A method for preparing a single crystal blade directional solidification flexible nested casting shell

Abstract

The invention relates to the technical field of casting of single crystal blades, and specifically discloses a method for preparing a single crystal blade directional solidification flexible nested casting mold shell, comprising: a pouring cup wax mold in a wax mold group tree is suspended above a center column wax mold through a pouring cup bracket wax mold support, the pouring cup wax mold is connected to the blade wax mold through a runner wax mold, a surface layer, a transition layer, an inner reinforcement layer, an interlayer and an outer reinforcement layer are prepared in sequence from the inside to the outside of the wax mold group after cleaning, ethyl silicate is coated on the inner reinforcement layer without sanding to form an interlayer, and the interlayer is baked after dewaxing to form a casting shell; the invention has good shell yieldability, excellent high temperature strength and good yieldability can be well compatible, recrystallization is avoided in the preparation of complex structure thin-walled single crystal blades, a preparation scheme in which the center column shell can be directly cast without blocking is realized, and a significant effect is achieved in improving metallurgical inclusions in the excessively long production of batch single crystal blades, thereby improving the metallurgical quality of the single crystal blades.

Description

Technical Field

[0001] The invention relates to the technical field of preparation of single crystal blades, in particular to a method for preparing a directional solidification flexible nested casting shell of a single crystal blade. Background Art

[0002] Aircraft engine and gas turbine high temperature alloy single crystal blades are known as the jewel in the crown of modern industrial technology due to their important performance and complex manufacturing technology, but many key technologies are still immature. Single crystal blades serve in extreme environments of high temperature (900-1100℃), high pressure (30-50bar), high speed (6000-1000 rpm) and gas corrosion. Therefore, single crystal blades have strict requirements on crystal structure and metallurgical inclusions.

[0003] Single crystal blades are prepared by directional solidification. Usually, the whole preparation process of single crystal blades, such as heat preservation and drawing, takes about 1-5 hours. Therefore, the shell must have excellent high temperature strength to ensure that the blade is completely formed during the long drawing process. At the same time, the shell must have good yieldability to reduce the casting stress of the blade and avoid recrystallization during the subsequent heat treatment. However, the high temperature strength and yieldability of the shell are two important and mutually balancing properties in the preparation of single crystal blades. Excellent high temperature strength can ensure the complete forming of the blade, but it makes its yieldability poor, which in turn causes a large stress concentration in the blade during the directional solidification process and recrystallization in the subsequent heat treatment process; when the shell has good yieldability, it will lead to insufficient high temperature strength, which is prone to alloy liquid leakage and incomplete forming. The current common technical route is to prioritize high temperature strength to achieve complete forming, and then gradually adjust the strength to increase yieldability. The shell is composed of slurry (binder + powder) and sand material alternating layer by layer. The simple way to increase the shell strength is to increase the number of shell layers, but this process will increase the manufacturing cycle and cost, and it is difficult for the blades to dissipate heat during the directional solidification process. More importantly, it is still impossible to ensure that there will be no alloy liquid leakage in large blades. Traditional methods to increase the shell strength also include optimizing the material combination. Patent CN118239759A improves the shell strength by adding a reinforcing agent (nano alumina particles + redispersible latex powder) to the reinforcement layer slurry. Patent CN114570879A uses epoxy resin and carbon fiber cloth to wrap when preparing the third layer to form a CC interconnected structure to increase the shell strength, and can reduce the number of shell layers. These methods all increase the shell strength in one direction, and lose the concession when ensuring the shell strength. Patent CN114570879A adopts fused quartz powder + silica sol binder as interlayer slurry to achieve compromise, so as to achieve both high strength and compromise of the shell through differentiation of the materials of the surface layer, transition layer and reinforcement layer. However, its compromise is still limited by the constraints of the overall shell geometry.

[0004] Since the directional solidification shell generally needs to withstand high temperature strength for several hours, the main technical route for shell preparation is based on ensuring high temperature strength, but the control of yield is insufficient. As the performance requirements of blades continue to increase, the structure of blade design becomes more complex and thinner, and the tendency of recrystallization gradually increases, then the control of shell yield becomes more important. However, the method of using differentiated materials for the surface layer, transition layer, and reinforcement layer is limited by the constraints of the overall shell geometry and has limited control over yield. Therefore, the excellent high temperature strength and good yield in the existing directional solidification shell preparation technology are not yet well compatible.

