Method for preventing internal hot cracks in spheroidal graphite cast iron

Abstract

The application provides a method for preventing and treating internal hot cracks of nodular cast iron, which comprises the following steps: sand mold molding, core making-coating paint-baking-matching box, synchronous molten iron smelting-pouring-cast cooling-flattening sand box-cooling-opening box-coarse cleaning polishing-heat treatment-fine cleaning; the original molten iron of the nodular cast iron comprises specific proportion chemical components; the coating paint is specifically as follows: no coating paint is coated on the lower core seat of the sand mold, sand filling treatment is carried out on the gap of the lower half part after the sand core is put in, coating paint is coated on the upper half part core head, and the corresponding core seat is also coated with coating paint; the timing of flattening the sand box is 2-3 hours after the cast cooling and then flattening the sand box. Through limiting the composition of the original molten iron of the nodular cast iron, the selective coating paint of the sand mold seat and the core and the timing of flattening the sand mold, the shrinkage stress during solidification is avoided from being large and the intergranular segregation is avoided, so that the formation tendency of the internal hot cracks is reduced.

Description

Technical Field

[0001] This invention relates to the field of ductile iron production, and more particularly to a method for preventing internal thermal cracking in ductile iron. Background Technology

[0002] A certain company produces a type of diesel engine ductile iron crankshaft made of QT800-2 material. After machining, magnetic particle inspection revealed a large number of fine cracks, which are curved lines with a length and depth of 5 to 30 mm. Shrinkage defects are also present in this area. Within six months, the scrap rate once reached 60%, which seriously affected the installation progress of the diesel engine and had a significant impact on the company's product quality and market.

[0003] Close observation revealed that the aforementioned cracks were not present on the surface of the casting, but were only detected during flaw detection after the crankshaft was precision-machined. Furthermore, they coexisted with shrinkage porosity and can be termed internal thermal cracks. Internal thermal cracks do not come into contact with air during solidification and therefore do not exhibit oxidation color; they can also be called stress shrinkage cavities, internal shrinkage cracks, or grain boundary cracks. The formation of internal thermal cracks primarily occurs during the solidification process of the casting. The outer layer first forms a crust, while the internal shrinkage thermal stress is high. Combined with intergranular segregation, significant thermal stress is generated in the last solidified areas of thick castings. At strain concentration points, the grain skeleton cracks. When the residual liquid phase in the last solidified portion of the casting is unable to fill these cracks, internal thermal cracks form. This phenomenon is even more severe in wide-solidification-temperature alloys like crankshafts. Summary of the Invention

[0004] The technical problem solved by this invention is to provide a method for preventing internal thermal cracks in ductile iron. The method provided in this application can reduce the tendency for internal thermal cracks to form.

[0005] In view of this, this application provides a method for preventing internal hot cracks in ductile iron, comprising: sand molding, core making, coating application, baking, mold assembly, simultaneous molten iron smelting, pouring, casting cooling, leveling the sand mold, cooling, mold opening, rough cleaning and grinding, heat treatment, and fine cleaning; characterized in that,

[0006] The original molten iron of the ductile iron comprises: C 3.8-3.9wt%, Si 1.05-1.2wt%, Mn 0.5-0.65wt%, P≤0.035wt%, S≤0.03wt%, Mo 0.2-0.3wt%, and Fe balance;

[0007] The specific process of applying the coating is as follows: the lower core seat of the sand mold is not coated; after the sand core is placed in, the gap in the lower half is filled with sand; the upper core head is coated; and the corresponding core seat is also coated.

[0008] The sand box is placed on the surface 2-3 hours after the casting has cooled.

[0009] Preferably, the pouring temperature is 1335-1345℃.

[0010] Preferably, the pouring sprue, runner and ingates adopt an open-closed-open gating system.

