Inspection Methods used to producing open die forged screw rotors with large diameter

Inspection is a crucial step in producing open-die forged screw rotors with large diameters (e.g., 580mm) to ensure the finished part meets the required dimensional accuracy, mechanical properties, and functional standards. Various inspection methods are employed throughout the manufacturing process to detect potential issues early and guarantee the quality of the final product. Below are the key inspection methods used for large-diameter open-die forged screw rotors:

1. Visual Inspection

Purpose: To identify surface defects such as cracks, inclusions, oxidation, and other visible issues that could affect the rotor’s performance.
When Used:

Pre-forging: Inspecting raw material for surface defects (e.g., cracks, pitting).
Post-forging: Inspecting the rotor after each open die forging stage for signs of improper deformation, flash formation, or irregularities.

Method: A thorough visual inspection is performed using handheld tools such as magnifying glasses or borescopes for internal features, depending on the size of the rotor.
Advantages: Simple, quick, and cost-effective method for detecting obvious defects.

2. Dimensional Inspection

Purpose: To verify that the rotor dimensions (e.g., diameter, length, groove profiles) conform to the design specifications.
When Used:

After each forging stage and post-machining to ensure that critical dimensions meet tolerance requirements.

Methods:

Calipers and Micrometers: Used for measuring the outer diameter, thickness, and other critical dimensions.
Height Gauges: To check vertical dimensions, such as face height or groove depth.
CMM (Coordinate Measuring Machine): For more precise dimensional measurement and to check complex geometries, CMMs are widely used. This machine measures the rotor's critical features by moving a probe along multiple axes to gather 3D measurements.
Laser Scanning: A laser scanner can be used to measure the part’s surface profile. It provides a high-precision 3D representation of the part, allowing for quick and accurate dimensional verification.
3D Measurement Systems: Advanced systems use laser triangulation or structured light for detailed dimensional analysis, offering accuracy in the micrometer range.

Advantages: Accurate and repeatable measurements, especially for large parts with complex geometry.

3. Ultrasonic Testing (UT)

Purpose: To detect internal defects like voids, inclusions, or cracks that may affect the rotor’s performance under operational conditions.
When Used:

Post-forging, after heat treatment, or post-machining to ensure the rotor is free of internal structural flaws.

Method: High-frequency sound waves are transmitted into the material, and the resulting echoes are analyzed to identify any irregularities in the material’s internal structure.

Through-Transmission UT: A probe on both sides of the part sends and receives waves.
Pulse-Echo UT: Uses one probe that sends and receives ultrasonic waves to detect flaws.

Advantages: Can detect internal flaws without cutting or damaging the part, providing a clear picture of material integrity.

4. Magnetic Particle Inspection (MPI)

Purpose: To detect surface and near-surface defects, such as cracks and surface stress fractures that are typically found in areas that undergo high-stress cycles.
When Used:

After forging and post-machining, especially on the surface areas that will undergo high-stress loading.

Method: The part is magnetized, and iron particles are applied to the surface. If there are any cracks or defects, the iron particles will collect around them, making them visible.
Advantages: Effective for detecting surface cracks and subsurface flaws close to the surface, especially on ferromagnetic materials like steel.

5. Dye Penetrant Inspection (DPI)

Purpose: To identify surface-breaking defects, such as cracks or pores, that are not visible under normal circumstances.
When Used:

Post-forging or after machining for final inspection of the rotor’s surface condition.

Method: A liquid dye penetrant is applied to the surface of the part. After a certain dwell time, the surface is cleaned, and a developer is applied. If there are cracks or defects, the dye will seep into the defect, making it visible under UV or visible light.
Advantages: Simple, low-cost, and effective for detecting surface flaws.

6. X-Ray Inspection (Computed Tomography or CT Scan)

Purpose: To detect internal defects, such as porosity, cracks, or inclusions, without disassembling or cutting the rotor.
When Used:

For large rotors or parts with complex internal geometries, post-forging or post-machining.

Method: X-rays are passed through the part, and the resulting image is captured and analyzed. The CT scan generates a 3D image of the internal structure, allowing detailed inspection of internal features.
Advantages: Non-destructive and can detect very fine internal defects that are invisible through other methods.

7. Hardness Testing

Purpose: To verify the rotor's material properties (hardness) after heat treatment to ensure it meets the specifications for strength and durability.
When Used:

Post-forging and post-heat treatment, particularly after quenching and tempering processes.

Methods:

Rockwell Hardness Testing: For testing the material’s hardness at specific points. A diamond indenter is pressed into the surface of the rotor under a specified load.
Vickers or Brinell Hardness Testing: Other methods of hardness testing used for different material types or more precise measurements.

Advantages: Provides direct insight into the mechanical properties of the rotor and ensures it will withstand the required stress.

8. Surface Roughness Measurement

Purpose: To verify the quality of the surface finish, especially for areas such as sealing surfaces and grooves where surface roughness can affect the performance.
When Used:

After machining or finishing operations on the rotor.

Method: Surface profilometers or roughness testers are used to measure the microscopic texture of the surface. The readings give a clear indication of the part’s finish.
Advantages: Ensures that the part meets the required specifications for smoothness and functional requirements.

9. Torque and Pressure Testing (Functional Testing)

Purpose: To test the performance of the rotor under realistic operational conditions, ensuring it meets the required torque, pressure, and vibration characteristics.
When Used:

After machining and final assembly, especially for rotors used in compressors, pumps, or blowers.

Method: The rotor is subjected to dynamic testing, such as simulating real-world operational conditions in a test rig, to verify that the rotor can handle the expected loads without failure.
Advantages: Validates the rotor's functional performance and longevity under operational conditions.

10. Dynamic Balancing

Purpose: To ensure the rotor is balanced and free of vibration, which is essential for smooth operation in screw compressors, blowers, or similar systems.
When Used:

Post-machining or post-assembly, particularly for large rotors that will experience rotational forces during operation.

Method: The rotor is placed on a balancing machine and rotated to check its mass distribution. If there is any imbalance, weight adjustments are made (e.g., by adding or removing material) to achieve a balanced condition.
Advantages: Essential for ensuring smooth operation, reducing wear, and increasing service life.

11. Non-Destructive Testing (NDT)

Purpose: A combination of the above-mentioned methods, like ultrasonic, magnetic particle, dye penetrant, and X-ray, to assess the structural integrity of the rotor without causing damage.
When Used:

Throughout the manufacturing process—pre-forging, post-forging, and post-machining—to ensure there are no material or structural defects that might compromise rotor performance.

Advantages: Provides a comprehensive view of potential issues without destroying the part.

Conclusion:

A combination of these inspection methods is typically used during the manufacturing process to ensure that open-die forged screw rotors with large diameters meet the required quality standards. Dimensional accuracy, surface quality, material integrity, and functional performance must all be verified to ensure that the rotor performs reliably and safely in its intended application.