[0005] On the other hand, the pouring preparation of single crystal blades after shell making has an important impact on the pouring quality and metallurgical inclusion control. Usually, the center column of the single crystal blade shell needs to be manually blocked before pouring. The blocking material and method have a significant impact on the metallurgical quality of the blade. During the pouring process, the high-temperature alloy melt flushes the blocked part, which is easy to cause damage and slag, thus leading to metallurgical inclusions in the blade. Therefore, the preparation of a non-blocked, integrated single crystal shell is of great significance for the control and improvement of blade metallurgical quality.

[0006] To this end, the present application proposes a method for preparing a single crystal blade directional solidification flexible nested casting shell to solve the above problems. Summary of the invention

[0007] In view of the existing problems, the present invention provides a method for preparing a single crystal blade directional solidification flexible nested casting shell, which can effectively solve the problems raised in the background technology.

[0008] To solve the above problems, the present invention adopts the following technical solutions:

[0009] A method for preparing a single crystal blade directional solidification flexible nested casting shell comprises the following steps:

[0010] S1, wax model tree assembly: prepare a pouring cup wax model, a base wax model, a center column wax model and a pouring cup bracket wax model, and combine them to form a tree assembly bracket, the pouring cup wax model is suspended above the center column wax model through the pouring cup bracket wax model support, the blade wax model is fixed on the tree assembly bracket, and the pouring cup wax model and the blade wax model are connected through the runner wax model to form a wax model group;

[0011] S2. Wax model processing: immerse the wax model set in detergent for cleaning, and then rinse with clean water;

[0012] S3. Shell preparation: The wax mold surface is sequentially prepared with a surface layer, a transition layer, an inner reinforcement layer, an interlayer and an outer reinforcement layer from the inside to the outside. After the inner reinforcement layer is dried, ethyl silicate is applied without sanding, and the interlayer is formed after air drying;

[0013] S4, shell dewaxing: polish and remove the shell on the top of the pouring cup and the bottom of the base of the wax mold to expose the wax discharge seal, and place the shell in a dewaxing kettle for dewaxing;

[0014] S5. Mold shell baking: The dewaxed mold shell is placed in a furnace and baked to form a casting mold shell.

[0015] As a further solution of the present invention: in step S1, a chassis is provided inside the base wax mold, a cylindrical steel is provided inside the middle column wax mold, and both ends of the cylindrical steel are respectively provided with an upper threaded section and a lower threaded section extending out of the middle column wax mold, and an arc-shaped bracket is provided inside the pouring cup bracket wax mold, the upper threaded section is threadedly connected to the arc-shaped bracket, and the lower threaded section is threadedly connected to the chassis.

[0016] As a further solution of the present invention: after the medium-sized shell is dewaxed in step S4, the chassis and the arc-shaped bracket are rotated to separate from the cylindrical steel, and the steel structure is dismantled.

[0017] As a further solution of the present invention: in step S1, the lower end of the blade wax mold is connected to the base wax mold through a spiral wax mold, and the lower end of the runner wax mold is connected to the spiral wax mold, and the upper end of the blade wax mold is connected to the pouring cup bracket wax mold through a reinforcing rib wax mold.

[0018] As a further solution of the present invention: in step S1, a filter screen is provided on the runner wax mold.

[0019] As a further scheme of the present invention: in the step S3, the surface layer slurry is prepared by mixing zirconium oxide powder with a particle size of 300-500 mesh and silica sol, the slurry viscosity is 40-70S, and the surface layer sand material is corundum sand with a particle size of 60-90 mesh; the transition layer slurry is prepared by mixing zirconium oxide powder with a particle size of 300-500 mesh and silica sol, the slurry viscosity is 8-20S, and the transition layer sand material is corundum sand with a particle size of 60-90 mesh; the inner reinforcement layer slurry is prepared by mixing 90% alumina, 10% mineralizer and silica sol, the slurry viscosity is 25-50S, the particle size of alumina is 300-500 mesh, the particle size of the mineralizer is 800-1200 mesh, the inner reinforcement layer sand material is corundum sand with a particle size of 24-40 mesh, and a wetting agent and a defoaming agent are added to the slurry.

