Investment casting process for martensitic stainless steel part

Abstract

The invention belongs to the technical field of casting, and provides an investment casting process of a martensitic stainless steel material part, according to the investment casting process of the martensitic stainless steel material part, through introduction of silicon carbide and metamorphism of a rare earth ferrosilicon alloy, the thermal expansion coefficient of steel is reduced, and the performance of the martensitic stainless steel part is improved. And meanwhile, structure grains in the martensitic stainless steel solidification process are remarkably refined, formation of thick and large matrix structures is reduced, and therefore internal stress generated in the casting solidification stage is reduced. And meanwhile, the plasticity and toughness of the material can be improved through the refined crystal grains, the adaptive capacity of the casting to martensite transformation volume change in the subsequent heat treatment process is enhanced, accumulation of internal stress is relieved, and finally cracks are effectively restrained. Compared with existing schemes such as casting temperature optimization and shell deformability adjustment, the process improves the material performance from the structure level through modification treatment, the anti-cracking capacity of the casting can be fundamentally improved, meanwhile, the size precision and the mechanical property of the casting are considered, and the bottleneck of the prior art is broken through.

Description

Technical Field

[0001] This invention belongs to the field of casting technology, specifically relating to an investment casting process for martensitic stainless steel parts. Background Technology

[0002] Investment casting, as a high-precision casting process, has been widely used in various high-end equipment manufacturing fields such as aerospace, machinery manufacturing, petrochemicals, and medical devices due to its ability to produce castings with complex shapes, high dimensional accuracy, and excellent surface quality. Martensitic stainless steel, with its excellent strength, hardness, wear resistance, and good corrosion resistance, has become an important material for manufacturing key components in these fields. By using investment casting to produce castings from martensitic stainless steel, the synergistic effect of material properties and process advantages can be fully utilized to meet the stringent requirements of high-end equipment for its components.

[0003] However, in the actual production process of investment casting of martensitic stainless steel, the castings are prone to cracking defects. This problem seriously restricts the product qualification rate and performance stability, and has become a technical bottleneck that urgently needs to be solved in the industry.

[0004] To address the aforementioned crack defects, industry professionals have explored various improvement methods, such as optimizing pouring temperature, adjusting shell collapsibility, and modifying heat treatment parameters. However, these methods all have limitations in practical applications: optimizing pouring temperature and adjusting shell collapsibility can only alleviate internal stress to a certain extent and cannot fundamentally improve the problem of coarse matrix structure; while modifying heat treatment parameters may result in the mechanical properties of the casting failing to meet design requirements, making it difficult to balance casting quality and performance. Summary of the Invention

[0005] This invention provides an investment casting process for martensitic stainless steel parts, aiming to solve the technical problems mentioned in the background art.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is: to provide an investment casting process for martensitic stainless steel parts, comprising the following steps: S01, Prepare wax model; S02, prepare the mold shell, and coat the inside of the mold shell with a layer of silicon carbide powder for casting; S03, molten steel, is produced by adding rare earth ferrosilicon alloy to molten steel during the molten steel smelting process to modify the molten steel. S04, the modified molten steel is poured into the mold shell; S05, After the casting has solidified, clean the mold shell; S06, perform heat treatment on the cleaned casting.

[0007] In one possible implementation of the investment casting process for martensitic stainless steel parts provided by the present invention, in step S03, the amount of rare earth ferrosilicon alloy added is 1% to 1.5% of the mass of the molten steel.

[0008] In one possible implementation of the investment casting process for martensitic stainless steel parts provided by the present invention, the rare earth ferrosilicon alloy is crushed and screened before use, with a particle size of 0.3~2.0mm.

[0009] In one possible implementation of the investment casting process for martensitic stainless steel parts provided by the present invention, the silicon carbide used for casting is SiC95, with a particle size of 200-400 mesh and a coating thickness of 0.1-0.3 mm.

[0010] In one possible implementation of the investment casting process for martensitic stainless steel parts provided by the present invention, a silicon carbide powder layer is coated onto the inner surface of the mold shell by dip coating.

[0011] In one possible implementation of the investment casting process for martensitic stainless steel parts provided by the present invention, step S01 includes: Wax injection is performed on a wax injection machine to obtain a wax pattern of the part. After cooling and shaping, the wax pattern is then welded to the casting mold head to obtain a wax model. The completed wax model is cleaned in cleaning solution and water and then hung to air dry.

