First, General Principles of Seamless Steel Pipe Fittings Processing Scheme
1. Scope of Application for Seamless Steel Pipe Fittings Processing: This scheme applies to the batch production of seamless steel pipe fittings for equipment such as construction cranes, excavators, and tower cranes. The pipe materials cover 20# and 45# carbon structural steel and Q345B low-alloy high-strength steel, with pipe diameters ranging from Φ25mm to Φ300mm and wall thicknesses from 3mm to 20mm.
2. Core Requirements for Seamless Steel Pipe Fittings Processing: Strictly adhere to GB/T 8162-2018 "Seamless Steel Pipes for Structural Use" and GB/T 12459-2017 "Steel Butt-Welded Seamless Pipe Fittings" standards, ensuring that the dimensional tolerance of the seamless steel pipe fittings is ≤±0.5mm, the weld strength is not lower than the base metal strength, and the surface roughness Ra≤6.3μm, meeting the heavy-duty, fatigue-resistant, and wear-resistant requirements of construction machinery.
3. Seamless Steel Pipe Fittings Batch Production Targets: Single batch capacity ≥ 5000 pieces, production qualification rate ≥ 99.2%, unit product processing cycle shortened by 15% compared to traditional processes, and production cost reduced by 10%.

Second, Raw Material Preparation and Pretreatment for Seamless Steel Pipe Fittings
1. Raw Material Screening for Seamless Steel Pipe Fittings: Standard-compliant hot-rolled seamless steel pipes are used. Upon arrival, material certificates and furnace batch numbers are checked. Samples are taken for chemical composition analysis and mechanical property testing. Unqualified raw materials are strictly prohibited from entering the warehouse. Pipe Appearance Inspection: The surface is free of defects such as cracks, folds, scars, and dents. Wall thickness uniformity deviation ≤ 0.3mm. Ultrasonic flaw detectors are used for full-length flaw detection to ensure there are no internal defects such as delamination or porosity.
2. Pre-treatment process for seamless steel pipe fittings:
- Cutting and blanking: According to the size requirements of the seamless steel pipe fittings, batch cutting is carried out using CNC plasma cutting or laser cutting machines. The perpendicularity deviation of the cut is ≤0.2mm, and the length tolerance is controlled within ±0.3mm to avoid burr residue.
- Surface treatment: Oxide scale and rust on the pipe surface are removed by shot blasting to achieve a rust removal grade of Sa2.5. Phosphating treatment is then performed to form a 5-8μm thick phosphate film, improving the lubricity and corrosion resistance of subsequent processing.
- Straightening treatment: Bending and deformed pipes are straightened using a hydraulic straightening machine. The straightness deviation is ≤0.5mm/m to ensure the accuracy of subsequent processing benchmarks.

Third, the core processing technology of seamless steel pipe fittings
(I) Elbow processing (batch processing using push-forming process)
1. Mold preparation: Custom-made molds are made according to the elbow specifications. The mold material is Cr12MoV alloy steel, which is quenched to ensure wear resistance and dimensional stability. 
2. Billet Heating: The pre-treated pipe billet is placed in a medium-frequency induction heating furnace. The heating temperature is controlled at 900-1050℃, and the heating rate is 5-8℃/s to ensure uniform heating and avoid local overheating or burning.
3. Push Forming: Batch forming is carried out using a hydraulic push forming machine at a push speed of 0.5-1m/min. The billet is bent and formed by mold guidance. The elbow angle and radius of curvature are monitored in real time, and the deviation is controlled within ±1%.
4. Shaping Process: The formed elbow is placed in a shaping mold, and a cold pressing forming process is used to correct dimensional deviations, ensuring that the parallelism of the elbow end face is ≤0.3mm and the wall thickness reduction is ≤10%.
(II) T-joint Processing (using the hot extrusion process)
1. Billet Pre-treatment: Seamless steel pipes are cut into billets of a preset length. Pre-drilled holes are made at the preset branch positions of the billet using a drilling machine. The hole diameter is determined according to the branch pipe diameter, and the hole wall roughness Ra≤3.2μm. 
2. Heating and Extrusion: The billet is heated in a resistance heating furnace to 850-950℃ and held for 30-60 minutes. Then, it is placed in an extrusion die, and axial pressure is applied through a hydraulic extruder to extrude branch pipes at pre-drilled holes, forming a tee structure.
3. Branch Shaping: The extruded branch pipes are cut and flared to ensure that the branch pipe diameter tolerance is ≤±0.4mm, the branch angle deviation is ≤0.5°, and the perpendicularity of the branch pipe to the main pipe is ≤0.3mm/m.
(III) Flange Processing (Forging + Machining)
1. Forging: Flange blanks are mass-produced using die forging technology. Round steel billets matching the pipe material are selected, heated to 1100-1200℃, and then die-forged to ensure a dense structure without porosity, shrinkage cavities, or other defects. After forging, normalizing treatment is performed to eliminate internal stress. 
2. Rough Machining: Batch machining of flange end faces and inner holes using CNC lathes, with a 0.5-1mm finishing allowance for the inner hole dimensions. End face roughness Ra≤12.5μm.
3. Finish Machining: Hole system machining using a machining center. Bolt hole position tolerance ≤±0.2mm, hole spacing tolerance ≤±0.15mm; flange sealing surfaces are machined by turning, with a roughness Ra≤3.2μm and flatness ≤0.1mm/m.
4. Pairing Machining: For flanges requiring paired use, a pairing grinding process is employed to ensure a ≥95% fit of the mating flange sealing surfaces.
(IV) Reducing Pipe Machining (Bottoming/Expanding Process)
1. Bottoming Processing: For reducing pipes from large to small diameters, a cold shrinking process is used. The pipe blank is placed in a shrinking mold, and radial pressure is applied using a hydraulic press to gradually shrink the blank into shape. The shrinkage rate is ≤20%, and the taper deviation is ≤±0.5%. 
2. Diameter Expansion Process: For reducing pipes from small to large diameters, a hot diameter expansion process is used. The billet is heated to 800-900℃, placed in an expansion mold, and expanded using a conical mandrel. The expansion rate is ≤30%, resulting in a smooth, wrinkle-free inner wall.

