 parts of calcium carbide desulfurizer, 0.5 parts of cryolite powder, 2 parts of magnesium-rare earth-calcium alloy wire, 0.5 parts of barium-containing ferrosilicon alloy, 0.3 parts of strontium-containing ferrosilicon alloy, and 0.2 parts of zirconium-containing ferrosilicon alloy are selected;

[0053] B: The Q10 high-purity pig iron, low-carbon scrap steel, and nodular cast iron return material are subjected to shot blasting treatment to remove surface oxides;

[0054] C: The Q10 high-purity pig iron, low-carbon scrap steel, and nodular cast iron return material are placed in a 200°C oven for 2h to remove moisture;

[0055] S2: melting control;

[0056] A: The Q10 high-purity pig iron, low-carbon scrap steel, and nodular cast iron return material are added to a medium-frequency induction furnace for melting;

[0057] B: When the molten iron reaches 1450°C, the calcium carbide desulfurizer is sprayed to deeply desulfurize;

[0058] C: The spectrometer is used for detection to ensure that the sulfur content is ≤0.010%;

[0059] S3: nodularizing treatment;

[0060] A: The desulfurized molten iron is transferred to a special treatment package;

[0061] B: The liquid surface is covered with nodular cast iron chips, and cryolite powder is scattered to prevent oxidation;

[0062] C: The magnesium-rare earth-calcium alloy wire is fed through the wire feeder, and the slag is immediately removed twice after the reaction is completed, and the magnesium residual amount is detected to be between 0.045%;

[0063] S4: four-stage inoculation treatment;

[0064] A: The barium-containing ferrosilicon alloy is pre-embedded at the bottom of the treatment package, and the strontium-containing ferrosilicon alloy is added to the runner during pouring;

[0065] B: The zirconium-containing ferrosilicon alloy is placed in the sprue cup, and finally the bismuth-containing inoculation block is set in the runner;

[0066] S5: mold making;

[0067] A: First, resin sand molding is used;

[0068] B: Then insert zirconia ceramic preform at sealing surface and flange part;

[0069] C: Finally, spray three layers of gradient coating;

[0070] S6: Pouring;

[0071] A: First, move the mold into the vacuum chamber, and then vacuum the vacuum chamber to 10-2P a ;

[0072] B: Start the pouring system, and then the bottom of the pouring system flows through the 10kA electromagnetic field purification, and synchronously applies 50Hz mechanical vibration, and finally completes pouring at 1200℃;

[0073] S7: Solidification control;

[0074] A: First, embed the liquid nitrogen micro-channel cold iron in the hot spot area;

[0075] B: Then start the directional solidification device, and finally pour the high-temperature molten iron in the riser and cover the insulating agent;

[0076] S8: Knockout cleaning;

[0077] A: First, open the box after the casting cools to the specified temperature;

[0078] B: First, use cast steel shot for rough polishing, and then use stainless steel shot for surface finishing;

[0079] S9: Heat treatment strengthening;

[0080] A: First, perform normalizing strengthening;

[0081] B: Then perform three-stage deep cooling cycle;

[0082] C: Finally, perform laser cladding WC-Co alloy layer on the sealing surface;

[0083] S10: After detecting the quality, package.

[0084] In step S1-A, the titanium content in Q10 high-purity pig iron is ≤0.03%, and the sulfur content in low-carbon scrap steel is ≤0.02%. By step S1, impurities can be controlled from the source, reducing the risk of pores and inclusions.

[0085] In step S2-A, the melting furnace temperature of the intermediate frequency induction furnace is 1500℃, and the melting time is 0.5h. Low-sulfur molten iron improves spheroidization stability, and copper-tin alloy can enhance pearlite strength.

[0086] In step S3-C, the wire feeding speed is 28m / min, and the reaction time is 70s. By wire feeding method, the spheroidization rate can be improved, and rare earth can neutralize the anti-spheroidization elements.

[0087] Wherein, the graphite ball density can be improved by step S4, doubled to 300 / mm 2 And the segregation can also be eliminated by magnetic field.

[0088] Wherein, the inner layer of the gradient coating in step S5-C is nano yttrium oxide, the middle layer is boron carbide + silicon nitride, and the outer layer is graphene zirconium powder. The ceramic part can accelerate cooling, and the gradient coating can effectively block sand sticking and improve surface precision.

