Australian Weld Inspection — AS/NZS 1554.1 Methods and Acceptance Criteria
Comprehensive reference for weld inspection, non-destructive testing (NDT), and acceptance criteria per AS/NZS 1554.1:2014 (Structural steel welding -- Part 1: Welding of steel structures). Covers inspection categories (GP, SP), visual inspection requirements, ultrasonic testing (UT), magnetic particle inspection (MPI), dye penetrant testing (PT), radiographic testing (RT), and inspector qualification requirements.
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Weld Categories and Inspection Levels
AS/NZS 1554.1 defines three weld categories that determine the required level of inspection:
GP (General Purpose) Category
GP is the highest category, applied to welds where failure would result in:
- Collapse of the structure or a major portion of it
- Risk to life safety
- Fracture of a primary tension member
- Failure of a member subject to dynamic or fatigue loading
GP welds are subject to the most stringent inspection: 100% visual + either 100% UT (butt welds) or MPI (fillet welds at sampling frequency).
SP (Structural Purpose) Category
SP is the standard category for primary structural welds in buildings and bridges where:
- Failure would impair the serviceability or function of the structure
- The weld connects primary load-bearing members
- The weld is subject to significant static loads
SP welds require 100% visual inspection + UT on 10-25% of butt welds depending on the connection type.
General Fabrication Category
Applied to welds in secondary members, non-structural attachments, and components where weld failure does not compromise the overall structural integrity. Visual inspection only (100%).
Selection Guide — AS/NZS 1554.1 Table 4.1
| Structural Application | Weld Type | Recommended Category |
|---|---|---|
| Full-pen butt weld in tension member | Butt | GP |
| Full-pen butt weld in compression member | Butt | SP |
| Fillet weld connecting primary beam to column | Fillet | SP |
| Web stiffener to beam web | Fillet | General Fabrication |
| Lifting lug attachment | Fillet | GP (safety-critical) |
| Purlins to cleats (roof secondary) | Fillet | General Fabrication |
| Moment-resisting frame beam-to-column connection | Butt + Fillet | GP |
Visual Inspection (VT) — Clause 6.2
Visual inspection is mandatory for 100% of all welds, regardless of category. It is the first and most important NDT method because many weld defects are visible on the surface, and the presence of surface defects often indicates sub-surface problems.
Visual Inspection Checklist
| Check Item | Acceptance Criterion (SP) | Measuring Method |
|---|---|---|
| Weld profile (convexity/concavity) | <= 1.5 mm (fillet), <= 3 mm (butt) | Weld gauge, straightedge |
| Undercut depth | <= 0.5 mm (SP), <= 1.0 mm (GP) | Weld gauge, feeler gauge |
| Surface crack | Not permitted (any category) | Visual, MPI confirmation |
| Surface porosity | Isolated pores <= 3 mm permitted | Visual, comparison gauge |
| Incomplete fusion at weld toes | Not permitted | Visual, dye penetrant if uncertain |
| Weld size (leg length, throat) | Not less than specified size | Fillet weld gauge |
| Weld length | Not less than specified length | Steel tape or rule |
| Arc strikes outside the weld zone | Not permitted (GP), avoid (SP) | Visual |
| Spatter | Remove if interferes with coating | Visual |
Fillet Weld Gauge Measurement
The standard fillet weld gauge measures the leg length (not the throat). For a specified 8 mm fillet weld:
- The convexity (face reinforcement) should not exceed 1.5 mm
- The concavity (undercut or face depression) should not exceed 0.5 mm
- The effective throat at the concavity must be at least 0.7 x leg length = 5.6 mm
Ultrasonic Testing (UT) — Clause 6.3
UT is the primary volumetric NDT method for butt welds in Australian structural fabrication. It can detect sub-surface defects including cracks, lack of fusion, slag inclusions, and porosity.
UT Principles
A piezoelectric transducer transmits high-frequency (2-5 MHz) sound waves into the steel through a couplant (gel or grease). The sound waves reflect from internal discontinuities, and the reflected signal (echo) is displayed on an A-scan screen. The amplitude, position, and shape of the echo indicate the size, depth, and type of defect.
UT Sensitivity and Calibration
The UT equipment is calibrated using a reference block (typically a DAC -- Distance Amplitude Correction block) with known side-drilled holes at different depths. The sensitivity is set so that the reference reflector (typically a 1.5 mm or 3 mm diameter side-drilled hole) produces an 80% full-screen-height echo. Any indication exceeding the reference level is investigated and assessed against the acceptance criteria.
Acceptance Criteria — AS/NZS 1554.1 Table 6.3 (UT)
| Defect Type | SP Category | GP Category |
|---|---|---|
| Crack | Not permitted | Not permitted |
| Lack of fusion | Not permitted | Not permitted |
| Slag inclusion | Length <= 3 mm, cumulative <= 6 mm | Length <= 6 mm, cumulative <= 12 mm |
| Isolated porosity | <= 3 mm diameter, <= 2 per 100 mm of weld | <= 6 mm diameter, <= 3 per 100 mm |
| Cluster porosity | <= 5% of projected area in any 50 mm zone | <= 10% of projected area |
Magnetic Particle Inspection (MPI) — Clause 6.4
MPI detects surface and near-surface cracks in ferromagnetic materials (all structural carbon steels). It is the preferred method for detecting cracks in fillet welds and for confirmatory inspection of suspect areas identified by visual inspection.
MPI Principle
A magnetic field is induced in the steel using a permanent magnet yoke or electromagnetic coil. Fine iron particles (dry powder or wet fluorescent suspension) are applied to the surface. The particles cluster at any discontinuity where the magnetic flux leaks from the surface, making the defect visible.
Sensitivity
Surface cracks as small as 0.5 mm long and 0.01 mm wide are detectable with fluorescent MPI under UV light in dark conditions (the most sensitive MPI configuration). For field inspection in daylight, dry powder MPI with a contrasting colour (red particles on grey steel) is used, which is less sensitive (minimum detectable crack approximately 2 mm long).
Dye Penetrant Testing (PT) — Clause 6.5
PT is used for non-ferromagnetic materials (stainless steel, aluminium) and as a supplementary method on carbon steel where MPI is impractical (tight corners, limited access). A coloured or fluorescent dye is applied to the surface, allowed to penetrate into surface-breaking defects by capillary action, and the excess is removed. A developer draws the penetrant out of the defect, making it visible.
Inspection Frequency — AS/NZS 1554.1 Table 6.5
| Weld Type / Category | Visual (VT) | UT | MPI |
|---|---|---|---|
| GP full-pen butt weld | 100% | 100% | As required |
| GP fillet weld | 100% | N/A | 20% sample |
| SP full-pen butt weld | 100% | 10-25% | As required |
| SP fillet weld | 100% | N/A | Spot check (5%) |
| General fabrication | 100% | N/A | N/A |
The sampling frequency for UT and MPI should be increased if defects are found: if one defect is found in a sample, the sample size is doubled; if further defects are found, 100% inspection is required.
Inspector Qualification — Clause 6.1
Weld inspectors performing UT must hold a current certification to:
- AINDT (Australian Institute for Non-Destructive Testing) Level 2 or 3 in Ultrasonic Testing
- or equivalent certification recognised under ISO 9712
Visual inspectors must be qualified by experience and training, with documented competency verification by the fabrication contractor. Weld inspectors on GP category welds must hold a minimum of AINDT Level 2 in the applicable method.
Worked Example: Inspection Plan for Structural Steel Frame
Problem: A multi-storey steel frame includes the following welds. Specify the inspection categories and NDT requirements per AS/NZS 1554.1.
