Coil Tube (Coiltube) — σ-Phase Precipitation Control Specification for Duplex Stainless Steel
1. Product Overview
The Coil Tube (Coiltube) product line offered by Wenzhou Jixing Metal Materials Co., Ltd. is engineered for high-performance applications where resistance to σ-phase (sigma phase) embrittlement is critical. σ-Phase is a hard, brittle intermetallic compound with a tetragonal crystal structure (approximately Fe₂₂Cr₂₅Mo₆N₂) that precipitates preferentially along grain boundaries and at phase interfaces within duplex stainless steel microstructures during prolonged thermal exposure in the range of 600–1000 °C.
This specification defines the metallurgical requirements, detection methodologies, and quality assurance protocols for controlling σ-phase precipitation in duplex stainless steel coil tubes. The product is classified under Materials & Metallurgy → Metallurgical Indicators → Microstructural Control (组织控制), with the critical knowledge point being σ-Phase Precipitation (σ相析出), requiring monitoring through metallographic examination and electrolytic extraction analysis.
2. Material & Properties
2.1 σ-Phase Formation Mechanism
σ-Phase is an ordered intermetallic compound belonging to the D8b crystal structure family, characterized by a body-centered tetragonal unit cell. In duplex stainless steels (UNS S31803, UNS S32750, UNS S32760, and related grades), σ-phase nucleation occurs when the combined effects of elevated temperature, prolonged dwell time, and microsegregation of Cr, Mo, W, and N exceed the thermodynamic stability threshold of the austenite-ferrite two-phase matrix.
The formation of σ-phase results in:
- Chromium and molybdenum depletion in the adjacent ferritic and austenitic matrix, reducing local pitting and crevice corrosion resistance
- Severe embrittlement, with impact toughness reductions exceeding 70% at fracture temperatures below 400 °C
- Grain boundary cracking along preferential precipitation paths, compromising fatigue life and ductility
- Progressive loss of machinability and weldability in affected zones
2.2 Critical Temperature and Time Parameters
σ-Phase precipitation kinetics in duplex stainless steels are governed by the Arrhenius-type relationship between temperature and time. The critical parameters are:
- Onset temperature: Approximately 600 °C for grades with high Cr + Mo content
- Maximum precipitation rate: 750–850 °C (the "σ-embrittlement window")
- Solubility limit (upper temperature boundary): Approximately 1000 °C, above which σ-phase dissolves back into the solid solution
- Time-to-embrittlement: As short as 100 hours at 700 °C for high-alloy grades; several thousand hours at 600 °C
2.3 Detection and Characterization Methods
The following analytical techniques are employed for σ-phase detection and quantification, as specified under the knowledge point 金相或电解萃取检测 (Metallographic Examination or Electrolytic Extraction Detection):
- Metallographic Examination (金相检测): Standard optical microscopy of etched cross-sections using specialized reagents (e.g., Beraha's reagent or picral) to reveal σ-phase morphology. σ-Phase appears as blocky, lath-shaped, or film-like precipitates along grain boundaries and δ-ferrite phase boundaries. Quantitative assessment follows ASTM E566 (visual area fraction) or SEM-based image analysis protocols.
- Electrolytic Extraction (电解萃取): A selective dissolution technique in which the matrix phase is dissolved in a controlled electrolyte solution, leaving the intermetallic precipitates (including σ-phase) as a residual deposit. The residual is then characterized by X-ray diffraction (XRD) or electron probe microanalysis (EPMA) to confirm σ-phase identity and estimate volume fraction. This method is particularly effective for detecting trace σ-phase (below 1 vol%) that may be missed by optical metallography.
- Supplementary Techniques: Scanning Electron Microscopy (SEM) with Energy-Dispersive X-ray Spectroscopy (EDS), Transmission Electron Microscopy (TEM), and X-ray Diffraction (XRD) for phase identification and crystallographic characterization.
