Design and management of investment casting process technology

1 Advantages and limitations of the investment casting process

1.1 Pros

1) The dimensional accuracy of investment castings is high and the surface roughness is small. Because investment casting uses fusible molds with high dimensional accuracy and smooth surfaces, the casting is closer to the final shape of the part, which can reduce the subsequent processing process and save the consumption of metal materials.
2) It is suitable for casting castings with complex shapes and precise shapes. The investment casting process can cast castings with complex structures that are difficult to process and produce.
3) Alloy materials can be selected in a wide range of ways. Carbon steel, alloy steel, stainless steel, high-temperature alloy, copper alloy, precious metal and cast iron can be cast according to different materials to meet more production needs.
4) It can be produced in large quantities and small batches, and the production organization is flexible.

1.2 Limitations

1) The investment casting process is complex, and there are many process factors that affect the quality of castings. Therefore, it is necessary to strictly control the quality of various raw materials and operating processes in order to achieve stable production.
2) The production cycle is relatively long.
3) The cooling speed of the casting is slow, which is easy to cause the coarse grain of the casting, resulting in Wechsler structure, which seriously affects the quality of the casting.
4) There are many production processes, and the labor intensity of employees is relatively large.

2 Design and research of process technology in key links

From the perspective of market demand, downstream customers have higher and higher requirements for casting materials, mechanical properties and surface finish, and the government’s requirements for enterprise safety, environmental protection and energy conservation are becoming more and more stringent processing and other links to improve the level of casting technology.

2.1 Moulding process

The mold making process is the basis of investment casting, which is directly related to the size and surface finish of the product. The water glass process uses a paraffin low molecular weight polyethylene mold of 95% paraffin wax (mass fraction) + 5% low molecular weight polyethylene (mass fraction). With a melting point of 66 °C and a softening point of about 34 °C, the proportion of polyethylene should be strictly controlled during the configuration. If the ratio is too low, it is easy to lead The model is softened and deformed, and the structure is unstable. If the ratio is too high, it is easy to cause
The shrinkage ratio increases, resulting in deformation of the product size. In the production process, polyethylene is easy to show aging characteristics for long-term use, which makes the performance of the mold material deteriorate. Therefore, polyethylene should be replenished according to the production cycle. The silica sol process uses medium-temperature waxes, which are mainly composed of paraffin, resin, and polymers. The melting point of medium-temperature wax is generally 70~100 °C, and compared with low-temperature wax, medium-temperature wax shrinks more It is small, the internal structure is more stable, the dimensional stability is good, the surface roughness of the investment mold pressed by liquid is fine, the replication performance is good, and the recycling process is more convenient. The traditional manual wax injection has low efficiency, poor working environment, high labor intensity, and the mold needs to be repaired manually after wax molding, which is labor-intensive and time-consuming. The use of a fully automated wax injection machine, through the use of detection switches, intermediate relays and PLC host and other automatic components, complete PLC programming, and complete automatic instructions according to the settings, the wax liquid is sent from the dewaxing kettle to the paste making machine, and the wax paste is automatically transported to the wax injection machine through the paste making machine, and then automatic wax injection is carried out to realize the automation of the whole process of wax injection. The use of automatic wax injection machine can improve the qualification rate and production efficiency of wax molds, cancel the mold repair process, and improve the labor environment of the mold making workshop, reduce the labor intensity of employees, and reduce the amount of labor.