[0012] In one possible implementation of the investment casting process for martensitic stainless steel parts provided by the present invention, in step S02, the mold shell is obtained by alternately coating the surface of the wax mold with multiple layers of adhesive and refractory material and firing it, and the thickness of the mold shell is 6.5~7.0mm. The adhesive for the surface layer of the shell is silica sol, and the refractory material is zircon powder; the adhesive for the back layer of the shell is silica sol, and the refractory material is mullite powder and fused silica powder; wherein the mass ratio of mullite powder to fused silica powder is 9:1.

[0013] In one possible implementation of the investment casting process for martensitic stainless steel parts provided by the present invention, in step S04, the casting temperature is 1600-1610℃; after pouring, the pouring cup needs to be replenished once, and then a self-heating heat preservation agent is sprinkled into the pouring cup.

[0014] In one possible implementation of the investment casting process for martensitic stainless steel parts provided by the present invention, in step S06, the surface of the casting is coated with an anti-oxidation coating before heat treatment, and methanol is dripped into the pit furnace during the normalizing process to create a reducing atmosphere, with a methanol dripping acceleration rate of 150-180 drops / minute.

[0015] In one possible implementation of the investment casting process for martensitic stainless steel parts provided by this invention, the initial heating rate after the casting is loaded into the furnace is 50°C / hour; after heating to 500°C, the heating rate is increased to 100°C / hour; the normalizing temperature is 880°C, held for 2 hours, and the casting is forced to cool by blowing air after being taken out of the furnace; the high-temperature tempering treatment temperature is 750°C, held for 2 hours, and the casting is air-cooled after being taken out of the furnace.

[0016] The beneficial effects of the investment casting process for martensitic stainless steel parts provided by this invention are as follows: Compared with the prior art, the investment casting process for martensitic stainless steel parts provided by this invention reduces the coefficient of thermal expansion of steel through the introduction of silicon carbide and the modification effect of rare earth ferrosilicon alloy. Simultaneously, it significantly refines the grain structure during the solidification process of martensitic stainless steel, reducing the formation of coarse matrix structures, thereby reducing the internal stress generated during the solidification stage of the casting. Furthermore, the refined grains enhance the plasticity and toughness of the material, strengthening the casting's adaptability to the volume changes of martensitic transformation during subsequent heat treatment, alleviating the accumulation of internal stress, and ultimately effectively suppressing crack formation. Compared with existing solutions such as optimizing pouring temperature and adjusting shell collapsibility, this process improves material properties at the microstructure level through modification treatment, fundamentally enhancing the crack resistance of the casting while simultaneously considering the dimensional accuracy and mechanical properties of the casting, breaking through the bottlenecks of existing technologies. Attached Figure Description

[0017] Figure 1 This is a schematic flowchart of the investment casting process for martensitic stainless steel parts provided in an embodiment of the present invention. Detailed Implementation

[0018] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and are not intended to limit the scope of this application.

[0019] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. The following description of at least one exemplary embodiment is actually illustrative only and is in no way intended to limit this application or its application or use. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0020] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0021] Unless otherwise specifically stated, the relative arrangement, numerical expressions, and values ​​of the components and steps described in these embodiments do not limit the scope of this application. It should also be understood that, for ease of description, the dimensions of the various parts shown in the drawings are not drawn to actual scale. Techniques, methods, and devices known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and devices should be considered part of the specification. In all examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not as limitations. Therefore, other examples of exemplary embodiments may have different values. It should be noted that similar reference numerals and letters in the following drawings denote similar items; therefore, once an item is defined in one drawing, it need not be further discussed in subsequent drawings.

[0022] In the description of this application, it should be understood that the orientation or positional relationship indicated by directional terms such as "front, back, up, down, left, right", "horizontal, vertical, horizontal" and "top, bottom" is usually based on the orientation or positional relationship shown in the accompanying drawings, and is only for the convenience of describing this application and simplifying the description. Unless otherwise stated, these directional terms do not indicate or imply that the device or element referred to must have a specific orientation or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on the scope of protection of this application; the directional terms "inner" and "outer" refer to the inner and outer contours relative to the outline of each component itself.

