A method for eliminating thin-wall corner cracks in large ring-shaped precision castings

Abstract

The present invention discloses a method for eliminating thin-wall corner cracks in large-sized ring precision castings, comprising the following steps: S1. Repair the size of the wax mold corresponding to the part on the casting surface prone to cracks; S2. Mark the outer surface part of the mold corresponding to the corner part of the casting prone to cracks with a marker pen; S3. Treat the outer surface of the mold; S4. Treat the inner surface of the mold; S5. Paste heat-insulating cotton on the inner and outer surfaces of the mold; S6. Adjust the preheating temperature of the mold and the pouring temperature of the molten steel; S7. Place the cast casting in an incubator and cool it slowly. The present invention can achieve that for the part with complex corners on the outer ring of large-sized ring precision castings, no crack problems will occur in the surface fluorescence and X-ray inspections after vacuum melting and casting, and the surface of the casting is well formed. On the premise of little change, on the basis of the existing technology, the crack problems on the surface of the casting are completely eliminated, and the product quality of the casting is improved.

Description

Technical Field

[0001] The present invention relates to the field of investment precision casting, and particularly relates to a method for eliminating thin-wall corner cracks in large-sized ring-shaped precision castings. Background Art

[0002] Due to the complex structure of large-sized ring-shaped castings, with alternating thick-wall and thin-wall parts and different cooling rates of the castings, crack defects will occur at the places where the thick and thin parts of the casting meet. Since the outer ring of the casting dissipates heat relatively quickly, the parts where cracks appear are mainly those close to the outer surface of the casting. For cracks generated in areas where the requirements of the casting are not important, they will not affect the quality of the casting, and the normal use of the casting can be restored after repair welding. However, after repair welding the crack part of the casting, different metallographic structures will be generated at this position, which is different from the state during the solidification of the casting. The cracks generated at this part of the casting will appear in both the surface inspection fluorescence and the internal inspection processes. For the cracks that appear in the surface inspection process, after surface treatment, they sometimes still appear during the internal inspection process, resulting in the need to grind and repair weld the same position of the casting twice. This affects the quality and delivery of the casting, and leads to a longer production process time for the casting. For some key castings, if cracks occur in the casting and no treatment is allowed, it will result in defective castings, affecting the delivery schedule of the castings and increasing the production cost of the castings.

[0003] Currently, the widely adopted measures for large-sized ring-shaped precision castings include replacing the mold shell material. For example, using a mold shell material with relatively low mold shell strength after pouring for the production of large-sized castings to reduce the influence of the mold shell strength on the generation of cracks in the casting. This requires replacing most of the mold shell refractory materials, which will increase the production cost and re-test and adjust the dimensions of the casting, resulting in an increase in a series of production materials and process methods. Summary of the Invention

[0004] In order to solve the problems existing in the prior art, the present invention provides a method for eliminating thin-wall corner cracks in large-sized ring-shaped precision castings. Without changing the material of the mold shell, measures are taken for the wax part itself, the wrapping method of the outer surface of the mold shell, and the cooling process after casting the casting, completely avoiding the generation of crack defects at the corner parts of the outer ring of the casting, reducing the waste loss of the casting caused by cracks, reducing the manual time and the detection time of the equipment, and improving the metallurgical quality of the casting.

[0005] To achieve the above object, the present invention includes the following steps:

[0006] S1. Repair the size of the wax mold corresponding to the part of the casting surface where cracks are likely to occur;

[0007] S2. Mark the outer surface part of the mold shell corresponding to the corner part of the casting where cracks are likely to occur;

[0008] S3. Treat the outer surface of the mold shell;

[0009] S4. Treat the inner surface of the mold shell;

[0010] S5. Paste heat-insulating cotton on the inner and outer surfaces of the mold shell;

[0011] S6. Adjust the preheating temperature of the mold shell and the pouring temperature of the molten steel;

[0012] S7. Place the castings after pouring into an incubator and cool them slowly.

[0013] In step S1, repair the size of the wax mold corresponding to the part of the casting surface prone to cracking, locally thicken the wax mold at this part, and increase the thickness from 1.5 - 2 mm to 2.5 - 3 mm.

[0014] In step S2, manufacture a mold shell for the group of castings with the gating system assembled, and mark the outer surface part of the mold shell corresponding to the corner part of the casting prone to cracking.