Welds in the frame:
- Full-pen butt weld in beam flange at moment connection splice (tension flange)
- Fillet weld connecting beam web to shear tab (web connection)
- Fillet weld connecting column base plate to column
- Stiffener fillet welds inside column at beam moment connection
Solution:
| Weld | Category | Visual | UT | MPI | Rationale |
|---|---|---|---|---|---|
| 1 | GP | 100% | 100% | As req | Tension butt weld in primary moment connection |
| 2 | SP | 100% | N/A | 5% spot | Standard shear connection, fillet weld |
| 3 | SP | 100% | N/A | 5% spot | Column base, compression dominant |
| 4 | General Fab | 100% | N/A | N/A | Internal stiffeners, secondary elements |
Inspection documentation:
- Weld maps prepared showing each weld with unique ID
- Visual inspection reports for each weld (signed by inspector)
- UT reports for Weld 1 with scan positions, reference level, and all indications above the reporting threshold
- MPI reports for Welds 2 and 3 with technique, sensitivity, and results
- Non-conformance reports (NCRs) issued for any defect exceeding acceptance criteria
- All NCRs closed out with repair procedure, re-inspection, and acceptance sign-off
Result: Inspection plan specified for all four weld types. GP for Weld 1 (100% UT), SP for Welds 2 and 3 (spot MPI), General Fabrication for Weld 4 (visual only).
Frequently Asked Questions
What is the minimum weld inspection required per AS/NZS 1554.1?
The absolute minimum is 100% visual inspection of all welds, regardless of category. For GP category butt welds, 100% UT is additionally required. For SP category butt welds, 10-25% UT is required. Fillet welds require MPI at 20% (GP) or 5% (SP) sampling rates. There is no provision in AS/NZS 1554.1 for uninspected welds in structural applications -- every weld must at minimum be visually inspected by a qualified inspector.
When is ultrasonic testing (UT) required versus magnetic particle inspection (MPI)?
UT is the primary method for butt welds (full or partial penetration) because it can detect sub-surface defects throughout the weld volume. MPI is used for fillet welds because the weld geometry (concave/convex surface, variable throat) makes UT unreliable. MPI detects only surface and near-surface defects in fillet welds (cracks and surface-breaking lack of fusion). For critical fillet welds where sub-surface inspection is essential, UT-amenable configurations (e.g., the beam web to end plate at full-pen corner joints) should be specified.
Are cracks ever acceptable in structural welds per AS/NZS 1554.1?
No. Cracks of any size, orientation, or type are never acceptable in any weld category (GP, SP, or General Fabrication). Per AS/NZS 1554.1 Table 6.3, "Not permitted" is the acceptance criterion for cracks in all categories. Any weld found to contain a crack must be repaired: the crack is gouged out (carbon arc gouging or grinding), the cavity is inspected to confirm complete removal, and the weld is remade and re-inspected.
How are weld inspection records maintained for Australian structural projects?
Per AS/NZS 1554.1 Clause 6.6, weld inspection records must be maintained and traceable to individual welds through a weld map or weld identification system. Records must include: the inspector's name and qualification, the inspection date, the method used, the acceptance criteria applied, the inspection results (pass/fail), and details of any non-conformances and their resolution. Records must be retained for the duration of the construction project plus the statutory limitation period (typically 6-10 years).
What qualifications does a weld inspector need in Australia?
Per AS/NZS 1554.1 Clause 6.1, weld inspectors must be qualified to AINDT (Australian Institute for Non-Destructive Testing) Level 2 in the relevant method (UT, MPI, PT, RT) for all inspection beyond visual. Visual inspectors must be certified by the fabrication contractor as competent, with documented training and experience. The minimum experience for an AINDT Level 2 inspector is typically 6-12 months of supervised practice after completing the theory and practical training courses, plus passing a certification examination.
Educational reference only. All inspection requirements must be verified against the current edition of AS/NZS 1554.1 and the project specification. This information does not constitute professional engineering advice.