[0023] For ease of description, spatial relative terms such as "above," "over," "on the upper surface of," "above," etc., are used herein to describe the spatial positional relationship of a device or feature as shown in the figures to other devices or features. It should be understood that spatial relative terms are intended to encompass different orientations in use or operation beyond the orientation of the device as described in the figures. For example, if the device in the figures were inverted, a device described as "above" or "above" other devices or structures would subsequently be positioned as "below" or "under" other devices or structures. Thus, the exemplary term "above" can include both "above" and "below." The device may also be positioned in other different ways, and the spatial relative descriptions used herein will be interpreted accordingly.

[0024] Furthermore, it should be noted that the use of terms such as "first" and "second" to define components is merely for the purpose of distinguishing the corresponding components. Unless otherwise stated, the above terms have no special meaning and therefore should not be construed as limiting the scope of protection of this invention.

[0025] Please refer to the following: Figure 1 The investment casting process for martensitic stainless steel parts provided by this invention will now be described. The investment casting process for the martensitic stainless steel parts includes the following steps: S01, Prepare wax model; S02, prepare the mold shell, and coat the inside of the mold shell with a layer of silicon carbide powder for casting; S03, molten steel, is produced by adding rare earth ferrosilicon alloy to molten steel during the molten steel smelting process to modify the molten steel. S04, the modified molten steel is poured into the mold shell; S05, After the casting has solidified, clean the mold shell; S06, heat treatment is performed on the cleaned casting.

[0026] It should be noted that in this embodiment, the process adds a step S02 to the conventional investment casting process, which involves coating the mold shell with a layer of silicon carbide powder for casting. During casting, the silicon carbide powder layer is washed away by the molten steel and mixed into the molten steel to reduce the coefficient of thermal expansion of the steel, thereby reducing deformation or cracking caused by thermal expansion. Furthermore, step S03 adds a rare earth ferrosilicon alloy modification treatment step, which works synergistically with the subsequent casting and heat treatment processes. The rare earth ferrosilicon alloy must be selected to match the melting temperature and composition system of the martensitic stainless steel. The modification treatment must be completed in the later stage of steel melting and before casting to ensure that the alloy can be fully dissolved and evenly distributed in the molten steel, avoiding premature addition that could lead to the burning and failure of rare earth elements.

[0027] The beneficial effects of the investment casting process for martensitic stainless steel parts provided in this embodiment of the invention are as follows: Compared with the prior art, the investment casting process for martensitic stainless steel parts provided in this embodiment of the invention reduces the coefficient of thermal expansion of steel by introducing silicon carbide and the modification effect of rare earth ferrosilicon alloy, while significantly refining the microstructure grains during the solidification process of martensitic stainless steel and reducing the formation of coarse matrix structure, thereby reducing the internal stress generated during the solidification stage of the casting.

[0028] Meanwhile, refined grains enhance the material's plasticity and toughness, improve the casting's adaptability to the volume changes of martensitic transformation during subsequent heat treatment, alleviate the accumulation of internal stress, and ultimately effectively suppress crack formation. Compared to existing solutions such as optimizing pouring temperature and adjusting shell collapsibility, this process improves material properties at the microstructure level through modification treatment, fundamentally enhancing the casting's crack resistance while maintaining dimensional accuracy and mechanical properties, thus overcoming existing technological bottlenecks.

[0029] In a specific embodiment of the investment casting process for martensitic stainless steel parts provided in this example, in step S03, the amount of rare earth ferrosilicon alloy added is 1% to 1.5% of the mass of the molten steel.

[0030] Preferably, the amount of rare earth ferrosilicon alloy added is 1.25% of the mass of the molten steel.

[0031] It should be noted that in this embodiment, the addition amount of 1% to 1.5% is the optimal value verified through multiple experiments. The determination of this value range is based on the compositional characteristics of martensitic stainless steel, melting temperature, and purity of rare earth ferrosilicon alloy.

[0032] In this embodiment, the beneficial effect of this limitation is to ensure the stability and effectiveness of the modification treatment: if the addition amount is less than 1%, the rare earth element content is insufficient, the grains cannot be sufficiently refined, the internal stress reduction effect is not obvious, and the crack suppression effect is limited; if the addition amount is more than 1.5%, the excess rare earth elements may form brittle compounds, which will reduce the mechanical properties of the casting and increase the production cost.

[0033] In one specific embodiment of the investment casting process for martensitic stainless steel parts provided in this example, the rare earth ferrosilicon alloy is crushed and screened before use, with a particle size of 0.3-2.0 mm.

[0034] It should be noted that in this embodiment, the purpose of crushing and screening the rare earth ferrosilicon alloy is to improve its dissolution efficiency and uniformity in molten steel. The particle size is controlled within the range of 0.3-2.0 mm, and it needs to be screened through a standard sieve to remove excessively large or small particles. Among them, particles smaller than 0.3 mm are easily carried away by flue gas during the addition process, resulting in waste and component loss, while particles larger than 2.0 mm are difficult to dissolve quickly in molten steel and may form undissolved inclusions, affecting the quality of castings.

[0035] In this embodiment, the beneficial effect of this particle size limitation is to ensure the uniformity and reliability of the modification treatment: a suitable particle size allows the rare earth ferrosilicon alloy to disperse quickly and dissolve completely in the molten steel, avoiding differences in local grain refinement effects due to insufficient dissolution, thereby reducing stress concentration points inside the casting. Simultaneously, it reduces the generation of undissolved inclusions, improves the density and material uniformity of the casting, further reduces the risk of crack formation, and ensures consistent performance across all parts of the casting.

[0036] In one possible implementation of the investment casting process for martensitic stainless steel parts provided by the present invention, the silicon carbide used for casting is SiC95, with a particle size of 200-400 mesh and a coating thickness of 0.1-0.3 mm.

[0037] It should be noted that in this embodiment, the SiC95 grade represents silicon carbide purity ≥95%. High purity can prevent impurities from reacting with molten steel at high temperatures and affecting the composition of the casting. The particle size range of 200 mesh to 400 mesh can ensure that the coating is uniform and dense. If the particle size is too coarse (less than 200 mesh), the coating surface will be rough. If the particle size is too fine (greater than 400 mesh), it will easily agglomerate and affect the coating effect.

[0038] In this embodiment, the beneficial effect of limiting the silicon carbide parameters is to accurately ensure the anti-cracking auxiliary effect of the coating and the quality of the casting, reduce the thermal expansion coefficient of steel, and synergistically improve the anti-cracking effect with rare earth modification treatment.

[0039] In one possible implementation of the investment casting process for martensitic stainless steel parts provided by the present invention, a silicon carbide powder layer is coated onto the inner surface of the mold shell by dip coating.

[0040] It should be noted that in this embodiment, the dip coating method requires a suitable silicon carbide coating (made from a mixture of SiC95 powder and industrial alcohol). During dip coating, the immersion speed and dwell time of the mold shell must be controlled to ensure that the inner surface is completely covered with the coating. After dip coating, drying treatment is required to ensure that the coating adheres tightly to the mold shell and prevent the coating from peeling off during casting. This method is suitable for mold shells with complex structures and can solve the problem of unevenly covering the dead corners of the mold shell cavity by methods such as spraying.

[0041] In a specific embodiment of the investment casting process for martensitic stainless steel parts provided in this example, step S01 includes: Wax injection is performed on a wax injection machine to obtain a wax model of the part. After the wax pattern of the part is cooled and shaped, it is then welded to the casting mold head to obtain the wax model. The completed wax model is then cleaned in cleaning solution and water and hung to air dry.

[0042] Specifically, the assembly and welding of the casting mold head can ensure the smooth filling of molten steel during the casting process, improve the density of the casting, and indirectly reduce the possibility of crack formation.

[0043] In a specific embodiment of the investment casting process for martensitic stainless steel parts provided in this example, in step S02, the shell is obtained by alternately coating the surface of the wax pattern with multiple layers of adhesive and refractory material and then firing it, with a shell thickness of 6.5~7.0mm.

[0044] It should be noted that, in this embodiment, the number of alternating coating layers needs to be determined based on the part size and structural complexity. The shell thickness is controlled between 6.5 and 7.0 mm, which is the optimal range considering both the shell's strength and resilience: too thin a thickness cannot withstand the impact and pressure of molten steel, easily leading to shell breakage; too thick a thickness results in insufficient resilience, exacerbating shrinkage stress during casting solidification. The firing temperature and time must be adapted to the shell material to ensure sufficient removal of moisture and binder decomposition products from the shell, improving its high-temperature stability.

