A method for improving the surface quality of ductile cast iron pipe

Abstract

The application discloses a method for improving the surface quality of nodular cast iron pipe, which comprises the following steps: (1) before centrifugal casting, a spraying method is used to replace a brushing method to spray release agent on the inner wall of a pipe mold, after spraying one layer or several layers, the next round of spraying is performed after water in the release agent is dried, and the spraying is performed back and forth for multiple rounds until the required spraying thickness is reached; a way of sleeving a slag resisting ring on the core head part of the socket end is combined with an online spheroidizing process to improve the mechanical properties and compactness of the inner surface of the nodular cast iron pipe, and to prevent slag inclusion and looseness of the inner surface of the socket; (2) the preheating temperature of the nodular cast iron pipe is controlled in a specified range during surface zinc spraying, and when a corrosion-proof layer and / or a heat preservation layer is sprayed on the outer surface of the nodular cast iron pipe, a top tip type rotary chuck is used to support the nodular cast iron pipe from the inner sides of two ports of the nodular cast iron pipe, so that the outer surface of the nodular cast iron pipe is ensured not to be in contact with other components.

Description

Technical Field

[0001] This invention relates to the field of centrifugal casting technology for ductile iron pipes, and in particular to a method for improving the surface quality of ductile iron pipes. Background Technology

[0002] Ductile iron pipes can be used as fluid transport pipelines or heat pipelines in water supply and drainage projects. Industrially, they are generally produced using centrifugal casting. For example, the patent document with publication number CN1119570A, entitled "A Method for Producing Centrifugal Ductile Iron Pipes," describes the basic process of heating a pipe mold, spraying a heat-insulating coating on the inner wall of the mold, centrifugally casting it into a ductile iron pipe, and then transferring the ductile iron pipe into an annealing furnace for annealing to obtain a ductile iron pipe with a high elongation. Before leaving the factory, ductile iron pipes also need to be sprayed with different protective or functional layers according to the working conditions and the technical requirements of the customer. Therefore, whether ductile iron pipes meet quality standards largely depends on the surface casting quality and the surface coating quality. In long-term production, the surface casting quality problems that are prone to recurrence in ductile iron pipes include many pits on the outer surface, poor flatness, slag inclusions and local looseness on the inner surface, and surface spraying quality problems include uneven zinc coating thickness, easy damage and peeling of the spray coating, and easy mechanical indentation on the outer anti-corrosion layer and / or insulation layer, which have not been completely eliminated. Summary of the Invention

[0003] To address the problems in the background art, this invention provides a method for improving the surface quality of ductile iron pipes. Starting from the two stages of casting and spraying, it adopts multiple effective measures to prevent and eliminate surface casting defects and surface spraying defects, thereby synergistically improving the surface quality of ductile iron pipes.

[0004] The technical solution of the present invention is as follows:

[0005] A method for improving the surface quality of ductile iron pipes includes:

[0006] (1) Prevention and elimination of surface casting defects

[0007] Before centrifugal casting, a spraying method is used instead of brushing to spray the release agent on the inner wall of the pipe mold. After each layer or several layers are sprayed, wait for the moisture in the release agent to dry before the next round of spraying. Repeat the spraying process multiple times until the required coating thickness is achieved. The uniform spraying of the release agent improves the smoothness of the outer surface of the ductile iron pipe and reduces pits on the outer surface.

[0008] The casting process uses a slag-blocking ring fitted at the core end of the socket, combined with an online spheroidizing process, to improve the mechanical properties and density of the inner surface of the ductile iron pipe and prevent slag inclusions and porosity on the inner surface of the socket.

[0009] (2) Prevention and elimination of surface coating defects

[0010] When controlling the surface zinc spraying, the preheating temperature of the ductile iron pipe is within the specified range. When spraying the anti-corrosion layer and / or heat insulation layer on the outer surface of the ductile iron pipe, the ductile iron pipe is supported from the inside of both ends by a center-type rotating chuck to ensure that the outer surface of the ductile iron pipe does not come into contact with other components.

[0011] Furthermore, the required coating thickness is 0.8-2.5 mm.

[0012] Furthermore, the main component of the release agent is zircon powder.

[0013] Furthermore, the slag-blocking ring is a ductile iron ring, which is fitted onto the part of the ductile iron pipe inner wall corresponding to the core head at the socket end where slag inclusions and loosening are prone to occur.

[0014] Furthermore, the ductile iron ring is a solid ring or formed by wrapping fine ductile iron fragments with iron sheet.

[0015] Furthermore, the online spheroidizing process is a spheroidizing treatment performed on the molten casting in the ladle on a casting trolley.

[0016] Furthermore, the online spheroidizing process is operated using a mobile spheroidizing frame on a ladle trolley.

[0017] Furthermore, the specified range of the preheating temperature is 150-200℃.

[0018] Furthermore, the specified range of the preheating temperature is 170-190℃.

[0019] Furthermore, the aforementioned top-mounted rotating chuck is also used to support the ductile iron pipe during zinc spraying.

[0020] The beneficial effects of this invention are as follows:

[0021] The method for improving the surface quality of ductile iron pipes provided by this invention starts from the two stages of casting and spraying, and adopts multiple effective measures to prevent and eliminate surface casting defects and surface spraying defects. In particular, it provides targeted improvement strategies for defects that are prone to recurrence in production, such as casting pits, slag inclusions, and local porosity, as well as defects in spraying, such as uneven coating distribution, easy damage and peeling, and easy mechanical indentation. These measures work together to improve the surface quality of ductile iron pipes. After a period of improvement and verification, the product qualification rate has been greatly improved. Attached Figure Description

[0022] Figure 1 This is a roadmap for the method of improving the surface quality of ductile iron pipes according to the present invention. Detailed Implementation

[0023] Example 1

[0024] A method for improving the surface quality of ductile iron pipes includes:

[0025] (1) Prevention and elimination of surface casting defects

[0026] The first measure adopted in this embodiment is to replace the previous brushing method with a spray method to spray the release agent on the inner wall of the pipe mold before centrifugal casting. The spray method uses compressed air to refine the release agent slurry into a spray, each layer is thinner and the spraying is more uniform, thereby reducing the probability of slurry lumps appearing after the release agent dries. As a result, the outer surface of the cast ductile iron pipe is smoother, with fewer pits and smaller size.

[0027] A high-temperature resistant release agent based on zircon powder micropowder is preferred. After each layer or several layers are sprayed, the moisture in the release agent is allowed to dry before the next round of spraying is carried out. Multiple rounds of spraying are carried out until the required coating thickness is achieved, preferably 0.8-2.5 mm, more preferably 1.2-2.0 mm. This improves the smoothness of the outer surface of the ductile iron pipe and reduces pits on the outer surface through uniform spraying of the release agent.

[0028] The second measure adopted in this embodiment is to use a slag-blocking ring fitted on the core head of the socket end in the casting process, combined with the online spheroidizing process, to improve the mechanical properties and density of the inner surface of the ductile iron pipe, and to prevent slag inclusions and porosity on the inner surface of the socket.

[0029] The socket is the area most prone to inclusions and porosity defects during the centrifugal casting process of ductile iron pipes, and these defects are most likely to occur near the surface. This is because the socket wall is relatively thick. After the molten iron is poured into the mold, under the action of centrifugal force, the molten iron slag and newly formed impurities in the thick-walled socket area are more likely to float towards the center, thus making the socket area more prone to inclusions. In addition to the same reasons as the inclusions mentioned above, the decline in surface mechanical properties is also largely caused by factors such as the spheroidization decay that occurs when the molten ductile iron waits too long before pouring. This factor will lead to a decline in the surface mechanical properties of not only the socket area but also the entire ductile iron pipe, and the newly formed magnesium slag will exacerbate the problems of inclusions and porosity.

[0030] This embodiment effectively addresses the aforementioned problems by setting a slag-blocking ring. Specifically, the slag-blocking ring in this embodiment is a ductile iron ring, which is fitted onto the inner wall of the ductile iron pipe corresponding to the core head at the socket end before mold closing, at the parts prone to slag inclusions and looseness. During centrifugal casting, the slag-blocking ring not only physically blocks the upward floating of molten iron and slag towards the center, but also further eliminates slag inclusion defects on the inner surface of the ductile iron pipe, especially at the socket end, by influencing the floating time and the formation mechanism of new slag. The ductile iron ring described in this embodiment is a solid ring or formed by wrapping fine ductile iron fragments with iron sheet. It can be made into a simple ring that is consistent with the specific placement position on the outer surface of the core head at the socket end. The gap size, cross-sectional area, etc., only need to be reasonably designed according to the space between the core head at the socket end and the external centrifugal casting mold to ensure that it does not affect the filling of molten iron and can be fused into the casting itself. Installing slag-blocking rings is a very effective way to control slag inclusions and looseness defects. Such problems that were previously stubborn in production can even be completely eliminated in some areas.

