Six Weld Defects That Compromise Gas System Purity

In a high-purity gas system, a bad weld is a contamination point and often a leak path. The acceptance standard for visual inspection of gas tungsten arc (GTA) welds in semiconductor fluid distribution systems is SEMI F81-1103. It defines what an inspector should accept, and what should be rejected and reworked.

Here are six defects an inspector should recognise on sight, with the limits the standard sets for each.

What SEMI F81-1103 covers, and what it does not

The standard covers autogenous GTA circumferential butt joint welds on fluid distribution system components 6 inches (150 mm) or less in diameter. It applies to automatic, mechanised, or machine GTA welding processes, and only to welds made without fillers or fluxes. Pressure vessels and process chamber welds are outside its scope.

The materials covered are austenitic and superaustenitic stainless steels, solid solution grades of nickel alloys, and solid solution grades of titanium alloys.

Defect 1: ID concavity

ID concavity is a depression in the inside diameter surface of the weld relative to the surface of the tube. SEMI F81-1103 does not permit ID concavity at any point on the weld. It is a profile defect, meaning it reduces wall thickness below that of the parent metal. Undercut and centerline shrinkage are also rejected on the same basis.

Defect 2: Excessive convexity

Both inside and outside diameter convexity carry a 10% limit. ID convexity must not exceed 10% of the nominal wall thickness T. OD convexity has the same 10% limit.

OD concavity rules are tighter on small tubing. On tubing under 1 inch (25 mm), no OD concavity is allowed. On tubing 1 inch and larger, OD concavity must not exceed 10% of T.

Defect 3: Weld bead meander

A weld bead that drifts side to side across the joint rather than tracking it cleanly is called meander. SEMI F81-1103 limits ID and OD bead meander to 35% of the nominal wall thickness T. Beyond that, the weld is rejected.

Defect 4: Weld width variation

Penetration and bead width should be uniform throughout the weld. Minimum ID bead width is 1.0 times T. Maximum ID bead width is 2.5 times T.

In any individual weld, the maximum ID bead width must not exceed 1.25 times the minimum bead width. A weld can be wide overall or narrow overall within the 1.0T to 2.5T window, but it cannot vary by more than 25% within itself.

Defect 5: Discoloration

Discoloration is any change in surface colour from the base metal, usually caused by oxidation during welding. Colours range from pale bluish-grey to deep blue, and from pale straw to a black crusty coating.

On the OD weld face, a light straw colour is acceptable. Light external oxidation may be removed with a stainless steel wire brush immediately after welding, unless the end user prohibits it.

On the ID of HP and UHP systems, no visible discoloration is allowed when viewed under a bright fluorescent light without magnification. There is one exception. Above 2 inches (50 mm) in diameter, a slight blue colour may be acceptable.

Defect 6: Unconsumed tack welds

Tack welds hold parts in alignment until the final weld is made. SEMI F81-1103 requires that they be totally consumed and undetectable in or around the OD or ID weld bead. If a tack weld is still visible, the weld fails.

How visual inspection catches these

SEMI F81-1103 lists the acceptable inspection tools: sight pipe, rigid borescope, calipers, V-blocks, dial indicators, and comparators. Magnification ranges from a hand magnifying glass up to an optical microscope at 2x to 40x. Illumination can be flashlight, bright fluorescent, or natural ambient light, with white paper for background.

Personnel qualifications referenced in the standard are ASNT SNT-TC-1A or AWS QC-1. Mechlink inspectors hold AWS CWI and ASNT Level II credentials.

Need weld inspection on a high-purity gas system?

Mechlink provides weld inspection services for high-purity gas distribution systems in Singapore, Taiwan and India. Boroscope inspection is available for visual inspection of internal pipe conditions. Response within 3 hours, subject to availability. Contact us to discuss your project.