[0020] As a further solution of the present invention: the surface layer, transition layer and inner reinforcement layer in step S3 are all one layer, the outer reinforcement layer is 3-14 layers, the slurry of the outer reinforcement layer is prepared by mixing 90% alumina, 10% mineralizer and silica sol, and is added with a wetting agent and a defoaming agent, the slurry viscosity is 25-50S, the particle size of alumina is 300-500 mesh, the particle size of the mineralizer is 800-1200 mesh, and the sand material of the outer reinforcement layer is corundum sand with a particle size of 24-40 mesh.

[0021] As a further solution of the present invention: the shell after dewaxing in step S5 is placed in a gas furnace or an electric furnace for baking, the baking time is 2-4 hours, and the baking temperature is 800-1100°C.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows: the present invention sets an interlayer of ethyl silicate material to generate a flexible nested structure after baking, the inner shell and the outer shell are separated from each other, the structure is fully adaptable to fit, and they are in physical contact without chemical bonding, thus reserving elastic space for stress and strain during directional solidification, and setting multiple multi-layer reinforcement layers to ensure that the shell of the single crystal blade does not collapse or the alloy liquid leaks during long-term high-temperature directional solidification, and has good shell yieldability, and excellent high-temperature strength and good yieldability can be well compatible, thus avoiding recrystallization in the preparation of thin-walled single crystal blades with complex structures; the pouring cup wax mold and the center column wax mold are suspended in the air through the pouring cup bracket wax mold, and the pouring cup in the prepared shell is not connected to the center column, thus realizing a preparation scheme in which the center column shell can be directly cast without blocking, which has a significant effect on improving metallurgical inclusions in the excessively long production of batch single crystal blades, thereby improving the metallurgical quality of single crystal blades. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a schematic diagram of the wax mold structure of the present invention;

[0024] Figure 2 This is a schematic diagram of the structure of the wax mold after shell making in the present invention;

[0025] Figure 3 for Figure 2 A magnified diagram of the details in the middle.

[0026] In the figure: 1. blade wax mold; 2. reinforcement rib wax mold; 3. spiral wax mold; 4. base wax mold; 5. center column wax mold; 6. pouring cup bracket wax mold; 7. pouring cup wax mold; 8. runner wax mold; 9. chassis; 10. cylindrical steel; 11. arc bracket; 12. lower thread section; 13. upper thread section; 14. filter screen; 15. shell; P1. surface layer slurry; S1. surface layer sand; P2. transition layer slurry; S2. transition layer sand; P3. inner reinforcement layer slurry; S3. inner reinforcement layer sand; P4. interlayer; P5. outer reinforcement layer slurry; S5. outer reinforcement layer sand. DETAILED DESCRIPTION

[0027] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0028] Combination Figures 1 to 3 This embodiment describes a method for preparing a single crystal blade directional solidification flexible nested casting shell, which includes the following steps:

