Heat treatment method of thin-wall casting

Abstract

The invention belongs to the technical field of casting, and mainly relates to a heat treatment method of a thin-wall casting, which specifically comprises the following steps: anti-deformation design of the casting: arranging an anti-deformation device on the inner ring of the casting during casting process design, and pouring with the casting at the same time. And a heat treatment tray is manufactured, and a plurality of clamping grooves with different diameter sizes are formed in the heat treatment tray and used for containing the castings. Casting heat treatment is conducted, specifically, the heat treatment tray where the casting is placed is placed in a heat treatment furnace for normalizing treatment, the casting is slowly heated to the set temperature at the set temperature rising speed, and heat preservation is conducted for the set time; and after the casting is cooled to the set temperature, tempering treatment is conducted. Through casting anti-deformation design, heat treatment tray manufacturing, casting heat treatment design and process control, size deformation of the lower ring casting is stably controlled, so that the mechanical property of the casting meets the standard, and casting production resources are remarkably saved.

Description

Technical Field

[0001] The invention belongs to the technical field of casting, and mainly relates to a heat treatment method for a thin-wall casting. Background Art

[0002] The material of the thin-walled lower ring casting of the turbine is hydropower martensitic stainless steel, which belongs to high-alloy steel. The outline size is φ3150mm×1120mm, and the cross-section is "1" shaped. The thickest part of the lower ring wall is 120mm, and the thinnest part is 40mm, with a large difference in thickness. The performance heat treatment of this type of lower ring casting currently adopts the heat treatment method of normalizing + tempering to eliminate the component segregation formed during the solidification process, further uniformize the composition, and re-dissolve the Cr carbide into the austenite, so as to obtain uniform supersaturated lath martensite later. However, in the heat treatment process, in order to dissolve the carbides, the normalizing process temperature is set relatively high. During the normalizing cooling process, the cooling rate of each part of the lower ring is different. At the same time, the specific volume of the quenched martensite structure obtained by normalizing is large, and the thermal stress and organizational stress are relatively large. In addition, the casting itself has poor restraint deformation force, so the size deformation of this lower ring during the heat treatment process is relatively large and it is very easy to deform. Therefore, how to prevent the deformation of thin-walled castings during heat treatment needs to be solved urgently. Summary of the invention

[0003] In view of the above description, the present invention provides a heat treatment method for thin-walled castings to solve the problem that the size of lower ring castings is easily deformed during the heat treatment process, resulting in waste of production resources. The heat treatment method specifically includes the following steps:

[0004] A heat treatment method for a thin-walled casting comprises the following steps:

[0005] Anti-deformation design of castings: When designing the casting process, an anti-deformation device is set on the inner ring of the casting and is cast and produced simultaneously with the casting.

[0006] Making a heat treatment tray: The heat treatment tray is provided with a plurality of slots with different diameters for placing the castings.

[0007] Heat treatment of castings: placing the heat treatment tray with the castings in a heat treatment furnace for normalizing treatment, slowly heating the castings to a set temperature at a set heating rate, and keeping the temperature for a set time; after the castings are cooled to the set temperature, tempering treatment is performed.

[0008] In one embodiment, in the step of designing the anti-deformation of the casting, the anti-deformation device includes a central frame and a plurality of tie bars, and the casting is connected to the central frame through the plurality of tie bars. Preferably, the central frame is arranged in a circular shape, and the plurality of tie bars are evenly arranged along the circular central frame, so that the supporting force of the tie bars on the casting is evenly distributed, which can effectively prevent the casting from being deformed.

[0009] In one embodiment, in the step of making the heat treatment tray, the tray includes a first surface and a second surface, and a plurality of concentric grooves are arranged on the first surface and the second surface. Preferably, the first surface and the second surface are arranged opposite to each other, and the grooves on the first surface and the grooves on the second surface are arranged in a staggered manner; both the first surface and the second surface can be used to place castings, and a plurality of grooves are arranged according to the sizes of castings of different standards, and the grooves on the two surfaces are arranged in a staggered manner, so that more castings of different sizes can be placed, thereby expanding the use range of the heat treatment tray.

[0010] In one embodiment, in the step of heat treating the casting, during normalizing, the casting is slowly heated to 1030° C. to 1050° C. at a heating rate of 30° C. / h to 50° C. / h. If the heating rate is too high, the temperature difference between various parts of the casting increases, the internal stress increases, and the yield strength of the high temperature area is low, which leads to deformation of the casting.