[0015] In step S3, first cut a piece of refractory fiber paper with a thickness of 1 - 3 mm into a size larger than the corner part of the casting by 30 - 50 mm. The 1 - 3 mm thick refractory fiber paper should have a thermal conductivity of 0.1 - 0.4 W / m×k and a bulk density of 100 - 400 kg / m 3 ; Secondly, mix it with refractory clay, and the ratio of the refractory clay is silica sol: 320 - mesh bauxite powder = 1: (1 - 2).

[0016] Furthermore, coat a layer of silica sol on the outer surface of the mold shell to wet the surface of the mold shell. Then coat the surface of the cut 1 - 3 mm thick refractory fiber paper with refractory clay, and bond the refractory fiber paper to the outer surface of the mold shell corresponding to the casting corner, so that the outer surface of the mold shell at the casting corner part is completely covered by the refractory fiber paper.

[0017] In step S4, similarly use a piece of refractory fiber paper with a thickness of 1 - 3 mm and the same size as in step S3, coat the surface with refractory clay, and paste it on the inner surface of the mold shell corresponding to the crack part of the casting. Make both the inner and outer surfaces of the mold shell covered by the refractory fiber paper.

[0018] In step S5, cut two pieces of heat-insulating cotton with a thickness of 20 - 25 mm, and the size is more than 20 - 50 mm larger than the part prone to cracking. The 20 - 25 mm thick heat-insulating cotton should have a thermal conductivity of 0.1 - 0.4 W / m×k and a bulk density of 80 - 200 kg / m 3 ;

[0019] Further, apply a layer of refractory clay evenly on the surface of the thermal insulation cotton with a thickness of 20 - 25 mm, and paste one piece on the inner and outer surfaces of the mold shell where cracks are likely to occur respectively. Wind and fix the surfaces of the bonded thermal insulation cotton on the inner and outer layers with iron wire of 0.8 - 1.2 mm.

[0020] Further, cover the other upper and lower surfaces and side surfaces of the mold shell except the pouring cup with one or two layers of thermal insulation cotton, and tighten the thermal insulation cotton with iron wire. For the local thick and large parts and the gating system parts of the casting, the covering thickness of the thermal insulation cotton can be increased or decreased.

[0021] In step S6, increase the preheating temperature of the mold shell by 10 - 50 degrees Celsius. On the premise of ensuring molding, reduce the pouring temperature of the casting and narrow the temperature difference between the pouring temperature of the molten steel and the temperature of the mold shell. Rapidly transfer the mold shell from the preheating furnace to the pouring furnace.

[0022] In step S7, make a heat preservation box with a cover. The heat preservation box is made of a metal frame and lined with thermal insulation cotton with a thickness of 20 - 50 mm. The thermal insulation cotton lining includes the bottom, side surfaces and cover part of the box. After the casting is poured, place the casting into the heat preservation box and fasten the cover for cooling.

[0023] Further, when the casting is cooled below the service temperature of the casting, take it out from the heat preservation box, and subsequent processing procedures of casting production can be carried out.

[0024] Compared with the prior art, the beneficial effects of the present invention are:

[0025] The present invention can achieve that for the parts with complex corners on the outer ring of large-scale ring-shaped precision castings, there will be no crack problems in the surface fluorescence and X-ray inspections after vacuum melting and casting, and the surface of the casting is well formed. By adopting this technology, it can directly work under the existing mold shell material system, increase necessary mold shell heat preservation measures, reduce the local cooling rate of the casting, and completely eliminate the crack problems on the surface of the casting on the basis of the existing technology with little change, improving the product quality of the casting. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic structural diagram of the casting prone to cracks of the present invention;

[0027] Figure 2 It is a schematic structural diagram of the mold shell with a pouring scheme of the present invention;

[0028] Figure 3 It is a schematic structural diagram of the mold shell surface pasted with refractory fiber paper of the present invention;

[0029] Figure 4 It is a schematic structural diagram of the mold shell surface pasted with thermal insulation cotton of the present invention;

[0030] Figure 5Schematic diagram of the mold shell structure of the present invention wrapped with thermal insulation cotton;

[0031] Figure 6 Schematic diagram of the mold shell being placed in an insulation box after casting in the present invention.