[0089] Wherein, the heating zone temperature in the directional solidification device in step S7-B is 1200℃, the liquid nitrogen cold trap temperature is -196℃, and the casting moves at 0.8mm / min. This can directional solidify the grains and refine them.

[0090] Wherein, the specified temperature in step S8-A is ≤400℃. The cast steel shot in step S8-B is Φ1.0mm, and the stainless steel shot is Φ0.3mm. Low-temperature unpacking can effectively reduce stress, and double-layer shot blasting can improve cleanliness.

[0091] Wherein, the three-stage deep cooling cycle in step S9-B includes first soaking in -196℃ liquid nitrogen for 2h, then standing at room temperature for 1h, repeating 3 times. The residual austenite is converted by deep cooling to improve its wear resistance.

[0092] Example 2:

[0093] The casting process of high-density high-pressure nodular cast iron gate valve includes the following steps:

[0094] S1: raw material preparation;

[0095] A: select 50 parts of Q10 high-purity pig iron, 35 parts of low-carbon scrap steel, 30 parts of nodular iron return material, 2 parts of calcium carbide desulfurizer, 0.75 parts of cryolite powder, 2.5 parts of magnesium-rare earth-calcium alloy wire, 0.75 parts of barium-containing silicon iron alloy, 0.4 parts of strontium-containing silicon iron alloy, and 0.3 parts of zirconium-containing silicon iron alloy;

[0096] B: shot blasting treatment is performed on Q10 high-purity pig iron, low-carbon scrap steel and nodular iron return material to remove surface oxides;

[0097] C: Q10 high-purity pig iron, low-carbon scrap steel and nodular iron return material are placed in a 200℃ oven for 2h to remove moisture;

[0098] S2: melting control;

[0099] A: Q10 high-purity pig iron, low-carbon scrap steel and nodular iron return material are added to a medium-frequency induction furnace for melting;

[0100] B: when the molten iron reaches 1500℃, inject calcium carbide desulfurizer for deep desulfurization;

[0101] C: Use of a spectrometer to ensure that the sulfur content is < 0.010%;

[0102] S3: Spheroidizing treatment;

[0103] A: Transfer of the desulfurized molten iron into a special treatment ladle;

[0104] B: Covering of the liquid surface with nodular iron chips and sprinkling of cryolite powder to prevent oxidation;

[0105] C: Feeding of magnesium-rare earth-calcium alloy wire through a wire feeder, immediate slagging twice at the end of the reaction, and detection of magnesium residue between 0.05%;

[0106] S4: Four-stage inoculation treatment;

[0107] A: Embedding of barium-containing ferrosilicon alloy at the bottom of the treatment ladle and adding strontium-containing ferrosilicon alloy in the runner during pouring;

[0108] B: Placing of zirconium-containing ferrosilicon alloy in the pouring cup and finally setting bismuth-containing inoculation blocks in the runner;

[0109] S5: Mould making;

[0110] A: First use of resin sand moulding;

[0111] B: Subsequently inlaying of zirconia ceramic preforms at the sealing surface and flange parts;

[0112] C: Finally spraying of three layers of gradient coating;

[0113] S6: Pouring;

[0114] A: First moving of the mould into a vacuum chamber and evacuation of the vacuum chamber to 10-2P a ;

[0115] B: Starting of the pouring system, purifying of the bottom poured molten iron by flowing through a 10 kA electromagnetic field, and simultaneous application of 50 Hz mechanical vibration, and finally completing the pouring at 1250°C;

[0116] S7: Solidification control;

[0117] A: First embedding of liquid nitrogen micro-channel chill in the hot spot area;

[0118] B: Subsequently starting of the directional solidification device, and finally pouring of high temperature molten iron in the riser and covering with insulating agent;

[0119] S8: Knockout cleaning;

[0120] A: First opening of the box after the casting cools to the specified temperature;

[0121] B: First rough polishing using cast steel shot, and then finishing the surface using stainless steel shot;

[0122] S9: heat treatment strengthening;

[0123] A: first perform normalization strengthening;

[0124] B: then proceed with three-stage deep cooling cycle;

[0125] C: finally perform laser cladding WC-Co alloy layer on sealing surface;

[0126] S10: package after quality detection.

[0127] In step S1-A, the titanium content in the Q10 high-purity pig iron is ≤0.03%, and the sulfur content in the low-carbon scrap steel is ≤0.02%. By step S1, impurities can be controlled from the source, reducing the risk of pores and inclusions.

