Duplex stainless steel refers to a kind of stainless steel grade containing both austenite and ferrite in steel. The volume fraction of two phases in duplex stainless steel which has been widely used in engineering is roughly equal presently. Therefore, it combines the features of austenitic stainless steel and ferritic stainless steel. Compared with ferritic stainless steel and martensitic stainless steel, it has the characteristics of high toughness, low brittle transition temperature and significantly improved intergranular corrosion resistance and weldability. Meanwhile,it retains the the ferritic stainless steel’s characteristics of high thermal conductivity, small expansion coefficient and superplasticity and it has been widely used in the petrochemical industry in China.
Duplex stainless steel 2205 alloy has the features of high strength, good impact toughness and good overall and local resistance to stress corrosion. Compared with austenitic stainless steel, the duplex stainless steel 2205 alloy has superior performance in resisting pitting corrosion and crevice corrosion and has high corrosion resistance. It has a lower coefficient of thermal expansion and higher thermal conductivity than austenite. It is twice as strong as austenitic stainless steel. After the solution treatment, the non-proportional extension strength Rp0.2b/(N/mm2)≥450, tensile strength Rm/(N/mm2) >655, section elongation A/%>25, hardness HBW：≤290.
The induction wheel is a quick-wear part in the petrochemical industry, duplex stainless steel 2205 was adopted in investment casting process , its chemical composition is shown in Table 1. Figure 1 is a simplified diagram of the induction wheel parts. The induction wheel has a complicated structure and uneven wall thickness. Its external dimension is φ72 mmxφ152 mmxφ175 mm, the blade wall thickness changes from 2.5 mm at the root to 0.5 mm and the casting weight is 0.6 kg. The product requirements are as follows: 1.The surface quality roughness is not more than 6.3, no cracks, no slag inclusions, no pores or other defects; 2. No shrinkage, shrinkage cavity or other defects in terms of internal quality; 3. Casting properties: tensile strength is not less than 655 MPa, the specified non-proportional extension strength is not less than 450 MPa, the section elongation is not less than 25%, and the hardness HBW is not more than 290.
For the purpose of ensuring product quality and shortening the production cycle of the product, ProCast software was used in investment casting to numerically simulate the filling and solidification process of the induction wheel and ensure the surface quality and internal quality requirements of the product. In the smelting and pouring process, the use of the non solution treatment agent ensures the mechanical properties of the product, reduces the heat treatment process and greatly lowers the production cost.
1. Stainless Steel Casting
1.1 Wax mold Production
The induction wax mold is laser sintered by 3D printing technology, which completely eliminates the process of mold design and processing and shortens the production cycle of the product. The rapid forming parts have better dimensional stability than medium temperature waxes. The shrinkage rate design of the rapid forming part can ensure that the casting size is within the tolerance range according to the structural characteristics of the induction wheel and the material characteristics of the duplex stainless steel 2205.
1.2 Pouring System Design
The investment casting system is designed according to the characteristics of the induction wheel structure and the casting properties of the material and the top pouring system is adopted (See Fig. 2). The molten metal is injected from the pouring cup, which is favorable for forming the progressive solidification and obtaining the dense casting, and has a good mold-filling capacity. It is conducive to filling the thin wall of the casting blade.
The casting defects such as shrinkage, shrinkage cavity and misrun are closely related to the filling and solidification processes in the casting process. In order to understand the casting defects of the induction wheel during the investment casting process, the casting simulation software ProCast is used to simulate and calculate the filling and solidification process of the induction wheel pouring system during the investment casting process. In the calculation process, the solidification phenomenon during the filling process and the flow of molten steel during the solidification process are fully considered in order to obtain the flow of molten metal during the filling process and get a more accurate temperature field distribution during solidification, which can imply the causes of various defects, thereby it improves the process conditions and high quality castings are obtained.
The setting of the boundary conditions during the numerical simulation as well as the selection of the thermophysical properties of the material determine the accuracy of the simulated calculation.
Based on the filling process analysis, it is possible to avoid defects such as misrun, shrinkage and shrinkage cavity in the forming process of the casting. The complete casting has been obtained. The casting temperature of the induction wheel casting process is selected to be 1 600 °C, and the shell temperature is 1 000 °C. The casting speed is 0.35 m/s.
1.3 Silica Sol Shell Making
The shell making material is zircon powder, zircon sand, molybdenum sand and muli powder. The shell surface, the transition and the reinforcement all has 2 layers according to the characteristics of the induction wheel weight, volume and structure. The shell should not be too thick for the purpose of ensuring the permeability of the shell. The dryness degree of each layer is measured by using a multimeter. A steam dewaxing kettle was used for rapid dewaxing after drying.
1.4 Melting and Pouring
The shell is preheated in a preheating furnace at a temperature of 1 100 °C for 40 to 50 mm before pouring. In order to guarantee the quality of the molten metal , firstly the melting furnace should be checked and dry the metal material, then melt with electricity. When the current is stable, the power can be increased gradually and start to melt the furnace burden. The slag-forming material is added to cover the entire molten surface after melting , then slag-forming and slagging-off are performed, the temperature is raised to 1580~1600°C for pre-deoxidation, 0.2% of the silicon-calcium alloy is added for deoxidation, it lasts for 1 to 2 minutes, and 0.4% electrolytic chromium is added, then add 0.2 kg non solution treatment agent, add covering slag removing agent after slagging-off. When the temperature is raised to 1 600~1 620 °C for final deoxidation, add 0.1% silicon calcium alloy to deoxidize, it lasts for 1-2 min, then add 0.2 kg non solution treatment agent; then clean the slag and sample. A rapid chemical analysis by direct reading spectrometer was conducted. The analysis results are shown in Table 2. After the ingredients are tested to qualified, then cut off the power and pour.
After the pouring is completed,insulation measures should be taken on the pouring gate to ensure the feeding effect, and the mechanical properties of the sample are cast with the furnace. The mechanical properties of the samples were tested and the test results are shown in Table 3.
2. Production Results and Analysis
With the simulated calculation of filling and solidification process of the induction wheel gating system during the stainless steel investment casting process by casting simulation software ProCast, the results showed that the castings had no defects such as shrinkage or shrinkage cavity. The chemical composition is spectrally analyzed after pouring and the composition is qualified. The mechanical properties are tested by tensile testing machine and hardness tester, which meets customers’ required indicators. It meets customers’ requirements without defects after the X-ray test on internal quality. After the refining and ball throwing on the casting surface, it is detected by fluorescence and the surface is free from defects.
Currently, these stainless steel castings are in volume production. The 2205 duplex stainless steel castings produced are shown in Figure 5, and the cutting parts are shown in Figure 6.
3. Summary
- By using the casting simulation software ProCast, the filling and solidification process of the duplex stainless steel 2205 induction wheel were simulated and calculated , the results show that to choose the proper casting temperature and casting speed during the stainless steel casting process can solve the defects such as misrun and internal shrinkage, the theoretical calculation is consistent with actual production results.
- With the non solution treatment agent,there is no need to perform solid solution treatmentduring the smelting process, it also has mechanical properties and corrosion resistance comparable to solid solution treatment.The problem of product deformation during the heat treatment process is avoided under the requirement of ensuring the mechanical properties of the product. Additionally, the production cost is greatly reduced.
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