What is the difference between MT and PT weld inspection?

Magnetic particle testing (MT) and liquid penetrant testing (PT) are both surface inspection methods, but they differ fundamentally in approach and capability. MT uses electromagnetic principles: a magnetic field is induced in the ferromagnetic part (all carbon and low-alloy steels), and when the field encounters a surface or near-surface discontinuity, flux leakage attracts iron particles (dry powder or wet fluorescent suspension) to form a visible indication. MT detects both surface-breaking and slightly subsurface defects (up to approximately 0.040 inch / 1 mm below the surface). PT uses capillary action: a low-viscosity penetrant dye (visible red or fluorescent) is applied to the surface, dwells for 10-60 minutes to seep into surface-breaking cracks, and after excess removal and developer application, the trapped dye bleeds out to form an indication. PT detects only surface-breaking defects. Key selection factors: (1) Material — MT requires ferromagnetic material (carbon steel, low-alloy steel). PT works on any non-porous material (stainless steel, aluminum, titanium, ceramics). (2) Speed — MT is faster because there is no dwell time; PT requires penetrant dwell (10-60 min) plus developer time. (3) Subsurface detection — MT detects near-surface defects; PT does not. (4) Surface condition — PT requires cleaner surfaces (no paint, scale, or rough profile). MT tolerates light surface contamination. (5) Sensitivity — MT with fluorescent wet method (visible under UV light) is the most sensitive surface NDE method for ferromagnetic materials. For structural steel per AWS D1.1, MT is the preferred surface method for carbon steel, while PT is reserved for non-magnetic materials or when MT equipment is impractical (confined spaces where AC yokes cannot fit).

When is ultrasonic testing (UT) required for structural welds?

Ultrasonic testing (UT) is required for complete joint penetration (CJP) groove welds subject to tension or transverse loading in building structures per AWS D1.1 Table 6.3. The specific requirements: (1) CJP groove welds in tension — UT required regardless of base metal thickness. These include beam flange-to-column flange welds in moment connections, tension chord splices in trusses, and column splice welds where net tension exists under any load combination. (2) CJP groove welds subject to transverse loading — UT required. This includes column flange splices in moment frames where shear is transferred across the joint. (3) CJP groove welds in compression only — UT may be required per project specification, but Table 6.3 provides minimum testing extent. (4) Fillet welds — UT is NOT required by AWS D1.1 for fillet welds because the UT beam path cannot reliably distinguish between the fillet leg and base metal. Fillet welds are inspected visually (VT) for size, profile, and surface defects only. (5) PJP groove welds — UT may be specified by project requirements but is challenging due to the unfused root face. For AISC 341 seismic applications, UT is required for all demand-critical welds in the Seismic Load Resisting System (SLRS), with enhanced acceptance criteria (typically Class A per AWS D1.1 Table 6.2). UT technicians must be qualified to ASNT SNT-TC-1A Level II minimum, and the UT procedure must be qualified per AWS D1.1 Annex K for building structures.

What weld defects does a CWI look for during visual inspection?

A Certified Welding Inspector (CWI) performing visual inspection (VT) per AWS D1.1 Section 6.9 checks for these defects in order of severity: (1) Cracks — any crack (hot crack, cold crack, crater crack, underbead crack) is rejectable regardless of size or location. Cracks are the most critical defect because they propagate under cyclic loading. (2) Undercut — depth must not exceed 1/32 inch (0.8 mm) for steel less than 1 inch thick in building structures, or 1/16 inch (1.6 mm) for statically loaded structures. For the beam flange-to-column flange CJP groove weld, undercut at the weld toe is a stress riser and must be carefully evaluated. (3) Overlap — cold lap (weld metal flowing onto base metal without fusion) is not permitted. This is a fusion discontinuity that can act as a crack initiator. (4) Porosity — visible surface porosity is evaluated against AWS D1.1 Table 6.1, which limits the frequency and size of gas pockets. The sum of diameters of visible porosity shall not exceed 3/8 inch in any linear inch of weld, and the maximum individual pore diameter is 3/32 inch. (5) Weld size — fillet weld leg length measured with a fillet weld gauge must meet or exceed the size shown on the drawings. Undersized welds are rejectable. (6) