Casting process deformation defects of bed castings

Confirmation of the main factors affecting the casting process deformation defects of bed castings, track and observe the actual production situation on site, and confirm the main factors affecting the above factors.

1) As the casting is a large one, only 1 ~ 2 pieces are produced every day. The mold lifting time of each mold is about 60min. The strength and hardness of the mold after mold lifting meet the requirements. Therefore, the cause of deformation due to early mold lifting is excluded.

2) The bottom plate of the model was inspected in the pattern group, and it was found that one of the four corners of the bottom plate was lower than the other three. Due to the poor stiffness of the model base plate, the pattern deformation is one of the reasons for the deformation.

3) When the strength of molding sand is low, the static pressure of molten iron will cause the mold wall to move outward and affect the dimensional accuracy of castings. After consulting the daily molding sand inspection records, and taking samples from the mold for tensile strength test, it is more than 0.6MPa, meeting the strength requirements. Therefore, the deformation cause of insufficient sand mold strength production is excluded.

4) The base casting material shall be HT300 and the hardness shall be 179 ~ 240hbw. Through the hardness test of defective castings, it is found that the hardness is generally 160 ~ 180hbw, which is lower than the required value. The chemical composition analysis showed that W C = 3.34%, w Si = 1.88%, w Mn = 0.68%, WP = 0.058%, WS = 0.060%. The composition requirements of ZHY casting for HT300 brand materials are WC = 3.10% ~ 3.25%, WSI = 1.8% ~ 2.1%, WMN = 0.8% ~ 1.0%, WP ≤ 0.1%, WS = 0.05% ~ 0.08%, and the chemical composition is unqualified. For gray cast iron, when the strength is low, the internal stress of the casting becomes small, which is not easy to cause casting deformation. However, if the hardness and strength of the casting are too low, it is easy to cause casting deformation in the process of lifting, sand blasting, grinding and handling. Therefore, the low grade of casting material is also a cause of deformation.

5) The casting has complex shape and large wall thickness difference, which will produce stress during cooling and needs to be cooled slowly. According to the actual tracking and confirmation, the unpacking time of castings is 8 ~ 12h, which meets the requirements of slow cooling process. Therefore, it is excluded that the early unpacking time of castings is the cause of deformation.

6) The casting adopts a two-stage bottom casting gating system from the track. Due to the large length and size of the casting, the uneven distribution of the inner gate, the cooling speed at the inner gate is slow during the cooling process, resulting in stress concentration. Therefore, another reason for deformation is the unreasonable gating system. The casting gating system is shown in the figure.

7) According to the actual inspection on the molding site, the molding site is 250mm thick concrete ground with good flatness. Therefore, the unevenness of the modeling site is not the cause of deformation.

Through the above cause analysis, it can be concluded that there are three main factors causing deformation: ① the stiffness of the model bottom plate is poor. ② The casting material grade is low. ③ The gating system is unreasonable.

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