As the core component of the blower, the material mechanical properties of the impeller directly affect the rotation speed and flow rate of the blower. In recent years, with the diversification of the market and the continuous improvement of customer requirements, it is necessary to develop impellers made of QT500-14.
Product technical requirements
The appearance of the impeller casting is shown in Figure 1. Its weight is 1.6 t, its outline size is 1 380 mm × 780 mm × 400 mm, its wall thickness is 75 to 125 mm, and its grade is QT500-14. The mechanical property requirements of the cast test block attached to the casting are: tensile strength ≥ 500 MPa, yield strength ≥ 400 MPa, elongation after fracture ≥ 14%, hardness 180 ~ 210 HBW; spheroidization grade above level 3.
Figure 1 Outline drawing of impeller casting
Molding methods and pouring systems
Furan resin sand is used for modeling, the furan resin nitrogen content is <3.0%, and the free phenol content is ≤0.5%. The resin addition amount accounts for 1.0%~1.2% of the weight of the molding sand, and the curing agent addition amount (accounting for the resin addition amount) is 30%~40% in summer, 50%~60% in winter, and 40%~50% in spring and autumn. The casting process is horizontal modeling and vertical pouring. The pouring system adopts bottom pouring type, and the cross-sectional area ratio of each component is ΣS straight: ΣS resistance: ΣS horizontal: ΣS inner = 1:0.6:1.4:1.3. Among them, there is 1 sprue with an area of 4 000 mm2; 2 chokes, located at the connection between the sprue and the runner, are made of φ40 mm ceramic tubes; 2 lateral runner are made of φ60 mm ceramic tubes; There are 4 gates, and φ40 mm ceramic pipes are used as gates. Two open heating risers with a diameter of 190 mm and a height of 280 mm are placed on the top of the casting. A 10PPi SiC filter is used in the middle of the sprue, with a filter size of φ200 mm and a thickness of 40 mm. The casting process design is shown in Figure 2.
Figure 2 Gating system design plan
MAGMA simulation software is used to perform filling and solidification simulation analysis on the above process. The filling speed is ≤100 mm/s, the molten iron is filled from bottom to top, and the filling process is smooth without turbulence and air entrainment, as shown in Figure 3.
(a) Fill 25%
(b) Fill 50%
（c）Fill 75%
（d）Fill 100%
Figure 3 Filling temperature analysis
From the analysis of the solidification process, when the liquid begins to shrink, the molten iron in the heating riser feeds the casting. After the liquid shrinkage of the casting is completed, the casting realizes self-feeding through graphitization expansion. According to the shrinkage porosity judgment analysis, the casting has no shrinkage porosity defects (Figure 4).
(a) Liquid phase 75%
（b）Liquid phase 50%
（c）Liquid phase 25%
(d) Shrinkage criterion
Figure 4 Liquid solidification analysis and shrinkage porosity criterion
Because ordinary ductile iron will greatly reduce the elongation after fracture as the tensile strength increases, while silicon solid solution strengthened ferrite-based ductile iron can maintain strength, the elongation after fracture will be significantly higher than that of materials with the same strength. improve. More importantly, due to the single matrix structure of solid solution-strengthened ferrite-based ductile iron, the casting hardness deviation is small, the processing and cutting performance is good, the tool life can be extended, and the processing cost can be reduced. Therefore, silicon solid solution strengthened ferrite-based ductile iron was selected as the material for the blower impeller casting.
The partial wall thickness of the impeller exceeds 100 mm, which is a thick and large-section casting. Since the module has a modulus of 2.86 cm, the solidification process is slow, which can easily cause spheroidization and growth recession, and form floating, blooming and broken graphite, etc., resulting in poor mechanical properties of the casting. Failure to meet technical requirements. Therefore, when designing the chemical composition, melting and pouring processes, it is important to focus on preventing these problems.
3.1 Determination of chemical composition
The selection principle of CE value is to increase it as much as possible without causing graphite floating or flowering graphite. According to the wall thickness of the impeller and solidification simulation calculation, the CE is determined to be between 4.3% and 4.4%. The molten iron in this range has good fluidity and small shrinkage tendency.
The essence of Si solid solution strengthening ferrite-based ductile iron is that after Si replaces Fe atoms in the original crystal lattice or enters the matrix lattice gap, the crystal lattice is distorted, which increases the dislocation slip resistance and reduces the strength of the material. and increased hardness. Therefore, the final ω (Si) value is set at 3.2%~3.4%.
C is a graphitizing element. In order to prevent graphite from floating, the original molten iron ω (C) range is set at 3.25% ~ 3.35%, and the final ω (C) is set at 3.10% ~ 3.30%.
Mn is a carbide-forming element. Due to the slow cooling rate of thick-walled castings, ω (Mn) > 0.35% will tend to segregate into the liquid phase and form network carbides at the boundaries of the eutectic clusters, which will eventually reduce the plasticity and toughness of the castings, so ω (Mn) is set to 0.25 %~0.35%.
P is a harmful element in molten iron. When ω (P) exceeds 0.05%, it will increase the tendency of segregation and form low-melting-point eutectic at the grain boundaries, thus reducing the mechanical properties of the material. It is required to control ω (P) ≤ 0.04%.