[0011] In one embodiment, in the step of heat treatment of the casting, during normalizing, the casting is slowly heated to 1030°C to 1050°C and then kept at this temperature for 3 to 4 hours. The holding time should be set to ensure that the casting is fully heated through, the structure of the casting is completely transformed into austenite, and Cr carbides are basically dissolved.

[0012] In one embodiment, in the casting heat treatment step, after the normalizing treatment is completed, the casting is cooled to a surface temperature less than 80°C and then stagnated for more than 3 hours before being tempered. Since the martensitic structure transformation end temperature of the casting material involved in the invention is about 85°C, after the surface structure transformation is completed, it is stagnated for a period of time to wait for the core structure transformation of the casting to be completed.

[0013] In one embodiment, in the casting heat treatment step, after the normalizing treatment is completed, air cooling is used for cooling. Since the material of the casting involved in the invention is GX4CrNi13-4+QT1, which is air-quenched steel, a martensitic structure can still be obtained if static air cooling is used, but the martensitic lath spacing obtained by air cooling is large, and the strength and hardness are not high; if water cooling is used, the cooling speed is too fast, and the stress and dimensional deformation of the casting are large. Therefore, air cooling is selected as the normalizing cooling method.

[0014] In one of the embodiments, in the step of heat treating the casting, during the tempering treatment, the casting is slowly heated to a set temperature at a set heating rate, kept at the temperature for a set time, and then taken out of the furnace and air-cooled to room temperature.

[0015] In one embodiment, during the tempering treatment, the casting is slowly heated to 595°C to 615°C at a heating rate of 30°C / h to 50°C / h. Since the austenitization temperature of the casting material involved in the invention is about 585°C, the tempering temperature needs to be set between the two-phase region, and after the tempering is completed, a certain amount of reverse austenite is obtained to improve its toughness; when the temperature is lower than the set temperature, the reverse austenite content obtained is less, which will lead to higher strength and hardness, and insufficient plastic toughness; when the temperature is higher than the set temperature, the reverse austenite content obtained is more, which reduces the strength and hardness.

[0016] In one of the embodiments, in the casting heat treatment step, during the tempering treatment, the casting is slowly heated to 595° C. to 615° C. and then kept warm for 6 h to 7 h.

[0017] The present invention provides a heat treatment method for thin-walled castings, which realizes the dimensional deformation of ring-type castings under stable control through anti-deformation design of castings, production of heat treatment trays, and casting heat treatment design and process control, so as to make the mechanical properties of the castings meet the standards and significantly save casting production resources. Description of the drawings:

[0018] Figure 1 It is a schematic structural diagram of a casting of an embodiment;

[0019] Figure 2 A schematic diagram of the structure of an anti-deformation device provided for a casting in an embodiment;

[0020] Figure 3 This is a schematic structural diagram of the first side of a heat treatment tray in an embodiment;

[0021] Figure 4 This is a schematic structural diagram of the second side of the heat treatment tray of the embodiment;

[0022] Figure 5 It is a schematic diagram of the structure of the embodiment casting arranged on the heat treatment tray. DETAILED DESCRIPTION

[0023] In order to facilitate the understanding of the present invention, the present invention will be described more comprehensively below, and preferred embodiments of the present invention are provided. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present invention more thoroughly and comprehensively understood.

[0024] This embodiment mainly relates to a heat treatment method for a thin-walled lower ring casting of a hydraulic turbine. The casting is made of hydropower martensitic stainless steel GX4CrNi13-4+QT1, which is a high-alloy steel. Its outline size is Ф3150mm×1120mm, and the cut section is in the shape of "1". For details, see the attached Figure 1As shown, the thickness of the lower ring wall is 120mm at the thickest part and 40mm at the thinnest part, with a large difference in thickness.

[0025] The above heat treatment method specifically comprises the following steps:

[0026] Step 01, anti-deformation design of casting: When designing the casting process, an anti-deformation device is set on the inner ring of the casting and cast at the same time as the casting. The anti-deformation device is set at one end close to the end of the riser of the casting.

[0027] Specifically, the anti-deformation device includes a central frame and a plurality of tie bars, and the casting is connected to the central frame through the plurality of tie bars. Figure 2 As shown, the central frame can be set to be circular, and a number of tie bars are evenly arranged along the circular central frame, that is, the casting body is a "large ring", and the central frame is a "small ring", forming a "small ring pulling a large ring" shape, and the cross-sectional diameter of the central frame is similar to the average wall thickness of the casting; the tie bar size is similar to the wall thickness of the matching casting part, and the root of the casting is preferably placed on the casting riser patch, not in contact with the casting body, so as to avoid the auxiliary tie bar defect from extending to the casting body, and the number of tie bars is the same as the number of riser patches, or the two are in multiple relationship, so as to finally make the casting and the anti-deformation device form a mutually constrained whole, thereby increasing the stability of the casting. The tie bars preferably use a reusable runner to realize the utilization of runner waste.