[0045] In this embodiment, the beneficial effects of the mold shell preparation are that it balances the strength and flexibility of the mold shell, alleviating the internal stress of the casting: the thickness of 6.5~7.0mm and the multi-layer coating and firing process ensure that the mold shell has sufficient high-temperature strength, avoiding mold shell breakage during pouring and thus preventing the casting from being scrapped. At the same time, the reasonable thickness and interlayer structure can give the mold shell a certain degree of flexibility, allowing it to deform appropriately during the solidification and shrinkage of the casting, reducing shrinkage resistance and the generation of internal stress. Combined with rare earth modification treatment, it further suppresses crack defects. In addition, the stable performance of the mold shell can improve the dimensional accuracy and surface quality of the casting, meeting the stringent requirements of high-end equipment components.

[0046] In a specific embodiment of the investment casting process for martensitic stainless steel parts provided in this example, the adhesive for the surface layer of the shell is silica sol, and the refractory material is zircon powder; the adhesive for the back layer of the shell is silica sol, and the refractory material is mullite powder and fused silica powder; wherein the mass ratio of mullite powder to fused silica powder is 9:1.

[0047] It should be noted that the surface layer is the layer that directly contacts the wax pattern, while the remaining layers are back layers. The zircon powder has a particle size of 270-320 mesh. Zircon powder has excellent high-temperature resistance, chemical stability, and low thermal conductivity, which can reduce the erosion of the mold shell by molten steel. Combined with a smaller particle size, it makes the internal surface of the mold shell smoother, ensuring casting accuracy and quality. The mullite powder and fused silica powder have a particle size of 200 mesh. Mullite powder can improve the strength and high-temperature stability of the mold shell, while fused silica powder can improve the shell's yielding properties. The combination of the two in a 9:1 ratio can achieve the optimal balance between strength and yielding properties. Combined with a larger particle size, it can ensure the overall strength of the mold shell.

[0048] In this embodiment, the beneficial effect of this shell material ratio lies in achieving precise matching of the properties of each layer of the shell, improving the casting quality and crack resistance: the surface layer material can ensure high precision and low defects on the casting surface, reducing surface stress concentration points; the reasonable ratio of the back layer material can provide sufficient flexibility while ensuring the overall strength of the shell, effectively alleviating the internal stress generated during the solidification shrinkage of the casting, and working synergistically with rare earth modification treatment to further reduce the probability of crack formation. At the same time, silica sol, as a universal and stable adhesive, can ensure tight bonding between the shell layers, reduce defects such as shell delamination and peeling, and improve process stability.

[0049] In a specific embodiment of the investment casting process for martensitic stainless steel parts provided in this example, in step S04, the casting temperature is 1600-1610℃; after pouring, the pouring cup needs to be replenished once, and then a self-heating heat-insulating agent is sprinkled into the pouring cup.

[0050] Specifically, self-heating insulation agents can be: Thermite (aluminum powder + iron oxide): High heating temperature (up to 2500℃), suitable for high-temperature alloy applications such as cast steel. It can release a large amount of heat through the thermite reaction, significantly prolonging the solidification time of the molten steel in the pouring cup, ensuring unobstructed feeding channels, and meeting the feeding needs of martensitic stainless steel with large shrinkage rate (2%~2.5%). Ferrosilicon powder + oxidant composite heating agent: Gentle and continuous heating, which can avoid local overheating, while also having the effects of heat preservation and shrinkage compensation, suitable for heat preservation of pouring cups in precision investment casting; Composite insulation and heating type: It uses low thermal conductivity materials such as expanded perlite and expanded graphite as the base, and adds heating components such as aluminum powder and ferrosilicon powder. It can reduce heat loss of molten steel by reducing heat through low thermal conductivity, and can also supplement heat by heating, further improving the insulation and shrinkage compensation effect.

[0051] It should be noted that in this embodiment, the pouring temperature is controlled at 1600-1610℃, which is determined based on the liquidus temperature and fluidity requirements of martensitic stainless steel. This temperature range ensures that the molten steel has good fluidity, ensuring that the mold cavity can be fully filled and avoiding defects such as incomplete pouring and cold shuts caused by insufficient fluidity. At the same time, compared with a higher pouring temperature, this temperature can reduce the superheat of the molten steel, reduce the tendency of grain growth, and avoid aggravating internal stress. The timing of the replenishment pouring should be when the molten steel level drops slightly after the first pouring, in order to replenish the shrinkage generated during the solidification process of the casting. The self-heating insulating agent should be selected with good heat preservation and heating performance, which can slow down the solidification rate of the molten steel in the pouring cup and provide a continuous replenishment of molten steel for the shrinkage of the casting.

