Thermocouple & RTD Protective Sheath Coil Tubes — 310S / Inconel 600 Seamless

Product Overview

Thermocouple and Resistance Temperature Detector (RTD) protective sheaths are precision-engineered stainless steel and superalloy seamless tubes designed to encase temperature-sensing elements in demanding industrial environments. These protective sleeves shield the delicate thermocouple wires and RTD sensing junctions from mechanical damage, chemical corrosion, and thermal degradation while maintaining high-fidelity thermal signal transmission.

Manufactured by Wenzhou Jixing Metal Materials Co., Ltd, this product line addresses the critical engineering trade-off between thermal response speed (favored by thin wall thickness) and mechanical/chemical durability (favored by thick wall thickness). The seamless coil tube format enables efficient winding into compact, high-density configurations ideal for multi-point temperature monitoring arrays in process instrumentation systems.

The primary application domain encompasses high-temperature and corrosive process media environments where conventional stainless steel grades (304, 316) are insufficient, necessitating the deployment of 310S austenitic stainless steel or Inconel 600 nickel-chromium-iron superalloy seamless tubing.

Material & Properties

Grade 310S (Austenitic Stainless Steel)

• Composition: Cr 24–26%, Ni 19–22%, C ≤ 0.08% (low-carbon variant of 310)
• Maximum Continuous Service Temperature: 1150 °C (2092 °F)
• Short-Term Peak Tolerance: Up to 1200 °C (2192 °F)
• Oxidation Resistance: Excellent in oxidizing atmospheres; forms a stable Cr₂O₃ scale layer
• Corrosion Resistance: Good against nitric acid, sulfuric acid (dilute), and organic acids
• Weldability: Excellent; low carbon content minimizes intergranular carbide precipitation
• Thermal Conductivity: ~26 W/(m·K) at 25 °C; moderate, enabling acceptable thermal response

Inconel 600 (Nickel-Chromium-Iron Superalloy)

• Composition: Ni 71–73%, Cr 14–17%, Fe 6–10%, Cu ≤ 0.3%
• Maximum Continuous Service Temperature: 1093 °C (2000 °F) in air
• Short-Term Peak Tolerance: Up to 1150 °C (2102 °F)
• Oxidation Resistance: Superior; forms protective NiO/Cr₂O₃ composite scale in air and steam
• Corrosion Resistance: Outstanding against sulfuric acid, hydrochloric acid, hydrofluoric acid, and mixed-acid environments
• Mechanical Strength at Elevated Temperature: Retains significant tensile strength up to 900 °C; superior to 310S above 800 °C
• Thermal Conductivity: ~15 W/(m·K) at 25 °C; lower than 310S, requiring careful wall-thickness optimization for response time

Specifications

Parameter	Specification
Product Form	Seamless Coil Tube (盘管)
Material Grades	310S (UNS S31008) / Inconel 600 (UNS N06600)
Outer Diameter (OD)	1.5 mm – 25 mm (customizable)
Wall Thickness	0.15 mm – 1.5 mm (optimized per application)
Inner Diameter (ID)	0.5 mm – 20 mm (derived from OD minus wall thickness)
Coil Length per Spool	50 m – 500 m (standard); custom lengths available
Surface Finish	Pickled & Passivated (P&P); Ra ≤ 0.8 μm; optional electropolished Ra ≤ 0.4 μm
Coil Inner Diameter (Winding)	≥ 200 mm (minimum bend radius: 5× OD)
Temperature Range (Continuous)	-50 °C to 1150 °C (310S); -50 °C to 1093 °C (Inconel 600)
Thermal Response Time (τ)	1 – 15 seconds (dependent on OD, wall thickness, and immersion depth)
Pressure Rating (Sealed End)	Up to 400 bar (40 MPa) depending on geometry
Manufacturing Standard	ASTM A269 / A213 / ASME SA-213; EN 10216-5; GB/T 12771
Corrosion Test	ASTM G48 (pitting), ASTM G154 (salt spray), custom acid immersion
Packaging	Wooden reel with moisture-proof wrapping; export-grade palletized

Manufacturing Process

1. Raw Material Selection: High-purity 310S or Inconel 600 seamless bar stock or wire rod, certified to ASTM/EN/GB chemical composition standards with low sulfur and phosphorus content.
2. Piercing & Hot Rolling: Billet piercing to produce seamless hollow tubes, followed by hot rolling to approximate dimensions. Inconel 600 requires controlled heating temperatures (≤ 1150 °C) to prevent grain coarsening.
3. Cold Drawing & Ironing: Multi-stage cold reduction through hardened tungsten carbide dies to achieve precise final OD, wall thickness, and surface finish. Reduction ratios of 30–60% per pass ensure uniform microstructure and cold-work hardening for enhanced mechanical properties.
4. Intermediate Annealing: Solution annealing at 1050–1100 °C (310S) or 1100–1150 °C (Inconel 600) in vacuum or controlled atmosphere to relieve cold-work stresses and restore ductility for subsequent drawing passes.
5. Coil Winding: Precision winding onto controlled-diameter mandrels to produce uniform coil geometry. Minimum bend radius is maintained at 5× OD to prevent ovalization or cracking.
6. Pickling & Passivation: Immersion in mixed acid (HNO₃ + H₂SO₄) solution to remove scale and surface contaminants, followed by nitric acid passivation to establish a protective chromium oxide layer.
7. Non-Destructive Testing (NDT): Eddy current testing (ECT) and ultrasonic testing (UT) for wall thickness verification and detection of internal/external discontinuities. Hydrostatic testing at 1.5× working pressure for leak integrity.
8. Dimensional Inspection: Laser micrometry for OD, wall thickness, and roundness verification at intervals along the coil length. Optical surface profiling for roughness confirmation.
9. Final Annealing (Optional): Stress-relief annealing at 870 °C for Inconel 600 or 1050 °C for 310S to achieve final mechanical property requirements.

