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

When a design requires a stainless steel capillary tube that maintains corrosion resistance and mechanical strength after welding or prolonged high-temperature exposure, the material selection narrows to stabilized austenitic grades. Engineers sourcing a 347 seamless capillary tube are typically solving for intergranular corrosion resistance in the as-welded condition, or for oxidation resistance up to 800 °C (1470 °F). This article provides the material properties, manufacturing tolerances, and application considerations needed to specify and procure 347 seamless capillary tubing for demanding industrial and instrumentation environments.

Applicable Scenarios and Industries

Type 347 stainless steel (UNS S34700) is a columbium (niobium)+tantalum stabilized austenitic alloy. The stabilization prevents chromium carbide precipitation at grain boundaries during heating in the 425–815 °C (800–1500 °F) range, making it suitable for applications that involve welding or service at elevated temperatures without post-weld heat treatment.

• Chemical and petrochemical processing – Thermowells, heat exchanger tubing, instrument impulse lines in processes involving corrosive media and temperature cycling.
• Power generation – Superheater and reheater tubes, boiler instrumentation, and sampling lines where sensitization cannot be tolerated.
• Aerospace and defense – Hydraulic and pneumatic sensing lines, fuel system capillaries, and engine bleed air tube assemblies.
• Instrumentation and process control – Thermocouple sheaths, pressure transmitter impulse lines, and gas chromatography columns that must resist oxidation and scaling.
• Heat exchange – Small-diameter double-pipe or coil heat exchangers for aggressive chemical streams.

Material Grade and Designations

347 seamless capillary tube is commonly supplied to ASTM A269 (general service) or ASTM A213 (boiler, superheater, heat exchanger) standards. The chemical composition and mechanical properties are defined per these specifications.

Chemical Composition (Weight %, per ASTM A269/A213)

Mechanical Properties (Annealed Condition, per ASTM A269)

Note: For higher creep-rupture strength at elevated temperatures, the 347H variant (UNS S34709, carbon 0.04–0.10) may be specified. Both grades share the same stabilization requirements.

OD / ID / Wall Thickness / Length Considerations

Capillary tube for instrumentation and precision fluid handling typically falls within the following dimensional ranges. All values are typical and depend on drawing specifications.

• Outside diameter (OD): 0.5 mm (0.020″) to 6.0 mm (0.236″). Common fractional sizes: 1/16″, 1/8″, 3/16″.
• Wall thickness: 0.05 mm (0.002″) to 1.0 mm (0.040″), depending on pressure rating and handling stiffness.
• Inside diameter (ID): Determined by OD minus 2× wall; typical IDs range from 0.3 mm to 4.0 mm.
• Length: Standard straight lengths up to 6 m (20 ft). For longer runs, coiled capillary (continuous length up to 300 m) can be produced per agreement.

Tolerances (Typical Ranges for Capillary Tube)

Capillary tube tolerances are tighter than conventional small-diameter tubing. The values below are representative of what is achievable with precision manufacturing and are subject to agreement based on drawing and process route.

All tolerances apply to the as-manufactured condition (annealed, straightened) unless otherwise stated on the drawing.

End Condition and Burr Control

For seamless capillary tube used in instrumentation and fluid systems, end finish is critical to avoid flow disruption, particle entrapment, or leakage at fittings. Standard options include:

• Square cut – Perpendicular to tube axis within 0.5°.
• Deburred – Inside and outside edge burrs ≤ 0.1 mm (0.004″). For OD < 1 mm, burr height depends on tooling and mutual agreement.
• Chamfered or beveled – For orbital welding applications, a 30–37.5° bevel is available as specified.

Burr control is verified by visual inspection (10× magnification) and, where required, by bore gauge or plug gauge.

Surface Finish and Cleanliness Requirements

The surface condition of 347 seamless capillary tube depends on the final process step. Common finishes include:

• As-drawn (mill finish) – Semi-bright, may contain light drawing lubricant residues unless specified otherwise.
• Bright annealed (hydrogen atmosphere) – Clean, oxide-free surface, typical Ra ≤ 0.8 µm (32 µin).
• Pickled or passivated – Removes heat tint and scale; matte appearance.

Cleanliness requirements (e.g., for oxygen service or ultra‑high‑purity gas lines) are specified separately and may include degreasing, end capping, and bagging. Standard cleanliness is oil‑free unless a lower residual carbon level is contractually defined.

Inspection Documentation

When sourcing 347 seamless capillary tube, the following inspection documents are commonly requested. Availability depends on the supplier’s quality system and should be confirmed at inquiry.

• Material test report (MTR) per EN 10204 Type 3.1 or 3.2, or per ASTM A269/A213 – includes heat analysis and mechanical properties.
• Dimensional inspection report – OD, ID, wall, straightness, and length measurements.
• Hydrostatic or pneumatic pressure test certificate (if required by specification).
• Positive Material Identification (PMI) report using XRF or OES – verifies chemistry against grade requirements.
• Surface roughness measurement (if Ra is called out on drawing).
• Third-party inspection – performed by an accredited agency per customer requirements.

Packaging Considerations

Packaging for capillary tube must protect against mechanical damage, corrosion, and contamination during transport and storage. Typical packaging options include:

• Individual poly sleeves or shrink wrap for straight lengths.
• Foam-lined wooden crates or cardboard boxes for multiple tubes, with foam spacers to prevent contact.
• Coil winding on spools for long continuous capillary, with tie-offs and protective wrapping.
• End caps for each tube to prevent ingress of debris.

For orders requiring cleanliness certification, tubes are double-bagged in nitrogen-purged pouches and marked with cleanliness lot numbers.

RFQ Information Checklist

To ensure an accurate quotation for 347 seamless capillary tube, the following information should be included in the request:

• Drawing or dimensional sketch with OD, wall thickness (or ID), and length.
• Material grade (e.g., 347, 347H) and applicable standard (ASTM A269, A213, or customer specification).
• Tolerance requirements (OD, wall, straightness, length).
• End condition (cut, deburred, chamfered, etc.).
• Surface finish and cleanliness level.
• Inspection documentation required (MTR, dimensional report, PMI, etc.).
• Quantity (total length or number of pieces).
• Packaging preference (straight vs. coil, level of protection).

Note: All dimensional, mechanical, and chemical values stated in this article are based on published ASTM specifications and common industry practice. Actual achievable capabilities depend on drawing, material condition, process route, and mutual agreement between parties.

To request a quotation for 347 seamless 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.