[0028] Step 02, make heat treatment tray: see attached for details Figure 3 and 4 As shown, a plurality of slots with different diameters are arranged on the heat treatment tray for placing castings. The heat treatment tray is made of Q345 and the outer contour shape can be an ellipse.

[0029] Specifically, the tray includes a first surface and a second surface, and a plurality of concentric grooves are arranged on the first surface and the second surface, that is, the centers of the grooves are the same but the sizes are different, and the annular castings of different diameters are placed, that is, the annular castings are clamped in the grooves of matching sizes. In order to stabilize the castings, wedge blocks can be added to further fill the gap between the castings and the grooves, so that the castings are firmly placed on the heat treatment tray. Preferably, the first surface and the second surface are arranged opposite to each other, and the grooves on the first surface are staggered with the grooves on the second surface; both the first surface and the second surface can be used to place the castings, and multiple grooves are arranged according to the sizes of castings of different standards, and the grooves on the two surfaces are staggered, so that more castings of different sizes can be placed, expanding the use range of the heat treatment tray. The size of the groove is designed according to the inner and outer surface allowances of the casting to be heat treated, so that castings of different sizes can be clamped on the tray.

[0030] It should be noted that the detailed structural design of the tray also needs to consider that the heat flow in the entire casting structure can diffuse smoothly during the heating and cooling processes to avoid holding back air and causing local temperatures to be too high or too low. That is, while ensuring that it can support a stable casting, the remaining structural parts can be set to a hollow structure, that is, the grooves can be discontinuous circular grooves, etc.

[0031] Step 03, Casting Heat Treatment: See attached for details Figure 5 As shown, the side of the casting away from the deformation device is placed on the heat treatment tray to prevent the side of the casting away from the tray from deformation, and to avoid interference between the tie rod and the protruding part of the tray; after ensuring the rigid connection between the casting and the tray, the heat treatment tray with the casting is placed in the heat treatment furnace for normalizing treatment, and the casting is slowly heated to the set temperature at a set heating rate and kept warm for a set time; after the casting is cooled to the set temperature, it is tempered.

[0032] Specifically, during normalizing treatment, the casting is slowly heated to 1030°C to 1050°C at a heating rate of 30°C / h to 50°C / h, and kept warm for 3h to 4h. The casting is cooled to a surface temperature of less than 80°C by air cooling, and then stagnated for more than 3h before tempering. Since the martensitic structure transformation end temperature of the casting material involved in the invention is around 85°C, after the surface structure transformation is completed, it is stagnated for a period of time to wait for the core structure transformation of the casting to be completed.

[0033] It should be noted that if the heating rate is too high, the temperature difference between various parts of the casting increases, the internal stress increases, and the yield strength in the high-temperature area is low, which leads to deformation of the casting. The holding time should be set to ensure that the casting is fully heated through, the casting structure is completely transformed into austenite, and the Cr carbides are basically dissolved. Since the material of the casting involved in this invention is GX4CrNi13-4+QT1, which is air-quenched steel, a martensitic structure can still be obtained if static air cooling is used, but the spacing between the martensitic laths obtained by air cooling is large, and the strength and hardness are not high; if water cooling is adopted, the cooling rate is too fast, and the stress and dimensional deformation of the casting are large. Therefore, air cooling is selected as the normalizing cooling method, and axial flow fan air cooling can be used. After the normalizing is completed, the heat treatment trolley is opened, and several fans are placed diagonally along the axis direction of the lower ring casting on both sides of the trolley, or arranged in a "meter" distribution for air cooling to ensure the consistency of casting cooling.

[0034] The holding time is calculated according to the following formula:

[0035] Insulation time t = δmax*(1.5~2.0) / 60

[0036] Where:

[0037] t is the holding time, in h;

[0038] δmax is the maximum wall thickness of the blade, in mm;

[0039] 1.5~2.0, is the constant coefficient, unit is min / mm;

[0040] 60 is the unit conversion factor, the unit is min / h.

[0041] Specifically, during the tempering treatment, the casting is slowly heated to the set temperature at a set heating rate, kept at the set temperature for a set time, and then taken out of the furnace and air-cooled to room temperature. The casting is slowly heated to 595°C to 615°C at a heating rate of 30°C / h to 50°C / h, and kept at the temperature for 6h to 7h.

[0042] The holding time is calculated according to the following formula:

[0043] t=δmax*(1.5~2.0) / 60+3

[0044] Where:

[0045] t is the holding time, in h;

[0046] δmax is the maximum wall thickness of the blade, in mm;

[0047] 1.5~2.0, is the constant coefficient, unit is min / mm;

[0048] 61 is the unit conversion factor, the unit is min / h;

[0049] 3 is the constant coefficient, with the unit of h. It is longer than the normalizing time because the martensitic structure transformation will be delayed during this temperature period. According to production experience, it is necessary to increase the time by 3h so that the slower heated parts can also obtain a certain amount of reversed austenite.

[0050] It should be noted that since the austenitizing temperature of the casting material involved in this invention is about 585°C, the tempering temperature needs to be set between the two-phase region. After the tempering is completed, a certain amount of reverse austenite is obtained to improve its toughness. When the temperature is lower than the set temperature, the reverse austenite content obtained is less, which will lead to higher strength and hardness, and insufficient plasticity and toughness. When the temperature is higher than the set temperature, the reverse austenite content obtained is high, which reduces the strength and hardness.

[0051] By adopting the heat treatment method provided by the present invention, the bottom of the casting is rigidly connected to the tray, and the top is dimensionally constrained by an anti-deformation device, so that the resistance to deformation is increased, and the size qualification rate of the final casting after marking can reach 100%; the heat treatment tray is widely used and can meet the needs of all lower ring castings with a casting size of less than 5400mm and a wall thickness of less than 300mm, meeting more than 90% of the sizes of such castings on the market, and having a high reuse rate; the method of the present application can significantly save costs and speed up production efficiency, not only saving the process of welding and fixing to the tray in the prior art, so that an average of 16 hours is saved in the production of each product; but also reducing the labor cost of welders and the production cycle cost.

[0052] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0053] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A method for heat treatment of thin-walled castings, characterized in that: The following steps are involved: Anti-deformation design of castings: When designing the casting process, an anti-deformation device is set on the inner ring of the casting and is cast and produced simultaneously with the casting; Making a heat treatment tray: the heat treatment tray is provided with a plurality of slots of different diameters for placing the castings; Heat treatment of castings: placing the heat treatment tray with the castings in a heat treatment furnace for normalizing treatment, slowly heating the castings to a set temperature at a set heating rate, and keeping the temperature for a set time; after the castings are cooled to the set temperature, tempering treatment is performed.

2. The heat treatment method for thin-walled castings according to claim 1, characterized in that: In the casting anti-deformation design step, the anti-deformation device includes a central frame and a plurality of tie bars, and the casting is connected to the central frame via the plurality of tie bars.

3. The heat treatment method for thin-walled castings according to claim 1, characterized in that: In the step of making a heat treatment tray, the tray comprises a first surface and a second surface, and a plurality of concentric grooves are arranged on the first surface and the second surface.

4. The heat treatment method for thin-walled castings according to claim 1, characterized in that: In the casting heat treatment step, during normalizing, the casting is slowly heated to 1030° C. to 1050° C. at a heating rate of 30° C. / h to 50° C. / h.

5. The heat treatment method for thin-walled castings according to claim 4, characterized in that: In the casting heat treatment step, during normalizing, the casting is slowly heated to 1030° C. to 1050° C. and then kept warm for 3 h to 4 h.

6. The heat treatment method for thin-walled castings according to claim 1, characterized in that: In the casting heat treatment step, after the normalizing treatment is completed, the casting is cooled to a surface temperature less than 80° C. and then stagnated for more than 3 hours before being tempered.

7. The heat treatment method for thin-walled castings according to claim 1, characterized in that: In the casting heat treatment step, after the normalizing treatment is completed, air cooling is adopted as the cooling method.

8. The heat treatment method for thin-walled castings according to claim 1, characterized in that: In the step of heat treatment of the casting, during the tempering treatment, the casting is slowly heated to a set temperature at a set heating rate, kept at the temperature for a set time, and then taken out of the furnace and air-cooled to room temperature.

9. The heat treatment method for thin-walled castings according to claim 7, characterized in that: During the tempering treatment, the casting is slowly heated to 595° C. to 615° C. at a heating rate of 30° C. / h to 50° C. / h.

10. The heat treatment method for thin-walled castings according to claim 8, characterized in that: In the casting heat treatment step, during the tempering treatment, the casting is slowly heated to 595° C. to 615° C. and then kept warm for 6 h to 7 h.

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