The sand suction pump casting is mainly used for river dredging, dredging and Embankment Consolidation, and the delivery medium can reach more than 40%. It is widely used in sand pumping and reclamation projects such as coastal reclamation. It is mainly composed of pump body casting, pump cover casting, impeller, guard plate, bracket and bearing components. As the motor adopts the oil chamber sealing method, it is necessary to prevent high-pressure water and impurities from entering the inner cavity of the motor, so the casting of the sand suction pump shall not have porosity and shrinkage defects.
The sand suction pump casting has a unit weight of 46 600 kg and an overall dimension of 5800 mm × 3 480 mm × 2 200 mm, the main wall thickness is 75 mm, the thickest part is 355 mm, and the material is QT500-7. There is a circle of threaded holes on the upper surface of the casting to match with the pump cover casting. It is required that there shall be no loose defects in the holes. The casting is difficult to cast due to irregular shape and strict quality requirements. Its three-dimensional structure is shown in Figure 1. After process analysis and evaluation, furan resin sand + foam mold is used to make the mold, and the foam mold is also called poly dragon mold, hereinafter referred to as mold casting.
1.Process scheme for mold retaining casting of pump body casting
Mold retention is very different from wood mold. In the process of mold design, there is no need to consider the mold starting inclination and positioning, but only need to increase the machining allowance and casting shrinkage. Because the pump body casting has many arcs and the mold making process is complex, the mold is divided into several modules (processed by machine tool) first, and then the blocks are spliced together. The assembled mold has high dimensional accuracy. However, due to the poor sand mold compactness during molding, the surface cleanliness of the mold cavity is poor, and it is easy to bring sand particles into the mold cavity during pouring.
1.1 Mold process
The shape of the mold is exactly the same as that of the casting. The mold is made after adding machining allowance and casting shrinkage on the basis of the part drawing. The mold is divided into several modules according to the three dimensional graph. On the one hand, it ensures the cutting operation and saves the foam board. On the other hand, it ensures the convenient assembly and assembly, and ensures the dimensional accuracy of the mold after assembly. Foam plate is processed by two ways: “CNC engraving + resistance wire cutting”, and then glued into a pump body casting mold. The fabricated mold is brushed with graphite coating twice. The brushing method is brush coating, and the thickness of each coating is 0.2 mm.
1.2 Process design
The pump body casting has large outline size, thick casting wall and heavy weight. The bottom injection double-layer stepped gating system is adopted (Fig. 2). External cooling iron and internal cooling iron are used in some important parts to prevent internal shrinkage. The riser and exhaust hole are set to ensure feeding, slag discharge and exhaust.
1.2.1 Gating system design
Through the analysis of the shape and internal structure of the casting, the stepped gating system is adopted, which is divided into upper and lower layers. The lower gating system is mainly used for mold filling, and the upper transverse sprue is mainly used for cushioning. The introduction position of the upper gating system is 1740 mm away from the bottom surface of the casting, which is convenient for mold closing, sand core lowering and relevant wall thickness inspection after core lowering.
The iron hydraulic head is calculated according to formula (1):
Where: HP is the iron hydraulic head, H box is the height of sand box, H cup is the height of pouring cup, C is the height of casting, and the pouring time is calculated according to formula (2):
Where: t is the pouring time and GL is the pouring weight.
A resistance is calculated according to formula (3):
Where: μ Take 0.4, ρ Take 7.3 kg / mm3, and G is the gravitational acceleration of 9.8 m / S2. Three ladle pouring is adopted, and the a resistance of each ladle can be calculated, a straight = a resistance / 0.8. According to the on-site sprue specification, the revised a straight and a transverse = a straight are obtained × 1.2, a in = a straight × 0.8, and then the dimensions of each sprue are obtained.
1.2.2 Cold iron design
The wall thickness of the pump body casting is thick, and the solidification time of large modulus nodular cast iron is long, which often reduces the graphite spheroidization rate, or leads to graphite distortion, graphite floating and coarse graphite defects. Therefore, cold iron should be used to shorten its solidification time. For castings with modulus less than 9 cm, cast iron external cooling iron is generally used, and its central thickness is greater than 300 mm. Therefore, formed internal cooling iron is required to control the solidification time within 50 min and the spheroidization rate reaches more than 80%. Since the assembly surface of the pump body casting and pump cover casting needs to be drilled with a circle of threaded holes without shrinkage, a circle of external cold iron shall be placed here. Place cold iron at the bearing installation to avoid defects.
