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

When designing or sourcing a capillary tube for high-temperature or corrosive fluid handling, engineers often require a material that resists intergranular corrosion and maintains strength at elevated temperatures. 347 stainless steel capillary tube offers that solution — a stabilized austenitic grade alloyed with niobium (columbium) to prevent chromium carbide precipitation during welding or service between 427–816 °C. This article provides a technical overview of 347 SS capillary tube properties, manufacturing tolerances, and procurement considerations for medical device, aerospace, and industrial applications.

Applicable Scenarios and Industries

347 SS capillary tube is selected where the assembly involves welding, brazing, or exposure to high temperatures. Common use cases include:

• Medical device components such as surgical tools, catheters, and sensor housings that require autoclaving or sterilization at high temperature.
• Aerospace fuel and hydraulic lines where resistance to thermal cycling and oxidation is critical.
• Chemical and petrochemical instrumentation lines carrying corrosive media at elevated pressures.
• Thermowells, heat exchangers, and exhaust gas sampling probes.

Unlike 304 or 316L, 347 retains stable mechanical properties after exposure to temperatures in the sensitization range, making it suitable for welded assemblies without post-weld heat treatment.

Material Grades and Designations

347 stainless steel corresponds to UNS S34700 and is covered under ASTM A269 (for welded and seamless small-diameter tubing) and ASTM A213 (seamless tubing for high-temperature service). Equivalent European standard is EN 1.4550 (X6CrNiNb18-10).

Compared to other grades:

Chemical Composition (Typical, per ASTM A213/A269)

• Chromium: 17.0–19.0%
• Nickel: 9.0–13.0%
• Niobium (Columbium): 10×%C to 1.10%
• Carbon: ≤ 0.08%
• Manganese: ≤ 2.00%
• Silicon: ≤ 1.00%
• Phosphorus: ≤ 0.045%
• Sulfur: ≤ 0.030%

Mechanical Properties (Annealed Condition)

• Tensile strength: ≥ 515 MPa (75 ksi)
• Yield strength (0.2% offset): ≥ 205 MPa (30 ksi)
• Elongation in 2 in. or 50 mm: ≥ 35% (per ASTM A269)

OD, ID, Wall Thickness, and Length Considerations

Capillary tube is typically defined by a small outside diameter. Common OD ranges for 347 SS capillary tube are 0.5 mm to 6.0 mm (0.020″ to 0.250″). Wall thickness can be as thin as 0.10 mm (0.004″) depending on drawability and final application ID requirements. Lengths are available from random mill lengths (2–6 m) up to custom cut-to-length pieces. For coiled capillary, continuous lengths of several hundred meters are possible with proper annealing between passes.

The ID tolerance is directly affected by manufacturing method (seamless drawn or welded and drawn). Tight concentricity is required for medical fluid delivery or sensor capillary applications.

Tolerances

Standard OD tolerances for capillary tube follow ASTM A269:

Wall thickness tolerance typically ±10% of nominal, but for critical applications tighter values can be negotiated. ID tolerance is derived from OD and wall tolerance; direct ID control is achievable with mandrel drawing. Straightness is typically 1 mm per 300 mm length unless specified tighter. All final tolerances depend on drawing, material condition, process route, and mutual agreement between parties.

End Condition and Burr Control

Capillary tube ends must be free of burrs, especially for applications involving fluid sealing or insertion into ferruges. Common end conditions:

• Deburred ends (inside and outside)
• Square cut (perpendicular to axis within 0.5°)
• Chamfered or radiused ends for assembly

Burr height is typically limited to 0.08 mm max unless otherwise specified. For Luer-lock or medical fittings, a secondary cleaning pass may be required.

Surface Finish and Cleanliness

347 SS capillary tube can be supplied in the following finishes:

• Mill finish (smooth, no further processing)
• Bright annealed (clean, oxide-free surface)
• Electropolished (Ra ≤ 0.4 μm, improved corrosion resistance and biocompatible finish)
• Pickled (removes heat treatment scale)

For medical or cleanroom applications, tubes are cleaned to remove oils, particulate, and residual processing compounds. Cleanliness levels per ASTM B912 or customer specification can be arranged. Do not assume “medical grade” – verify that cleaning methods meet your device requirements.

Inspection Documentation

Standard documentation for 347 SS capillary tube includes:

• Material Test Report (MTR) per ASTM A213 or A269 listing heat number, chemistry, and mechanical results.
• Dimensional inspection report showing OD, ID, wall, length, straightness, and ovality.
• Surface finish measurement report (if electropolished or specified Ra).
• Flattening, flange, and hydrostatic test reports (when required by standard or order).
• Certificate of Compliance (C of C).

Third-party inspection (e.g., by customer representative or accredited laboratory) can be arranged at additional cost.

Packaging Considerations

Capillary tubes are packaged to prevent damage during transit and handling:

• Straight lengths: placed in poly bags and bundled with foam or paper wraps, then boxed or crated.
• Coils: wound on spools with internal support, protected by shrink wrap.
• Each bundle or spool is labeled with material grade, size, heat number, and quantity.
• Custom labeling per customer specification available.

RFQ Information Checklist

To receive an accurate quotation for 347 SS capillary tube, provide the following details:

1. Desired OD, wall thickness (or ID), and length (or coil length).
2. Material grade: 347 SS and applicable standard (ASTM A269, A213, etc.).
3. Quantity (pieces or total length).
4. Required tolerances (if tighter than standard).
5. End condition (deburred, chamfered, square cut).
6. Surface finish (mill, bright annealed, electropolished, etc.).
7. Cleanliness requirements (e.g., medical clean, ISO grade 5 or better).
8. Inspection and documentation needs (MTR, dimensional report, third-party).

To request a quotation for 347 SS 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.