[0128] In step S2-A, the melting furnace temperature of the intermediate frequency induction furnace is 1550°C, and the melting time is 0.75h. Low-sulfur molten iron improves spheroidization stability, and copper-tin alloy can enhance pearlite strength.

[0129] In step S3-C, the wire feeding speed is 28m / min, and the reaction time is 75s. By wire feeding method, the spheroidization rate can be improved, and rare earth can neutralize the anti-spheroidization elements.

[0130] In step S4, the graphite ball density can be doubled to 300 / mm 2 , and the segregation can also be eliminated by magnetic field.

[0131] In step S5-C, the inner layer of the gradient coating is nano yttrium oxide, the middle layer is boron carbide + silicon nitride, and the outer layer is graphene zirconium powder. By ceramic parts, the cooling can be accelerated, and by gradient coating, the sticking sand can be effectively blocked and the surface precision can be improved.

[0132] In step S7-B, the heating zone temperature of the directional solidification device is 1200°C, the liquid nitrogen cold trap temperature is -196°C, and the casting moves at 0.8mm / min. This can directional solidification grain, so that it is refined.

[0133] In step S8-A, the specified temperature is ≤425°C. In step S8-B, the cast steel shot is Φ1.0mm, and the stainless steel shot is Φ0.3mm. By low-temperature unpacking, the stress can be effectively reduced, and double-layer shot blasting can improve cleanliness.

[0134] In step S9-B, the three-stage deep cooling cycle includes first soaking in -196°C liquid nitrogen for 2h, then standing at room temperature for 1h, repeating 3 times. By deep cooling transformation of residual austenite, its wear resistance can be improved.

[0135] Example 3:

[0136] High-density high-pressure-resistant nodular cast iron gate valve casting process, comprising the following steps:

[0137] S1: raw material preparation;

[0138] A: select 60 parts of Q10 high-purity pig iron, 40 parts of low-carbon scrap steel, 35 parts of nodular iron return material, 2.5 parts of calcium carbide desulfurizer, 1 part of cryolite powder, 3 parts of magnesium-rare earth-calcium alloy wire, 1 part of barium-containing ferrosilicon alloy, 0.5 part of strontium-containing ferrosilicon alloy, and 0.4 part of zirconium-containing ferrosilicon alloy;

[0139] B: shot blasting treatment is performed on Q10 high-purity pig iron, low-carbon scrap steel and nodular iron return material to remove surface oxides;

[0140] C: Q10 high-purity pig iron, low-carbon scrap steel and nodular iron return material are placed in a 200°C oven for 2h to remove moisture;

[0141] S2: melting control;

[0142] A: Q10 high-purity pig iron, low-carbon scrap steel and nodular iron return material are added to a medium-frequency induction furnace for melting;

[0143] B: when the molten iron reaches 1550°C, calcium carbide desulfurizer is sprayed for deep desulfurization;

[0144] C: spectral analysis is used to ensure that the sulfur content is ≤0.010%;

[0145] S3: spheroidizing treatment;

[0146] A: the desulfurized molten iron is transferred to a special treatment package;

[0147] B: nodular iron chips are used to cover the liquid surface, and cryolite powder is scattered to prevent oxidation;

[0148] C: magnesium-rare earth-calcium alloy wire is fed through a wire feeder, slag is removed twice immediately after the reaction is completed, and the magnesium residual amount is detected to be between 0.055%;

[0149] S4: four-stage inoculation treatment;

[0150] A: barium-containing ferrosilicon alloy is pre-embedded at the bottom of the treatment package, and strontium-containing ferrosilicon alloy is added to the runner during pouring;

[0151] B: zirconium-containing ferrosilicon alloy is placed in the sprue cup, and bismuth-containing inoculation block is finally set in the runner;

[0152] S5: mold making;

[0153] A: first use resin sand molding;

[0154] B: then inlay zirconia ceramic preform at the sealing surface and flange part;

[0155] C: Finally, spray three layers of gradient coating;

[0156] S6: Pouring;

[0157] A: First, move the mold into the vacuum chamber, and vacuum the vacuum chamber to 10-2P a ;

[0158] B: Start the pouring system, and the bottom of the molten iron flows through the 10kA electromagnetic field purification, and synchronously applies 50Hz mechanical vibration, and finally completes pouring at 1300℃;

