Views: 0 Author: Site Editor Publish Time: 2026-05-26 Origin: Site
Manufacturing Process of Stainless Steel Boiler Coils
Stainless steel boiler coils are the most important component of gas-fired boilers and the only pressure-bearing component. The outer wall of the coil absorbs the chemical heat generated by fuel combustion, and the inner wall transfers the heat to the heat transfer oil inside the coil.
The core of the manufacturing process for stainless steel boiler coils is: tube selection → material cutting → welding → tube bending → heat treatment → surface treatment → hydrostatic/non-destructive testing → assembly. Strict control over materials, welds, and residual stress is maintained throughout the process to ensure safety and durability under high temperature and pressure.
Common Materials: 304/304L, 316/316L, 310S (310S is preferred for high-temperature boilers, with a temperature resistance of 1100℃).
Tube Type: Seamless tubes or high-quality welded tubes. Boiler coils mostly use seamless tubes for higher pressure resistance.
Specifications: Common outer diameter φ16–φ57mm, wall thickness 2–4mm, standard length 6m/piece, ensuring strength and ductility.
Incoming Inspection: Material certificate, no cracks/rust on appearance, dimensional tolerances, eddy current/ultrasonic flaw detection passed.
CNC Cutting to Length: Precisely cut to length according to drawings, with an error ≤ ±1mm, removing burrs and oxide scale.
Beveling: Cut a 30°–37° V-shaped bevel at the tube end, leaving a 1–1.5mm blunt edge to ensure weld penetration and prevent burn-through.
Cleaning: Polish the bevel and the inner and outer walls within a 20mm range to remove oil and oxide scale, exposing the metallic luster.
Welding method: Fully automatic argon arc welding (GTAW), argon gas protection inside the pipe, 360° all-position welding.
Welding parameters: Current 80–120A, voltage 10–14V, argon flow rate 8–12L/min, single-layer single-pass welding.
Quality control: Weld reinforcement 0–1.5mm, uniform width; 100% radiographic (RT) or ultrasonic (UT) flaw detection, no cracks, incomplete penetration, or slag inclusions.
Process Selection: Cold bending (room temperature) protects the metallographic structure and provides high strength; hot bending of stainless steel requires 1050–1150℃, which easily leads to coarse grains and is rarely used.
Equipment: Fully automatic CNC tube rolling machine/tube bending machine, multi-tube simultaneous rolling, uniform tube coil spacing.
Key Parameters: Minimum bending radius ≥ 3 times the tube diameter (to prevent cracking/wrinkling); wall thickness reduction rate after bending ≤ 10%, ellipticity ≤ 5%.
Forming: Manufactured into spiral coils, serpentine tubes, or U-shaped tube arrays according to design, with precise dimensions and neat arrangement.
Solution Treatment (304/316): 1050–1100℃, hold for 1–2 hours, water quench; eliminates work hardening, restores corrosion resistance, and releases residual stress.
Stabilization Treatment (316L/310S): 850–900℃, hold for 2 hours, air cool; stabilizes the microstructure and prevents intergranular corrosion.
Results: Hardness ≤ HB 200, improved plasticity and toughness, preventing deformation and cracking during operation.
Acid Pickling and Passivation: Immersion in a mixture of nitric acid and hydrofluoric acid removes oxide scale and weld spatter; a dense passivation film forms on the surface, improving corrosion resistance.
Polishing (Optional): Mechanical or electrolytic polishing achieves an inner and outer wall finish Ra≤1.6μm, reducing scaling and improving heat exchange efficiency.
Marking: Stamped with material, specifications, furnace number, and hydrostatic test pressure for traceability.
Hydraulic Pressure Test: 1.5–2.0 times the design pressure (e.g., boiler operating pressure 1.6MPa, test pressure 2.4–3.2MPa), hold pressure for 30 minutes, no leakage, no plastic deformation.
Non-Destructive Testing:
Welds: 100% RT/UT, Class II qualified.
Pipes: 100% eddy current testing (ET), no cracks or corrosion defects.
Bends: Random UT inspection, no delamination or cracking.
Assembly: Welding of coils to headers, flanges, and inlet/outlet pipes; precision machining of flange sealing surfaces to ensure reliable sealing.
Final Inspection: Dimensional verification, visual inspection, and document review (material certificate, flaw detection report, pressure test record).
Packaging: Sealing after drying, wrapping with moisture-proof film, and securing in a wooden crate to prevent deformation and corrosion during transportation.
Material: 310S for high-temperature furnaces, 304/316L for medium and low-temperature zones.
Welding: Fully automated argon arc welding + internal argon shielding, 100% weld inspection.
Bending: Cold bending + controlled thinning rate/ovality to prevent cracking and deformation.
Heat Treatment: Solution treatment/stabilization to relieve stress and improve corrosion resistance.
Pressure Testing: 1.5–2.0 times design pressure, pressure holding with no leakage.
