Precision Manufacturing Methods for Oversized Lemniscate Tubing
In addressing the challenge of achieving high-precision curved surface processing for large-diameter stainless steel thin-walled parts, research was conducted on the manufacturing process. By optimizing the blank forming method and selecting an appropriate processing scheme, along with employing precision turning and grinding techniques, we ensured that both the surface accuracy and surface roughness requirements of the double-curved surface were met. This work provides a valuable reference for the processing of similar thin-walled parts.
01 Preface
Independent research and development in gas turbine technology can significantly enhance the industry’s competitiveness and optimize the energy structure. The double-curved flow tube, which connects to the gas turbine intake transition section, is utilized for testing the compressor’s intake flow performance. The inlet features a trumpet-shaped diffuser section, while the outlet is a cylindrical straight pipe section. Both sections share the same diameter, and the diffuser’s inlet is joined to the flow tube with a flange structure.
The inlet shape of the double-curved flow tube follows a specific geometric design known as the double-curved curve. This configuration is intended to create an ideal flow, ensuring a stable, uniform, and axisymmetric velocity field near the bell mouth outlet. Given the working conditions, both the straight pipe and diffuser sections of the flow tube are constructed from stainless steel, characterized by thin walls and low rigidity. To provide support, the outer side is reinforced with transverse and longitudinal welded ribs.
The inner surface of the tube has stringent requirements regarding machining dimensions, shape tolerance, and surface roughness, resulting in a large overall machining volume. Machining stainless steel is challenging due to its low efficiency and propensity to deform during processing. To achieve a high-quality flow field, rolling and polishing are necessary after fine turning the inner cavity to meet the high precision standards.
Manufacturing large-caliber stainless steel thin-walled components with high-precision curved surfaces poses significant difficulties and carries a high risk of defects. Consequently, researching and exploring effective machining process solutions is of considerable technical and economic importance.
02 Analysis of part structure and rough forming difficulties
As illustrated in Figure 1, the double-negative wire flow tube features a combination of the left straight tube section and the right diffuser section. The overall length of the tube is 2856 mm, with a wall thickness of 32 mm, and it has a net weight of 5 tons. The outer surface is reinforced by horizontal and vertical process plate ribs.
The diffuser section is shaped by fitting together dozens of coordinate points, ensuring that each point creates an arc transition. The difference in inner diameter between the left and right sides is 1200 mm. The surface profile on the diffuser section measures 0.15 mm, and its surface roughness is rated at Ra=0.8 μm. This component is a typical large-diameter, thin-walled part.
If this part is cast at the blanking stage, sand holes may form, adversely affecting surface quality and leading to rust in the future, which further impacts its overall surface condition. In the case of painting, there is a risk of paint peeling off under high flow rate conditions.
Forging the part as a whole presents challenges as well. The excessive axial length and significant difference in inner diameter between the left and right sides of the diffuser section lead to large forging margins and elevated costs. Alternatively, if the material is cut into plates and rolled using a plate rolling machine, the diffuser section will encounter difficulties. The considerable wall thickness will create challenges during the rolling process, and excessive residual stress could lead to deformation during subsequent processing.
To ensure a high-quality and smoothly structured inner cavity as proposed in the design, we aim to reduce the scrap rate during processing, lower production costs, and improve overall production efficiency. The double-twist flow tube will be made from 06Cr19Ni10 austenitic stainless steel, which is known for its excellent corrosion resistance and good resistance to intergranular corrosion. This material can withstand high temperatures ranging from 1000 to 1200°C and is widely used in the aerospace, chemical, and petroleum industries.
During the blank manufacturing stage, the straight pipe section will be cut from 40mm thick plates and then shaped using a plate rolling machine. The expansion section will be created from forgings, consisting of two circular rings that are forged and then welded together. This dual approach to manufacturing the two sections significantly reduces both the blank costs and production expenses.
03 Analysis of Difficulties in Part Processing
The diffuser section is made of stainless steel and features a large-diameter, thin-walled, curved surface with a finished wall thickness of 32mm. The design requires a smooth transition at the flange connection, and it has high standards for surface accuracy and roughness. The following processing challenges arise:
1) The design necessitates a smooth transition at the flange joint. Initially, the diffuser section and the straight pipe section are processed separately, leaving a margin. These components are then assembled and combined using flanges for final processing. Challenges arise from excessive overhang and the weak rigidity of the vertical lathe spindle, requiring careful control of the cutting amount to minimize cutting force.
2) The contour of the inner cavity surface of the diffuser section must remain within a tolerance range of 0.15mm, with a surface roughness value of Ra=0.8μm across a large processing area. Ensuring the precision of large-diameter profiles, approximately 2m in size, is difficult and results in long processing cycles. The turning capabilities of large vertical lathes are insufficient to meet the surface roughness requirements.
3) The expansion angle of the diffuser section surface exceeds 90°. This large angle makes it impossible to complete the turning process in one operation. Both the inner and outer sides will require clamping, and the issue of tool setting must be addressed.
4) The significant amount of overall welding results in uneven processing allowances, and factors like residual stress during processing can lead to deformation. Austenitic stainless steel is particularly challenging to work with due to its high toughness, poor thermal conductivity, strong chip adhesion, and susceptibility to hardening and deformation during processing, all of which can negatively impact the surface quality.
04 Process analysis and processing practice
4.1 Process route of blank stage
The expansion section is illustrated in Figure 2. The thickness of the forging measures 45 mm, with an outer margin of 5 mm and an inner margin of 8 mm.