2.2 Shell-making process

The shell is made of binder, refractory powder and sand sprinkler material mixed into a coating in proportion, and then made by dipping, sanding, drying and hardening. The coating layer is divided into a surface layer and a reinforcing layer. The surface layer is used to accurately replicate the surface shape of the model, forming a dense and smooth surface cavity The result is a smooth surface finish. The reinforcing layer is used to thicken and reinforce the shell, so that the shell has good strength and other comprehensive properties. At present, there are two main types of shell-making process: water glass process and silica sol process, the former shell is made of quartz powder and quartz sand as refractory materials, water glass is used as binder, and chloride salt is used as chemical hardening agent. The latter uses zircon powder, molai powder and corundum powder as refractory materials, and silica sol as a binder. The two processes mainly have the following characteristics: 1) The dimensional accuracy of the castings cast by the water glass shell is not high, and the surface roughness is large. The castings of the colloidal silica shell are dimensionally precise and have a smooth surface finish.
2) The manufacturing cycle of the shell of water glass is short, and the investment in raw materials and equipment has a great price advantage compared with the silica sol shell.
3) The strength, air permeability, thermal expansion, thermal shock stability and other properties of silica sol shell are better than those of water glass shell.
4) The silica sol type shell is thin and strong, which can save time and energy in the process of dewaxing and roasting.
5) The water glass shell produces a large amount of hydrogen chloride gas during the roasting process, which will cause air pollution.
6) The silica sol process is better than the water glass process in terms of casting precision, quality stability, environmental protection and energy saving, and has gradually become the mainstream process of green casting. However, the cost of equipment is high, the cost of raw materials is high, and it is still difficult for some small and micro foundry enterprises to completely eliminate the water glass process.

The environmental problems caused by the use of hardeners such as ammonia chloride and aluminum chloride in the original water glass shell making process can be improved by using a new hardening agent YS-28 (i.e., a hardening ester fused with a carboxyl organic compound with an appropriate amount of hydrocarbon-based modifier).

The new hardener has the following advantages:

1) The mold shell has high strength at room temperature and high temperature, low residual strength, good shelling, smooth surface of the casting, and its natural drying and hardening characteristics are similar to those of silica sol.
2) Improve the working environment, the shell-making process is odorless, non-toxic, and non-corrosive to the equipment.
3) Reduce water pollution, eliminate acid gas emissions, and contribute to environmental protection.
4) The price is moderate, after the new hardener and sand are evenly stirred, it is still effective for many days, and the cost is similar to the cost of chloride salt. In the shell-making process, the shell-making assembly line composed of mechanical sand spreader, electric mixer, automatic conveying line and dust removal and dust suppression equipment is adopted, which has the advantage of greatly reducing the amount of labor in manual shell-making in the production process, which can save half of the labor expenditure, reduce the workload of workers, and improve production efficiency. Mechanized operation can ensure that the operator can operate in strict accordance with the process requirements, avoiding quality problems caused by a large number of human factors. At the same time, the working environment is improved, the workers’ bodies are avoided from direct contact with the production materials, and harmful factors such as dust are avoided from causing harm to the workers. Centralized collection of dust and recycled materials in the production process The set treatment is conducive to protecting the environment and meeting the requirements of green casting.

2.3 Shell roasting process

After making a qualified shell, it is also necessary to use a good alloy liquid and the correct casting process to obtain a high-quality casting. This process includes shell roasting, molten steel smelting and molten steel pouring before casting, and the three processes are interlocked and closely coordinated. The shell roasting is mainly to put the dewaxed shell into the roasting furnace, and the roasting temperature is controlled at 870~1 050 °C, which is mainly used to roast the dewaxed shell into a ceramic shell, so as to eliminate impurities in the shell. The roasting process generates a large amount of heat energy, and the untreated roasting process is accompanied by a large amount of heat loss, resulting in a waste of energy. can By installing energy-saving devices such as automatic ignition and automatic temperature control above the roaster, the amount of natural gas and electricity consumption can be controlled according to the actual temperature requirements, so as to reduce energy consumption, improve energy utilization efficiency and reduce production costs. At the same time, the exhaust gas circulating water heating system can also be installed, including the exhaust gas boiler device, the circulating water pool and the hot water circulation pump, etc., and the exhaust gas can be used to heat the boiler. After obtaining the steam, the circulating water pool is heated, and the hot water is transported through the hot water circulating pump for the preheating of the wax pot heating, dewaxing heating and other processes, and it is convenient for employees to heat in winter. In this way, it can not only reduce the temperature of exhaust gas discharge, facilitate the environmental treatment and discharge of exhaust gas in the later stage, but also reduce the consumption of other energy sources through the use of waste heat.

