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

When designing or sourcing capillary tubing for high-temperature or corrosive environments, one recurring challenge is selecting a material that resists intergranular corrosion after welding or prolonged thermal exposure. Type 347 stainless steel seamless capillary tubing addresses this problem through stabilization with niobium (columbium) and tantalum, which prevents chromium carbide precipitation at grain boundaries. Engineers and procurement teams seeking 347 seamless capillary tubing are typically solving for applications involving repeated heating cycles, oxidizing atmospheres, or chloride-bearing media where standard 304 or 316L would sensitize.

Applicable Scenarios and Industries

347 seamless capillary tubing is specified where the component must remain corrosion-resistant after welding or service between 425°C and 900°C. Primary industries include:

• Chemical and petrochemical processing – heat exchangers, thermowells, instrumentation lines in reformers and crackers.
• Aerospace and power generation – fuel injection lines, sensor probes, bleed air tubes in gas turbines and exhaust systems.
• Pharmaceutical and specialty chemical – small-diameter transfer lines for high-purity, elevated-temperature fluids.
• Oil and gas – downhole gauging, hydraulic control lines in sour service (with proper NACE consideration).

347 is not recommended for strong reducing acids or environments with hot chlorides above critical stress levels; always verify compatibility with the process fluid.

Material Grades and Designations

347 seamless capillary tubing corresponds to UNS S34700 and is covered by the following standards:

• ASTM A213 – Seamless ferritic and austenitic alloy-steel boiler, superheater, and heat-exchanger tubes.
• ASTM A269 – Seamless and welded austenitic stainless steel tubing for general service.
• EN 10216-5 – Seamless steel tubes for pressure purposes (grade X6CrNiNb18-10).

Chemical composition (weight %, typical per ASTM A213):

Mechanical properties (annealed condition, per ASTM A213): tensile strength 515 MPa min, yield strength 205 MPa min, elongation 35% min.

Compared to 321 (stabilized with titanium), 347 offers better weldability and high-temperature strength above 540°C due to the niobium carbide phase. 316Ti is an alternative where molybdenum is required for pitting resistance.

OD / ID / Wall Thickness / Length Considerations

Capillary tubing is defined by a small outer diameter, typically 0.5 mm to 6.35 mm (1/4 inch) OD. Common sizes include 1/16 in, 1/8 in, 3/16 in, and 1/4 in, with wall thicknesses from 0.10 mm to 1.65 mm.

Wall thickness selection must balance pressure rating (use ASME B31.3 or similar) against internal diameter requirements for fluid flow or wire passage. For seamless 347 capillary, heavier walls (e.g., 0.035 in / 0.89 mm) are preferred when the tubing must withstand internal pressure at elevated temperatures.

Lengths are supplied as straight lengths (up to 6 m or by agreement) or coils (continuous up to several hundred metres). Coiled capillary minimizes joints in long instrumentation runs but requires careful handling to avoid kinking.

Tolerances

Tolerances for 347 seamless capillary tubing depend on the specific standard referenced, manufacturing process, and final drawing requirements. Typical industry ranges are provided below; the exact values must be specified in the procurement documents.

• Outer diameter: ±0.05 mm to ±0.15 mm for OD under 6 mm (per ASTM A269 or customer spec).
• Wall thickness: ±10% of nominal thickness, or as agreed between supplier and buyer.
• Length: ±5 mm on cut-to-length straight pieces; coil length tolerance depends on target and variability in the draw.
• Ovality and straightness: For capillary, ovality is controlled to within 50% of the diameter tolerance; straightness is typically 1 mm per 1000 mm unless otherwise specified.

Critical applications may require tighter tolerances; these are achievable but must be evaluated case by case.

End Condition and Burr Control

Finished ends on 347 seamless capillary are typically deburred inside and outside to prevent particulate generation during assembly. Standard end conditions include square cut (perpendicularity within 1 degree) or chamfered for easier insertion into fittings. For high-purity systems, ends may be reamed or electropolished to remove micro-burrs. Specify the desired end condition in the RFQ.

Surface Finish and Cleanliness Requirements

Surface finish for 347 seamless tubing is defined by average roughness (Ra). Typical as-drawn finish ranges from 0.4 µm to 1.6 µm Ra. Where lower roughness is needed (e.g., for reduced friction or cleanability), the tubing can be electropolished, achieving 0.2 µm Ra or better. For medical or semiconductor applications, cleanliness levels such as degreased, passivated, and packaged in a clean room must be explicitly requested. Standard mill annealing leaves an oxide scale that can be removed by pickling or bright annealing.

Inspection Documentation

Each shipment of 347 seamless capillary tubing should include:

• Material test report (MTR) per EN 10204 Type 3.1 or ASTM A1016, listing heat number, chemical analysis, and mechanical properties.
• Dimensional inspection report: OD, ID, wall thickness, and length measurements (sampling basis agreed in contract).
• Surface finish certificate (if specified Ra value).
• Hydrostatic or eddy current test report (if applicable per ASTM A213).
• Positive Material Identification (PMI) report when required.

Independently requested third-party inspection (TPIA) can be arranged at additional cost.

Packaging Considerations

Straight lengths are bundled with protective end caps or plastic sleeves to prevent damage. Coiled tubing is wound on reels or in standard coil boxes with inner support to maintain coil integrity. For sensitive applications (e.g., cleanroom, medical), tubing is double-bagged in anti-static polyethylene. Desiccant packs are added for export shipments to control humidity.

RFQ Information Checklist

To ensure accurate quotation and timely delivery of 347 seamless capillary tubing, provide the following in your request:

• Dimensions: OD, wall thickness, and length (or coil weight/run length).
• Material grade: UNS S34700 (or equivalent EN/ISO).
• Quantity: total metres, pieces, or coils.
• Tolerance requirements: OD, wall, straightness, ovality.
• Surface finish and cleanliness level.
• End condition: square cut, chamfered, deburred.
• Inspection / documentation requirements: MTR, dimensional report, third-party inspection.
• Packaging: domestic or export, straights or coils, protective measures.

To request a quotation for 347 seamless capillary tubing, 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.