1.2.3 Riser and exhaust design
The shrinkage volume of nodular iron castings is larger than that of gray iron castings under the condition of ordinary sand box, so risers are generally set. 6 at the top Φ The 180 mm thermal insulation and heating riser sleeve is evenly arranged on the end face of the bearing seat. Through the thermal insulation effect of the thermal insulation material, it can delay the heat transfer between the metal liquid in the riser and the external environment such as sand mold and air, prolong the solidification time of the metal liquid, and enhance the feeding effect of the riser on the casting. Two Φ 140 mm Common riser is placed on the foot convenient for processing and leveling to compensate the volume shrinkage caused by liquid metal cooling and prevent shrinkage cavity and porosity of castings. It also has the functions of exhaust, slag collection and guiding mold filling. For this thick and large casting, in addition to setting an air outlet channel at the rib, an exhaust channel is also set at the corresponding high point on the top surface of the casting to ensure that the gas in the mold cavity can be discharged in time at the beginning of pouring.
2.Molding process control
The addition amount of resin is 1.2%. First, the foam mold is placed on the modeling platform. Because of the symmetry of the upper and lower type, a box is first built and another box is rebuilt. Four box modeling is selected, which is convenient to observe the matching degree of each layer of sand core during mold closing. A total of 6 pairs of core boxes are put into the core bone, filled with resin sand and tamped. One of the largest sand cores is a special sand core, and two sand cores at the central layer are combined into one sand core. There are two kinds of core bones, of which the largest sand core is cast by casting, and the other sand core bones are 50mm × 60mm welded core bone. The outside of the sand core is coated with zircon powder, and the Baume degree of the coating is 65 ~ 70.
Wood baffle, sodium silicate sand and resin sand are used for the bottom parting surface, welded baffle, sodium silicate sand, resin sand and angle iron are used for the middle parting surface, and anti rise box steel plates are welded on both sides of the width direction.
3.Spheroidizing inoculation treatment
Spheroidizing treatment is one of the key processes in the production of ductile iron castings, and its treatment process directly affects the properties of ductile iron. Dy-7f heavy rare earth spheroidizing agent is used as the spheroidizing agent, with the addition amount of 1.05%; calballoy inoculant is used as the inoculant, with the addition amount of 0.4%; yfy1a high-efficiency inoculant is used as the inoculant, with the addition amount of 0.15%. The spheroidizing inoculation effect is ensured by multiple inoculation.
Due to the large weight of the pump body casting, in order to obtain the required performance requirements and strengthen the anti recession ability of molten iron, the molten iron is pretreated. 0.4% pretreatment agent is added before tapping, in order to increase the nucleation core and improve the internal quality and fluidity of molten iron. The casting method is used to throw the molten iron, add 0.4% inoculant, follow the flow method is used during pouring, and add 0.15% inoculant with a discharge bucket.
4.Melting and pouring process control
Three ladle pouring is adopted. During pouring, large flow rate and fast uninterrupted flow are required, and the pouring temperature is 1330 ~ 1350 ℃. The composition of molten iron before spheroidization is 3.4% ~ 3.5% C, 1.5% ~ 1.6% Si, 0.35% ~ 0.45% Mn, P ≤ 0.04%, s ≤ 0.03%; After spheroidization, the composition is controlled as 3.2% ~ 3.4% C, 2.3% ~ 2.5% Si, 0.35% ~ 0.45% Mn, P ≤ 0.04%, s ≤ 0.02%, 0.65% ~ 0.75% Cu and 0.035% ~ 0.055% Mg.
Before pouring, place 4 10 t pressure irons and pad them with steel pipes.
5.Cooling and unpacking
After pouring, make use of the good thermal insulation and collapsibility of resin sand to let the castings cool slowly in the mold, so as to achieve the purpose of stress relief and aging annealing. According to the experience of thick ductile iron castings, open the box and drop the sand after pouring for 240 h to remove the top pouring riser system. Loosen the box several times two days in advance, and control the unpacking temperature below 300 ℃.
6.Mechanical properties and metallographic structure of castings
The pump body castings produced are shown in Figure 3. The metallographic and mechanical properties of the test block are tested. The test results are shown in Table 1 and the metallographic structure results are shown in Figure 4. It is required that the tensile strength is 420 MPa, the elongation is 5%, the spheroidization level is not lower than level 3, that is, the spheroidization rate is greater than 80%, and the actual test block meets the requirements.
7.Conclusion
Through the first production of the pump body casting of thick and large nodular iron castings, the effectiveness and rationality of the gating and riser system are verified in the production process, the design of the gating and riser system integrating filling, exhaust, slag collection and feeding is further improved, and the defects such as shrinkage cavity and porosity of thick and large section nodular iron are solved. For pump body castings, due to the thick wall thickness, long solidification time and high casting quality requirements, it is very difficult in the production process. Through strict process control, selecting appropriate raw materials, appropriate spheroidizing inoculation treatment process and pretreatment technology, high-quality large-scale nodular cast iron pump body castings have been produced.