Fourth, Welding Process of Seamless Steel Pipe Fittings (for fittings requiring splicing)
1. Welding Methods for Seamless Steel Pipe Fittings: For mass production, submerged arc welding or gas shielded welding is used. Welding materials and fluxes matching the base material are selected to ensure the weld composition is consistent with the base material.
2. Welding Parameters for Seamless Steel Pipe Fittings: Submerged arc welding current 500-600A, voltage 32-36V, welding speed 30-40cm/min; Gas shielded welding current 200-250A, voltage 24-28V, shielding gas flow rate 20-25L/min.
3. Post-weld treatment of seamless steel pipe fittings:
- Slag removal: Remove weld slag and spatter from the weld surface. Grind the weld reinforcement using an angle grinder to a minimum of 3mm.
- Heat treatment: Perform post-weld stress-relieving heat treatment on seamless steel pipe fittings with a wall thickness ≥10mm. Heating temperature: 600-650℃, holding for 2-3 hours, cooling rate ≤5℃/min, to eliminate welding internal stress.
- Weld inspection: Batch inspection using ultrasonic and radiographic testing. Weld pass rate ≥99.5%, free from cracks, incomplete penetration, slag inclusions, and other defects.

Fifth, finished product inspection and packaging of seamless steel pipe fittings.
1. Dimensional inspection of seamless steel pipe fittings: Batch inspection of key dimensions of seamless steel pipe fittings using a coordinate measuring machine, calipers, and angle gauges. Randomly sample 3% of each batch for full-dimensional inspection. Defective products are immediately isolated and processed.
2. Mechanical property testing of seamless steel pipe fittings: Three samples are taken from each batch for tensile strength, yield strength, and impact testing. Test results must meet relevant standard requirements.
3. Surface quality inspection of seamless steel pipe fittings: Surface defects are detected using visual inspection and magnetic particle testing. The surface must be free of cracks, scratches, rust, etc., and magnetic particle testing must show no exceedances.
4. Corrosion protection treatment of seamless steel pipe fittings: According to customer requirements, finished products undergo galvanizing, painting, or plastic coating for corrosion protection. The galvanized layer thickness is ≥85μm, the paint film thickness is ≥120μm, and the adhesion of the anti-corrosion layer meets the Class 1 standard in GB/T 9286-1998.
5. Packaging and warehousing: Waterproof packaging paper + woven bags or wooden boxes are used for packaging. Each seamless steel pipe fitting is labeled with specifications, material, batch number, and production date. A ledger is maintained upon batch warehousing to ensure traceability.

Sixth, Process Control and Quality Assurance of Seamless Steel Pipe Fittings
1. Process Control of Seamless Steel Pipe Fittings Processing: Quality control points are set up for key processes, and dedicated inspectors are assigned to record process parameters in real time to ensure process stability.
2. Equipment Maintenance for Seamless Steel Pipe Fittings Processing: Production equipment is regularly maintained and calibrated monthly to prevent equipment failures from affecting product quality.
3. Personnel Training for Seamless Steel Pipe Fittings Processing: Operators receive pre-job training and regular assessments to ensure they are familiar with process requirements and equipment operating procedures, and are certified to work.
4. Quality Traceability for Seamless Steel Pipe Fittings Processing: A batch production quality traceability system is established to record information such as raw material procurement, processing procedures, and test results. In the event of quality problems, the cause can be quickly traced and corrective measures taken.