3. Specifications
| Parameter | Specification / Requirement | Test Method / Standard | Acceptance Criteria |
|---|---|---|---|
| Applicable Grades | Duplex Stainless Steel: UNS S31803, UNS S32750, UNS S32760, UNS S32751, and equivalent grades | ASTM A790 / ASTM A791 / EN 10217-7 | Grade designation confirmed by chemical analysis |
| σ-Phase Volume Fraction (as-delivered) | ≤ 1.0 vol% (optimal: < 0.5 vol%) | ASTM E566 / Electrolytic Extraction + XRD | No detectable σ-phase by metallography; electrolytic residual below 0.5 vol% |
| σ-Phase Volume Fraction (after simulated service) | ≤ 2.0 vol% after 100 h at 700 °C; ≤ 5.0 vol% after 1000 h at 650 °C | ASTM A923 / ISO 3506 | Impact energy retention ≥ 50% of baseline |
| Operating Temperature Range | 600–1000 °C (σ-embrittlement monitoring zone) | — | Service documentation requires periodic σ-phase re-assessment |
| Chemical Composition (Cr) | 22.0–25.0 wt% (S31803); 25.5–27.5 wt% (S32750) | ASTM E1268 (OES) / ASTM E1410 (ICP) | Within grade-specific specification limits |
| Chemical Composition (Mo) | 3.0–4.0 wt% (S31803); 3.0–4.5 wt% (S32750) | ASTM E1268 / ASTM E1410 | Within grade-specific specification limits |
| Chemical Composition (N) | 0.14–0.20 wt% (S31803); 0.25–0.35 wt% (S32750) | ASTM E1019 (Carbide Sulfur & Nitrogen) | Within grade-specific specification limits |
| Phase Balance (Austenite : Ferrite) | 40–60% each phase (by area fraction) | ASTM E562 / Magnetic or Metallographic Method | 35–65% ferrite (magnetic method) |
| Impact Toughness (Charpy V-Notch) | ≥ 47 J at 20 °C; ≥ 27 J at -40 °C (S31803) | ASTM A703 | Values indicative of absence of significant σ-phase embrittlement |
| Tensile Strength (UTS) | ≥ 550 MPa (S31803); ≥ 620 MPa (S32750) | ASTM A370 | Within specified range; no evidence of σ-phase-induced hardening |
| Elongation (at fracture) | ≥ 25% (S31803); ≥ 20% (S32750) | ASTM A370 | Values consistent with absence of brittle intermetallic phases |
| Pitting Resistance (PREN) | PREN = %Cr + 3.3 × %Mo + 16 × %N ≥ 34 (S31803); ≥ 38 (S32750) | ASTM G48 | PREN within specification; no local depletion zones detected |
| Electrolytic Extraction Residual | ≤ 0.5 vol% intermetallic precipitate residue (as-delivered condition) | Internal standard / ASTM E566 adapted | Residue characterized by XRD; σ-phase peaks absent or below detection limit |
| Coil Dimensions (Typical) | OD: 6.0–114.3 mm; WT: 0.5–8.0 mm; Coil ID: 100–500 mm | ASTM A791 / EN 10217-7 | Dimensions within ±0.1 mm tolerance |
4. Manufacturing Process
4.1 Steelmaking and Cast Composition Control
The base duplex stainless steel is produced via Argon Oxygen Decarburization (AOD) or Vacuum Arc Remelting (VAR) followed by Vacuum Induction Remelting (VIM) to achieve precise chemical composition control. Key alloying elements — Cr, Mo, N, and Ti — are carefully balanced to optimize the austenite-ferrite phase ratio while minimizing σ-phase susceptibility. Nitrogen content is elevated to promote austenite formation and increase PREN, while titanium additions (0.05–0.15%) are introduced to tie up carbon and reduce free carbon availability for chromium carbide precipitation that could accelerate σ-phase nucleation.
4.2 Hot Rolling and Coiling
Hot-rolled coil tubes are produced through continuous hot-rolling mills with controlled reduction schedules. The rolling process is designed to achieve a homogeneous grain structure with a target grain size of ASTM 4–8 (100–315 μm). Intermediate reheating temperatures are maintained below 1050 °C to prevent excessive grain growth and minimize the risk of σ-phase or other intermetallic precipitation during the rolling cycle. The coil is cooled at a controlled rate (typically air cooling or controlled water cooling) to achieve the target 40–60% austenite-ferrite phase balance.
4.3 Solution Treatment (Stress Relief)
Following hot rolling, the coil tube undergoes solution annealing (stress relief) at 1050–1100 °C for 30–60 minutes, followed by rapid water quenching. This treatment dissolves any σ-phase or other intermetallic precipitates that may have formed during hot rolling, restoring the full solid-solution strengthening and ensuring the as-delivered condition is free of brittle phases. The post-annealing phase balance is verified by magnetic permeability measurement or metallographic examination.
4.4 Cold Working and Cold Reduction (if applicable)
For cold-worked coil tube variants, controlled cold reduction (typically 20–50%) is applied to achieve desired mechanical properties and dimensional accuracy. The cold working process is monitored to ensure that the strain-induced martensite does not create localized zones susceptible to σ-phase nucleation during subsequent service exposure. Stress relief at 300–400 °C is performed after cold working to eliminate residual stresses without promoting intermetallic precipitation.
4.5 In-Process Microstructural Monitoring
Throughout the manufacturing process, periodic microstructural examinations are conducted to verify the absence of σ-phase and other detrimental phases:
- Post-hot-rolling: Metallographic examination of representative samples to confirm phase balance and absence of intermetallic phases
- Post-solution-treatment: Full metallographic and electrolytic extraction analysis on qualification samples
- Post-cold-working (if applicable): Verification that cold working has not introduced localized compositional variations that could promote σ-phase nucleation
- Final inspection: 100% dimensional inspection; representative sampling for full metallurgical certification
5. Applications
Duplex stainless steel coil tubes with verified σ-phase control are specified for the following high-temperature and high-corrosion-resistance applications:
5.1 Oil and Gas Industry
- Downhole tubing and casing in sour service (H₂S-containing environments)
- Subsea production lines and export piping operating at elevated temperatures
- Heat exchanger tubes in offshore platforms where σ-embrittlement monitoring is required for long-term service integrity
5.2 Chemical and Petrochemical Processing
- Reactor internals and heat exchanger tubes in chloride-containing process streams
- Distillation column internals and condenser tubes operating at 400–800 °C
- Acid pickling and processing equipment requiring resistance to both corrosion and thermal embrittlement
5.3 Power Generation
- Boiler tubes and superheater tubes in ultra-supercritical (USC) power plants
- Condenser tubes in marine-cooled power plants exposed to chlorinated seawater at elevated temperatures
5.4 Marine and Desalination
- Seawater piping and heat exchanger tubes in desalination plants (MED and RO systems)
- Submarine cable armor and undersea structural components
5.5 Pulp and Paper Industry
- Recovery furnace tubes and evaporator tubes in kraft pulping processes
- Chlorine dioxide generation equipment exposed to high-temperature chloride environments
6. Quality Certifications and Compliance
6.1 Applicable Standards
- ASTM A790: Standard Specification for Seamless and Welded Austenitic and Austenitic-Ferritic (Duplex) Stainless Steel Bars and Shapes
- ASTM A791: Standard Specification for Seamless and Welded Austenitic and Austenitic-Ferritic (Duplex) Stainless Steel Pipe
- EN 10217-7: Pressure Vessels — Non-Heat-Resistant Steels — Part 7: Seamless Tubes for Heat Exchangers
- ISO 3506: Fasteners of Stainless Steel — Mechanical Properties (reference for σ-embrittlement testing)
- ASTM A923: Standard Test Methods for Determining Susceptibility to Intergranular Cor