Process Analysis of Lost Foam Casting Ductile Iron Pipe Fittings


There are many foundries in China using lost foam casting process to produce ductile iron pipe fittings, the output is quite large, and the product quality can meet the requirements of international standards. Our company produces various specifications and models of oil pipeline valve fittings for Iran HANYCO company. We have accumulated a lot of experience in the production process. In this process, we are willing to discuss and analyze with our colleagues in the casting industry, learn from each other and make common progress. At the same time, I hope you can criticize and correct the shortcomings.

Casting Process Analysis of Ductile Iron Pipe Fittings

The lost foam casting process has considerable advantages in the production of ductile iron pipe fittings, a wide variety of pipe fittings, and the versatility of the lost foam casting sand box solves the problem of burying the pipe fittings of different specifications and models. At the same time, the white mold produced by the lost foam casting mold can be assembled interchangeably to reduce the production cost. At the same time, the end face of the model does not need to be divided, and the end face hole of the flange plate can be cast without processing, which is suitable for mass production and greatly reduces the labor cost. On the whole, the cost of lost foam casting ductile iron pipe fittings is relatively low. In the pouring process of lost foam casting, the foam will vaporize rapidly after encountering molten iron, and the temperature of molten iron will drop. If the pouring temperature is too low, defects such as cold isolation and insufficient pouring will easily occur. Sometimes, incomplete gasification of foam model will occur, causing some foam residues to remain in the casting or on the casting surface, resulting in carbon defects. Such defects will lead to loose and non-dense internal structure of the casting and affect the pipe wall of ductile pipe, the appearance of these defects will lead to the phenomenon of water seepage in the process of casting pressure. In this regard, I through continuous efforts and try to improve the process, take appropriate measures to produce qualified castings.

1. White area

(1) Advance

Use an electric steam pre-generator, the foaming raw material is Longwang material, the cylinder temperature is controlled at about 90 ℃, the bead density after pre-generator is 22~23g/L, and the pre-generator is dried and cured for at least 5h before use. The electric steam pre-generator is mainly heated by steam and assisted by electric heating to effectively control the uniformity of the temperature of the cylinder. It has the advantages of high working efficiency, uniform foaming and good bead curing effect.

(2) Molding

Use semi-automatic molding machine, running smoothly. Setting of external conditions: steam pressure 0.4~0.6MPa, water pressure 0.3~0.4 MPa, air pressure 0.6~0.8 MPa, manual charging. Adjust and set the running parameters, and pay attention to prevent the deformation of the white mold.

(3) White model

Put the qualified white mold into the drying room for drying. The drying room adopts automatic control of temperature and humidity, steam main drying and electric heating auxiliary. The temperature of the drying room is set to 45 ℃ and the humidity is set to below 16%. The white mold is placed in the drying room for drying for 1 day, and the weight of the white mold shall be subject to the same weight before entering the painting process.

(4) bonding, group type

Grind the burrs on the outer surface of the model, and use paper tape, double-sided adhesive tape and repair paste to repair pits, damage and other defects. Uniformly apply cold glue to bond the white mold segments together. Whether bonding or repairing, the amount of glue should be strictly controlled. Under the condition of ensuring the bonding strength, the less glue is used, the better, so as to reduce its influence on the casting. The bonding gap is sealed with double-sided adhesive, and the position on the inner side of the casting that cannot be used with double-sided adhesive can be replaced by newspaper. This method can effectively reduce the defect of water seepage of the casting. As shown in the group Figure 1.

 

Fig. 1 White mold repair

When bonding modules, attention must be paid to concentricity to ensure that the form and position tolerance is within the tolerance range. When necessary, tires should be used. Cold glue is used for bonding. Cold glue has the characteristics of firm bonding and long solidification time. It is convenient to have enough time to adjust the concentricity of flange when bonding white mold. When bonding, attention should be paid to the gap. When the gap cannot be avoided, repair paste can be used for repair, as shown in Figure 2.