[0052] In this embodiment, the beneficial effects of the pouring parameters and replenishment measures are to improve the density of the casting and reduce internal stress: precise pouring temperature control can suppress grain coarsening while ensuring the fluidity of the molten steel, and further refine the grains with rare earth modification treatment; the application of replenishment and self-heating insulating agent can effectively reduce defects such as shrinkage cavities and porosity inside the casting, improve the density of the casting, and avoid stress concentration caused by internal defects; at the same time, the self-heating insulating agent can slow down the solidification rate of the molten steel, making the solidification process of the casting more uniform, reducing the thermal stress caused by the temperature gradient, thereby reducing the risk of crack formation. Compared with the existing schemes that simply optimize the pouring temperature, this scheme further improves the crack resistance of the process and the quality of the casting through the synergy of temperature control and replenishment and insulation measures.

[0053] In a specific embodiment of the investment casting process for martensitic stainless steel parts provided in this example, in step S06, the surface of the casting is coated with an anti-oxidation coating before heat treatment, and methanol is dripped into the pit furnace during the normalizing process to create a reducing atmosphere, with a methanol dripping rate of 150-180 drops / minute.

[0054] Specifically, anti-oxidation coatings can be of the following types: Glass-based anti-oxidation coating: With borosilicate glass as the main component, it can form a dense glassy protective film at high temperature, which isolates the air and prevents the casting from oxidation and decarburization. It is also easy to remove by mechanical cleaning or warm water rinsing after heat treatment and is suitable for normalizing temperature of 880℃. Ceramic-based anti-oxidation coating: Composed of high-temperature resistant ceramic powders such as alumina and zirconium oxide and binder, it has excellent high-temperature resistance, high coating strength, and can adhere stably during heat treatment, effectively preventing oxidation of casting surfaces. It also has good compatibility with methanol reducing atmosphere. Nickel-chromium composite coating material: Ni+Cr composite coating formed by electrochemical deposition or powder infiltration has excellent high-temperature oxidation resistance and corrosion resistance for martensitic stainless steel, which can significantly improve the surface quality of castings, avoid the formation of decarburized layer, and ensure stable mechanical properties.

[0055] It should be noted that in this embodiment, the anti-oxidation coating must be selected to match the heat treatment temperature of martensitic stainless steel, possess good high-temperature anti-oxidation properties, and be easy to clean after heat treatment. Methanol is dripped into the pit furnace to create a reducing atmosphere because methanol decomposes at high temperatures to produce carbon monoxide and hydrogen, forming a reducing environment that prevents oxidation and decarburization of the castings during normalizing. The methanol dripping rate is controlled at 150-180 drops / minute, which is the optimal range verified by experiments. This amount ensures a stable reducing atmosphere in the furnace, avoiding insufficient dripping leading to poor anti-oxidation and decarburization effects, or excessive dripping leading to incomplete methanol decomposition and carbon buildup, thus affecting the casting quality.

[0056] In this embodiment, the beneficial effects of this heat treatment auxiliary measure are to protect the surface quality of the casting and ensure the heat treatment effect: the synergistic effect of the anti-oxidation coating and the reducing atmosphere can effectively prevent the formation of oxide scale and decarburized layer during the high-temperature normalizing process of the casting, avoid stress concentration caused by surface defects, and reduce the inducing factors for crack formation; at the same time, the reduction of the decarburized layer can ensure that the core mechanical properties such as hardness and strength of martensitic stainless steel are not weakened, solving the problem that improving heat treatment process parameters in the prior art may lead to substandard mechanical properties. A stable reducing atmosphere can also improve the uniformity of the heat treatment process, ensure consistent microstructural transformation in all parts of the casting, reduce structural stress, and further improve the performance stability of the casting.

[0057] In a specific embodiment of the investment casting process for martensitic stainless steel parts provided in this example, the initial heating rate of the casting after being loaded into the furnace is 50°C / hour; after heating to 500°C, the heating rate is increased to 100°C / hour; the normalizing temperature is 880°C, held for 2 hours, and the casting is forced to cool by blowing air after being taken out of the furnace; the high-temperature tempering treatment temperature is 750°C, held for 2 hours, and the casting is air-cooled after being taken out of the furnace.