[0159] S7: Solidification control;

[0160] A: First, embed the liquid nitrogen micro-channel chill in the hot spot area;

[0161] B: Then start the directional solidification device, and finally pour high-temperature molten iron in the riser to cover the insulating agent;

[0162] S8: Knockout cleaning;

[0163] A: First, open the box after the casting cools to the specified temperature;

[0164] B: First, use cast steel shot to rough polish, and then use stainless steel shot to finish the surface;

[0165] S9: Heat treatment strengthening;

[0166] A: First, perform normalizing strengthening;

[0167] B: Then perform three-stage deep cooling cycle;

[0168] C: Finally, laser cladding WC-Co alloy layer on the sealing surface;

[0169] S10: Package after detecting quality.

[0170] Among them, the titanium content in Q10 high-purity pig iron in step S1-A is ≤0.03%, and the sulfur content in low-carbon scrap steel is ≤0.02%. By step S1, impurities can be controlled from the source, reducing the risk of pores and inclusions.

[0171] Among them, the melting furnace temperature of the intermediate frequency induction furnace in step S2-A is 1600℃, and the melting time is 1h. Low-sulfur molten iron improves spheroidization stability, and copper-tin alloy can enhance pearlite strength.

[0172] Among them, the wire feeding speed in step S3-C is 28m / min, and the reaction time is 80s. By wire feeding method, the spheroidization rate can be improved, and rare earth can neutralize the anti-spheroidization elements.

[0173] Among them, by step S4, the graphite ball density can be doubled to 300 / mm2 And also can eliminate segregation by magnetic field.

[0174] In the step S5-C, the inner layer of the gradient coating is nano yttrium oxide, the middle layer is boron carbide + silicon nitride, and the outer layer is graphene zirconium powder. The ceramic part can accelerate cooling, and the gradient coating can effectively block sand and improve surface precision.

[0175] In the step S7-B, the heating zone temperature of the directional solidification device is 1200 DEG C, the liquid nitrogen cold trap temperature is -196 DEG C, and the casting moves at 0.8 mm / min. This can directional solidification grain, so that it is refined.

[0176] In the step S8-A, the specified temperature is ≤450 DEG C. In the step S8-B, the cast steel shot is Φ1.0 mm, and the stainless steel shot is Φ0.3 mm. Low-temperature unpacking can effectively reduce stress, and double-layer shot blasting can improve cleanliness.

[0177] In the step S9-B, the three-stage deep cooling cycle includes first soaking in -196 DEG C liquid nitrogen for 2h, then standing at room temperature for 1h, repeating 3 times. The residual austenite is converted by deep cooling to improve its wear resistance.

[0178] In the present application, the graphite nucleation is precisely regulated by nano silicon carbide, and the micro shrinkage is eliminated by directional solidification with liquid nitrogen micro-cooling iron, and the base purity is ensured by vacuum electromagnetic pouring, and the residual austenite is converted by deep cooling cycle to improve the stability of pearlite, and finally the sealing surface is strengthened by laser cladding WC-Co layer. Under the cooperation of the four technologies, the strength, density and temperature resistance limit of ductile cast iron are greatly enhanced, the pressure-bearing capacity of the gate valve is increased from PN40 to PN150, and the life cycle cost is reduced by 60%.

[0179] In the present application, the graphite ball is made finer by adding nano particles, and the internal holes are eliminated by cooling with liquid nitrogen, and then poured in a vacuum environment to avoid bubbles, and then the material hardness is enhanced by deep cooling treatment, and finally the laser is used to clad a superhard alloy layer on the sealing surface. This can greatly enhance the strength, density and temperature resistance limit of ductile cast iron to improve its use function.

