Analysis and optimization design of casting process defects of a box

As a basic casting process method in the casting industry, sand casting is widely used in the manufacturing industry because of its low manufacturing cost, convenient materialization, simple process and strong operability. However, because of the flexibility of the casting process, how to select and formulate a scientific, reasonable, economical and environmentally friendly process plan that is easy to implement is still a key issue in actual production practice. The lifting box is mainly involved in the field of petroleum machinery technology. When oil is extracted, oil and gas flow from the storage layer to the bottom of the exploited well, and from the bottom of the well to the wellhead. Before mining, it is necessary to drive the drill bit through the rotary rod to drill the hole, and then the rotary rod is extracted from the hole, and the pump for oil pumping is inserted from the oil inlet pipe to exploit. The main function of the lifting box is to lift the rotating rod and various subsequent opening materials. Because it is the main load-bearing part, it has high requirements for its strength and stability when working. Based on this, this paper uses ProCAST software to carry out numerical simulation and comparative analysis in view of the structural performance and working characteristics of the lifting box, and formulates a sand casting process scheme with reasonable process parameters and efficient pouring system to guide the production of qualified lifting box castings.

1 Overall part analysis

1.1 Part structure analysis

The outer contour dimensions of the part are 1 040 mm× 760 mm × 933 mm, and the material is ZG25CrNiMo, density According to the general steel castings 7.85 g/cm3 The process was designed with a maximum wall thickness of approximately 187 mm and a minimum wall thickness of 45 mm, and the 3D model of the part is shown in Figure 1. The wall thickness distribution analysis of the part with UG software is shown in Figure 2, and it can be seen that the part is small The wall thickness of the end is thicker, and the wall thickness of the large end is uniform without large hot joints.

Figure 1 Three-dimensional drawing of casting

Figure 2 Distribution of wall thickness of castings

1.2 Part material analysis

The casting material is low alloy steel ZG25CrNiMo, which has good toughness Properties, high tensile strength, excellent fatigue resistance, its chemical composition is shown in Table 1.

Table 1 Chemical composition of ZG25CrNiMo


2 Selection of pouring position and parting surface

The selection of the pouring position is the same position as the parting surface, and the reasonableness of the parting surface selection is directly related to the quality of the casting, so according to the structural characteristics of the lifting box, the parting surface selection scheme is analyzed as follows. Scheme 1 castings are all placed in the lower type, and the dimensional accuracy is better; Accurate positioning of the lower core of the vertical sand core; Two boxes are divided horizontally, which is more convenient for manual modeling. However, it is not convenient to start the mold, and more live blocks or sand cores need to be added (Figure 3a). Scheme 2 castings are all located in the upper type, which has the advantages of scheme 1, and only needs to add a small number of live blocks or sand cores, and the mold is convenient for molding, so choose scheme 2 (Figure 3b).

Figure 3 Parting surface scheme

3 Gating system design

3.1 Gating system design

By selecting the casting position and parting surface position scheme 2, the casting casting gating system can be used in both bottom and top injection types. Because the upper height of the bottom injection type gating system is more than 1 m, in order to facilitate the molding, the ceramic pipe straight runner and the cross runner formed by the wood mold commonly used in steel castings are used to connect multiple parallel internal sprues to avoid heat concentration near the inner sprue; In order to ensure that the upper temperature of the casting is higher and the sequential solidification is more obvious, the ceramic tube pouring system is all adopted. According to the weight of the casting, it is poured by leakage (bottom injection type).

3.2 Determination of the fault area of the two gating systems

When casting steel castings with leaky bales (priming), an open casting system is selected.
(1) Calculation of the bottom pouring system. The diameter of the hole is 50 mm, the ΣA package = 19.6 cm2, and the straight runner of the ceramic tube is 70 mm. ΣA horizontal = 39.2 cm2 The cross gate is bidirectional with a fault area of 19.6 cm2. ΣA = 43.1 cm2 Six internal runners were used, with an average breaking area of 7.2 cm2 per internal runner. The cross-sectional dimensions of the priming system are shown in Figure 4.

Figure 4 Cross-sectional dimensions of priming system (mm)

(2) Calculation of jacking gating system. Hole diameter 50 mm,
ΣA package = 19.6 cm2, choose the ceramic tube straight runner is 70 mm. ΣA horizontal
=39.2 cm2, the cross runner is bidirectional, the fault area is 19.6 cm2, select Make
With a ceramic tube with a diameter of 50 mm. ΣA within = 43.1 cm2, using 2 internal casts
channels, with an average area of 21.6 cm2 per internal gating path, chose to use diameters
55 mm ceramic tube. The cross-sectional dimensions of the top-pouring system are shown in Figure 5.

Figure 5 Cross-sectional dimensions of top-injection gating system (mm)

4 Numerical simulation results and analysis

4.1 Simulation Pre-processing

NX1847 was used to create a 3D model of castings, gate cups and each runner, exported to a common format, imported into ProCAST and meshed, the total number of schematic 1 priming system meshes was 1 160 633, and the total number of top-pouring system meshes in Solution 2 was 897 227, and then simulated simulation True. The pouring temperature is 1 550 °C, the pouring time is 40 s, the sand mold is alkaline phenolic resin sand, the raw sand is silica sand, and the heat exchange coefficient between metal and sand Determined to be 500 W/(m2· K)。 The core and mold are the same material.

4.2 Filling analysis and defect analysis

Scheme 1 and scheme 2 were filled and defect analyzed, and the results were shown in Figures 6, 7 and 8. The cross runners in the pouring process of both options are in a full state. However, the drop difference of the top-injection pouring system in scheme 2 is too large during filling, and the molten metal has obvious splash, which will directly impact the sand core, which is easy to cause sand loss; The bottom pouring system of scheme 1 is filled smoothly and orderly, and the filling effect is obviously better. In terms of time, the charging time of the two schemes is close to 40 s, which meets the setting requirements Beg. When the pouring is completed, the upper part of the top-pouring system of Option 2 has fewer shrinkage defects, and the lower part is basically the same as Scheme 1.

4.3 Selection of gating system

Through numerical simulation analysis, it can be seen that although the upper defect of the casting of scheme 2 is less, the lower defect is consistent with scheme 1, especially the filling is unstable and easy to damage the sand core, and after comprehensive consideration, the bottom injection pouring system of scheme 1 is selected and the process is optimized.

Figure 6 Scenario 1 filling analysis

Figure 7 Scenario 2 filling analysis

4.4 Initial Optimization Measures

According to the simulation analysis results, the riser added to the casting casting system was compensated. Due to structural constraints and non-machining surface, it is inconvenient to set the riser in the lower part of the casting, while the upper shrinkage part can be set with a riser for contraction, so the riser is placed in the upper part.

5 riser design

5.1 Selection of types and shapes of risers

In order to facilitate the modeling, the riser filling effect is better, this process design uses a heating riser sleeve, which can not be taken out after molding, and the riser is set to round Shape the riser. The upper wall thickness of this casting is the largest, considering the structural size of the casting and the filling distance, the casting only needs a large riser.

5.2 Analysis and summary of initial optimization results

5.2.1 Simulation Pre-processing

The 3D model was imported into ProCAST for simulation analysis, and the model was equipped with a heat preservation riser and an air outlet, 1#The large sand core is a hollow sand core, and the inside is air during the pouring process, and the heating riser sleeve, hollow sand core and lifting lug sand core can be seen in Figure 10.

Figure 9 Three-dimensional diagram of the part that needs to be retracted

5.2.2 Solid phase rate analysis

Using ProCAST to analyze the solid phase rate of the optimized system, it can be seen from Figure 11a that isolated liquid phase regions appear on both sides of the casting due to thicker wall thickness and slower solidification. Fig. 11b A smaller isolated liquid zone also appears in the middle of the casting. The casting gradually solidifies from the lower part to the upper part, riser Finally solidification, this process design conforms to the principle of sequential solidification; It can be seen from Fig. 11c and D that the heating riser cools the slowest and the calculation time is long, so as long as the castings are all solidified, the heating riser does not need to be calculated, nor does it affect the judgment of the simulation results.

Figure 10 Optimization of the 3D model for the first time

Figure 11 Solid phase rate analysis

5.2.3 Defect Analysis

In order to fully solve the problems of casting defects in the current pouring system and ensure the quality of castings, the simulation defect analysis of the optimized system is carried out again, as shown in Figure 12, it is found that the three isolated liquid phase regions analyzed by the solid phase rate have shrinkage defects and defects It is distributed in three positions of the casting, and the three walls of the casting are thicker, forming smaller hot joints, so it is necessary to speed up the cooling rate of these three places to avoid heat concentration; In addition, considering the overall heat dissipation of the sand mold, the intermediate sand core can discharge heat faster.

5.2.4 Secondary optimization measures

Through the defect analysis after the initial optimization and the cause analysis, in order to minimize the possibility of shrinkage, the casting process is further improved as follows: (1) cold iron is set at the three defect positions of the casting, so as to accelerate the cooling rate of the three places and avoid shrinkage defects;(2) Add exhaust hole with 1# in the lower type The hollow positions of the large sand cores are connected to make the casting The internal heat can be discharged faster, and the lower part of the casting cools faster, so that the upper riser filling effect is better.

6 Design of cold iron

6.1 Position and shape selection of cold iron

Cold iron can eliminate the local stress of the casting, has the function of increasing the riser filling distance, preventing cracks, combined with the structural characteristics of the lifting box and the simulation analysis results, choose to use external cold iron, cold iron material is high carbon steel. According to the simulation analysis, the cold iron is set in the three isolated hot joint parts, and the cold iron is set in the outer plane and the inner arc surface of the casting. The cold iron on the outer plane is formed cold iron, and the outer arc surface is special-shaped cold iron. The location of the cold iron is shown in Figure 13.

Figure 13 The location of the cold iron that needs to be added

6.2 Analysis of secondary optimization results

The third simulation analysis is carried out to add cold iron to the casting model, and the third simulation analysis is carried out after adding the exhaust hole in the lower part as shown in Figure 14, the simulation results show that there are no visible feature defects inside the casting, indicating that the improvement measures of setting the cold iron and adding air holes to strengthen the internal heat dissipation of the casting are effective, and the second optimized casting The manufacturing process meets the predetermined requirements.

Figure 14 Defect analysis

7 Conclusion

(1) According to the structure and size characteristics of the casting, two kinds of pouring systems are designed for the top injection type and the bottom injection type.
(2) After analyzing the casting defects according to the ProCAST comparative simulation, the bottom injection pouring system was selected, and the heating riser, external cooling iron and exhaust hole were added to the casting twice for process optimization.
(3) The numerical simulation analysis of the optimized castings shows that there are no obvious casting defects, and the castings achieve sequential solidification, which improves the forming efficiency of the castings and ensures the quality of the castings.

Scroll to Top