[0058] In this embodiment, the beneficial effect of the heat treatment parameters lies in achieving an optimal balance between mechanical properties and internal stress: segmented heating rate control can effectively reduce the thermal stress generated during heat treatment, avoiding crack formation caused by the superposition of thermal stress and structural stress; precise control of normalizing and tempering temperatures, holding times, and cooling methods can enable the casting to obtain a uniform and stable martensitic structure, ensuring high strength and high hardness while eliminating internal stress through tempering, thus improving the toughness and crack resistance of the casting. This heat treatment scheme solves the problem in the prior art where improving heat treatment process parameters is difficult to balance casting quality and performance. Working synergistically with the preceding rare earth modification treatment, shell optimization, and pouring control processes, it ultimately achieves a significant improvement in casting yield and performance stability, meeting the stringent requirements of high-end equipment for components.

[0059] Example 1 Taking the production of a valve body as an example, the investment casting process for martensitic stainless steel parts provided in this embodiment includes the following steps: S01, Prepare wax model; Perform wax injection operation on wax injection machine to obtain part wax pattern; After cooling and shaping, the part wax pattern is then welded to the casting rod mold head to obtain wax model; After welding, the wax model is cleaned in cleaning solution and water and then hung to air dry.

[0060] S02, Prepare the mold shell. The mold shell is obtained by alternately coating multiple layers of adhesive and refractory material on the surface of a wax mold and then firing it. The thickness of the mold shell is 6.5~7.0mm. The adhesive for the surface layer of the mold shell is silica sol, and the refractory material is zircon powder with a particle size of 270~320 mesh. The adhesive for the back layer of the mold shell is silica sol, and the refractory material is mullite powder and fused silica powder with a particle size of 200 mesh. The mass ratio of mullite powder to fused silica powder is 9:1. Coat the inside of the mold shell with a layer of silicon carbide powder for casting. The silicon carbide for casting is SiC95 with a particle size of 200 mesh and a coating thickness of 0.2mm.

[0061] S03, molten steel, is a steel melt where rare earth ferrosilicon alloy is added during the steel melt smelting process to modify the steel melt. The amount of rare earth ferrosilicon alloy added is 1.25% of the mass of the steel melt. The rare earth ferrosilicon alloy is crushed and screened before use, with a particle size of 1 mm.

[0062] S04 involves pouring the modified molten steel into the mold shell at a casting temperature of 1600-1610℃. After pouring, the pouring cup needs to be filled once, and then a composite heat-insulating and heat-generating agent is sprinkled into the pouring cup.

[0063] S05, After the casting has solidified, clean the mold shell; S06. The cleaned castings are heat-treated. Before heat treatment, the surface of the castings is coated with a ceramic-based anti-oxidation coating. During the normalizing process in the pit furnace, methanol is dripped to create a reducing atmosphere, with a methanol dripping rate of 150-180 drops / minute. The initial heating rate of the castings after loading into the furnace is 50℃ / hour; after reaching 500℃, the heating rate is increased to 100℃ / hour; the normalizing temperature is 880℃, held for 2 hours, and the castings are forced to cool by blowing air after being removed from the furnace; the high-temperature tempering treatment temperature is 750℃, held for 2 hours, and the castings are air-cooled after being removed from the furnace.

[0064] Of the 3,000 pieces produced using the above casting process, 103 pieces cracked, resulting in a cracking rate of 3.42%.

[0065] Comparative Example 1 Taking the production of a valve body as an example, the traditional investment casting process includes the following steps: S01, Prepare wax model; Perform wax injection operation on wax injection machine to obtain part wax pattern; After cooling and shaping, the part wax pattern is then welded to the casting rod mold head to obtain wax model; After welding, the wax model is cleaned in cleaning solution and water and then hung to air dry.

[0066] S02, Preparation of the mold shell, the mold shell is obtained by alternately coating the surface of the wax mold with multiple layers of adhesive and refractory material and then firing it. The thickness of the mold shell is 6.5~7.0mm. The adhesive for the surface layer of the mold shell is silica sol, and the refractory material is zircon powder with a particle size of 270~320 mesh. The adhesive for the back layer of the mold shell is silica sol, and the refractory material is mullite powder and fused silica powder with a particle size of 200 mesh. The mass ratio of mullite powder to fused silica powder is 9:1.