[0031] Online spheroidizing is a crucial measure to ensure effective spheroidizing timing. In this embodiment, spheroidizing treatment of the molten casting in the ladle is preferably performed on the ladle trolley. This can be achieved using a mobile spheroidizing frame on the ladle trolley. The mobile spheroidizing frame can be made into a simple support, carrying a spheroidizing container filled with spheroidizing agent, and is movably installed on the ladle trolley. During the ladle transfer process or after the ladle trolley has transferred the ladle to the pouring station but before pouring begins, the mobile spheroidizing frame moves above the ladle and pushes the spheroidizing container downwards into the ductile iron in the ladle. The spheroidizing container has multiple molten iron inlets and outlets, which can fully spheroidize the ductile iron in the ladle. Because online spheroidizing of the ductile iron can be performed, it not only allows for more flexible postponement of pre-mold spheroidizing time but also avoids the uneven spheroidizing caused by the operational difficulty of in-mold spheroidizing. This avoids exacerbating defects such as slag inclusions, porosity, and decreased surface quality due to improper spheroidizing timing and also prevents potential new and unnecessary quality problems.

[0032] (2) Prevention and elimination of surface coating defects

[0033] Zinc spraying is an indispensable coating process for ductile iron pipe products. In this embodiment, by controlling the preheating temperature of the ductile iron pipe during zinc spraying within a specified range, the uniformity and adhesion strength of the coating can be effectively improved. The specified range of preheating temperature can be controlled between 150-200℃, and the coating quality is best when the temperature is in the range of 170-190℃.

[0034] Ductile iron pipes typically require multiple protective or functional layers. After zinc spraying, anti-corrosion and insulation layers are usually applied. Conventional spraying equipment, needing to constantly support the weight of the ductile iron pipe, can easily leave mechanical indentations on the outer surface when applying these layers. By improving the spraying equipment and incorporating the center-type rotating chuck structure used in zinc spraying, the center-type rotating chuck supports the ductile iron pipe from the inside of both ends when applying the anti-corrosion and / or insulation layers, ensuring that the outer surface of the ductile iron pipe does not contact other components.

[0035] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for improving the surface quality of ductile iron pipes, characterized in that, include: (1) Prevention and elimination of surface casting defects Before centrifugal casting, a spraying method is used instead of brushing to spray the release agent on the inner wall of the pipe mold. After each layer or several layers are sprayed, wait for the moisture in the release agent to dry before the next round of spraying. Repeat the spraying process multiple times until the required coating thickness is achieved. The uniform spraying of the release agent improves the smoothness of the outer surface of the ductile iron pipe and reduces pits on the outer surface. The casting process uses a slag-blocking ring fitted at the core end of the socket, combined with an online spheroidizing process, to improve the mechanical properties and density of the inner surface of the ductile iron pipe and prevent slag inclusions and porosity on the inner surface of the socket. (2) Prevention and elimination of surface coating defects When controlling the surface zinc spraying, the preheating temperature of the ductile iron pipe is within the specified range. When spraying the anti-corrosion layer and / or heat insulation layer on the outer surface of the ductile iron pipe, the ductile iron pipe is supported from the inside of both ends by a center-type rotating chuck to ensure that the outer surface of the ductile iron pipe does not come into contact with other components. The specified range for the preheating temperature is 150-200℃.

2. The method for improving the surface quality of ductile iron pipes according to claim 1, characterized in that, The required coating thickness is 0.8-2.5 mm.

3. A method for improving the surface quality of ductile iron pipes according to claim 1 or 2, characterized in that, The main component of the release agent is zircon powder.

4. The method for improving the surface quality of ductile iron pipes according to claim 1, characterized in that, The slag-blocking ring is a ductile iron ring, which is fitted onto the part of the ductile iron pipe inner wall corresponding to the core head at the socket end where slag inclusions and loosening are prone to occur.

5. The method for improving the surface quality of ductile iron pipes according to claim 4, characterized in that, The ductile iron ring is a solid ring or formed by wrapping fine ductile iron fragments with iron sheet.

6. A method for improving the surface quality of ductile iron pipes according to claim 4 or 5, characterized in that, The online spheroidizing process is a spheroidizing treatment performed on the molten casting in the ladle on a casting trolley.

7. The method for improving the surface quality of ductile iron pipes according to claim 6, characterized in that, The online spheroidizing process is operated using a mobile spheroidizing frame on a ladle trolley.

8. The method for improving the surface quality of ductile iron pipes according to claim 1, characterized in that, The specified range for the preheating temperature is 170-190℃.

9. A method for improving the surface quality of ductile iron pipes according to claim 8, characterized in that, The aforementioned top-type rotating chuck is also used to support the ductile iron pipe during zinc spraying.

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