FIG. 2-40 is an impeller casting with an uncomplicated appearance, but it has four curved ribs inside and a narrow flow path.
In the original process, after coating two layers, 120 mesh zirconium sand was poured into the flow passage to make the zirconium sand fill the entire inner cavity of the flow passage, and then the outer round mouth was sealed with refractory mud, and then continued to crust.
In the current process, 90 ~ 120 mesh brown special casting sand is used to replace zirconium sand, and sand is also poured into the flow passage to make the special sand fill the entire inner cavity of the flow passage. The following operations are the same as above. During casting, mold shell cracking, shell swelling and steel leakage did not occur, and the quality of the inner cavity of the casting remained normal.
- Figure 2-40 Schematic diagram of the impeller
- Process control of turbine investment casting production
The structural characteristics of turbine castings are that there is a great difference between shell wall thickness and blade wall thickness. The blades are not only numerous in number and thin in wall thickness, but also connected with half national rings. Therefore, turbine casting is very difficult. In the process of turbine trial production, by strengthening the control of process details, under the conditions of non-vacuum melting and non-vacuum casting, the quality of production castings meets the requirements of customers.
The turbine shell has 21 curved blades with a thickness of 1mm. In particular, a semicircular ring is connected to the blade. The wall thickness of the semicircular ring is 1.6mm, the outer diameter of the shell is 270mm, the inner diameter of the ring is 230mm, and the height is 62mm. Single weight of casting is 2.3kg.
- Figure 2-41 Stamped and welded turbine
- Figure 2-42 Local status of stamped and welded blades
Figure 2-41 shows the stamped and welded turbine, and Figure 2-42 shows the partial state of stamped and welded blades. The shell of the turbine is made of 4mm steel plate, which is stamped and welded. The turbine blades are made of thin 1mm steel plates that are stamped and welded to the inner cavity of the casing. The semicircular ring is also a stamping part, which is welded to 21 blades. Obviously, the stamping component assembly welding method not only has a large workload, long production cycle and low efficiency, but also cannot meet the requirements of turbine design performance and working parameters. Therefore, it is required to change the turbine manufacturing method to investment casting.
Control of process details
Pressure type design.
From the perspective of turbine modeling, the difficulties of investment casting focus on the inner cavity, and the curved blade is connected to a semicircle ring. It is problematic to press the wax mold at one time, so two pressure molds must be set up separately, and two wax molds must be pressed, and then combined. See Figure 2-43 to Figure 2-45.
- 2-43 Three-dimensional profile of blade and half ring anastomosis
- Figure 2-44 Three-dimensional diagram of blade state in the inner cavity of the housing
- Figure 2-45 Three-dimensional diagram of the position of the embedded ring in the blade
- Set three stages of wax filtration. The low-temperature moulds are dewaxed with hot water, and the wax liquid is filtered for the first time before it flows from the dewaxing tank to the treatment tank. After acid treatment, the wax solution is filtered for the second time before flowing into the stationary precipitation bucket. Strain the wax solution a third time before pouring into the mold.
- Add wax planing process. Using casting mold ingot, a cylinder with a diameter of 450mm and a length of 800mm, placed on the wax planer to process into thin pieces of wax. Stirring the wax paste is fast, uniform and delicate, and there are no particles in the wax paste.
- Cooling of wax mold. The temperature of the molding room shall be strictly controlled to be less than 25℃. After the wax mold of blade and ring is removed from the pressing mold, it shall not be put into water for cooling. The wax mold shall be stored on the plate in pairs, and the wax mold shall not be stacked.
- Stitching of wax mold. The traditional welding process with ferrochrome is abandoned and bonded wax is adopted, as shown in Figure 2-46 and Figure 2-47. The heating temperature of bonding wax is generally 60°C, under which the bonding wax liquid is more dense, and a circle of “wax accumulation” often appears on the bonding surface when the wax mold is glued. Therefore, the heating temperature was raised to 70°C, and the semi-circular ring wax mold was immersed in the adhesive wax solution for less than 2s. After the wax was glued, the wax solution was not immediately glued. The wax solution was evenly divided with a row of brushes, and the wax solution was stopped for 5-7s. Then smoothly place the semi-circular ring wax mold into the blade wax mold, as shown in Figure 2-48.
- Figure 2-46 Blade wax mold
- Figure 2-47 semi-circular ring wax mold
- Figure 2-48 Bonded wax mold
Gating system design.
The design scheme of the first pouring feeding system is to use spherical riser and centrifugal pouring, following the radius of the riser, as shown in FIG. 2-49 and FIG. 2-50. The purpose of setting three thick feeding runner on the spherical riser is to facilitate wax discharge, exhaust, shell feeding and improve the shell rigidity of the module.
- Figure 2-49 Spherical die head
- Figure 2-50 Three exhaust bars on the spherical mold head
The design scheme of the second pouring feeding system, spherical riser with four inner runner centrifugal pouring, is shown in Figure 2-51.
- Figure 2-51 Connecting the runner to the half-ring
- Figure 2-52 Integrated inner runner
The third design scheme of pouring feeding system adopts an integral inner runner. The whole inner runner body is set at the top of the turbine housing, and the upper part is set with 5 liquid steel channels. The upper part is similar to the horizontal washing channel, which is made into an overall circle considering the characteristics of centrifugal pouring.
In order to ensure the complete filling of the semicircular ring in the turbine inner cavity, a filling mode combining inside and outside is formed. A direct washing channel is drawn from the center of the lower end of the circular cross runner, and a four-pronged inner gate is adopted to connect with the inner wall of the through-hole with a diameter of 74mm, as shown in Figure 2-53.
- Figure 2-53 four-pronged inner runner
Shell making process
The first layer: Apply mullet powder for 35s, sprinkle 80-100 mesh mullet sand, dry for 10h, drying chamber temperature 23°C, relative humidity 65%.