[0011] Preferably, the molten iron smelting specifically comprises:

[0012] According to the component proportion of the nodular cast iron, the ingredients are proportioned according to mass percentage: Q10 pig iron 15-35wt%, recycled material 25-45wt%, scrap steel 25-45wt%, manganese iron 0.3-0.5wt%, molybdenum iron 0.2-0.4wt%, carbon additive 1.2-2wt%, the above components are mixed and melted to obtain molten iron;

[0013] The inoculant is added in two times, the first inoculant is added in an amount of 0.4-0.5wt% when the molten iron is poured into the spheroidizing ladle for 1 / 2, the second inoculant is added in an amount of 0.2-0.3wt% when the molten iron is poured, and the inoculant is added uniformly through the funnel at the nozzle of the pouring cup; the furnace temperature is 1520-1540℃, and the pouring temperature is 1335-1345℃;

[0014] The spheroidizing agent, rare earth magnesium silicon iron alloy 1.0-1.2wt%, 0.5-0.7wt% copper, is added into the spheroidizing ladle, the spheroidizing ladle adopts a dam ladle, the spheroidizing agent is poured into one side of the dam, the copper plate and the crushed silicon iron are pressed on the spheroidizing agent, and the spheroidizing agent is flattened and compacted.

[0015] Preferably, the pouring mode is flat pouring.

[0016] Preferably, the casting cooling is sand box vertical cooling at 45°.

[0017] Preferably, the sand mold molding adopts furan resin sand molding.

[0018] Preferably, the final molten iron of the smelting comprises: C 3.5-3.9wt%, Si 2.0-2.35wt%, Mn 0.5-0.65wt%, P≤0.035wt%, S≤0.025wt%, Cu 0.5-0.7wt%, Mo 0.2-0.3wt%, Re 0.015-0.025wt%, Mg 0.04-0.05wt%, and Fe the balance.

[0019] The application provides a method for preventing and treating internal hot cracks of nodular cast iron, which comprises sand mold molding, core making-coating material brushing-baking-matching box, synchronous hot metal smelting-pouring-cast cooling-flatting sand box-cooling-opening box-coarse cleaning polishing-heat treatment-fine cleaning in sequence; in the above steps, the application limits the composition of the original hot metal of the nodular cast iron, selectively brushes coating material on the sand mold base and core, and limits the timing of flatting the sand mold, thereby avoiding the increase of shrinkage stress during solidification and intergranular segregation, and reducing the formation tendency of internal hot cracks. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 The application provides a method for preventing and treating internal hot cracks of nodular cast iron, which comprises sand mold molding, core making-coating material brushing-baking-matching box, synchronous hot metal smelting-pouring-cast cooling-flatting sand box-cooling-opening box-coarse cleaning polishing-heat treatment-fine cleaning in sequence; in the above steps, the application limits the composition of the original hot metal of the nodular cast iron, selectively brushes coating material on the sand mold base and core, and limits the timing of flatting the sand mold, thereby avoiding the increase of shrinkage stress during solidification and intergranular segregation, and reducing the formation tendency of internal hot cracks. DETAILED DESCRIPTION

[0021] In order to further understand the application, the preferred embodiments of the application are described below in combination with examples, but it should be understood that the description is only for further illustrating the features and advantages of the application, and is not a limitation on the claims of the application.

[0022] The application provides a method for preventing and treating internal hot cracks of nodular cast iron, which comprises sand mold molding, core making-coating material brushing-baking-matching box, synchronous hot metal smelting-pouring-cast cooling-flatting sand box-cooling-opening box-coarse cleaning polishing-heat treatment-fine cleaning in sequence; in the above steps, the application limits the composition of the original hot metal of the nodular cast iron, selectively brushes coating material on the sand mold base and core, and limits the timing of flatting the sand mold, thereby avoiding the increase of shrinkage stress during solidification and intergranular segregation, and reducing the formation tendency of internal hot cracks.

[0023] The original hot metal of the nodular cast iron comprises C 3.8-3.9 wt%, Si 1.05-1.2 wt%, Mn 0.5-0.65 wt%, P≤0.035 wt%, S≤0.03 wt%, Mo 0.2-0.3 wt%, and Fe balance;

[0024] The coating material is specifically that the lower core base of the sand mold is not coated with coating material, the gap of the lower half part is filled with sand after the sand core is put in, the core head of the upper half part is coated with coating material, and the corresponding core base is also coated with coating material;

[0025] The timing of flatting the sand box is 2-3 h after the cooling of the cast.