[0067] S03, molten steel.

[0068] S04, pour molten steel into the mold shell at a casting temperature of 1600-1610℃; after pouring, the pouring cup needs to be filled once, and then a composite heat-insulating and heat-generating agent is sprinkled into the pouring cup.

[0069] S05, After the casting has solidified, clean the mold shell; S06. The cleaned castings are subjected to heat treatment. The initial heating rate after the castings are loaded into the furnace is 50℃ / hour. After the temperature reaches 500℃, the heating rate is increased to 100℃ / hour. The normalizing temperature is 880℃, and the holding time is 2 hours. After the castings are taken out of the furnace, they are forced to cool by blowing air. The high-temperature tempering temperature is 750℃, and the holding time is 2 hours. The castings are then air-cooled after being taken out of the furnace.

[0070] Of the 3,000 pieces produced using the above casting process, 495 pieces cracked, resulting in a cracking rate of 16.5%.

[0071] Therefore, the investment casting process for martensitic stainless steel parts provided in this application embodiment can greatly reduce the cracking rate of the parts.

[0072] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A loss casting process for martensitic stainless steel parts, characterized in that, include: S01, Prepare wax model; S02, prepare the mold shell, and coat the inside of the mold shell with a layer of silicon carbide powder for casting; S03, molten steel, is produced by adding rare earth ferrosilicon alloy to molten steel during the molten steel smelting process to modify the molten steel. S04, the modified molten steel is poured into the mold shell; S05, After the casting has solidified, clean the mold shell; S06, perform heat treatment on the cleaned casting.

2. The investment casting process for martensitic stainless steel parts as described in claim 1, characterized in that, In step S03, the amount of rare earth ferrosilicon alloy added is 1% to 1.5% of the mass of the molten steel.

3. The investment casting process for martensitic stainless steel parts as described in claim 2, characterized in that, The rare earth ferrosilicon alloy is crushed and screened before use, with a particle size of 0.3~2.0mm.

4. The investment casting process for martensitic stainless steel parts as described in claim 1, characterized in that, In step S02, the silicon carbide used for casting is SiC95, with a particle size of 200-400 mesh and a coating thickness of 0.1-0.3 mm.

5. The investment casting process for martensitic stainless steel parts as described in claim 4, characterized in that, In step S02, a silicon carbide powder layer is coated onto the inner surface of the shell by dip coating.

6. The investment casting process for martensitic stainless steel parts as described in claim 1, characterized in that, Step S01 includes: Wax injection is performed on a wax injection machine to obtain a wax pattern of the part. After cooling and shaping, the wax pattern is then welded to the casting mold head to obtain a wax model. The completed wax model is cleaned in cleaning solution and water and then hung to air dry.

7. The investment casting process for martensitic stainless steel parts as described in claim 1, characterized in that, In step S02, the shell is obtained by alternately coating the surface of the wax mold with multiple layers of adhesive and refractory material and then firing it. The shell thickness is 6.5~7.0mm. The adhesive for the surface layer of the shell is silica sol, and the refractory material is zircon powder; the adhesive for the back layer of the shell is silica sol, and the refractory material is mullite powder and fused silica powder; wherein the mass ratio of mullite powder to fused silica powder is 9:

1.

8. The investment casting process for martensitic stainless steel parts as described in claim 1, characterized in that, In step S04, the casting temperature is 1600-1610℃; after pouring, the pouring cup needs to be replenished once, and then a self-heating heat preservation agent is sprinkled into the pouring cup.

9. The investment casting process for martensitic stainless steel parts as described in claim 1, characterized in that, In step S06, the casting is coated with an anti-oxidation coating before heat treatment, and methanol is dripped into the pit furnace during the normalizing process to create a reducing atmosphere, with a methanol dripping rate of 150-180 drops / minute.

10. The investment casting process for martensitic stainless steel parts as described in claim 9, characterized in that, The initial heating rate of the casting after loading into the furnace is 50℃ / hour; after heating to 500℃, the heating rate is increased to 100℃ / hour; the normalizing temperature is 880℃, held for 2 hours, and the casting is forced to cool by blowing air after being taken out of the furnace; the high-temperature tempering temperature is 750℃, held for 2 hours, and the casting is air-cooled after being taken out of the furnace.

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