2.4 Molten steel smelting

In the smelting process of general carbon steel, low-alloy steel and stainless steel, most of the melting equipment adopts coreless fast intermediate frequency induction furnace, which has the advantages of high power, fast melting speed, less burning loss of alloying elements, and fewer porosity and inclusion defects caused by heavy melting. The disadvantage is that the gas and non-metallic inclusions in the molten iron are easy to cause the purity of the molten iron to not meet the standard and the accidental splashing of the molten iron. The extent to which molten iron splashes cause harm depends on the form and content of non-metallic inclusions present in the steel, as well as H2, O2 and N2 in the gas, with H2 being the most hazardous. H2, O2 and N2 are mainly derived from the charge, and at the same time, the molten steel is also drawn from the furnace gas during the melting process. In addition, poorly sintered linings, poorly dried alloy melters, undried ladles and unbaked shells will increase the gas in the molten steel Content. In order to avoid the mixing of these gases, the following measures can be taken: 1) Clean the return material. In particular, the return material with serious rust should be derusted, and not only iron oxide is brought into the rust, but also moisture is brought in, and the decomposition of water causes the molten steel to inhale H2.
2) Shorten the high-temperature smelting time to prevent the gas from being involved in the molten steel tumbling process.
3) Adequate deoxygenation. Aluminum is used in ordinary carbon steel smelting to achieve final deoxidation, and insufficient addition of aluminum will lead to insufficient deoxidation, and excessive addition will cause low fluidity and deteriorate the machining performance of castings.
4) The roasting temperature of the shell meets the requirements, and the holding time meets the requirements, so as to reduce the gas generation of the shell and inhibit the inflow of invasive gas.

With the popularization of VOD (argon oxygen refining furnace) and AOD (vacuum refining furnace) refining methods, the rapid remelting of high-quality charges has appeared in China, making the melting level of investment casting a big step towards the goal of pure steel. At the same time, many precision casting manufacturers use the melting technology of the inert gas cover protection method, trying to drip liquid argon or liquid nitrogen and other liquid inert gases to let the liquid The inert gas rapidly vaporizes and diffuses, replacing the air layer above the melt pool to form an inert gas protection. This technology can play a role in blocking the air and preventing it from entering the smelting fluid, thus ensuring the purity of the molten iron. At the same time, the traditional deoxidation method can be changed, the deoxidizing elements can be placed in the thin-walled metal deoxidation tube, and then inserted into the molten metal in the furnace, the deoxidized elements react in the molten metal in the furnace to produce kinetic energy, drive the molten steel to roll, make the gas and impurities in the molten steel float, and more easily remove the air and impurities in the molten steel.

2.5 Casting link

In the casting process, whether the casting temperature and casting speed meet the standard are the key factors affecting the yield of castings. In order to change the traditional smelting and casting environment, improve production efficiency and casting quality, the introduction of automatic casting machine has become the trend of the development of the foundry industry. The automatic casting machine can be controlled by remote control The detection device measures the temperature of the smelting furnace, and after the temperature of the molten iron reaches the casting requirements, according to the position of the automatic casting chassis of the shell, the casting is carried out at a fixed time, at a fixed speed and quantitatively, so that the casting process is more stable and the finished product rate of casting is higher. At the same time, the casting operation is carried out by the manipulator, which reduces the amount of labor in the casting process, and maintains a certain safety between the workers and the shell and the melting furnace distance, which can effectively avoid high temperature scalding and direct damage to the human body caused by harmful gases in molten steel.