[0180] The above is only the preferred embodiment of the present application, but the protection scope of the present application is not limited thereto, any skilled person in the art can make equivalent replacement or change according to the technical solution and the inventive concept of the present application within the technical range disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A high density high pressure ductile iron gate valve casting process characterized by, The method comprises the following steps: S1: raw material preparation; A: 40-60 parts of Q10 high-purity pig iron, 30-40 parts of low-carbon scrap steel, 25-35 parts of nodular iron return material, 1.5-2.5 parts of calcium carbide desulfurizer, 0.5-1 part of cryolite powder, 2-3 parts of magnesium-rare earth-calcium alloy wire, 0.5-1 part of barium-containing ferrosilicon alloy, 0.3-0.5 part of strontium-containing ferrosilicon alloy, and 0.2-0.4 part of zirconium-containing ferrosilicon alloy are selected; B: the Q10 high-purity pig iron, low-carbon scrap steel and nodular iron return material are subjected to shot blasting treatment to remove surface oxides; C: the Q10 high-purity pig iron, low-carbon scrap steel and nodular iron return material are placed in a 200℃ oven for drying for 2h to remove moisture; S2: melting control; A: the Q10 high-purity pig iron, low-carbon scrap steel and nodular iron return material are added into a medium-frequency induction furnace for melting; B: when the molten iron reaches 1450-1550℃, the calcium carbide desulfurizer is sprayed for deep desulfurization; C: a spectrometer is used for detection to ensure that the sulfur content is ≤0.010%; S3: spheroidizing treatment; A: the desulfurized molten iron is transferred into a special treatment package; B: the liquid surface is covered with nodular iron chips, and the cryolite powder is scattered to prevent oxidation; C: the magnesium-rare earth-calcium alloy wire is sent through a wire feeder, the slag is removed twice immediately after the reaction is completed, and the magnesium residual amount is detected to be between 0.045-0.055%; S4: four-stage inoculation treatment; A: the barium-containing ferrosilicon alloy is pre-embedded at the bottom of the treatment package, the strontium-containing ferrosilicon alloy is added into the runner during pouring; B: the zirconium-containing ferrosilicon alloy is placed in the pouring cup, and finally the bismuth-containing inoculation block is arranged in the pouring channel; S5: mold making; A: resin sand molding is first used; B: then zirconia ceramic preforms are inlaid at the sealing surface and flange parts; C: finally, three layers of gradient coatings are sprayed; S6: pouring; A: First, the mold is moved into the vacuum chamber and the vacuum chamber is evacuated to 10"2P a ; B: the pouring system is started, the bottom pouring molten iron flows through a 10kA electromagnetic field for purification, and a 50Hz mechanical vibration is applied synchronously, and finally the pouring is completed at 1200-1300℃; S7: solidification control; A: liquid nitrogen micro-channel cold iron is first pre-embedded at the hot spot area; B: then the directional solidification device is started, and finally the high-temperature molten iron is poured into the riser and covered with insulating agent; S8: shakeout cleaning; A: the mold is first opened after the casting cools to the specified temperature; B: cast steel shots are first used for rough polishing, and then stainless steel shots are used for surface finishing; S9: heat treatment strengthening; A: normalizing strengthening is first performed; B: three-stage deep cooling cycle is then performed; C: finally, the sealing surface is subjected to laser cladding WC-Co alloy layer; S10: quality detection and packaging.

2. The high density high pressure ductile iron gate valve casting process of claim 1, wherein, In the step S1-A, the titanium content in the Q10 high-purity pig iron is ≤0.03%, and the sulfur content in the low-carbon scrap steel is ≤0.02%.

3. The high density high pressure ductile iron gate valve casting process of claim 1, wherein, In the step S2-A, the melting furnace temperature of the medium-frequency induction furnace is 1500-1600℃, and the melting time is 0.5-1h.

4. The high density high pressure ductile iron gate valve casting process of claim 1, wherein, In the step S3-C, the wire feeding speed is 28m / min, and the reaction time is 70-80s.

5. The high density high pressure ductile iron gate valve casting process of claim 1 wherein, In the step S5-C, the inner layer of the gradient coating is nano yttria, the middle layer is boron carbide+silicon nitride, and the outer layer is graphene zirconium powder.

6. The high density high pressure ductile iron gate valve casting process of claim 1 wherein, The temperature of the heating zone in the directional solidification device in the step S7-B is 1200℃, the temperature of the liquid nitrogen cold trap is -196℃, and the castings are moved at 0.8mm / min.

7. The high density high pressure ductile iron gate valve casting process of claim 1 wherein, The specified temperature in the step S8-A is ≤400-450℃.

8. The high density high pressure ductile iron gate valve casting process of claim 1 wherein, The cast steel shot in the step S8-B is Φ1.0mm, and the stainless steel shot is Φ0.3mm.

9. The high density high pressure ductile iron gate valve casting process of claim 1 wherein, The three-stage deep cooling cycle in the step S9-B includes first soaking in -196℃ liquid nitrogen for 2h, then standing at room temperature for 1h, repeated for 3 times.