This paper focuses on the improvement of the casting process for steel castings of axle box body. It analyzes the problems existing in the production process, such as sand shedding, shrinkage, sand holes, and insufficient machining allowance, and proposes corresponding improvement measures. The improved process is verified through production, and qualified castings are obtained.
Introduction:
With the continuous improvement of the speed and load capacity of locomotives, higher requirements are put forward for the cast steel parts of locomotive bogies, especially the stressed components. The axle box body of subway vehicles is a thin-walled box structure casting and an important stressed component for axle box assembly. In the previous production and processing, there were defects such as shrinkage, sand holes, cracks, tilting of the sand core during the mold closing and pouring process, and black skin during machining. These defects are closely related to the casting process of the axle box body, so it is necessary to analyze and adjust the process of the axle box body.
Casting Structure and Production Requirements:
The structure of the axle box body product is shown in Figure 1. The casting material is ZG25MnCrNiMo (Class C steel), the main wall thickness is 20 – 30 mm, the rough weight of the casting is 95 kg, the linear shrinkage of the casting is 2%, and CO2 hardened sodium silicate sand is used for molding and core making. Two pieces are cast in one box. The melting is carried out in a 10 t electric arc furnace, and the bottom pouring method is used. The pouring temperature is controlled at 1560 – 1580 °C. The design requires that the entire surface of the casting be subjected to magnetic particle inspection. According to the requirements of locomotive assembly parts, there shall be no shrinkage or crack defects on all machined surfaces and main stressed parts of the axle box body, and the castings shall be subjected to ultrasonic inspection by batch.
Original Process Scheme and Existing Problems:
- Original Casting Process Introduction:
The three-dimensional model of the casting process scheme for the axle box body is shown in Figure 2. The axle box is divided along the middle of the axle tube. An oval-shaped is set in the large axle tube to ensure the sequential solidification and feeding of the axle tube. A semicircular blind riser is set at the top near the connection part between the small axle tube and the rib plate of the axle box to ensure the internal quality. A 16 mm thick cold iron plate is set on the lower box of the large axle tube and the spring plate surface to enhance the chilling effect. The inner runner of the casting is selected at the center rib plate position. The original process scheme ensures the quality of the machined surfaces and main stressed parts of the casting in the general direction, but in the mass production process, the defects mainly exist in the large axle tube, inner runner, spring plate, and other parts, and there are also problems such as sand shedding and uneven machining allowance. - Existing Problems:
The main problems in the production process of the axle box body are as follows:
(1) Defects caused by sand shedding appear on the connecting rib plate of the large and small axle tubes during the molding process of the axle box body, as shown in Figure 3. The shed sand is wrapped in the molten steel and cannot be processed, resulting in a small batch of rejects. The same problem also exists at the ear-shaped sand core for the hanging sand.
(2) A certain proportion of sand holes and shrinkage defects appear in the large axle tube, as shown in Figure 4. Welding repair is required after rough machining, which increases the production cost and affects the product quality.
(3) The sand core of the spring plate surface tilts during the mold closing and pouring process, resulting in uneven machining allowance of the spring plate, as shown in Figure 5.
(4) There are shrinkage crack defects at the position of the inner runner of the casting, and chipping is required during cleaning.
Cause Analysis and Process Improvement:
- Sand Shedding:
The possible reasons for sand shedding are: low strength of the molding sand, poor bonding ability, uneven mixing; few or no flask bands; no hooks or iron nails for reinforcement at the upper box surface or hanging sand area; insufficient ramming during molding, uneven compaction or partial missed ramming; too deep cavity depth, unstable mold closing resulting in collision; and knocking and vibration during the alignment of the pinhole or fixture fastening after mold closing.
For the CO2 hardened sodium silicate sand on the outer shape of the axle box, the sand shedding occurs in two situations: the sand core at the ear of the axle tube is made by molding and hanging with adhesive, and collision may occur during the turnover and lifting process, and the adhesive strength of the core paste does not meet the requirements, which will lead to sand shedding at this place. To solve the problem and meet the production requirements, the method of reserving a hole in the center of the sand core during core making is adopted, and nails are embedded from the reserved hole for reinforcement after hanging the sand core; the height of the connecting rib plate cavity between the large and small axle tubes is 50 mm, and the depth is relatively deep. After the sand mold is vibrated and compacted by the vibration table, the strength of the sand mold at this position is insufficient, and the molding sand is shed due to the scouring of the molten steel during the pouring process. During molding, the welding rod is folded into a core bone to strengthen the core strength of this part through the core bone. - Shrinkage and Sand Holes in the Large Axle Tube:
The main defects in the large axle tube are shrinkage and sand holes.
The reason for the sand holes during processing is that the operation is not careful, and the cleaning of the cavity is not clean, resulting in sand hole defects in the axle tube. Since the inner wall of the cylinder is a machined surface, most of the sand holes can be removed after processing. Therefore, by increasing the machining allowance at this part, the sand hole defects can be effectively solved.