2.6 Heat treatment

The heat treatment process refers to the metal treatment process in which the cast castings are subjected to subsequent heat treatment according to the requirements of process parameters and mechanical performance parameters, and cooled at an appropriate rate, so as to change the overall mechanical properties of the castings. The heat treatment process is generally quenching, tempering, normalizing and
There are four basic processes of annealing. Quenching is a heat treatment process in which steel is heated to a critical temperature, kept warm for a period of time, fully or partially austenitized, and then cooled below the Ms point at a cooling rate greater than the critical cooling rate, so that the casting undergoes martensite or bainite transformation. Usually aluminum is also combined The solution treatment of materials such as gold, copper alloys, titanium alloys, and tempered glass or the heat treatment process with a rapid cooling process is called quenching. Tempering is a heat treatment process that heats the quenched metal or parts to a certain temperature, keeps warm for a certain period of time, and cools in a certain way, and tempering is the follow-up treatment process of quenching. Normalizing is to heat the casting to a critical temperature, keep it warm for a certain period of time, and then carry out rapid cooling to achieve the purpose of crystalline grain refinement. Normalizing can not only make the casting get satisfactory strength, but also significantly improve the toughness and reduce the cracking tendency of the casting. Some low-alloy hot-rolled steel plates, low-alloy steel forgings and castings are normalized After treatment, the comprehensive mechanical properties of the material can be greatly improved, and the cutting performance can also be improved. Annealing is to heat the casting to a certain temperature, take appropriate heat preservation measures, and then cool it at the appropriate temperature. The use of heating electric furnace instead of traditional coal and gas heating furnace can accurately control according to the set temperature, which not only ensures the accuracy of heating temperature, the effectiveness of the aging of heat preservation effect, the efficient use of heat energy, but also avoids the exhaust gas emission in the process of coal and gas combustion.

2.7 Experiments and testing

In addition to the transformation of the above-mentioned traditional links, foundry enterprises can also use CAE software (i.e., analysis software for computer solution and analysis of the structural performance of complex engineering products) to carry out static analysis, dynamic analysis and structural fluid analysis. The CAE software can simulate the casting filling process, solidification process, microstructure, stress and strain and heat treatment process for improvement Manufacturing and upgrading traditional casting technology plays an irreplaceable role in reducing the production cost of products and improving the competitiveness of foundry enterprises. In terms of defect prediction in the casting process, it is possible to effectively predict and quantify the defects of casting porosity formed during the solidification process. The numerical simulation algorithm in the filling process has been perfected, and the defects in the filling process, such as insufficient pouring, cold separation, air entrapment and inclusions, can be qualitatively predicted. Tissue and stress simulations can predict possible problems in actual production. Through these technical means, workers can quickly evaluate the feasibility and operability of the process according to the simulation results, and carry out it in a timely manner The corresponding process optimization can also accumulate rich design experience in the implementation process. The application of CAE software can speed up the development process of new products, change the traditional trial-and-error experiment to predictive experiment, shorten the trial production cycle of new products, and save a lot of manpower and material resources. The use of spectrometer, metallographic microscope, tensile testing machine and hardness tester and other testing equipment is also a must for the promotion and development of modern foundry enterprises To measure. Through these testing methods, batching can be carried out according to the test results, the casting process can be designed, and the heat treatment process can be improved, so as to meet the requirements of high-end products, improve the quality of products, and improve the competitiveness of enterprises.

3 Conclusion

Modern investment casting enterprises have achieved technological upgrading by eliminating backward production capacity and production equipment, and met the requirements of local safety production, standard emissions and energy conservation. Upgrading and transformation of key links such as molding, shell making, shell roasting, molten steel smelting, casting and subsequent heat treatment, coupled with modern experiments and testing methods, improve product quality and product value, effectively reduce production costs, meet higher market requirements, and improve enterprises The market competitiveness has laid a solid foundation for the company to achieve breakthrough development in the future.

6 Conclusion

Based on the good application effect of YCXE-1 locomotive in Yangchun New Iron and Steel, it is confirmed that the pure electrification of the locomotive is suitable for the working conditions of the shunting locomotive of metallurgical enterprises. Pure electric locomotives have obvious cost and environmental protection advantages, which are in line with the national strategic decision of carbon peak and carbon neutrality, and are also in line with equipment and facilities The trend of informatization and informatization has the value of promoting and applying to the whole industry, which is the development direction of shunting locomotives in the domestic metallurgical industry.

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