The processing route is as follows:
- Welding of two forgings
- Weld inspection
- Low-temperature annealing and vibration stress relief
- Removal of tooling
- Machining the outer margin to achieve the outer diameter size
- Re-installation of tooling
- Welding of the outer rib plate and ring plate
- High-temperature annealing
- Removal of tooling
- Rough machining of the inner side to a diameter of 1064 mm
- Inspection of the margin (table data includes 12 data points for each section, with no fewer than 6 sections)
- Pickling and passivation.
The straight pipe section is illustrated in Figure 3. The outer margin of the rolled plate measures 5 mm, while the inner diameter is 1056 mm. The processing route is as follows:
- Check for shrinkage at the weld.
- Pre-process the outer side.
- Assemble the internal tooling.
- Weld the flanges at both ends, along with the outer ring plate and the rib plate.
- Conduct a weld inspection.
- Perform high-temperature annealing to relieve stress.
- Remove the tooling.
- Rough machine the inner cavity to approximately f 1064 mm.
- Check the margin (record the data, detecting 12 measurements for each section, with no fewer than 6 sections).
- Carry out pickling and passivation.
4.2 Processing technology method at the finished product stage
The surface accuracy and roughness of the diffuser section are significantly higher than those of the larger parts, making the processing more challenging. This article focuses solely on the processing technology at the finished product stage, analyzing both quality and efficiency improvements.
The processing steps for the diffuser section on a CNC vertical lathe are as follows: rough turning, semi-finishing turning, finishing turning, and rolling. Due to the large expansion angle of the curved surface, two tool setups are required to avoid interference with the tool holder. The inner side is processed using the vertical surface of the connecting flange, while the outer side is left with a margin at the flange to create a platform for tool setting on the vertical lathe.
To address the insufficient rigidity caused by the overhang of the machine tool spindle, the straight pipe section and the diffuser section are processed separately during the rough turning and semi-finishing turning stages, both using 93° rough turning tools (see Figure 4). After rough machining, most of the blank margin is removed, leaving a 3mm margin on each side. The connecting holes on each flange surface are then drilled and tapped to the design size on a boring machine, followed by checks for any deformation caused by rough turning and drilling.
The rough turning parameters are a cutting speed of 120 m/min, a feed rate of 0.5 mm/z, and a back cutting depth of 2.5 mm.
Semi-finishing turning involves using the template gap method to detect the surface profile based on rough turning. The accuracy of the surface machining is ensured by measuring the gap between the template and the workpiece. Once the machining program’s accuracy is verified, an additional allowance is removed, leaving a 1mm allowance after semi-finishing turning. The transition area is machined after assembling the diffuser section and the straight pipe section to ensure a smooth transition at the flange fitting surface. During this CNC process, the clamping is loosened to relieve machining stress naturally, resulting in a deformed size. The cutting line speed for semi-finishing turning is set at 140 m/min, with a feed rate of 0.5 mm/z and a back cutting amount of 1 mm.
Finishing turning occurs after the diffuser section and the straight pipe section are assembled, utilizing an R6mm arc cutter (see Figure 5). Due to the increased height and mass of the workpiece, the cutting line speed is reduced, and the surface roughness of the inner cavity must meet grinding requirements. It is essential to reduce both the feed rate and back cutting amount during this stage to lower cutting force and enhance surface quality. The finishing cutting speed is 60 m/min, with a feed rate of 0.25 mm/z and a back cutting depth of 0.5 mm. Additionally, support fixtures are placed on both sides (see Figure 6) during the assembly process to ensure the rigidity of the workpiece at the top bell mouth.
During the inspection of the workpiece using a roughness meter, it was determined that the surface roughness value did not meet the requirement of Ra = 0.8 μm. This was primarily due to the limitations of the rotation speed of the large vertical lathe. To address this issue, it was decided to incorporate a polishing process following the fine turning.
Rolling polishing was selected as the method for this additional processing. This technique, which utilizes carbide balls for rolling polishing, is both cost-effective and efficient, offering a modern approach to improving the surface quality of the workpiece. The rolling process is illustrated in Figure 7. The rolling device is securely attached to the spindle of the vertical lathe using screws in a tooling connector. During the rolling process, the metal surface is subjected to both rotational extrusion and high-frequency impact treatment by the roller, significantly reducing the surface roughness.
Rolling processing not only enhances the surface quality of the workpiece but also improves its wear resistance and fatigue strength through work hardening. Furthermore, the method involves squeezing the metal surface and utilizing plastic deformation to achieve a smoother finish. With the use of cutting fluid, issues such as grinding heat and cracks are eliminated.
The design of the roller allows it to process not only flat surfaces but also various contour shapes. In this instance, a semicircular roller with a cross-section radius of 6 mm was employed. This radius matches that of the R6 mm arc cutter used during the fine turning process, enabling the same program and tool compensation to be applied. The rolling polishing operation was conducted at a linear speed of 60 m/min, with a feed rate of 0.2 mm/z and a press-in amount of 0.01 mm.
The surface roughness and profile of the inner cavity of the part were evaluated using a roughness meter and a sample gap detection method. The surface processing successfully met the design specifications outlined in the drawing.
05 Conclusion
The high-precision curved surface of the diffuser section in a lemniscate flow tube is crucial for achieving a high-quality wind speed flow field. This study focuses on the processing technology of large-diameter lemniscate flow tubes. By employing an economical and efficient blank cutting method, along with multiple turning and rolling polishing techniques, we take full advantage of the processing capabilities of large vertical lathes. This approach effectively addresses the challenges of high-precision curved surface processing for large-diameter stainless steel thin-walled components, providing valuable practical experience for similar part manufacturing.
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