AWS D18.1 Weld Inspection Criteria for Hygienic Piping

Quick Reference

AWS D18.1/D18.2 is the welding standard that governs austenitic stainless steel tube and pipe systems in sanitary (hygienic) applications -- food processing, beverage, dairy, pharmaceutical, and biotechnology facilities. If you weld sanitary tubing and your project spec calls out D18.1, this page gives you the acceptance criteria, inspection requirements, and documentation expectations in one place.

D18.1 covers the welding procedure and performance qualification requirements. D18.2 covers the acceptance criteria and inspection methods. In practice, most people reference them together. The criteria below are drawn from the current editions. Always confirm against the specific edition called out in your contract documents.

Scope: What AWS D18.1/D18.2 Covers

AWS D18.1 applies to autogenous (no filler metal) and filler-added GTAW (TIG) butt welds on austenitic stainless steel tubing and pipe used in hygienic service. This includes:

• Food and beverage processing piping (3-A Sanitary Standards applications)
• Dairy processing systems
• Pharmaceutical and biotech process piping (though many pharma projects specify ASME BPE instead or in addition)
• Personal care and cosmetic manufacturing systems

The standard applies to manual and orbital (mechanized) GTAW. It does not cover structural welds, pressure vessel welds, or non-austenitic materials -- those fall under other codes.

Weld Classification

D18.1/D18.2 defines two weld categories based on how the weld is made:

Classification	Description	Typical Application
Autogenous	No filler metal added. Base metal fuses to base metal.	Orbital tube welding, most sanitary tubing joints 4" and under
Filler Added	Filler wire deposited during welding (manual or mechanized)	Larger diameter joints, repair welds, fittings where autogenous fusion is impractical

The acceptance criteria differ slightly between these two categories. Autogenous welds have tighter limits because the process is more controlled and the resulting bead profile is more predictable.

Visual Inspection Acceptance Criteria

This is the table you will use most often. These are the dimensional limits for visual inspection of completed welds per AWS D18.2.

Autogenous Welds (No Filler Metal)

Defect / Feature	Acceptance Limit	Notes
ID Penetration (Convexity)	Flush to 1/16" (1.6 mm) maximum above the parent metal ID surface	Must blend smoothly. No sharp edges or crevices that trap product.
ID Concavity (Suck-Back)	Maximum 10% of wall thickness, not to exceed 1/32" (0.8 mm)	Measured from the original tube ID contour. Must transition smoothly -- no sharp undercut.
OD Concavity	Maximum 10% of wall thickness	OD must not drop below the adjacent parent metal surface by more than this limit.
OD Convexity (Reinforcement)	Maximum 1/16" (1.6 mm)	Must blend smoothly into the parent material on both sides of the weld.
ID Discoloration (Heat Tint)	Per D18.2 color reference chart. Commonly limited to light straw (#4) or lighter for hygienic service.	The D18.2 color chart ranges from #1 (no color) through #8 (black). Owner specification determines the acceptable level.
Misalignment (Hi-Lo)	Maximum 20% of wall thickness, not to exceed 1/32" (0.8 mm)	Measured as the radial offset between the two tube IDs at the joint.
Lack of Penetration	Not permitted	The weld must show full penetration around the entire circumference on the ID.
Cracks	Not permitted	Any crack is rejectable regardless of size, length, or location.
Porosity	Not permitted on the ID surface. OD: no more than one pore per linear inch, max diameter 1/32" (0.8 mm).	Zero porosity tolerance on the product-contact surface.
Undercut	ID: not permitted. OD: maximum 10% of wall thickness, not to exceed 1/32" (0.8 mm).	Any undercut on the ID is rejectable -- it creates a crevice.
Overlap / Cold Lap	Not permitted	Unfused metal on either surface is rejectable.
Incomplete Fusion	Not permitted	Both tube ends must be fully fused across the entire joint.

Filler-Added Welds

Filler-added welds follow the same general criteria, with these differences:

Feature	Filler-Added Limit	Difference from Autogenous
OD Reinforcement	Maximum 3/32" (2.4 mm)	Slightly more reinforcement permitted due to filler deposit
ID Penetration	Flush to 3/32" (2.4 mm)	Wider tolerance reflects the filler addition
Root Pass Concavity	Maximum 1/32" (0.8 mm)	Same as autogenous

All other criteria (cracks, porosity, undercut, misalignment) are the same as for autogenous welds.

Borescope Inspection Requirements

Visual inspection of the weld ID is a core requirement of D18.1/D18.2 for hygienic service. Because the ID surface is the product-contact surface, you cannot accept a weld based solely on OD appearance.

When borescope inspection is required:

• On all welds where the ID surface cannot be directly viewed
• 100% inspection is the default for hygienic piping. Some project specifications allow sampling plans (e.g., 10% or 25%) after a proven track record, but this must be agreed upon in the contract documents.
• At minimum, start-of-day and end-of-day coupons should be sectioned or borescoped even when production welds are on a sampling plan.

Borescope specifications:

• Minimum resolution sufficient to identify defects at the acceptance limit sizes (porosity at 1/32", discoloration differences)
• Video borescopes with recording capability are preferred. Still images are acceptable.
• The inspector must have sufficient training to distinguish between acceptable and rejectable conditions on a curved ID surface viewed through an optic
• Lighting must be adequate to evaluate heat tint color accurately. LED borescopes with consistent color temperature are preferred over fiber optic illumination for color evaluation.

For equipment recommendations, see our weld inspection camera guide.

