Stainless Steel Coil Tube for HVAC Systems — Anti-Freeze Cracking Specification

Product No.: 1108 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.

This technical specification document addresses the critical engineering challenge of coil tube freezing and cracking failure in air-conditioning (HVAC) units during low-temperature winter operation. Drawing upon metallurgical principles of cryogenic ductility, fatigue resistance, and corrosion fatigue behavior in austenitic stainless steels, this document defines the material, manufacturing, and performance requirements for coil tubes that are engineered to withstand repeated freeze-thaw cycles without catastrophic structural failure.

1. Product Overview

Stainless steel coil tubes are helically wound tubular elements used as heat exchange media in air-conditioning units, chillers, refrigeration systems, and HVAC (Heating, Ventilation, and Air Conditioning) assemblies. During winter operation, coil tubes are susceptible to freeze-induced cracking (frozen burst) when residual water within the tube lumen expands upon solidification (volumetric expansion of approximately 9% upon ice formation). This phenomenon generates internal hoop stresses that may exceed the tensile yield strength of the tube material, resulting in transverse or longitudinal fracture.

The coil tubes supplied by Wenzhou Jixing Metal Materials Co., Ltd. are specifically engineered to mitigate this failure mode through optimized material selection (high cryogenic toughness austenitic grades), controlled cold-work parameters, and precision manufacturing tolerances that ensure uniform wall thickness and consistent mechanical properties throughout the coil length.

2. Material & Properties

2.1 Material Grades

The primary material grade for HVAC coil tube applications is Austenitic Stainless Steel (奥氏体不锈钢), selected for its exceptional cryogenic toughness, corrosion resistance, and weldability. The following grades are specified:

2.2 Mechanical Properties

The mechanical properties of the coil tube material are governed by the degree of cold work (cold reduction) applied during forming. The following table presents the typical mechanical property ranges:

Property Unit 304 (Annealed) 304 (1/2 Hard) 316 (Annealed) 316L (Annealed)
Tensile Strength (UTS) MPa ≥ 515 ≥ 750 ≥ 515 ≥ 515
Yield Strength (0.2% Offset) MPa ≥ 205 ≥ 550 ≥ 205 ≥ 205
Elongation (A50) % ≥ 40 ≥ 15 ≥ 40 ≥ 40
Cold Bend (180°, d = 1×a) — No Cracking No Cracking No Cracking No Cracking
Impact Toughness (Charpy V-Notch, −196°C) J ≥ 80 ≥ 60 ≥ 80 ≥ 80
Thermal Conductivity (25°C) W/(m·K) 16.3 16.3 14.6 14.6
Linear Thermal Expansion (20–100°C) 10-6/K 17.3 17.3 16.0 16.0

2.3 Chemical Composition

Element 304 (wt%) 316 (wt%) 316L (wt%)
C (Carbon) ≤ 0.08 ≤ 0.08 ≤ 0.03
Cr (Chromium) 18.0–20.0 16.0–18.0 16.0–18.0
Ni (Nickel) 8.0–10.5 10.0–14.0 10.0–14.0
Mo (Molybdenum) — 2.0–3.0 2.0–3.0
Mn (Manganese) ≤ 2.0 ≤ 2.0 ≤ 2.0
S (Sulfur) ≤ 0.03 ≤ 0.03 ≤ 0.03
P (Phosphorus) ≤ 0.045 ≤ 0.045 ≤ 0.045
Si (Silicon) ≤ 1.0 ≤ 1.0 ≤ 1.0

2.4 Cryogenic Performance

A critical differentiator for HVAC coil tubes operating in winter conditions is their cryogenic mechanical behavior. Austenitic stainless steels exhibit a unique property: unlike ferritic and martensitic steels, they do not undergo a ductile-to-brittle transition (DBTT). Instead, the yield strength increases with decreasing temperature while elongation remains high. This is attributed to the face-centered cubic (FCC) crystal structure of austenite (γ-phase), which provides multiple active slip systems at all temperatures.

Key Metallurgical Insight: At temperatures as low as −196°C (liquid nitrogen temperature), AISI 304 and 316 retain elongation values exceeding 40%, compared to carbon steels which may fracture in a brittle manner below −40°C. This inherent cryogenic ductility is the primary defense mechanism against freeze-induced cracking in coil tube applications.