[0032] Among them, 1 - casting, 2 - mold shell, 3 - 3mm thick refractory fiber paper, 4 - 25mm thick thermal insulation cotton, 5 - insulation box. Specific implementation manners

[0033] The following further elaborates on the present invention in conjunction with the accompanying drawings and specific embodiments. As Figures 1-6 shown, the present invention provides a method for eliminating thin - walled corner cracks in large - sized annular precision castings, including the following steps:

[0034] S1. Repair the size of the wax mold corresponding to the part on the surface of the casting 1 where cracks are likely to occur;

[0035] S2. Mark the outer surface part of the mold shell 2 corresponding to the corner part of the casting 1 where cracks are likely to occur with a marker pen;

[0036] S3. Treat the outer surface of the mold shell 2;

[0037] S4. Treat the inner surface of the mold shell 2;

[0038] S5. Paste 25mm thick thermal insulation cotton 4 on the inner and outer surfaces of the mold shell 2;

[0039] S6. Adjust the preheating temperature of the mold shell 2 and the pouring temperature of the molten steel;

[0040] S7. Place the cast casting 1 into the insulation box 5 and cool it slowly.

[0041] In step S1, repair the size of the wax mold corresponding to the part on the surface of the casting 1 where cracks are likely to occur, and increase the thickness of the wax mold at this part from 2mm to 3mm. This can increase the local heat storage capacity of the casting and enhance the tensile strength during solidification. If necessary, the thickness can be thinned during subsequent processing or surface treatment.

[0042] In step S2, manufacture the mold shell 2 for the casting 1 with the gating system assembled, and mark the outer surface part of the mold shell 2 corresponding to the corner part of the casting 1 where cracks are likely to occur with a marker pen.

[0043] In step S3, first cut a piece of 3mm thick refractory fiber paper 3 that is more than 50mm larger than the corner part of the casting. The 3mm thick refractory fiber paper 3 has a thermal conductivity of 0.3W / m×k and a bulk density of 300kg / m 3 ; secondly, mix it with refractory clay, and the proportion of the refractory clay is silica sol: 320 - mesh bauxite powder = 1:2.

[0044] Furthermore, a layer of silica sol is applied to the outer surface of the mold shell 2 to wet the surface of the mold shell 2. Then, refractory mud is applied to the surface of the cut 3 mm thick refractory fiber paper 3, and the refractory fiber paper 3 is bonded to the outer surface of the mold shell 2 corresponding to the corner of the casting 1, so that the outer surface of the mold shell 2 at the corner of the casting 1 is completely covered by the 3 mm thick refractory fiber paper 3.

[0045] In step S4, a 3 mm thick refractory fiber paper 3 of the same size as in step S3 is coated with refractory mud and pasted on the inner surface of the mold shell 2 corresponding to the cracked part of the casting 1. Both the inner and outer surfaces of the mold shell 2 are covered with the 3 mm thick refractory fiber paper 3.

[0046] In step S5, two pieces of 25 mm thick insulation cotton 4 are cut, the thermal conductivity is 0.3 W / m×k, and the volume density should reach 150 kg / m 3 , the size exceeds the area where cracks are prone to occur by 50mm.

[0047] Furthermore, a layer of refractory mud is evenly applied on the surface of the 25 mm thick insulation cotton 4, and a piece is respectively pasted on the inner and outer surfaces of the shell 2 where cracks are prone to occur, in order to further reduce the cooling rate of the thin-walled parts of the casting. In order to prevent the bonded 25 mm thick insulation cotton 4 from falling off, it is fixed with a 1.2 mm iron wire.

[0048] Furthermore, the upper and lower surfaces and the side surfaces of the mold shell 2 except the pouring cup are covered with two layers of heat-insulating cotton 4, which are tightened with iron wire to prevent the mold shell 2 from falling off during the preheating and turnover process. The cooling rate of the casting 1 after the entire casting can also be reduced. The thickness of the heat-insulating cotton 4 can be increased or decreased for the locally thick parts and the pouring system parts of the casting 1.

[0049] In step S6, the preheating temperature of the mold shell 2 is increased by 50 degrees Celsius. Under the premise of ensuring molding, the pouring temperature of the casting 1 is reduced, and the temperature difference between the pouring temperature of the molten steel and the temperature of the mold shell 2 is reduced, which can help reduce the solidification speed of the molten steel in the mold shell 2 and reduce crack defects in the casting 1. And the mold shell 2 can be quickly turned over from the preheating furnace to the pouring furnace.

[0050] In step S7, a heat preservation box 5 with a cover is made. The heat preservation box 5 is made of a metal frame and lined with 25 mm thick heat preservation cotton 4. The heat preservation cotton 4 is lined at the bottom, sides and cover of the box. After the casting 1 is poured, the casting 1 is placed in the heat preservation box 5 and the cover is closed for cooling.

