According to thespecification, the design structure of cast steel gear is divided into full circle structure, 1 / 2 structure and 1 / 4 structure. During casting, full circle casting and split casting can be adopted according to the size of modeling pit, melting capacity, on-site lifting and other equipment capacity.
1. Riser design
The riser design of steel castings must meet two requirements: first, the riser solidification must be later than the casting solidification; The second is to have enough feeding liquid steel. As we all know, the riser weight of steel castings is about 50% ~ 100% of the casting weight, that is, 1 / 3 ~ 1 / 2 of the liquid steel produced in the cast steel workshop is consumed in the riser. In order to meet the use requirements of large gears, the riser design must ensure the quality of gear tooth area and inner connecting flange, and meet the flaw detection requirements after rough machining. The large gear riser is mainly arranged at the outer rim and inner flange. The riser size is determined according to the hot pitch circle and modulus. The solidification process is numerically simulated by magma software to adjust and optimize the riser. Through years of production research, CITIC Heavy Industry has continuously optimized the design form of the riser. In order to ensure the quality of the tooth area, the outer rim riser has been optimized from the early dispersed riser to the ring riser; At the same time, due to the large diameter and high pouring height of the dispersed riser, when the process is optimized to the whole ring riser, theyield does not decrease, but the feeding efficiency of the ring riser to the casting is greatly improved. The schematic diagram before and after optimization of riser form of casting process is shown in Figure 1.
2. Subsidy and cold iron design
Casting subsidies are often designed in the casting process of steel castings. There are usually two functions of subsidies:
① The casting structure is complex and diverse, and it is often unable to realize sequential solidification. In order to meet the requirements of sequential solidification, the local shape of the casting needs to be changed. By increasing the thickness at the riser, the wall thickness near the riser and the wall thickness far away from the riser, the solidification time near the riser can be prolonged;
② Setting subsidies in steel castings can increase the expansion angle of feeding channel and increase the feeding channel of riser, which is conducive to the smooth flow of feeding liquid steel in riser to the feeding place of castings and improve the feeding efficiency of riser. The large gear casting subsidy is generally designed at the outer rim, and the subsidy forms are divided into external subsidy and internal subsidy. The external subsidy can be removed by hot cutting and processing in the later stage, but the thickness of the external circle is increased in the design. After tooth opening, the tooth area is located in the center of the casting, the organization is not dense, and the quality is difficult to guarantee. At the same time, the external subsidy is not conducive to the layout of process chill.
CITIC Heavy Industry’s large gear casting subsidies now mostly use internal subsidies. The comparison of magma temperature field solidification simulation with or without internal subsidies for large gears is shown in Figure 2 and 3. It can be seen from the comparison that setting reasonable subsidies makes the solidification feeding of castings more sufficient and the temperature field more reasonable.
Chilled iron has chilling effect, and the setting of chilled iron can improve the thickness of dense layer on the surface of casting. Casting chill is often set in the key area of parts. During the use of large gear outer circle tooth area, it is not only the key meshing area, but also the area prone to fatigue failure. In order to ensure the compactness of tooth area after tooth opening, sand hanging chill is often set in the outer circle of gear casting process. The layout of large gear casting chill is shown in Figure 4.
3. Pouring Scheme Design
One or two-layer gating system is selected according to the gear height, and the lower gating system adopts the bottom return form, which can ensure the smooth rise of molten steel, reduce turbulence and ensure the internal quality of castings. In order to realize rapid pouring, reduce the time for the cavity to be radiated by high-temperature molten steel, and ensure the stable filling of molten steel, the gating system must be set as an open gating system.
4. Model making and modeling
The large gear of mining mill has a large diameter. In order to ensure the cavity size and casting quality, the large gear modeling method adopts the form of solid sample + core assembly. During modeling, chromite sand with high fire resistance is selected on the surface of the sand core, and the surface of the sand mold is coated with alcohol-based zircon powder.