Shrinkage is a void defect or loose structure caused by the inability of the casting to be fed by the molten steel during solidification and the inability to achieve sequential solidification. The wall thickness of the large cylinder is 25 mm before processing, and the distance of the feeding end zone is about 65 mm, the riser zone is about 50 mm, and the vertical feeding distance is 115 mm. The height of the axle tube is 245 mm. To achieve sequential solidification and sufficient feeding, the required depth value of the 补贴 is 130 mm. The distance between the two insulating risers of the casting is 120 mm, and the feeding distance between the two risers is 100 mm. In summary, the reason for the shrinkage defects in the large axle tube is the insufficient feeding distance between the risers, and the difficulty in controlling the pouring temperature of the molten steel, resulting in shrinkage defects. The tentative improvement plan is to replace the original two oval insulating risers with three circular insulating exothermic risers, and reduce the distance between the risers to 90 mm, which can theoretically achieve a good feeding effect without affecting the process yield of the casting. - Insufficient Machining Allowance of the Spring Plate:
The reason for the insufficient machining allowance of the spring plate is that in the original production, the sand core at the spring plate position is a disc-shaped sand core with a lower core head. There is an error between the draft angle of the core box core head position and the core seat. The wooden mold deforms during the production and use process, resulting in a large gap between the lower core head and the core seat. During mold closing, the sand core tilts forward, and the machining allowance of the blank spring plate surface after pouring is uneven. The upper half of the machining allowance is insufficient and requires welding repair. The positioning effect of the lower core head is not good. The lower core head scheme is adjusted to the left and right core heads, and the size of the core head is reduced. The adjusted core head and the core seat fit better, and the positioning effect is better than the former. - Shrinkage and Crack at the Inner Runner:
The size of the inner runner of the axle box is 40/35 mm × H, where H is the height of the inner runner section. The height dimension transitions from the horizontal runner to the position of the casting rib plate is 15 mm, and the thickness of the rib plate at the inner runner is 7.5 mm, there is a thickness difference. The rib plate of the casting will solidify first, generating tensile stress on the contact position, and cracks will appear during the solidification of the casting.
Due to the structural limitation of this part, to solve the shrinkage problem, the thickness of the inner runner and the casting rib plate needs to be made more uniform, and the open gating system is required for the inner runner, and the inner runner is made into a flat shape. However, the molten steel has the characteristics of high melting point and poor fluidity. After being changed to a flat structure, if the pouring temperature of the molten steel is not well controlled during the pouring process, the problem of insufficient pouring may occur.
The process considers adjusting the position of the inner runner, and the inner runner is adjusted to the left slot mouth. The slot mouth position is a machined surface with a thickness of 25 mm. It is more reasonable to select the inner runner at this position, and even if there are cracks or other defects, they can be eliminated during machining.
New Process Field Production Control:
Based on the above improvement measures, a new casting process is designed, and the process diagram is shown in Figure 6. The new casting process is simulated, and the results are consistent with the analysis. The mold is made for trial production, and the control of the molding link is strengthened during production. During production, cold irons, gate rods, insulating riser sleeves, etc. are placed according to the regulations of the process card. The cold irons shall not be skewed or padded with sand. The sand eating amount between the core bone and the mold wall shall be controlled within 15 – 50 mm (or more than this sand eating amount). The mold lifting shall be stable, and the compactness of each part of the sand mold and sand core shall be uniform, and the compactness shall meet the regulations of the process card. If there is no process card, the compactness shall be controlled above 45 units. When the core is brushed with coating, the depth shall be appropriate, there shall be no accumulation or flow marks, and the thickness shall be uniform and smooth. Before lifting the core, the cavity of the sand mold must be cleaned up. The loose sand around the air hole and the air hole shall be scraped flat and blown clean, and the air outlet shall be unobstructed. The oxygen addition hole in the cavity shall be blocked and repaired with molding sand. The surface of the sand core must be cleaned up, the flash and burrs shall be repaired flat, and the sharp edges shall be rounded.
Using the improved process scheme to produce 10 sample pieces, after the castings are cleaned and polished, the appearance inspection is carried out. The improved castings have no sand shedding defects. The dimensional marking inspection is carried out, and the main inspection dimensions of the castings are good. The machining allowance at the spring plate position is 7 mm and the thickness is uniform. After cleaning, the ultrasonic inspection is carried out on the axle tube, and no shrinkage defects are found inside all the castings, and the external inclusions and sand holes are significantly reduced. The magnetic particle inspection is carried out on the position of the inner runner, and there are no surface defects such as cracks, and the inspection is qualified. Each of the first batch of castings is inspected and inspected one by one, and all are qualified, which verifies the rationality of the cause analysis and the feasibility of the improvement measures.
Conclusion:
Through the analysis and improvement of the casting process of the axle box body, the problems of sand shedding, shrinkage, sand holes, and insufficient machining allowance have been effectively solved. The improved process has been verified through production, and qualified castings have been obtained. This provides a reference for the improvement of the casting process of similar castings in the future.
| Defect | Cause | Improvement Measure |
|---|---|---|
| Sand Shedding | Low strength of molding sand, poor bonding ability, uneven mixing; few or no flask bands; no hooks or iron nails for reinforcement at the upper box surface or hanging sand area; insufficient ramming during molding, uneven compaction or partial missed ramming; too deep cavity depth, unstable mold closing resulting in collision; knocking and vibration during the alignment of the pinhole or fixture fastening after mold closing. | Using the core reserved hole to reinforce with nails and using the welding rod to break into the core bone to strengthen the core strength. |
| Shrinkage and Sand Holes in the Large Axle Tube | Insufficient feeding distance between the risers, difficulty in controlling the pouring temperature of the molten steel. | Replacing the original two oval insulating risers with three circular insulating exothermic risers, and reducing the distance between the risers to 90 mm. |
| Insufficient Machining Allowance of the Spring Plate | Error between the draft angle of the core box core head position and the core seat; deformation of the wooden mold during production and use. | Adjusting the lower core head scheme to the left and right core heads, and reducing the size of the core head. |
| Shrinkage and Crack at the Inner Runner | Thickness difference between the inner runner and the casting rib plate; difficulty in controlling the pouring temperature of the molten steel after changing the inner runner to a flat shape. | Adjusting the position of the inner runner to the left slot mouth. |