Response Speed vs. Strength — Engineering Optimization

The fundamental design challenge in thermocouple/RTD protective sheath engineering is the inverse relationship between wall thickness and thermal response time. A thinner wall reduces the thermal mass (m·cₚ) of the sheath, accelerating heat transfer from the process medium to the sensing element. However, excessive thinning compromises structural integrity under pressure, mechanical vibration, and erosive flow conditions.

The optimized design philosophy employed by Wenzhou Jixing Metal Materials Co., Ltd addresses this trade-off through:

• Material-Grade Selection: Inconel 600 permits thinner walls than 310S at equivalent pressure ratings due to its superior high-temperature tensile strength (~620 MPa at 900 °C vs. ~345 MPa for 310S), enabling faster response in the same pressure environment.
• Geometric Optimization: Selection of the minimum OD compatible with the sensing element diameter, combined with the thinnest wall thickness that satisfies the pressure differential and mechanical loading criteria (per ASME BPV Section VIII calculations).
• Immersion Depth Design: Extension of the sheath immersion length beyond the minimum required to improve thermal contact while maintaining the response time advantage of thin-wall construction.
• Surface Treatment: Electropolishing to reduce surface roughness and eliminate surface defects that could accelerate localized corrosion, thereby extending service life without increasing wall thickness.

Applications

Primary Application: Temperature Measurement Instrumentation

• Thermocouple Protective Sheaths: Housing Type K, Type N, Type R, Type S, and Type B thermocouple assemblies in high-temperature process environments.
• RTD (Resistance Temperature Detector) Protective Sheaths: Encasing Pt100, Pt500, and Pt1000 RTD sensing elements in corrosive process streams.
• Multi-Point Temperature Arrays: Coil tube format enables winding of multiple sheath elements into compact configurations for distributed temperature monitoring.

Industry Applications

• Petrochemical & Refining: Crude oil distillation columns, reforming furnaces, hydrogen production reactors, and catalytic cracking units operating at 600–1200 °C in reducing/oxidizing atmospheres.
• Power Generation: Boiler superheater tubes, steam turbine inlet temperature monitoring, and flue gas temperature measurement in coal-fired and gas-fired power plants.
• Chemical Processing: Acid concentration plants, chlor-alkali production, and specialty chemical synthesis involving sulfuric acid, hydrochloric acid, and hydrofluoric acid media.
• Metallurgical & Steel Industry: Continuous casting temperature monitoring, annealing furnace control, and hot rolling mill process instrumentation.
• Aerospace & Defense: Jet engine exhaust temperature monitoring, rocket engine combustion chamber instrumentation, and high-altitude sensor deployment.
• Food & Pharmaceutical Processing: Sterilization temperature validation (SIP/CIP systems), CIP (Clean-in-Place) temperature monitoring, and hygienic process temperature control requiring electropolished surfaces.
• Oil & Gas Downstream: Subsea production facility temperature monitoring, offshore platform instrumentation, and LNG (Liquefied Natural Gas) processing temperature measurement.

Quality Certifications

• ISO 9001:2015 — Quality Management System Certification
• ISO 14001:2015 — Environmental Management System Certification
• ISO 45001:2018 — Occupational Health and Safety Management System
• ASME PE Stamp — Pressure Equipment certification for high-pressure applications
• EN 10204 3.1 Material Test Certificates — Third-party verified chemical composition and mechanical property certification
• NACE MR0175 / ISO 15156 — Compliance for sour service (H₂S-containing) environments
• ASTM A269 / A213 Compliance — Seamless stainless steel tube product standards
• GB/T 12771 Compliance — Chinese national standard for stainless steel seamless tubes
• CE Marking — European Conformity for pressure equipment and instrumentation
• RoHS / REACH Compliance — Environmental and hazardous substance regulations

Unique Selling Points

• Dual-Material Platform: Industry-leading capability to supply both 310S and Inconel 600 seamless coil tubes from a single source, enabling customers to optimize material selection per application without supply chain fragmentation.
• Response-Time Engineering: Proprietary wall-thickness optimization methodology combining thermal modeling (Fourier heat conduction analysis) with pressure-vessel design codes to deliver the fastest achievable thermal response at the minimum viable wall thickness for each operating condition.
• Coil Format Advantage: Continuous coil tube format eliminates the need for field welding of multiple short sheath segments, reducing leak points, improving thermal continuity, and enabling rapid deployment in multi-sensor arrays.
• Ultra-Thin Wall Capability: Production of walls as thin as 0.15 mm enables thermal response times as low as 1–3 seconds, critical for process control loops requiring fast temperature feedback.
• Customization Flexibility: Full customization of OD, wall thickness, coil length, surface finish, and end configurations (open, closed, welded, threaded) to match specific instrument vendor requirements.
• Traceability: Complete batch-level material traceability from raw bar stock through all manufacturing stages, supported by EN 10204 3.1 certification documentation.