 

Fig. 2 Type of white mold bonding group

2. Yellow Zone

(1) The paint shall be stirred in accordance with the specified ratio. The brushing is carried out by dipping, stirring for 120min, adjusting the viscosity, and the Baomei degree can reach about 1.6.

(2) The white mold of the group is painted 2-3 times. The painting method is mainly dipping. The temperature of the drying room is set at 50 ℃ and the humidity is set below 16%, so that each layer of paint can be completely dried. The first drying time is 1 day, and the exposed part is recoated. After the second drying time is 12 hours, the gate and inlet are thickened and brushed, and the drying time is about 12-24 hours. After two times, the thickness of the yellow mold coating reaches about 1.3mm, and the gate and inlet reaches about 1.6mm.

(3) The main points of white mold dipping coating. First, the coating on the pipe fitting pasting part and the flange end face needs to be brushed with a brush to prevent bubbles at the edges and corners of the groove from causing defects such as sand and meat sticking to the casting. Secondly, paint must not be accumulated at the socket groove. This is the position where sand is easy to stick. For smaller pipe fittings, resin sand or paint mud can be embedded to solve the problem. For larger pipe fittings, graphite paint can be brushed once more to solve the problem. Finally, the pipe fittings after painting shall be allowed to stand for 1min until the surface coating is uniform, and then the placement position shall be adjusted and placed in the drying room for drying to prevent paint accumulation on the yellow mold.

(4) The white mold of the pipe fittings after painting is extremely prone to deformation, especially the socket and socket, and the anti-deformation measures are essential. There are many kinds of anti-deformation measures. Let me introduce some of the more commonly used methods.

The first is the sand liner anti-deformation measures, the operation method is to use furan resin sand mixing machine mixing or manual mixing.

For the sand lining, the size of the sand ring shall be determined according to the inner diameter of the socket or socket where deformation prevention is required. The sand liner can choose different positions to prevent deformation according to the diameter of the pipe, as shown in Figure 3. Installation of sand liner When the last coating is applied, the sand liner also needs to be dip coated before installation.

 

Fig. 3 Anti-deformation measures

The second is the iron lining anti-deformation measures, iron lining is made of 20mm flat iron, flat iron thickness according to the size of the pipe selection. During production, the flat iron is fixed to the tooling to lap weld the interface, and the cross support reinforcement is welded. The roundness of the iron lining must be ensured. The ovality of the DN300-500 is not more than 2mm, and the ovality of the DN600-900 is not more than 3mm. Install the iron liner when applying the paint for the last time. The iron liner shall be brushed with a layer of paint in advance. The thickness of the paint shall be controlled at 0.5-0.7mm and dried, as shown in Figure 4.

 

Fig. 4 Anti-deformation Measures for Iron Liner

3. Black area

(1) In terms of smelting process, one ton electric furnace is used for smelting. During the smelting process, the composition of molten iron should be strictly controlled. The design composition of molten iron is C:3.6~3.8; Si:1.2~2.5; Mn:≤ 0.4%;P:≤ 0.07%;S:≤ 0.03%. In the smelting process, the amount of raw materials added and the quality of raw materials must be strictly controlled. Under normal circumstances, the amount of pig iron added is about 65%, the amount of iron returned to the furnace is about 22%, the amount of scrap steel added is about 12%, the amount of spheroidizing agent added is about 1%, and the amount of inoculant added is about 0.6. Minimize the use of severely rusted steel, slag should be removed during smelting, and the temperature of the furnace is about 1580 ℃. After the molten iron is discharged from the furnace, slag shall be beaten again to ensure that there is no residue in the molten iron. During the pouring process, slag retaining cotton shall be used to effectively stop slag to ensure that no slag enters the casting.

(2) The spheroidization treatment adopts the method of spheroidization, the inoculant adopts QRMg8Re2, and the addition amount is 1.0~1.3, and the 75 ferrosilicon inoculant is used, and the inoculation and addition amount are controlled at about 0.6. Put the well-called inoculant and nodulizer into the special ladle for ductile iron and cover it with iron filings to prevent the nodulization reaction from proceeding prematurely, resulting in poor water nodulization. The nodulization process must be strictly controlled and the nodulization must be complete.

