Difficulties in machining large-diameter flanges
Difficulty in preparing raw materials and blanks:
Difficulty: Large sized steel ingots or forgings are prone to defects such as segregation, looseness, and slag inclusion inside. During the forging process, it is extremely difficult to confirm the uniform refinement of metal flow lines and grains. During heat treatment, there is a large temperature difference between the inside, outside, and various parts, which can easily generate residual stresses.
Consequence: It may lead to deformation during subsequent processing or failure due to the expansion of internal defects during use.
Difficulty in controlling dimensional accuracy and geometric tolerances:
Difficulty: 'Big' itself is a challenge. Under the huge diameter and weight, the clamping deformation during the machining process, the thermal deformation caused by cutting heat, and the accuracy limitations of the machine tool itself will be amplified. Confirming the flatness of the sealing surface, perpendicularity to the center circle of the bolt hole, parallelism of the two flange surfaces, and other geometric tolerances is much more difficult than small flanges.
Consequence: Uneven sealing surface can cause uneven stress on the gasket, leading to leakage; Misalignment of bolt holes can lead to installation difficulties or additional stress.
The sealing surface is difficult to process:
Difficulty: Processing a concentric circular texture with uniform roughness and no cutting marks on a huge annular surface is a huge challenge. The wear of cutting tools, the flatness of machine tool guides, and the stability of the spindle will all be "recorded" on the machining surface.
Consequence: The substandard quality of the sealing surface is the direct cause of the leakage.
Difficulty in heat treatment and residual stress control:
Difficulty: Uneven heating and cooling of large-diameter workpieces can easily generate significant thermal and structural stresses. If residual stress is not fully relieved, it will gradually release after processing or during use, leading to flange deformation, also known as "aging deformation".
Consequence: Measurement during processing is qualified, but warping and loss of accuracy occur after placement or use.
Difficulty in lifting, transportation, and measurement:
Difficulty: Natural deformation caused by self weight (such as sagging) can affect measurement and machining benchmarks. Traditional measuring tools such as calipers are not suitable and require one-on-one measuring tools or three-dimensional measurements. Improper lifting can also cause collisions or deformations.
Whole process quality control method
To address the above difficulties, it is necessary to implement quality control throughout the entire process:
1、 Raw materials and blanks stage
Method:
Using vacuum degassing steel ingots to reduce impurities and gas content from the source.
Using forging or free forging+ring rolling process, confirm the distribution of metal flow lines along the flange contour to improve mechanical properties.
Implement strict ultrasonic testing (UT), conduct a 90% scan of the interior of the forging, and confirm that there are no defects exceeding the standard.
Implement standardized heat treatment such as normalizing and tempering, use computer-controlled furnace temperature uniformity, and use "stress relief annealing" to specifically eliminate residual stress.
2、 Processing technology stage
Method:
Rough semi fine fine stage processing: leaving sufficient time between each stage for natural aging or arranging stress relief annealing to gradually release stress.
One on one fixture: using multi-point support, hydraulic auxiliary support and other methods to simulate a free state and reduce fixture deformation. When processing sealing surfaces, it is common to use the method of attaching the back of the flange to the fixture and tightening it with bolts to simulate its final working state.
Optimize cutting parameters: Adopt low speed, small cutting depth, and large feed rate to reduce cutting heat and cutting force, prevent thermal deformation and tool yielding.
Careful processing of sealing surface: Use wide and sharp precision cutting tools or adopt scraping and grinding technology to protect the continuous texture and roughness of the sealing surface to meet the standard. For particularly important flanges, a "pre tightening state simulation" machining can be performed before final finishing.
3、 Inspection and Measurement Stage
Method:
One on one measuring tools: Use bridge type coordinate measuring machines, laser trackers, electronic level gauges, etc. to accurately measure aperture, hole spacing, flatness, verticality, etc.
Important inspection of sealing surface:
Coloring method to check flatness: Paint on a standard flat plate and rub it against the flange sealing surface to see if the contact spots are uniform.
Optical flat crystal or plane interferometer: detects small fluctuations in flatness.
Multi point measurement of roughness meter: Confirm that the roughness of the entire annular sealing surface is consistent.
Bolt hole inspection: Confirm that all bolt hole sizes are consistent and can be smoothly threaded through with standard bolts.
4、 Packaging and Protection Stage
Method:
Rigid protection of sealing surface: Clean immediately after processing, apply rust inhibitor, and install a sturdy wooden or plastic protective cover. It is strictly prohibited for the sealing surface to come into direct contact with any object.
Technical placement and transportation: Vertical placement or flat placement using the same support method as during processing to prevent deformation due to self weight. Secure firmly and prevent impact during transportation.