347 Thin Wall Capillary Tube: Material Properties, Manufacturing Tolerances, and Application Guide

When designing or sourcing a 347 thin wall capillary tube, engineers are typically addressing a specific engineering challenge: the need for a small-diameter tube that retains mechanical strength and corrosion resistance at elevated temperatures while minimizing weight and internal volume. Grade 347 stainless steel, stabilized with niobium (columbium), offers resistance to intergranular corrosion and creep up to 800°C, making it suitable for applications such as thermocouple sheaths, pressure sensing lines, and high-temperature fluid transfer in demanding industrial and medical device environments.

Applicable Scenarios and Industries

Grade 347 thin wall capillary tubing is selected for environments where exposure to intermittent or continuous high temperatures (up to 800°C) is combined with corrosive media. Common applications include:

• Thermocouple and temperature sensor sheaths in furnaces, exhaust systems, and process reactors.
• Pressure-sensing capillary lines in hydraulic and pneumatic systems operating at elevated temperatures.
• Fluid transfer lines in chemical processing, petrochemical, and power generation equipment.
• Instrumentation and control lines in aerospace and gas turbine systems.
• Medical device components (e.g., endoscopic tools, catheter reinforcement) where resistance to repeated sterilization and high-temperature processing is required.

Material Grades and Designations

347 stainless steel is defined by several standards. The most relevant for capillary tube are:

Compared to 304 and 316L, 347 offers superior resistance to intergranular corrosion after welding or exposure to sensitizing temperatures (427–860°C) due to niobium stabilization. 321 (titanium-stabilized) is an alternative, but 347 (niobium-stabilized) often provides better mechanical properties at very high temperatures. 316Ti is less common in capillary dimensions.

Typical chemical composition (ASTM A240) for 347: C ≤ 0.08%, Mn ≤ 2.00%, Si ≤ 1.00%, P ≤ 0.045%, S ≤ 0.030%, Cr 17.0–19.0%, Ni 9.0–13.0%, Nb + Ta 10 × C min to 1.00%. Mechanical properties in the annealed condition per ASTM A269: tensile strength ≥ 75 ksi (515 MPa), yield strength ≥ 30 ksi (205 MPa), elongation ≥ 40%.

OD / ID / Wall Thickness / Length Considerations

For 347 thin wall capillary tube, the demand is for small outer diameters with a thin wall to reduce mass, increase flexibility, or minimize internal volume. Typical ranges:

• Outer diameter (OD): 0.020″ to 0.125″ (0.51 mm to 3.18 mm)
• Wall thickness: 0.002″ to 0.020″ (0.05 mm to 0.51 mm)
• Inner diameter (ID): Calculated from OD minus 2 × wall thickness; ID tolerances are derived from OD and wall tolerances.
• Length: Typically supplied in cut lengths from 0.5″ to 120″ (12.7 mm to 3048 mm), or coiled for longer runs.

Wall thickness selection depends on burst pressure, collapse resistance, and heat transfer requirements. For high-temperature applications, wall thickness must account for creep strength reduction at operating temperature.

Tolerances

Tolerances for 347 thin wall capillary tube are typically specified according to ASTM A632 (for small-diameter tubing) or by mutual agreement. Representative values for capillary sizes:

Final tolerances depend on drawing, material condition, process route, and mutual agreement between parties.

End Condition and Burr Control

For 347 thin wall capillary tube, end conditions must be specified based on assembly requirements. Common options:

• Cut as received: Sheared or saw-cut ends with minimal burr.
• Deburred ends: Inside and outside edges chamfered or smoothed to avoid blockage and facilitate fitting.
• Square cut: Ends cut perpendicular to the tube axis within ±1°.
• Special ends: Notches, slots, or custom profiles per drawing.

Burr control is critical in capillary tubes to prevent particle entrapment and flow restriction. Standard deburring processes can achieve surface roughness at the cut edge below 32 μin Ra if required.

Surface Finish and Cleanliness

Surface finish for 347 thin wall capillary tube is typically specified as:

• As-drawn (matte): Ra 20–40 μin (0.5–1.0 μm) – suitable for general industrial use.
• Annealed and pickled: Smoother surface, Ra 10–20 μin (0.25–0.5 μm) – for improved corrosion resistance and cleanability.
• Electropolished or mechanically polished: Ra < 10 μin (0.25 μm) – for medical or ultra-clean applications.

Cleanliness requirements may include degreasing, passivation, or packaging in clean-room conditions. The level should be agreed between buyer and supplier.

Inspection Documentation

When sourcing 347 thin wall capillary tube, typical documentation includes:

• Material test report (MTR) / certificate of conformance per ASTM A269, A632, or A213, listing heat number, chemical composition, and mechanical properties.
• Dimensional inspection report – recorded OD, ID, wall thickness, and length measurements for each lot.
• Surface roughness report (if specified).
• Non-destructive testing (NDT) reports – e.g., eddy current, hydrostatic, or pneumatic tests as required by standard or customer specification.
• Traceability documentation from raw material through final tube.

All numbers in reports should be traceable to the applicable ASTM/EN standard and the actual production conditions.

Packaging Considerations

Because 347 thin wall capillary tube is often used in sensitive assemblies, packaging must protect against physical damage, moisture, and contamination. Typical packaging methods:

• Individually sleeved or poly-bagged tubes.
• Foam-lined wooden or corrugated boxes for small quantities.
• Layer winding on spools for longer lengths (coiled).
• Desiccant and vapor barrier packaging for export or high-humidity environments.
• Clean-room compatible packaging for medical or semiconductor applications.

RFQ Information Checklist

To ensure an accurate quotation for 347 thin wall capillary tube, provide the following details:

• Complete drawing or specification with OD, ID, wall thickness, length, and tolerances.
• Material grade: 347 per ASTM A269, A632, or A213 (include condition – annealed, as-drawn).
• Quantity in pieces or total length (feet/meters) per order.
• Required tolerances (OD, wall, length, straightness).
• End condition and burr control requirements.
• Surface finish and cleanliness level.
• Inspection documentation needed (MTR, dimensional report, NDT).
• Packaging and delivery requirements.

To request a quotation for 347 thin wall capillary tube, submit your drawings, material grade, quantity, tolerance, surface finish, and inspection requirements through the Healsmed website. The engineering team will review and respond with a technical evaluation.