Stainless Steel Coil Tube — Power Boiler Superheater Application (Coiltube Series)

1. Product Overview

The Stainless Steel Coil Tube (Coiltube) is a precision-formed tubular component specifically engineered for high-temperature steam service in power boiler superheater sections. Supplied in continuous coil form, this product is manufactured by Wenzhou Jixing Metal Materials Co., Ltd and is designed to withstand the extreme thermal cycling, oxidizing environments, and mechanical stresses inherent in utility-scale power generation systems. The coil configuration facilitates seamless on-site installation, reducing welding joints and minimizing points of potential failure in critical heat-transfer circuits.

This product line draws upon extensive metallurgical failure analysis research, particularly the investigation into cracking failure mechanisms in power boiler superheater steam coil tubes, ensuring that material selection, microstructural control, and manufacturing tolerances are optimized to prevent the dominant degradation modes observed in field service — including intergranular cracking (IGC), stress corrosion cracking (SCC), creep rupture, and thermal fatigue cracking (TFC).

2. Material & Properties

The coil tube is fabricated from austenitic and super-austenitic stainless steel grades selected for their superior resistance to high-temperature oxidation, carburization, and sulfidation. The following material grades are available:

2.1 Mechanical Properties (at 20 °C, Annealed Condition)

Property SAE 310S SAE 304 SAE 321 SAE 347
Tensile Strength (MPa) ≥ 515 ≥ 515 ≥ 515 ≥ 515
Yield Strength, 0.2% (MPa) ≥ 205 ≥ 205 ≥ 205 ≥ 205
Elongation (%) ≥ 40 ≥ 40 ≥ 40 ≥ 40
Hardness (HBW, max) 200 200 200 200
Maximum Service Temperature (°C) 1150 870 900 900

2.2 Chemical Composition (wt%, Typical)

Element C Mn S P Cr Ni Si Ti Nb
SAE 310S ≤ 0.08 ≤ 2.0 ≤ 0.03 ≤ 0.045 24.0–26.0 19.0–22.0 ≤ 1.0 — —
SAE 304 ≤ 0.08 ≤ 2.0 ≤ 0.03 ≤ 0.045 18.0–20.0 8.0–12.0 ≤ 1.0 — —
SAE 321 ≤ 0.08 ≤ 2.0 ≤ 0.03 ≤ 0.045 17.0–19.0 9.0–13.0 ≤ 1.0 5×C–0.70 —
SAE 347 ≤ 0.08 ≤ 2.0 ≤ 0.03 ≤ 0.045 17.0–19.0 9.0–13.0 ≤ 1.0 — 8×C–1.00

3. Specifications

Parameter Specification
Outer Diameter (OD) Φ 12.7 mm – Φ 63.5 mm (custom sizes available upon request)
Wall Thickness 0.8 mm – 4.0 mm (tolerance: ±10% or ±0.1 mm, whichever is greater)
Coil Inner Diameter (ID) Φ 300 mm – Φ 1200 mm
Coil Length (continuous) 15 m – 120 m per coil
Surface Finish 2B cold-rolled / No. 1 annealed / Pickled and polished
End Condition Plain cut / Beveled / Threaded (per customer drawing)
Heat Treatment Solution annealed at 1050–1100 °C, followed by controlled air or water quench
Applicable Standards ASTM A213, ASTM A269, ASME SA-213, GB/T 12771, EN 10216-5
Forming Tolerance Roundness: ≤ 1% of OD; Straightness: ≤ 2 mm/m

4. Manufacturing Process

The production of stainless steel coil tubes follows a rigorously controlled multi-stage manufacturing sequence designed to ensure microstructural homogeneity, dimensional accuracy, and freedom from manufacturing-induced defects that could initiate cracking in service.

  1. Raw Material Selection & Certification — Virgin austenitic stainless steel strip is procured from certified mills with full mill test certificates (MTC) per EN 10204 Type 3.1. Incoming material is verified for chemical composition (spectrographic analysis), grain size, and freedom from centerline segregation or inclusion clusters.
  2. Cold Rolling & Tubular Forming — Flat strip is slit to precise width and cold-rolled into a tubular blank using a roll-forming process. The forming ratio and bend radius are carefully controlled to prevent localized strain concentration that could serve as a crack initiation site under cyclic thermal loading.
  3. Seam Welding (TIG / Orbit) — The tubular blank is joined via continuous TIG (Tungsten Inert Gas) or orbit welding under argon shielding. Weld parameters are calibrated to achieve full penetration with a narrow heat-affected zone (HAZ), minimizing carbide precipitation at grain boundaries.
  4. Solution Annealing & Pickling — Welded tubes are solution-annealed at 1050–1100 °C to dissolve any chromium carbide (Cr₂₃C₆) precipitates formed during welding, restoring full solid-solution strengthening and corrosion resistance. Subsequent acid pickling (HNO₃/HF blend) removes surface scale and restores chromium oxide passivity.
  5. Coiling & Dimensional Inspection — Annealed tubes are coiled at controlled tension to prevent springback deformation. Each coil undergoes dimensional verification (OD, wall thickness, roundness) using ultrasonic thickness gauging and laser micrometry.
  6. Non-Destructive Testing (NDT) — 100% ultrasonic testing (UT) of weld seams per ASTM E213, supplemented by magnetic particle inspection (MPI) for surface defect detection where applicable.
  7. Final Quality Assurance & Packaging — Each coil is assigned a unique heat traceability number, accompanied by a complete inspection dossier. Coils are wrapped in moisture-proof polyethylene film and secured on wooden pallets for export-ready shipping.

5. Applications

The stainless steel coil tube is purpose-designed for the following critical applications:

Engineering Insight: The investigation into power boiler superheater steam coil tube cracking failure has identified that the dominant degradation mechanism in conventional straight-tube designs is the synergistic action of creep deformation, thermal fatigue, and intergranular oxidation. The coil tube geometry, combined with the stabilized austenitic microstructure of grades 321 and 347, effectively decouples these damage mechanisms by ensuring more uniform wall temperature distribution and reducing peak circumferential stress.

6. Quality Certifications

All products are manufactured under a certified quality management system and comply with the following international standards and certifications:

7. Unique Selling Points