[0029] S1. Wax model tree assembly: prepare the pouring cup wax model 7, the base wax model 4, the middle column wax model 5 and the pouring cup bracket wax model 6, and combine them to form a tree assembly bracket. In the specific preparation, first select the metal chassis 9, the cylindrical steel 10 and the arc bracket 11. The two ends of the cylindrical steel 10 are respectively provided with an upper thread segment 13 and a lower thread segment 12. Put them into the mold and press the wax model to form the base wax model 4 wrapped with the chassis 9, the middle column wax model 5 wrapped with the cylindrical steel 10 The upper threaded section 13 of the cylindrical steel 10 is threadedly connected to the arc-shaped bracket 11, and the lower threaded section 12 of the cylindrical steel 10 is threadedly connected to the middle part of the chassis 9 to form a supporting steel structure wax mold. The outer edge bottom surface of the pouring cup wax mold 7 is connected to the pouring cup bracket wax mold 6. A gap is left between the pouring rod wax mold 7 and the pouring cup bracket wax mold 6 to facilitate the shell coating, so that the pouring cup wax mold 7 passes through the pouring cup bracket. The wax mold 6 is supported and suspended above the middle column wax mold 5, and a reinforcing rod can also be set at the upper end of the pouring cup wax mold 7. The blade wax mold 1 is fixed on the tree support to form a wax mold group. Specifically, the lower end of the blade wax mold 1 is connected to the base wax mold 4 through the spiral wax mold 3, and the upper end of the blade wax mold 1 is connected to the pouring cup bracket wax mold 6 through the reinforcing rib wax mold 2. The pouring cup wax mold 7 is connected to the spiral wax mold 3 through the runner wax mold 8. The runner wax mold 8 does not contact the pouring cup bracket wax mold 6. After shell dewaxing, the pouring cup and the bottom of the blade form an independent runner, the middle column will not enter the alloy liquid, and the pouring cup wax mold is an arc structure. The upper end of the runner wax mold 8 is connected to the side wall of the pouring cup wax mold 7, and in order to improve the purity of the molten metal, a filter screen 14 is pre-embedded in the runner wax mold 8. The filter screen 14 extends out of the side wall of the runner wax mold 8. After the shell is formed, the filter screen 14 can be fixed to the shell to ensure high-quality casting of the blade.

[0030] S2. Wax model processing: Immerse the wax model set in detergent for cleaning, and then rinse it with clean water to ensure that there is no wax residue on the surface of the wax model and it has good coating properties.

[0031] S3, preparation of shell 15: the wax mold surface is sequentially prepared with a surface layer, a transition layer, an inner reinforcement layer, an interlayer P4 and an outer reinforcement layer from the inside to the outside. After the inner reinforcement layer is dried, ethyl silicate is applied without sanding, and the interlayer P4 is formed after air drying.

[0032] The preparation of the shell 15 specifically includes the following:

[0033] Preparation of surface layer: The surface layer slurry P1 is prepared by mixing zirconium oxide powder with a particle size of 300-500 mesh and silica sol (silicon 30), and a wetting agent and a defoaming agent are added to the slurry. The viscosity of the prepared slurry is 40-70S. The surface sand material S1 is corundum sand with a particle size of 60-90 mesh. The high-viscosity zirconium oxide slurry has good coating and high-temperature stability. The surface roughness of the prepared blade is good, and zirconium oxide is compatible with a certain degree of concession. After sanding, it is dried in the shade;

[0034] Preparation of transition layer: The transition layer slurry P2 is prepared by mixing zirconium oxide powder with a particle size of 300-500 mesh and silica sol (silicon 30), and a wetting agent and a defoaming agent are added to the slurry. The slurry viscosity is 8-20S. The transition layer sand material S2 uses corundum sand with a particle size of 60-90 mesh. The low-viscosity zirconium oxide slurry has good yieldability and is shade-dried after sanding.

[0035] Preparation of inner reinforcement layer: The inner reinforcement layer slurry P3 is prepared by mixing 90% alumina, 10% mineralizer and silica sol (silicon 30), and a wetting agent and a defoaming agent are added to the slurry. The slurry viscosity is 25-50S, the particle size of alumina is 300-500 mesh, the particle size of the mineralizer is 800-1200 mesh, and the mineralizer is mullite. The inner reinforcement layer sand material S3 uses corundum sand with a particle size of 24-40 mesh. The ultrafine mullite-reinforced alumina slurry can make the shell have excellent high-temperature strength. The coating is air-dried after sanding, and the interlayer P4 treatment is carried out after drying;

[0036] Preparation of external reinforcement layer: The external reinforcement layer slurry P3 is prepared by mixing 90% alumina, 10% mineralizer and silica sol, and is added with a wetting agent and a defoaming agent. The slurry viscosity is 25-50S, the particle size of alumina is 300-500 mesh, the particle size of the mineralizer is 800-1200 mesh, and the mineralizer is mullite. The external reinforcement layer sand S5 uses corundum sand with a particle size of 24-40 mesh. The ultrafine mullite-reinforced alumina slurry can make the shell have excellent high-temperature strength. The paint is air-dried after sanding. According to the blade size, the external reinforcement layer is 3-14 layers, and each layer is dried before preparing the next layer.