[0051] Furthermore, when the casting 1 is cooled to a temperature below the use temperature of the casting 1 , it is taken out from the heat preservation box 5 , and subsequent processing steps for the production of the casting 1 can be performed.

Claims

1. A method for eliminating thin-wall corner cracks in large-sized ring-shaped precision castings, characterized in that: It includes the following steps: S1. Repair the size of the wax mold corresponding to the part on the casting surface where cracks are likely to occur; S2. Mark the outer surface part of the mold shell corresponding to the corner part of the casting where cracks are likely to occur; S3. Treat the outer surface of the mold shell; S4. Treat the inner surface of the mold shell; S5. Paste heat-insulating cotton on the inner and outer surfaces of the mold shell; S6. Adjust the preheating temperature of the mold shell and the pouring temperature of the molten steel; S7. Place the cast casting in a heat-insulating box and cool it slowly; In step S1, repair the size of the wax mold corresponding to the part on the casting surface where cracks are likely to occur, locally thicken the wax mold at this part, and increase the thickness from 1.5 - 2 mm to 2.5 - 3 mm; In step S3, first, cut a piece of refractory fiber paper with a thickness of 1-3 mm into a size larger than the corner part of the casting by 30-50 mm. The refractory fiber paper with a thickness of 1-3 mm should have a thermal conductivity of 0.1-0.4 W / m×k and a bulk density of 100-400 kg / m 3 ; secondly, mix it with refractory mud. The ratio of the refractory mud is silica sol: 320-mesh bauxite powder = 1: (1-2); apply a layer of silica sol to the outer surface of the mold shell to wet the surface of the mold shell; then coat the surface of the cut refractory fiber paper with a thickness of 1-3 mm with refractory mud, and bond the refractory fiber paper to the outer surface of the mold shell corresponding to the corner of the casting, so that the entire outer surface of the mold shell at the corner part of the casting is covered with refractory fiber paper; In step S5, cut two pieces of thermal insulation cotton with a thickness of 20 - 25 mm, and the size should exceed the crack - prone part by 20 - 50 mm; for the thermal insulation cotton with a thickness of 20 - 25 mm, the thermal conductivity should reach 0.1 - 0.4 W / m×k, and the bulk density should reach 80 - 200 kg / m 3 ; evenly apply a layer of refractory clay on the surface of the 20 - 25 mm thick thermal insulation cotton, and paste one piece on the inner and outer surfaces of the mold shell prone to cracking respectively; wind and fix the surface of the bonded thermal insulation cotton on the inner and outer layers with a wire of 0.8 - 1.2 mm; cover the other upper, lower and side surfaces of the mold shell except the pouring cup with 1 - 2 layers of thermal insulation cotton, and tighten the thermal insulation cotton with a wire; for the local thick - large parts and gating system parts of the casting, increase or decrease the covering thickness of the thermal insulation cotton; In step S6, increase the preheating temperature of the mold shell by 10 - 50 degrees Celsius. On the premise of ensuring molding, reduce the pouring temperature of the casting and narrow the temperature difference between the pouring temperature of the molten steel and the temperature of the mold shell; Rapidly transfer the mold shell from the preheating furnace to the pouring furnace.

2. The method for eliminating thin-wall corner cracks in large-sized ring-shaped precision castings according to claim 1, characterized in that: In step S2, manufacture the mold shell for the casting group with the gating system assembled, and mark the outer surface part of the mold shell corresponding to the corner part of the casting where cracks are likely to occur.

3. The method for eliminating thin-wall corner cracks in large-sized ring-shaped precision castings according to claim 1, characterized in that: In step S4, also use a refractory fiber paper with a thickness of 1 - 3 mm and the same size as in step S3, coat the surface with refractory mud, and paste it on the inner surface of the mold shell corresponding to the crack part of the casting; Make both the inner and outer surfaces of the mold shell covered with refractory fiber paper.

4. The method for eliminating thin-wall corner cracks in large-sized ring-shaped precision castings according to claim 1, characterized in that: In step S7, make a heat-insulating box with a cover. The heat-insulating box is made of a metal frame and lined with heat-insulating cotton with a thickness of 20 - 50 mm. The heat-insulating cotton lining includes the bottom, side, and cover parts of the box; After the casting is poured, place the casting in the heat-insulating box and fasten the cover for cooling.

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