3. Specifications

Parameter Specification Standard / Reference
Outer Diameter (OD) 6.35 mm (1/4"), 9.53 mm (3/8"), 12.70 mm (1/2"), 15.88 mm (5/8"), 19.05 mm (3/4") ASTM B411 / EN 10217
Wall Thickness 0.50 mm – 1.50 mm (standard); 0.30 mm – 0.50 mm (thin-wall, special order) ASTM A269 / ASTM B411
Coil Length 10 m – 50 m (standard); up to 100 m (custom) Manufacturer Specification
Coil Diameter 300 mm – 600 mm (depends on OD and wall thickness) Manufacturer Specification
Surface Finish No. 4 Brushed, 2B Annealed, or Mirror Polish (Ra ≤ 0.4 μm) ASTM A480
Tolerance (OD) ±0.05 mm (±0.002 in) ASTM B411
Tolerance (Wall Thickness) ±10% of nominal ASTM B411
Pressure Rating (Hydrostatic Test) 1.5 × Maximum Working Pressure (MWP); minimum 1.0 MPa ASME B31.5 / EN 12163
Operating Temperature Range −196°C to +425°C ASME VIII Div. 1
Corrosion Resistance Passive film (Cr₂O₃) self-healing; pitting resistance (PREN) ≥ 18 (304) / ≥ 24 (316) ISO 11997
Freeze Cycle Resistance ≥ 1,000 freeze-thaw cycles without wall thinning or crack initiation Internal Test Protocol

4. Manufacturing Process

The manufacturing process for stainless steel coil tubes follows a rigorous multi-stage sequence designed to ensure dimensional accuracy, mechanical integrity, and surface quality:

  1. Hot Rolling (热轧): Stainless steel slabs are hot-rolled into seamless tube blanks at temperatures above the recrystallization temperature (approximately 1,100°C–1,200°C for austenitic grades). This establishes the initial tube geometry and homogenizes the microstructure.
  2. Piercing & Enlarging (穿孔与扩大): The hot-rolled billet is pierced and enlarged to form a hollow tube blank with approximate final dimensions.
  3. Pickle & Anneal (酸洗与退火): The tube blank is pickled in a mixed acid bath (HNO₃/HF) to remove scale and oxide, followed by solution annealing at 1,050°C–1,150°C in a controlled atmosphere (vacuum or argon) furnace. This dissolves carbides (particularly chromium carbides, Cr₂₃C₆) and restores full austenitic microstructure, ensuring maximum corrosion resistance and ductility.
  4. Cold Drawing / Cold Reduction (冷拔/冷轧): The annealed tube is cold-drawn through precision dies to achieve final OD and wall thickness. The degree of cold work is carefully controlled to balance strength and ductility. For freeze-resistant coil tubes, a minimum elongation of 35% is maintained to ensure adequate cryogenic ductility.
  5. Intermediate Annealing (中间退火): If multiple cold-drawing passes are required, intermediate annealing is performed to relieve accumulated cold-work stresses and prevent cracking during subsequent forming operations.
  6. Coiling (盘卷): The finished tube is helically coiled using precision winding equipment. Tension control during coiling is critical to prevent ovality distortion and residual stress concentration at the coil ends. The coil is wound with consistent pitch and maintained within specified coil diameter tolerances.
  7. Final Annealing (终退火): A final solution anneal may be performed post-coiling to relieve coiling-induced residual stresses, particularly at the coil's inner and outer radii where bending strains are highest.
  8. Hydrostatic Testing (水压试验): Each coil tube undergoes 100% hydrostatic pressure testing at 1.5× MWP for a minimum hold time of 30 seconds to verify weld integrity and absence of pinhole defects.
  9. End Sealing & Packaging (端部封堵与包装): Tube ends are sealed with corrosion-resistant caps to prevent moisture ingress during storage. Coils are packaged in moisture-proof wrapping with desiccant to maintain surface passivity.

5. Applications

Stainless steel coil tubes manufactured to this specification find application in the following domains: