Stainless Steel Coil Tube for Heating Furnace Applications — Repair Welding Process Specification

1. Product Overview

The Stainless Steel Coil Tube (Product No. 1001) is a precision-engineered tubular component designed for high-temperature industrial heating furnace systems. Manufactured and supplied by Wenzhou Jixing Metal Materials Co., Ltd., this coil tube product is engineered to withstand sustained thermal cycling, mechanical vibration, and corrosive environments inherent in metallurgical and chemical processing operations. The coil tube is formed into helical or serpentine configurations to maximize heat transfer surface area within confined furnace geometries, making it a critical component in radiant heating, waste heat recovery, and process heating systems.

This technical specification integrates the company's proprietary Heating Furnace Coil Tube Repair Welding Process, a comprehensive methodology developed through extensive field experience and academic research. The repair welding process ensures that damaged or degraded coil tubes can be restored to full structural and functional integrity, extending service life and reducing overall lifecycle costs.

2. Material & Properties

2.1 Base Material Grades

The coil tubes are manufactured from austenitic and ferritic stainless steel grades selected for their superior oxidation resistance, thermal stability, and weldability. The following grades are commonly specified:

2.2 Mechanical Properties

Property Grade 304 (UNS S30408) Grade 321 (UNS S32108) Grade 310 (UNS S31008)
Tensile Strength (Rm), MPa ≥ 520 ≥ 520 ≥ 515
Yield Strength (Rp0.2), MPa ≥ 205 ≥ 205 ≥ 205
Elongation (A5), % ≥ 40 ≥ 40 ≥ 40
Maximum Service Temperature, °C 870 980 1150
Thermal Conductivity at 100 °C, W/(m·K) 16.3 14.6 16.3
Linear Thermal Expansion, 10-6/°C (20–800 °C) 17.3 17.5 18.0

2.3 Chemical Composition (Typical, wt%)

Grade C Cr Ni Ti Mn Si S P
304 ≤ 0.08 18.0–20.0 8.0–10.5 — ≤ 2.0 ≤ 0.75 ≤ 0.03 ≤ 0.045
321 ≤ 0.08 17.0–19.0 9.0–12.0 5×C to 0.70 ≤ 2.0 ≤ 0.75 ≤ 0.03 ≤ 0.045
310 ≤ 0.15 24.0–26.0 19.0–22.0 — ≤ 2.0 ≤ 1.00 ≤ 0.03 ≤ 0.045

3. Specifications

Parameter Specification
Product No. 1001
Product Name Stainless Steel Coil Tube (盘管 Coiltube)
Outer Diameter (OD) φ14 mm – φ89 mm (customizable up to φ159 mm)
Wall Thickness (WT) 0.5 mm – 6.0 mm (tolerance ±10%)
Coil Diameter 300 mm – 3000 mm (configurable per furnace design)
Coil Length 5 m – 50 m per continuous coil
Surface Finish 2B, No. 4 Brushed, BA (Bright Annealed), or Custom
Manufacturing Standard ASTM A213, ASTM A269, GB/T 14976, EN 10216-5
Welding Standard ISO 15614, ASME IX, AWS D10.9
Maximum Operating Temperature Up to 1200 °C (grade-dependent)
Maximum Operating Pressure Up to 6.0 MPa (design-dependent)
Minimum Bend Radius 3D (D = outer diameter)
Delivery Condition Solution Annealed, Pickled & Passivated

4. Manufacturing Process

4.1 Coil Tube Fabrication

  1. Raw Material Inspection: Incoming stainless steel seamless tubes or welded tubes undergo 100% visual inspection, dimensional verification, and chemical composition analysis via Optical Emission Spectroscopy (OES). Positive Material Identification (PMI) testing is performed using X-Ray Fluorescence (XRF) spectroscopy.
  2. Heat Treatment: Tubes are solution-annealed at 1040–1120 °C followed by controlled air or furnace cooling to achieve a fully austenitic microstructure with optimal corrosion resistance and ductility.
  3. Pickling & Passivation: The surface oxide scale is removed via a mixed acid pickling solution (HNO3/HF or HNO3/H2SO4), followed by passivation in a nitric acid solution to establish a protective chromium oxide (Cr2O3) film.
  4. Coil Forming: The straight tubes are cold-formed into the required helical or serpentine geometry using a precision mandrel-driven coiling machine. The forming process is performed with in-tube mandrel support to prevent ovalization and wrinkling. The minimum bend radius is maintained at 3D to avoid excessive work hardening.
  5. Post-Forming Stress Relief: Formed coils are stress-relieved at 600–700 °C to eliminate residual stresses introduced during cold forming, ensuring dimensional stability during thermal cycling in service.
  6. Non-Destructive Testing (NDT): 100% Visual Testing (VT) and Magnetic Particle Testing (MT) or Dye Penetrant Testing (PT) are performed on weld seams. Positive Material Identification (PMI) and dimensional checks are conducted on all coils prior to dispatch.

4.2 Repair Welding Process for Heating Furnace Coil Tubes

This section details the proprietary repair welding methodology developed by Wenzhou Jixing Metal Materials Co., Ltd. through systematic study and field validation. The process is designed to restore the structural integrity and metallurgical compatibility of coil tubes that have experienced localized damage during furnace service, including burn-through, pitting corrosion, mechanical breach, and thermal fatigue cracking.

4.2.1 Damage Assessment and Classification

4.2.2 Pre-Welding Preparation

  1. Inspection & Mapping: The damaged area is thoroughly inspected using ultrasonic testing (UT) or eddy current testing (ECT) to determine the full extent of the defect. A repair zone is demarcated with a minimum 15 mm clearance from the defect boundary.
  2. Surface Preparation: The repair zone is ground to bare metal using a flap disc or dedicated grinding tool. The ground surface must reveal the base metal luster and be free of oxide scale, rust, and contamination. The ground area is cleaned with acetone or isopropyl alcohol immediately prior to welding.
  3. Backing & Support: For burn-through repairs, a matching-diameter stainless steel backing ring or a ceramic backing ring is installed on the interior of the tube to provide root reinforcement. The backing material must be electrically insulated from the tube to prevent unintended heat paths.
  4. Fixture & Clamping: The coil tube section is securely clamped in a dedicated fixture that simulates the in-service geometry. This prevents distortion during welding and ensures the repair weld maintains the original tube alignment and dimensional tolerances.

4.2.3 Welding Parameters and Technique

The repair welding process employs Tungsten Inert Gas (TIG) Welding (Gas Tungsten Arc Welding, GTAW) as the primary method, with Submerged Arc Welding (SAW) available for thick-wall applications. The following parameters are specified for typical repairs on 2–4 mm wall thickness tubes:

Parameter Specification
Welding Process TIG (GTAW) — AC or DCEN
Filler Wire ER308L (for 304), ER347 (for 321), ER310 (for 310)
Filler Wire Diameter 1.6 mm – 2.4 mm
Shielding Gas Argon (99.995% purity) or Ar/He (75/25) for thick sections
Gas Flow Rate 12–15 L/min (primary), 5–8 L/min (back purge)
Welding Current 80–160 A (DCEN)
Travel Speed 20–40 mm/min
Interpass Temperature ≤ 150 °C (strictly controlled)
Preheat Temperature 100–150 °C (for wall thickness ≥ 3 mm)

4.2.4 Welding Sequence

  1. Root Pass: A single-pass root weld is deposited using a low current (60–80 A) with precise tungsten control. The back purge gas (pure argon) is maintained throughout to prevent internal oxidation. The root bead must achieve full penetration with no undercut, porosity, or incomplete fusion.
  2. Filler Pass