(3) pouring system design generally adopts open pouring system to ensure the filling speed and prevent the collapse of the box. The pouring temperature should not be too low, otherwise it will lead to incomplete gasification of foam and excessive residual substances, resulting in carbon defects in castings and water leakage during pressure. As shown in Figure 5 for the various types of pipe casting system design, to provide a reference.

  

Fig.5 Design of gating system

4. CASTING CLEANING

After casting is poured, the heat preservation time is selected according to the size and structure of the casting, which is generally controlled at more than 30min. The casting after turning over the box needs to be placed in a place without wind for natural cooling to prevent unnecessary losses caused by casting deformation due to uneven cooling. After natural cooling, workers cut the runner with a plasma cutting machine, and knock down the gate part with a hammer, as shown in Figure 6. After the casting is removed from the gate, subsequent workers are required to perform shot blasting. After shot blasting, the gate part and the sand-sticking part are polished with an angle grinder. This process requires skilled workers to operate.

 

Fig. 6 Casting Cleaning Process

5. Casting suppression

The pipe fittings are pressed with special pressing equipment. The DN40-300 minimum experimental pressure is 2.5Mp, the DN350-600 minimum experimental pressure is 1.6Mp, and the DN1100-2000 minimum experimental pressure is 1.0Mp. The operation method is to put the experimental pipe fittings into the hydraulic press, press the pipe fittings with the oil cylinder, open the drain port and the water pump to add water, wait for the pipe fittings to be filled with water, close the exhaust port, open the booster pump to pressurize to the required pressure, and maintain the pressure for more than 15s. Observe whether there is leakage around the pipe fittings and mark it for further repair.

6. Casting repair

Pipe fitting repair is an important work link. Gas cutting is used to heat the leakage part of the pipe fitting with light leakage. After heating, sulfuric acid powder is sprinkled on the surface of the pipe fitting and applied evenly. After heating, sulfuric acid powder will liquefy and leak into the pipe fitting, thus achieving the effect of leakage repair, as shown in Figure 7.

 

Fig. 7 Casting Repair Process

For the parts with serious leakage of pipe fittings, repair welding shall be used to repair them. First, the leaking parts shall be polished, and J422 welding rod shall be used to repair them. After polishing, cast iron welding rod can be used to repair the sand holes of castings, as shown in Figure 8.

 

Fig. 8 Casting Repair Welding Process

7. Standard operation of lining cement

(1) Make cement according to the proportion: sand cement water

Summer: 2 : 1 : 0.5

Winter: 1.5 : 1 : 0.5

(2) cement thickness according to customer demand lining mud thickness, if the customer has no demand, in accordance with international standards for cement lining.

(3) After pressing the qualified castings, the diluted construction glue (801 glue) shall be applied to the parts that need to be lined with cement. Then put the right amount of cement.

(4) the use of tools to smooth the cement, must be uniform.

(5) Wait for more than 24 hours to dry.

(6) the use of tools: 1/5 PVC pipe, spoon.

(7) Precautions: The construction glue is too thin and the cement is easy to fall off.

8. Pipe fittings grinding and painting

After qualified pressing, cleaning is completed, and the cement lining reaches the final painting process. In order to ensure the smoothness of the surface of the pipe fittings after painting, the cement remaining on the surface of the casting shall be ground off with a polishing machine. For the grinding marks generated in the cleaning process, a layer of putty, commonly known as putty, shall be coated on the surface of the casting. Appropriate curing agent shall be added during use. After the polishing putty is dried, the casting surface shall be polished, then enter the painting link, as shown in group figure 9.

 

Fig. 9 Putty on Castings

   9. Summary

Lost foam casting production is interlocking, pay attention to all processes forward, all problems are solved in the previous link, so as to finally produce qualified castings. Therefore, technicians and workers need to have rich operation experience and serious attitude to ensure that each link is carried out in an orderly manner and the system of responsibility to people is in place, so as to avoid some unnecessary low-level mistakes leading to the scrapping of castings and affecting the efficiency of the workshop.

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