ID Discoloration (Heat Tint) Color Reference

The AWS D18.2 color chart is the standard visual reference for weld ID discoloration. Heat tint colors result from oxide layer growth on the stainless steel surface during welding, caused by exposure to residual oxygen in the purge atmosphere.

D18.2 Color #	Appearance	Typical Purge Quality
#1	No discoloration (bright, shiny)	Excellent purge, < 10 ppm O2
#2	Very light straw	Good purge, 10-25 ppm O2
#3	Light straw	Adequate purge, 25-50 ppm O2
#4	Dark straw	Marginal purge, 50-100 ppm O2
#5	Light blue	Poor purge, 100-500 ppm O2
#6	Dark blue	Inadequate purge, 500-1000 ppm O2
#7	Gray / Black	No effective purge
#8	Heavy black oxide, flaking	No purge or severe contamination

What does the standard require? D18.2 itself does not mandate a specific color limit. It provides the color chart as a reference tool. The owner specification (or contract documents) must state which color number is acceptable. Common requirements:

• Food/beverage (3-A): Typically #4 or lighter
• Pharmaceutical: Typically #2 or lighter (many specs require #1)
• Semiconductor: #1 only

When the spec says "no visible heat tint," that means color #1. If the spec references "D18.2 color #4 maximum," colors #1 through #4 pass; #5 and above are rejectable.

AWS D18.1 vs. ASME BPE: Key Differences

Both standards address sanitary/hygienic welding, but they serve different industries and have different acceptance criteria. Understanding the distinction matters when a project spec references one or both.

Feature	AWS D18.1/D18.2	ASME BPE
Primary industry	Food, beverage, dairy	Pharmaceutical, biotech
ID convexity (penetration)	Maximum 1/16" (1.6 mm)	Maximum 10% of wall, not to exceed 0.015" (0.38 mm)
ID concavity	Maximum 10% of wall, not to exceed 1/32" (0.8 mm)	Maximum 10% of wall, not to exceed 0.015" (0.38 mm)
Misalignment	Maximum 20% of wall, not to exceed 1/32" (0.8 mm)	Maximum 20% of wall, not to exceed 0.020" (0.51 mm)
Heat tint requirement	Per owner spec (uses D18.2 color chart)	Per owner spec (often references D18.2 chart)
Surface finish	Not defined in D18.1	Defined in detail (SF1-SF6)
Borescope inspection	Required for ID evaluation	100% default for product-contact welds
Documentation	WPS, PQR, WPQ required	More extensive traceability, weld logs, material tracking

The key takeaway: BPE criteria are tighter. If your project calls out both standards, BPE acceptance limits govern because they are more restrictive. See our full ASME BPE welding requirements reference for detailed BPE criteria.

Inspector Qualification

AWS D18.1/D18.2 requires that personnel performing visual inspection are qualified. The standard accepts:

• AWS Certified Welding Inspector (CWI) per AWS QC1
• Personnel qualified per the employer's written practice, which must include training on the D18.1/D18.2 acceptance criteria, borescope operation, and documented evidence of capability
• Owner-approved inspectors per project specification

The inspector must demonstrate the ability to distinguish between acceptable and rejectable weld conditions at the dimensional limits. For borescope inspection, this means hands-on training with the specific equipment being used and demonstrated ability to evaluate heat tint color under borescope lighting.

Documentation Requirements

A D18.1-compliant welding program requires the following documentation:

Document	Purpose
Welding Procedure Specification (WPS)	Defines every essential variable: base metal, joint design, gas type and flow, amperage, travel speed, position, electrode type and size
Procedure Qualification Record (PQR)	Documents the test results (visual, mechanical, metallographic) from qualifying the WPS
Welder/Operator Performance Qualification (WPQ)	Proves the operator can produce welds meeting the acceptance criteria using the WPS
Weld Log	Production record: joint ID, date, operator, WPS, equipment, inspection results
Inspection Reports	Visual and borescope findings per the acceptance criteria
Material Certifications	Mill test reports for base materials, traceable to heat numbers

For orbital welding, the WPQ must demonstrate that the operator can properly set up the equipment, prepare the joint, execute the programmed weld cycle, and evaluate the result. It is an operator qualification, not a manual skill test.

Common Rejection Reasons

Based on field experience, these are the most frequent causes of weld rejection under D18.1 criteria:

1. Excessive heat tint -- Inadequate purge is the single most common problem. Verify O2 levels with a calibrated analyzer before initiating the arc. See our purge monitoring guide.
2. Lack of penetration -- Usually caused by insufficient amperage, excessive travel speed, or inconsistent material chemistry (low sulfur content). Check your material MTRs for sulfur range.
3. ID concavity (suck-back) -- Excessive amperage or too-slow travel speed. Can also result from excessive purge pressure pulling the molten pool inward.
4. Misalignment (hi-lo) -- Poor joint preparation or inadequate clamping. Orbital weld heads with proper collet inserts reduce this significantly.
5. Porosity -- Contaminated base material (oil, residue, moisture), contaminated gas supply, or gas turbulence at the weld zone.
6. Incomplete tie-in -- The stop point of an orbital weld does not overlap sufficiently with the start. Adjust the overlap angle in the weld schedule.

Need Help With D18.1 Compliance?

TechSouth provides orbital welding equipment, borescope inspection tools, and technical support for hygienic piping projects. Whether you need help developing weld procedures to meet D18.1 acceptance criteria, selecting inspection equipment, or sourcing stainless steel tubing for sanitary service, Contact TechSouth to discuss your project requirements.