First, what are the raw material preparation and pretreatment processes for seamless steel pipe fittings?
Raw material screening: 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. The wall thickness uniformity deviation is ≤0.3mm. Ultrasonic flaw detectors are used for full-length flaw detection to ensure there are no internal defects such as delamination or porosity.
Pretreatment process:
Cutting and blanking: According to the pipe fitting size requirements, CNC plasma cutting machines or laser cutting machines are used for batch cutting. The perpendicularity deviation of the cut is ≤0.2mm, and the length tolerance is controlled within ±0.3mm to avoid burr residue.
Surface Treatment: Shot blasting removes oxide scale and rust from the pipe surface, achieving a rust removal grade of Sa2.5. Phosphating is then applied to form a 5-8μm thick phosphate film, improving lubrication and corrosion resistance in subsequent processing.
Straightening Treatment: Bending and deformed pipes are straightened using a hydraulic straightening machine, with a straightness deviation of ≤0.5mm/m, ensuring the accuracy of subsequent processing standards.
Second, what are the core processing technologies for seamless steel pipe fittings?
(I) Elbow Processing of Seamless Steel Pipe Fittings (Batch production using push-forming process)
Die Preparation: Custom-made dies are used according to the elbow specifications. The die material is Cr12MoV alloy steel, hardened to ensure wear resistance and dimensional stability.
Bill Heating: The pre-treated pipe billet is placed in a medium-frequency induction heating furnace. The heating temperature is controlled at 900-1050℃ (adjusted according to material), with a heating rate of 5-8℃/s, ensuring uniform heating of the billet and avoiding localized overheating or burning.
Push Forming: Batch forming is performed using a hydraulic push forming machine at a speed of 0.5-1 m/min. The blank is bent and formed by mold guidance, with real-time monitoring of the elbow angle and radius of curvature, controlling deviations within ±1%.
Shaping Process: The formed elbows are placed in a shaping mold and cold-pressed to correct dimensional deviations, ensuring elbow end face parallelism ≤0.3 mm and wall thickness reduction ≤10%.
(II) Seamless Steel Pipe Fittings T-joint Processing (Hot Extrusion Process)
Blank Pretreatment: Seamless steel pipes are cut into blanks of a predetermined length. Pre-drilled holes are made at predetermined branch positions on the blanks using a drilling machine. The hole diameter is determined according to the branch pipe diameter, and the hole wall roughness Ra ≤3.2 μm.
Heating and Extrusion: The blanks are heated in a resistance heating furnace to 850-950℃ and held for 30-60 min. Then, they are placed in an extrusion mold, and axial pressure is applied using a hydraulic extruder to extrude the branch pipe at the pre-drilled hole position, forming a tee structure. Branch Forming: 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 of Seamless Steel Pipe Fittings (Forging + Machining Process)
Forging Forming: Flange blanks are mass-produced using die forging technology. Round steel billets matching the pipe material are selected and heated to 1100-1200℃ for die forging to ensure a dense structure in the flange blanks, free from defects such as porosity and shrinkage cavities. Normalizing treatment is performed after forging to eliminate internal stress.
Rough Machining: The flange end faces and inner holes are mass-machined using CNC lathes. A 0.5-1mm finishing allowance is reserved for the inner hole dimensions, and the end face roughness Ra ≤ 12.5μm.
Finishing: Hole system machining (bolt holes, locating holes) is performed using a machining center. Bolt hole position tolerance ≤ ±0.2mm, hole spacing tolerance ≤ ±0.15mm; flange sealing surfaces are machined by turning, with a surface roughness Ra ≤ 3.2μm and flatness ≤ 0.1mm/m.
Matching Machining: For flanges requiring pairing, a pairing grinding process is used to ensure a sealing surface fit of≥ 95%.
(IV) Reducing Process for Seamless Steel Pipe Fittings (Bottoming/Expanding Process)
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 by a hydraulic press to gradually shrink the blank into shape. The shrinkage rate is ≤ 20%, and the taper deviation is ≤ ±0.5%. Expanding Diameter: For reducing pipes from small to large diameters, a hot expanding process is used. The billet is heated to 800-900℃, placed in an expanding mold, and expanded using a conical mandrel. The expanding rate is ≤30%, resulting in a smooth, wrinkle-free inner wall.
Third, Welding Process of Seamless Steel Pipe Fittings.
Welding Method: For mass production, submerged arc welding or gas shielded welding is used. Welding materials are selected using welding wire and flux that match the base metal to ensure the weld composition is consistent with the base metal.
Welding Parameters: 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 (Ar+CO₂) flow rate 20-25L/min.
Post-Welding 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 depth of ≤3mm. Heat Treatment: For pipe fittings with a wall thickness ≥10mm, post-weld stress-relieving heat treatment is performed. The heating temperature is 600-650℃, held for 2-3 hours, and the cooling rate is ≤5℃/min to eliminate welding internal stress.
Weld Inspection: Batch inspection is conducted using ultrasonic testing (UT) and radiographic testing (RT). The weld pass rate is ≥99.5%, with no cracks, incomplete penetration, slag inclusions, or other defects.
Fourth, Finished Product Inspection and Packaging of Seamless Steel Pipe Fittings.
Dimensional Inspection: Key dimensions of pipe fittings are batch inspected using a coordinate measuring machine, calipers, and angle gauges. 3% of each batch is randomly sampled for full-dimensional inspection, and non-conforming products are immediately isolated.
Mechanical Property Testing: Three samples are taken from each batch for tensile strength, yield strength, and impact testing (-20℃). Test results must meet relevant standard requirements.
Surface Quality Inspection: Surface defects are inspected visually and using magnetic particle testing (MT). The surface is free of cracks, scratches, rust, and other defects, and MT testing shows no exceedances.
Corrosion Protection: Finished products undergo galvanizing, painting, or plastic coating for corrosion protection according to customer requirements. 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.
Packaging and Warehousing: Waterproof packaging paper + woven bags or wooden crates are used for packaging. Each pipe fitting is labeled with specifications, material, batch number, and production date. Records are maintained for batch warehousing to ensure traceability.
Fifth, Process Control and Quality Assurance of Seamless Steel Pipe Fittings
Process Control: 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.
Equipment Maintenance: Production equipment is regularly maintained and calibrated monthly to prevent equipment failures from affecting product quality.
Personnel Training: Operators receive pre-job training and undergo regular assessments to ensure they are familiar with process requirements and equipment operating procedures, and are certified to work.
Quality Traceability: A batch production quality traceability system is established to record information such as raw material procurement, processing, and testing results. In the event of a quality problem, the cause can be quickly traced and corrective measures taken.
Post time: Feb-24-2026