[0037] S4, dewaxing of the mold shell 15: Grind and remove the mold shell 15 on the top of the pouring cup and the bottom of the base on the wax module to expose the wax drainage seal, and place the mold shell in a dewaxing kettle for dewaxing. After the mold shell is dewaxed, the chassis 9 and the arc-shaped bracket 11 are rotated to separate them from the cylindrical steel 10, and the chassis 9 and the cylindrical steel 10 are separated from the mold shell in the f direction, and the arc-shaped bracket 11 is separated from the mold shell in the g1 direction and the g2 direction. The arc-shaped bracket 11 adopts a four-claw or two-claw structure, which is convenient for disassembly and extraction, and the supporting steel structure is removed, and then roasting is carried out.

[0038] S5. Firing of the shell 15: The dewaxed shell is placed in a gas furnace or an electric furnace for 2-4 hours at a temperature of 800-1100°C. After calcination, the surface layer, transition layer, inner reinforcement layer and outer reinforcement layer of the shell 15 are separated from each other at the interlayer P4, in physical contact without chemical bonding. Due to the natural and uniform coating of the slurry, the shell structure at the interlayer P4 is fully adapted to fit, forming a flexible nested structure, and reserving elastic space for stress and strain. The calcined shell 15 does not need to block the center column, and can be directly poured. The center column is connected to the pouring cup bracket, and the pouring cup bracket is connected to the pouring cup at the top. After pouring, the center column will not enter the alloy liquid, and the pouring cup is directly connected to the pouring channel with the filter 14, which can ensure high-quality casting of the blade.

[0039] The present invention is currently applied in two embodiments. One case is in the preparation of the first set of quadruple single-crystal blades in China. The large overall size of the blades requires high high-temperature strength of the shell, and the quadruple thin-wall design requires the shell to have good yieldability. The shell preparation method of the present invention is used to effectively advance the project progress. The other case is applied in the batch production of E-class heavy-duty gas turbines, which increases the production qualification rate of large-scale single-crystal blades from 50% to 70%.

[0040] The working principle of the present invention is: in terms of high-temperature strength: ultrafine mineralizer is used in the outer reinforcement layer to strengthen the alumina slurry to ensure that the shell has excellent high-temperature strength; in terms of yieldability: the shell has good shell yieldability through the differentiation of each layer of shell material and the flexible nested structure design, and different viscosities of zirconium oxide slurry are used in the surface layer and the transition layer. The strength of the single material slurry after roasting is relatively low. After the preparation of the third layer of the inner reinforcement layer is dried, ethyl silicate is coated without sanding to form an interlayer P4. After roasting, a flexible nested structure is generated. The first three layers of the shell are separated from the subsequent outer reinforcement layer shell, and the structure is completely adapted to fit, and is in physical contact without chemical bonding. Elastic space is reserved for stress and strain during directional solidification, ensuring that the shell of the single crystal blade does not collapse or alloy liquid leaks during long-term high-temperature directional solidification, and has good shell yieldability. Excellent high-temperature strength and good yieldability can be well compatible, avoiding recrystallization in the preparation of complex structure thin-walled single crystal blades. The pouring cup wax mold 7 and the center column wax mold 5 are suspended in the air through the pouring cup bracket wax mold 6. The pouring cup in the prepared mold shell is not connected with the center column, realizing a preparation scheme in which the center column mold shell can be directly poured without blocking. This has a significant effect on improving the metallurgical inclusions in the excessively long production of batch single crystal blades, and improves the metallurgical quality of single crystal blades.

[0041] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or apparatus that includes a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or apparatus.

