Stainless Steel Coil Tube — Grain Size Control & Metallurgical Specification
Product Category: Materials & Metallurgy → Metallurgical Indicators → Microstructure Control
Supplier: Wenzhou Jixing Metal Materials Co., Ltd
1. Product Overview
The stainless steel coil tube (盘管, Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd is a precision-wound tubular product formed from cold-rolled stainless steel strip, designed for high-performance applications in chemical processing, pharmaceutical manufacturing, food-grade piping systems, and heat exchanger assemblies. A defining metallurgical characteristic of this product is its rigorously controlled grain size (晶粒度), evaluated in accordance with the internationally recognized standard ASTM E112 — Standard Test Methods for Determining Grain Size.
Grain size represents the average linear dimension of individual crystalline grains within the polycrystalline microstructure of the stainless steel. The grain size directly governs the mechanical properties, corrosion resistance uniformity, and long-term fatigue performance of the coil tube. Wenzhou Jixing enforces a minimum grain size rating of ASTM E112 Grade 5 or finer (≥ G5), ensuring superior material integrity across all production batches.
2. Material & Properties
2.1 Grain Size and Its Metallurgical Significance
The grain size of austenitic stainless steel is fundamentally determined during the hot rolling, recrystallization annealing, and cold working stages of production. A finer grain structure — corresponding to a higher ASTM E112 grade number — yields the following material advantages:
- Enhanced Yield and Tensile Strength: Finer grains increase the Hall-Petch relationship contribution, where yield strength (σy) is inversely proportional to the square root of the average grain diameter (d): σy = σ0 + ky·d−1/2. A grain size of G5 or finer provides a measurable strength premium over coarser-grained alternatives.
- Improved Fracture Toughness: Reduced grain size increases the number of grain boundaries per unit area, which impede crack propagation and promote more uniform energy absorption during fracture events.
- Extended Fatigue Life: Fine-grained microstructures delay the initiation and propagation of fatigue cracks under cyclic loading conditions, which is critical for coil tubes deployed in dynamic or vibrating piping systems.
- Uniform Corrosion Resistance: Consistent grain size distribution minimizes intergranular corrosion susceptibility and ensures homogeneous passivation behavior, particularly important for 304, 316, and 321 austenitic stainless steel grades.
- Superior Formability at Reduced Temperatures: Fine grain structures exhibit lower anisotropy and more predictable springback behavior during cold forming operations.
2.2 ASTM E112 Grain Size Rating Scale
The ASTM E112 standard employs a logarithmic numbering system where each increment in grade number corresponds to a doubling of grain count per unit area. The relationship between ASTM grain size number (G) and the number of grains per square inch at 100× magnification (N) is defined as:
N = 2(G−1)
| ASTM E112 Grade (G) | Grains per Square Inch (100×) | Average Grain Diameter (μm) | Classification |
|---|---|---|---|
| G1 | 2 | ~1,500 | Very Coarse |
| G2 | 4 | ~1,060 | Coarse |
| G3 | 8 | ~750 | Coarse |
| G4 | 16 | ~530 | Moderate |
| G5 | 32 | ~375 | Standard (Minimum Required) |
| G6 | 64 | ~265 | Fine |
| G7 | 128 | ~187 | Fine |
| G8 | 256 | ~132 | Very Fine |
| G9 | 512 | ~93 | Very Fine |
| G10 | 1,024 | ~66 | Extra Fine |
3. Technical Specifications
| Parameter | Specification / Requirement | Test Standard |
|---|---|---|
| Grain Size Rating | ≥ ASTM E112 Grade 5 (G5) | ASTM E112 |
| Minimum Average Grain Diameter | ≤ 375 μm (corresponding to G5) | ASTM E112 | Grain Size Uniformity | ≤ 1 grade variation across sample field of view | ASTM E112 |
| Microstructure Evaluation Method | Etched metallographic examination at 100× magnification | ASTM E112, Section 6 (Linear Intercept Method) |
| Etching Agent | 10% picric acid in ethanol (aqueous or alcoholic solution) | ASTM E112 |
| Applicable Stainless Steel Grades | 304, 304L, 316, 316L, 321, 310S, 904L (austenitic series) | ASTM A213 / ASME SA-213 |
| Yield Strength (Typical, G5+) | ≥ 205 MPa (304); ≥ 240 MPa (316) | ASTM A240 |
| Tensile Strength (Typical, G5+) | ≥ 515 MPa (304); ≥ 580 MPa (316) | ASTM A240 |
| Elongation (Typical, G5+) | ≥ 40% (304); ≥ 35% (316) | ASTM A240 |
| Impact Toughness (Charpy V-Notch, 25°C) | ≥ 200 J (304); ≥ 175 J (316) | ASTM E23 |
| Intergranular Corrosion Resistance | Pass — Acetic Acid / Copper Copper Sulfate Test (ASTM A978) | ASTM A978 |
| Coil Diameter Range | 500 mm – 1,200 mm (OD) | Manufacturer Specification |
| Tube Outer Diameter Range | 6.35 mm – 168.3 mm (¼" – 6") | ASTM A213 |
| Tube Wall Thickness Range | 0.5 mm – 3.0 mm (0.020" – 0.118") | ASTM A213 |
| Surface Finish | Mirror Polish (Ra ≤ 0.1 μm) / Brushed / Pickled & Passivated | ASTM A473 |
| Temperature Operating Range | −196°C to +870°C (continuous service) | Material Grade Dependent |
4. Manufacturing Process
4.1 Upstream: Steelmaking and Hot Rolling
The production chain begins with vacuum arc remelting (VAR) or electro-slag remelting (ESR) of primary stainless steel ingots to achieve ultra-low sulfur (S ≤ 0.015%) and phosphorus (P ≤ 0.030%) content, ensuring a clean base material. The ingot is subsequently hot-rolled into strip form at temperatures above the austenitization range (typically 1,100°C – 1,250°C), followed by controlled cooling to establish the initial austenitic microstructure.