[0026] In the application, the molding process, smelting process and pouring process are limited; specifically, the first day is used for molding and core making, the second day is used for brushing and baking, and the third day is used for matching and pouring. Figure 1The core head and the core base are brushed in the molding. The lower core base is not brushed, and the gap of the lower half of the sand core is filled with sand after the sand core is put in. The sand core base and the sand core are brushed. The above processing is because the gap of the upper core head is not convenient for sand filling treatment, and the core head is easily wrapped when the molten iron is poured. Therefore, the brushing is performed to close the sand core exhaust, so that the lower half is left to exhaust the resin decomposition gas of the sand core, to avoid the gas in the sand mold (core) entering the molten iron when the molten iron is poured, causing volume expansion and secondary oxidation inclusions, so that the shrinkage stress is large when solidified and intergranular segregation occurs, thereby reducing the internal thermal stress crack. The gap between the core head and the core base is filled with sand, and the place where the molten iron is easily contacted is brushed with paint, to avoid the gas in the sand mold (core) entering the molten iron when the molten iron is poured, causing volume expansion and secondary oxidation inclusions, so that the shrinkage stress is large when solidified and intergranular segregation occurs, thereby reducing the internal thermal stress crack.

[0027] In the process of molten iron smelting, the original molten iron includes: C 3.8-3.9wt%, Si 1.05-1.2wt%, Mn 0.5-0.65wt%, P≤0.035wt%, S≤0.03wt%, Mo 0.2-0.3wt%, and Fe balance; the final molten iron includes: C 3.5-3.9wt%, Si 2.0-2.35wt%, Mn 0.5-0.65wt%, P≤0.035wt%, S≤0.025wt%, Cu 0.5-0.7wt%, Mo 0.2-0.3wt%, RE 0.015-0.025wt%, Mg 0.04-0.05wt%, and Fe balance.

[0028] The ingredient ratio is: Q10 pig iron 15-35%, recycled material 25-45%, scrap steel 25-45%, manganese iron 0.3-0.5%, molybdenum iron 0.2-0.4%, copper 0.5-0.7%, and carbon additive 1.2-2%; the inoculant (ferrosilicon+silicon barium) is added twice.

[0029] According to the composition ratio of nodular cast iron, the ingredients are mixed according to the mass percentage: Q10 pig iron 15-35wt%, recycled material 25-45wt%, scrap steel 25-45wt%, manganese iron 0.3-0.5wt%, molybdenum iron 0.2-0.4wt%, and carbon additive 1.2-2wt%. The above ingredients are melted to obtain molten iron.

[0030] The inoculant is added twice. The first inoculant is added in an amount of 0.4-0.5wt% when the molten iron is poured into the spheroidizing package 1 / 2. The second inoculant is added in an amount of 0.2-0.3wt% when the molten iron is poured, and is added uniformly at the nozzle of the pouring cup using a funnel. The furnace temperature is 1520-1540℃, and the pouring temperature is 1335-1345℃.

[0031] The spheroidizing agent rare earth magnesium ferrosilicon alloy 1.0-1.2wt%, 0.5-0.7wt% copper plate is added into the spheroidizing ladle, and the spheroidizing ladle adopts the dam ladle, the spheroidizing agent is poured into the dam side, the copper plate and the crushed ferrosilicon are pressed on the spheroidizing agent, and the spheroidizing agent is flattened and appropriately tamped.

[0032] In the above smelting process, the inoculant, the spheroidizing agent and the copper plate are all added based on the total amount of raw materials.

[0033] In the above smelting process, the high C content of the original molten iron is beneficial to the graphite expansion in the solidification process of the casting, self feeding is carried out, casting stress is reduced, and the shrinkage tendency is reduced; the inoculant performs secondary inoculation, slows down spheroidization recession and reduces component segregation, which is beneficial to reducing intergranular segregation and reducing the tendency of internal heat cracking; the pouring temperature is 1335-1345℃, the lower pouring temperature is not easy to remove slag and exhaust, and is easy to cold lap, too high is easy to cause large shrinkage tendency, this temperature meets the requirements of slag removal, exhaust reduction, less shrinkage tendency and less internal heat cracking tendency; further, the P and S contents of the molten iron are controlled, the intergranular segregation degree is reduced, and the internal heat cracking tendency is reduced; after spheroidization, the Re and Mg contents of the casting molten iron are controlled, the spheroidization rate is ensured, the intergranular segregation degree is prevented, and the internal heat cracking tendency is reduced.