[0042] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a single crystal blade directional solidification flexible nested casting shell, characterized in that: The steps include: S1, wax model tree assembly: prepare a pouring cup wax model, a base wax model, a center column wax model and a pouring cup bracket wax model, and combine them to form a tree assembly bracket, the pouring cup wax model is suspended above the center column wax model through the pouring cup bracket wax model support, the blade wax model is fixed on the tree assembly bracket, and the pouring cup wax model and the blade wax model are connected through the runner wax model to form a wax model group; S2. Wax model processing: immerse the wax model set in detergent for cleaning, and then rinse with clean water; S3. Shell preparation: The wax mold surface is sequentially prepared with a surface layer, a transition layer, an inner reinforcement layer, an interlayer and an outer reinforcement layer from the inside to the outside. After the inner reinforcement layer is dried, ethyl silicate is applied without sanding, and the interlayer is formed after air drying; S4, shell dewaxing: polish and remove the shell on the top of the pouring cup and the bottom of the base of the wax mold to expose the wax discharge seal, and place the shell in a dewaxing kettle for dewaxing; S5. Mold shell baking: The dewaxed mold shell is placed in a furnace and baked to form a casting mold shell.

2. The method for preparing a single crystal blade directional solidification flexible nested casting shell according to claim 1, characterized in that: In step S1, a chassis is provided inside the base wax mold, a cylindrical steel is provided inside the middle column wax mold, and both ends of the cylindrical steel are respectively provided with an upper threaded section and a lower threaded section extending out of the middle column wax mold, and an arc-shaped bracket is provided inside the pouring cup bracket wax mold, the upper threaded section is threadedly connected to the arc-shaped bracket, and the lower threaded section is threadedly connected to the chassis.

3. The method for preparing a single crystal blade directional solidification flexible nested casting shell according to claim 2, characterized in that: After the medium shell is dewaxed in step S4, the chassis and the arc-shaped bracket are rotated to separate from the cylindrical steel and the steel structure is removed.

4. The method for preparing a single crystal blade directional solidification flexible nested casting shell according to claim 1, characterized in that: In step S1, the lower end of the blade wax mold is connected to the base wax mold through the spiral wax mold, and the lower end of the runner wax mold is connected to the spiral wax mold, and the upper end of the blade wax mold is connected to the pouring cup bracket wax mold through the reinforcing rib wax mold.

5. The method for preparing a single crystal blade directional solidification flexible nested casting shell according to claim 4, characterized in that: In the step S1, a filter screen is provided on the sprue wax mold.

6. The method for preparing a single crystal blade directional solidification flexible nested casting shell according to claim 1, characterized in that: In step S3, the surface slurry is prepared by mixing zirconium oxide powder with a particle size of 300-500 meshes and silica sol, the slurry viscosity is 40-70S, and the surface sand material is corundum sand with a particle size of 60-90 meshes; The transition layer slurry is prepared by mixing zirconium oxide powder with a particle size of 300-500 mesh and silica sol, and the slurry viscosity is 8-20S. The transition layer sand material is corundum sand with a particle size of 60-90 mesh; the inner reinforcement layer slurry is prepared by mixing 90% alumina, 10% mineralizer and silica sol, and the slurry viscosity is 25-50S, the particle size of alumina is 300-500 mesh, the particle size of the mineralizer is 800-1200 mesh, the inner reinforcement layer sand material is corundum sand with a particle size of 24-40 mesh, and wetting agents and defoaming agents are added to the slurry.

7. The method for preparing a single crystal blade directional solidification flexible nested casting shell according to claim 6, characterized in that: In the step S3, the surface layer, transition layer and inner reinforcement layer are all one layer, the outer reinforcement layer is 3-14 layers, the slurry of the outer reinforcement layer is prepared by mixing 90% alumina, 10% mineralizer and silica sol, and is added with a wetting agent and a defoamer, the slurry viscosity is 25-50S, the particle size of alumina is 300-500 mesh, the particle size of the mineralizer is 800-1200 mesh, and the sand material of the outer reinforcement layer is corundum sand with a particle size of 24-40 mesh.

8. The method for preparing a single crystal blade directional solidification flexible nested casting shell according to claim 1, characterized in that: The dewaxed shell in step S5 is placed in a gas furnace or an electric furnace for baking, the baking time is 2-4 hours, and the baking temperature is 800-1100°C.

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