High ω (S) will neutralize the Mg in the molten iron. At this time, if you want to ensure graphite spheroidization, you need to add more spheroidized cored wire, causing waste. In addition, if ω (S) in the original molten iron is too high, it will also cause slag inclusion defects in castings. However, ω (S) should not be too low. If it is too low (≤0.005%), it will cause poor incubation effect after spheroidization, resulting in insufficient number of graphite balls and easy generation of carbide. The final ω(S) is controlled within the range of 0.006%~0.015%.
When ω (Sb) is between 0.003% and 0.008%, it can form a heterogeneous core with RE and other elements in the molten iron, thus improving the nucleation rate of the ductile core. Controlling ω (Sb) between 0.005% and 0.007% according to the impeller wall thickness can effectively suppress the effects of harmful elements such as excessive rare earths and Ti in the molten iron, and can increase the spheroidization rate in the center of the section, making graphite nodules With small diameter and large number of balls, the as-cast mechanical properties are significantly improved.
3.2 Determination of spheroidized cored wire and inoculant
When producing ductile iron castings with thick and large sections, the types of spheroidized cored wire and inoculant must be carefully selected:
(1) The spheroidized core-covered wire must have strong resistance to recession and graphite distortion. Due to the large modulus of thick and large-section castings, it takes a long time to solidify, which can easily cause spheroidization and growth recession. Research and practice have shown that the spheroidization decay time of heavy rare earths is longer than that of light rare earths. Therefore, spheroidized cored wire containing heavy rare earth yttrium is selected, in which ω (total RE) is 1% to 3% and ω (Mg) is 24% to 31%.
(2) The inoculant must have the ability to increase the number of graphite nodules and resist recession. Both primary inoculation and secondary inoculation use long-acting inoculants containing Ca and Ba. Among them, ω (Ca) is 1%~2%, ω (Ba) is 4%~6%, and the particle size is 3~8 mm. Flow inoculation uses a sulfur and oxygen flow inoculant with a particle size of 0.2~0.7 mm to increase the number of graphite nucleation, improve the roundness of graphite balls, and delay decay.
3.3 Smelting and pouring process
A 3,000 kg medium frequency induction electric furnace is used for smelting. The charge ratio is 50% return charge + 50% scrap steel. The composition of the original molten iron is adjusted through carburizing agent and ferrosilicon until the composition of the original molten iron is qualified. After the temperature continues to rise until the temperature meets the requirements, the molten iron begins to come out of the furnace. .
When coming out of the furnace, add silicon barium inoculant with a particle size of 3~8 mm along with the molten iron in an amount of 0.5%. After the molten iron weight reaches the requirement, it is moved to the spheroidization chamber as soon as possible for wire feeding and spheroidization. After the spheroidization is completed, the slag is removed immediately. The spheroidized bag is transferred to the resin sand pouring line for secondary bag inoculation. Silicon barium inoculant with a particle size of 3~8 mm is used, and the addition amount is 0.5%.
After removing the scum on the surface of the molten iron, take a spectral sample to detect the composition of the molten iron. The pouring temperature is 1 330 ~ 1 300 ℃. During pouring, 0.2 ~ 0.7 mm of sulfur and oxygen flow inoculants are added with the molten iron. This completes the three inoculation treatments. The pouring and filling time is 80~100 s.
After cooling in the mold for 23 to 24 hours, unpack.
Performance test results
The tensile properties and body hardness test results of the cast test block attached to the casting are shown in Table 2, and the body hardness test location is shown in Figure 5.
Figure 5 Body hardness detection position
Samples were taken from the attached cast test blocks to examine the metallographic structure. Figures 6 and 7 show the metallographic structure before and after corrosion. As can be seen from Figure 6, the spheroidization rate has reached more than 85%, which is above level 3 in GB/T 9441-2021; Figure 7 The matrix structure is mainly ferrite, and the pearlite content is less than 5%.
Figure 6 Uncorroded metallographic phase
Figure 7 After corrosion
in conclusion
(1) Using MAGMA simulation analysis software to optimize the gating system can minimize filling and solidification defects in castings, shorten the development cycle, and improve process yield.
(2) When producing thick ductile iron castings, problems such as spheroidization, growth recession, blooming, broken graphite, and graphite floating are prone to occur. Therefore, chemical composition needs to be determined with caution. The chemical composition of the solid solution strengthened ferrite-based ductile iron QT500-14 material is determined to be: CE 4.3%~4.4%, and ω(C) 3.10%~3.30%, ω(Si) 3.2%~3.4%, ω(P ) ≤ 0.04%, ω (Mn), ω (S) and ω (Sb) are within the appropriate range, using yttrium-containing heavy rare earth spheroidized core-covered wire and three inoculations, and a low-temperature casting process at 1 330 ~ 1 300 ℃ to produce A thick and large-section impeller casting that meets the technical requirements of QT500-14.
(3) The mass-produced impeller castings have good chip processing performance and have passed the installation verification. This shows that in order to improve the efficiency of the blower, the selected impeller material and the preparation process used are reasonable.