4.2 Intermediate: Cold Rolling and Recrystallization Annealing
The hot-rolled strip undergoes multi-pass cold rolling to achieve the target coil tube wall thickness. Cold working introduces dislocation density and strain energy, producing a heavily deformed martensitic or deformed austenitic structure. A critical recrystallization annealing cycle is then applied:
- Heating: The cold-rolled coil is heated to a recrystallization temperature typically in the range of 1,050°C – 1,150°C (grade-dependent), held for sufficient soak time to achieve complete recrystallization.
- Grain Growth Control: The dwell time and peak temperature are precisely calibrated to promote nucleation of new strain-free grains while limiting excessive grain coarsening. This is the primary process lever for achieving the target ASTM E112 ≥ G5 rating.
- Quenching: Rapid water quenching from the annealing temperature locks in the recrystallized grain structure and prevents post-annealing grain growth.
- Acid Pickling: The oxidized surface scale is removed using a mixed acid solution (typically HNO₃/HF or HNO₃/H₂SO₄) to restore the passive chromium oxide layer.
4.3 Forming: Coil Tube Production
The annealed and pickled strip is fed through a series of roll-forming stations where progressive bending and welding (typically TIG or laser welding) form the tubular geometry. The coil tube is wound onto a spool in a continuous production line, maintaining the fine grain structure established during annealing. Post-forming stress relief may be applied at low temperatures (≤ 400°C) to minimize residual stresses without triggering grain boundary migration.
4.4 Grain Size Verification
Every production batch undergoes metallographic grain size verification per ASTM E112:
- Test coupons are sectioned from representative locations (head, body, tail) of the coil.
- Specimens are mounted, ground (SiC papers 120–4000 grit), and polished to a mirror finish (diamond paste 1 μm → 0.05 μm).
- The polished surface is etched in 10% aqueous or alcoholic picric acid solution for 5–15 seconds.
- Grain boundaries are examined under an optical microscope at 100× magnification.
- The Linear Intercept Method (ASTM E112, Section 6) is employed: multiple traverse lines are superimposed on micrographs, and the average number of grain boundary intersections per unit length is recorded.
- The ASTM grain size number (G) is calculated from the average intercept length (L̄) using the formula: G = −3.322·log₁₀(L̄) + 6.644, where L̄ is in mm.
- A minimum rating of G5 is confirmed before batch release.
5. Applications
The fine-grained stainless steel coil tube (ASTM E112 ≥ G5) finds application in demanding industrial environments where material reliability is paramount:
- Chemical Processing: Reactor internals, heat exchanger tubes, and process piping handling aggressive acids, alkalis, and chlorinated solutions (particularly 316L and 904L grades).
- Pharmaceutical Manufacturing: CIP (Clean-in-Place) piping systems, sanitary process lines, and bioreactor components requiring mirror-polished, corrosion-resistant tubing.
- Food and Beverage Industry: Dairy processing lines, brewing equipment, and food-grade conveying systems (304 and 316L grades with Ra ≤ 0.4 μm finish).
- Power Generation: Boiler tubes, superheater elements, and nuclear-grade cooling circuits where fatigue resistance and thermal cycling durability are critical.
- Oil and Gas: Subsea pipelines, wellhead assemblies, and offshore platform piping exposed to chloride-induced stress corrosion cracking environments.
- Aerospace: Hydraulic lines, fuel system tubing, and engine exhaust components requiring certified grain structure for airworthiness compliance.
- Medical Devices: Endoscope tubing, implantable stent substrates, and surgical instrument shafts.
6. Quality Certifications and Compliance
Wenzhou Jixing Metal Materials Co., Ltd maintains a comprehensive quality management system ensuring traceability and conformance to international standards:
- ISO 9001:2015 — Quality Management System certification for manufacturing operations.
- ISO 14001:2015 — Environmental Management System certification.
- EN 10204 3.1 — Third-party inspection certification with full material test reports (MTR) including grain size verification data.
- EN 10204 3.2 — Third-party inspection certification with independent laboratory test certificates available upon request.
- ASTM E112 Compliance — Grain size testing performed and documented per ASTM E112 for every production lot.
- ASME SA-213 / SA-269 — Conformance for stainless steel tubing used in heat exchangers and heat-transfer applications.
- ASTM A269 / A213 — Conformance for seamless and welded austenitic stainless steel tubing.
- NACE MR0175 / ISO 15156 — Compliance for materials used in sour service environments (H₂S-containing oil and gas applications).
- 3-A Sanitary Standards — Compliance for food-grade and pharmaceutical-grade tubing applications.
- Mill Test Reports (MTR) — Provided with every shipment, documenting chemical composition, mechanical properties, grain size rating, and dimensional tolerances.
7. Unique Selling Points
Wenzhou Jixing's grain size control program delivers a measurable performance advantage:
- Consistent Fine-Grain Guarantee: A minimum ASTM E1