[0034] In the casting process, flat pouring and vertical cooling are adopted, and the straight sprue, the cross sprue and the inner cross sprue adopt an open-closed-open type pouring system casting process, this casting method has good slag blocking and gas gathering, and is not easy to be oxidized and inclusions, the hot junction and the R angle of the casting are provided with cold iron, which balances the temperature field distribution, balances the solidification, avoids large local shrinkage tendency, and reduces the crack and shrinkage defects. Further, after the casting is completed, the sand box is placed horizontally after 2-3h at 45°, the molten iron paste state of the crankshaft is prevented from shaking, the outer layer is first solidified, the internal force changes, and the internal cracks are formed.

[0035] In the process of preparing nodular cast iron, except for the special description above, the technical means adopted in other steps is carried out according to the method familiar to those skilled in the art, and the present application has no special limitation.

[0036] The prevention method provided by the present application is for nodular cast iron, especially for the internal heat cracking defects of the nodular cast iron crankshaft of the diesel engine.

[0037] In order to further understand the present application, the prevention method of the internal heat cracking of the nodular cast iron provided by the present application is described in detail in combination with the examples, and the protection scope of the present application is not limited by the following examples.

[0038] Example 1

[0039] The preparation of the marine nodular cast iron crankshaft is prepared according to the following steps: sand mold molding, core making - coating - baking - closing, synchronous molten iron smelting - pouring - casting cooling - putting the sand box flat - cooling - opening the box - rough cleaning and polishing - heat treatment - fine cleaning; wherein the original molten iron composition: C 3.85wt%, Si 1.20wt%, Mn 0.62wt%, P 0.020wt%, S 0.028wt%, Mo 0.22wt%; the treatment of the core head and the core seat at the parting surface, the sand core core head is not coated, the lower core seat is not coated, the gap of the lower half part after putting the sand core is filled with sand, the upper half part core head is coated, and the corresponding core seat is also coated, to prevent the iron liquid from covering the core head and the sand core exhaust entering the iron liquid; the pouring temperature is 1336℃, the sand box is put flat after 2.5 hours of flat pouring and vertical cooling. 5 pieces are produced according to the above method; the results are all qualified, and no internal hot crack is formed.

[0040] Comparative Example 1

[0041] The preparation of the marine nodular cast iron crankshaft is prepared according to the following steps: sand mold molding, core making - coating - baking - closing, synchronous molten iron smelting - pouring - casting cooling - putting the sand box flat - cooling - opening the box - rough cleaning and polishing - heat treatment - fine cleaning; wherein the original molten iron composition: C 3.75wt%, Si 1.15wt%, Mn 0.6wt%, P 0.025wt%, S≤0.030wt%, Mo 0.2wt%; the treatment of the core head and the core seat at the parting surface, the sand core core head is not coated, the upper and lower core seats are not coated, and the gap of the lower half part after putting the sand core is filled with sand; the pouring temperature is 1362℃, and the sand box is put flat after 1 hour of flat pouring and vertical cooling. 5 pieces are produced according to the above method; the results are that 4 pieces all have internal hot cracks, and are accompanied by shrinkage.

[0042] Comparative Example 2

[0043] The preparation of the marine nodular cast iron crankshaft is prepared according to the following steps: sand mold molding, core making - coating - baking - closing, synchronous molten iron smelting - pouring - casting cooling - putting the sand box flat - cooling - opening the box - rough cleaning and polishing - heat treatment - fine cleaning; wherein the original molten iron composition: C 3.72wt%, Si 1.12wt%, Mn 0.58wt%, P 0.024wt%, S≤0.055wt%, Mo 0.21wt%; the treatment of the core head and the core seat at the parting surface, the sand core core head is not coated, the upper and lower core seats are not coated, and the gap of the lower half part after putting the sand core is filled with sand; the pouring temperature is 1342℃, and the sand box is put flat after 2.5 hours of flat pouring and vertical cooling. 5 pieces are produced according to the above method; the results are that 2 pieces all have internal hot cracks, and are accompanied by shrinkage and surface porosity of the casting blank.

[0044] Comparative Example 3

[0045] The preparation of the marine nodular cast iron crankshaft is prepared according to the following steps: sand moulding, core making - coating - baking - closing, synchronous molten iron smelting - pouring - cooling of the casting - placing the sand box - cooling - opening the box - rough cleaning and polishing - heat treatment - fine cleaning; wherein the original molten iron composition: C 3.76wt%, Si 1.12wt%, Mn 0.58wt%, P 0.034wt%, S 0.038wt%, Mo 0.2wt%; the treatment of the core head and core seat at the parting surface, the sand core head is not coated, the lower core seat is not coated, the gap in the lower half part is filled with sand after the sand core is placed, the upper half part of the core head is coated, and the corresponding core seat is also coated, so as to prevent the core head from being covered by the molten iron and the sand core exhaust entering the molten iron; the pouring temperature is 1338 DEG C, the sand box is placed after 2.5 hours of flat pouring and vertical cooling. According to the above method, 5 pieces are produced; and the result is that 1 piece has slight internal hot crack and is accompanied by shrinkage.

[0046] The above description of the embodiments is only used to help understand the method of the present application and its core idea. It should be noted that for those skilled in the art, some improvements and modifications can be made to the present application without departing from the principles of the present application, and these improvements and modifications also fall within the protection scope of the claims of the present application.

[0047] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method of preventing internal heat cracks in ductile cast iron, comprising: Sand mold molding, core-making - coating - baking - closing, synchronous molten iron smelting - pouring - casting cooling - flat sand box - cooling - opening - rough cleaning polishing - heat treatment - fine cleaning; characterized in that, The raw molten iron of the ductile cast iron comprises: C 3.8-3.9wt%, Si 1.05-1.2wt%, Mn 0.5-0.65wt%, P≤0.035wt%, S≤0.03wt%, Mo 0.2-0.3wt%, and Fe balance. The coating material is specifically: no coating material is applied to the lower core seat of the sand mold, the gap of the lower half part is filled with sand after the sand core is put in, the upper half part core head is coated with coating material, and the corresponding core seat is also coated with coating material. The timing of the flat sand box is 2-3h after the casting cooling, and the casting cooling is the sand box with 45° vertical cooling.

2. The control method according to claim 1, characterized by, The pouring temperature is 1335-1345℃.

3. The control method according to claim 1, characterized by, The pouring sprue, runner and inner runner adopt an open-closed-open pouring system.

4. The control method according to claim 1, characterized by, The molten iron smelting is specifically: According to the component proportion of the ductile cast iron, the ingredients are proportioned according to mass percentage: Q10 pig iron 15-35wt%, recycled material 25-45wt%, scrap steel 25-45wt%, manganese iron 0.3-0.5wt%, molybdenum iron 0.2-0.4wt%, and carbon additive 1.2-2wt%, the above components are mixed and melted to obtain molten iron; The inoculant is added in two times, the first time inoculant is added in an amount of 0.4-0.5wt%, and is added when the molten iron is poured into the spheroidizing package 1 / 2, the second time inoculant is added in an amount of 0.2-0.3wt%, and is added uniformly at the nozzle of the pouring cup by using a funnel when the molten iron is poured; the furnace temperature is 1520-1540℃, and the pouring temperature is 1335-1345℃; The spheroidizing agent rare earth magnesium silicon iron alloy 1.0-1.2wt% and 0.5-0.7wt% copper are added into the spheroidizing iron package, the spheroidizing iron package adopts a dam package, the spheroidizing agent is poured into one side of the dam, the copper plate and the crushed silicon iron are pressed on the spheroidizing agent, and the spheroidizing agent is leveled and properly tamped.

5. The control method according to claim 1, characterized by, The pouring mode is flat pouring.

6. The control method according to claim 1, characterized by, The sand mold molding adopts furan resin sand molding.

7. The control method according to claim 1, characterized by, The final molten iron of the smelting comprises: C 3.5-3.9wt%, Si 2.0-2.35wt%, Mn 0.5-0.65wt%, P≤0.035wt%, S≤0.025wt%, Cu 0.5-0.7wt%, Mo 0.2-0.3wt%, Re 0.015-0.025wt%, Mg 0.04-0.05wt%, and Fe balance.

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