Optimization of lost foam casting process for grate bar


The grate bar for sintering machine is used to support the high-temperature coke and ash after combustion. The material is required to be heat-resistant and wear-resistant. The brand isCr20Mn14Si2N. The outline size of the casting is 70mm high, 40mm wide, 800mm long, 2200mm long, 1200mm wide and 800mm high, and the single weight of the casting blank is 19kg. It is required to make full use of the sand box space. 36 pieces or more are poured in one box. The casting technology requires full processing and no casting defects are allowed on the working surface. Therefore, although the structure of the casting is simple, it requires high casting technology and production. The structure of the casting is shown in Figure 1.

Figure1 grate bar casting structure

1 Original process and production status

1.1 original process production form

The lost foam casting production method is adopted, and the box type is used for modeling, and the break is filled with wet paint. Sand box size is long2200mm wide 1200mm high 800mm, each box of castings buried in a cluster, each cluster model out of 12 pieces, gating riser process as shown in Figure 2. The pouring temperature is 1620 ℃, the negative pressure degree of pouring is-0.055MP, the negative pressure degree during pressure holding is-0.02MP, and the pressure holding time is 30min.

Figure2 The casting model cluster produced by the original process.

1.2 of the original process characteristics and production of casting quality

1.2.1 Original process characteristics

In the original process, the casting has two risers, the molten metal is poured from one riser, the riser at the distal end is a cold riser, and the section near the gate is a hot riser. Considering that the feeding distance of the riser is equal to three times the diameter of the riser, this feeding process is unreasonable, the feeding effect of the cold riser at the distal end is poor, and the middle part of the casting cannot be fed beyond the feeding distance.

Due to the relatively large size of the sand box, it can accommodate more castings, thereby improving production efficiency; therefore, the model cluster is produced in one box.12 pieces, the process of burying only one layer in the buried box is unreasonable, there is no effective use of the sand box space, and the production efficiency is too low.

1.2.2 Casting quality condition

1): sand and slag defects.

The castings are fully machined castings and require a higher casting quality. However, due to the use of the box group type, the strength of the part filled with wet paint is not enough, the bonding is not firm, and it is a fragile part that is prone to problems. During pouring, these bonded parts are washed away by high-temperature liquid metal, resulting in slag and sand defects in castings. In addition, in the formulation of the casting process, because the actual use of the casting was not taken into account, the actual working face of the casting was upward in the direction of the pouring position, and the working face of the casting was not allowed to have defects, resulting in the scrap of slag and sand inclusion of the casting, as shown in the figure.3 is shown.

Figure3 Defect diagram of slag and sand inclusion after full processing

2) Air hole

There are wrinkled pores on the surface of the casting. According to the defect shape, it is determined that the water vapor caused by the paint is not completely dried. The defect shape is shown in the figure.4 is shown. Due to the production process, the production workers in the paint drying and inspection links to do not carefully, the paint is not completely dried.

Figure4 Porosity Defect Chart

2 Process optimization

According to the problems in actual production and the reasons for casting scrap, the casting process of grate bar castings was optimized.

2.1: improvement of the riser.

In order to increase the stability of the process and change the design of the casting riser, the original two risers are close to the middle of the casting length direction. According to the calculation of the feeding distance, the center of the riser can be obtained from both ends of the casting.The position of 260mm can realize feeding in place, the riser is 300mm long, 110mm wide and 60mm high, and three grate bars are spanned under one riser. In addition, the inner runner is directly connected with the two risers to realize the feeding of the hot risers and increase the feeding efficiency, so that the temperature field of the casting tends to be reasonable. After optimization, the sprue runner of the gating system is 60X60mm, the runner is 60X50mm high, the inner runner is 50X40mm high, the model cluster has three layers, and the distance between each layer is 60mm.

Control of slag inclusion, sand inclusion and porosity defects in 2.2

The casting faces downward in the direction of the pouring position to ensure the quality of the working face. The outer box type is adopted to ensure the tightness of the model after coating, and to prevent slag and sand inclusion defects caused by damage to fragile parts of the coating caused by replenishment at the butt joint. The pouring cup adopts foam pouring cup. When coating, the pouring cup should be brushed more. When burying the box, some resin sand should be placed on the periphery of the pouring cup to ensure the strength of the pouring cup, thus controlling the connection between the pouring cup and the sprue.

In terms of pores, strict control is required from management, process implementation and supervision and inspection to ensure that the paint is dried to prevent the paint from returning to moisture. For such problems, the factory needs to train the workers and relevant management personnel, and ensure that the responsibility is assigned to the people, the supervision is in place, and the production process is meticulous, so as to prevent such low-level mistakes from affecting the efficiency of the foundry.

2.3: Production efficiency improvement

According to the original white mold placement method, it can be seen that the utilization rate of the sand box is low, and the casting can be placed in two more layers in the height direction to realize two model clusters and three layers.Pouring of 36 castings. According to the above two ideas, the improved form of the grate bar casting process can be obtained as shown in Figure 5.

Figure5 Model cluster diagram after the improved process.

3 Production application and effect

After process optimization, the production efficiency is improved.3 times, but also reduce the cost. In the process of small batch production, the slag sand inclusion defect of the casting is completely controlled, the current casting scrap rate is less than 4%, the casting processing did not find the defect of shrinkage shrinkage loose slag sand inclusion, has been mass production, process stability.

4 Conclusions

1): By optimizing the casting process, the casting production efficiency has been successfully improved and the production cost has been reduced.

2): the box group type, to avoid the defect of bonding damage into the sand. Adjust the pouring position of the casting and optimize the casting riser reasonably to make the casting process more stable. The factory has started mass production, and the practice has proved that the process is mature.

3): Casting production is linked to many links, pay attention to all processes forward, all problems are solved in the previous links, so as to produce qualified castings in the end. 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. Only in this way can we avoid some unnecessary low-level mistakes that lead to the scrapping of castings and affect the efficiency of the workshop.

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