The second layer: apply Mullet powder 22s, sprinkle 60-80 mesh mullet sand, dry for 12h, drying chamber temperature 23°C, relative humidity 65%.
The third layer: apply mullet powder for 15s, sprinkle 60-80 mesh mullet sand, dry for 12h, drying room temperature 23°C, relative humidity 50%, blow air, tie wire.
The fourth layer: apply mullet powder for 14s, sprinkle 30-60 mesh mullet sand, dry for 12h, drying room temperature 23°C, relative humidity 50%, blow.
The fifth and sixth layers: apply mullet powder for 14S, sprinkle 16-30 mesh mullet sand, dry for 12h, drying room temperature 23℃, phase: humidity 50%, blow.
Sealing layer: Apply Mo Shui slurry for 14s, dry for 16h, drying room temperature 23°C, relative humidity 50%, blow.
Current production process
In order to facilitate sand cleaning, the first and second layers remain unchanged. Before coating the third layer, the inner cavity of the blade is filled with sand (60~80 mesh mullite) and closed sand (mullite slurry and mullite powder are mixed into mud and blocked), and then coated with the third, fourth and fifth layers and sealing layer. After pouring, the cleaning difficulty is significantly improved.
Aafter dewaxing, take out the shell, immediately rinse it with boiling water twice, and thoroughly remove the remaining wax and debris in the shell.
Two roasting of the mold shell
- Pre-roasting: The cross shell is pre-roasted in a baking furnace at 950℃. After the pre-calcined mold shell cools, the inner cavity of the mold shell is cleaned with water.
- Packing roasting: packing roasting means that the pre-roasted mold shell is put into a round iron box, which is filled with coarse sand, and the surface of the sand is brushed with a layer of thin silica sol. The purpose is to make the mold shell heated evenly and ensure that the casting temperature of the mold shell is increased, so as to facilitate the centrifugal casting.
- Roasting temperature and holding time. The roasting temperature of the mold shell is set at 1100~1150°C, the insulation temperature of the mold shell is 1100~1150°C, and the insulation time of the mold shell is ≥30min.
Smelting and pouring
The trial production practice has proved that the turbine must adopt centrifugal pouring to meet the requirement of complete filling. See Figure 2-55 for the self-made adjustable speed centrifuge.
- Figure 2-54 About to cast after packing roasting
- Figure 2-55 Self-made adjustable centrifuge
Melting temperature and pouring temperature
Adopt high-power furnace to melt the charge, wait for all the charge to melt, temperature rise to 1560~1570℃, add the preheated mass fraction of 0.20% ferromangmanganese and mass fraction of 0.10% ferrosilicon as pre-deoxidizing agent, remove the slag, cover the slag, remove the slag, add the mass fraction of 0.03% pure aluminum deoxidizing, liquid steel sedate, slag removal. The turbine is made of ZG310-570, and the steel output temperature is generally 1570~1590°C. Considering the complete filling of the turbine, the steel output temperature is raised to 1610~1620°C.
Pouring speed and centrifuge speed
According to the formula of centrifugal force F’=0.112Rr (n/100)² and gravity coefficient G=0.112(n/100)²R, through calculation and production practice, the centrifuge speed is set at 293r/min, and the pouring time is controlled at 5~8s. When the molten steel is poured close to the nozzle neck, the rotation stops immediately.
Casting ladle and baking of casting ladle
A small teapot casting ladle of 10kg is made and dried naturally for more than 1 day after the casting ladle is built. The casting ladle shall be pre-baked at the same time when the mold shell is pre-roasted. In order to minimize the drop of pouring temperature of the molten steel, the molten steel should be returned to the furnace immediately after pouring into the tea shell bag. After pouring the molten steel into the teapot bag for the second time, the molten steel should be poured immediately. See Figure 2-56 for the teapot bag.
- Figure 2-56 Baked 10kg teapot bag
Strict control: Pour one set of type shells in one pot.
Strict control: If there is any residual liquid steel in the ladle, it must be poured back into the furnace.
The purpose is to facilitate wax discharge, exhaust, shell feeding and improve the shell rigidity of the module.
Clear division of labor and coordination
Roasting, smelting and pouring are the key processes for turbine production. In on-site production, unified command, close cooperation and collaborative operation are emphasized in addition to strict compliance with process regulations.
Heat preservation and cooling to prevent cracking.
Strictly limit the unpacking time, so that the casting just cast good out of the iron box (if the winter to delay the unpacking time), continue to cool naturally at room temperature conditions, until the hand touch the mold shell without the feeling of hot, can shake the shell, so as to effectively prevent the crack, in the whole cooling
Do not water cooling during the process.
Civilized cleaning and standardized operation.
In the process of removing the mold shell on the casting, cutting the pouring riser and process bars, and removing the sand and oxide skin in the inner cavity of the casting, it is necessary to handle lightly, stack neatly, prevent leaf damage, adhere to civilized cleaning and standard operation. Crawler type shot blasting machine is adopted. The particle size of the shot batch shall not be greater than 0.3mm. The residual coating which is not easy to be removed in the groove of the inner cavity shall be soaked with ballast removing liquid.
Casting results By strengthening
The control of process details in each process of investment casting, high-difficulty and high-demand turbines can be produced under the condition of non-vacuum melting and non-vacuum casting. It is reported that the production of the same type of turbine, Japan is still using stamping, welding process.
Under the same process control conditions, for the filling integrity, the integral inner runner pouring system is superior to the spherical riser and cross runner pouring system, as shown in Figure 2-57 and Figure 2-58.
- Figure 2-57 Casting cast by spherical riser pouring system
- Figure 2-58 Castings cast by the integrated inner runner pouring system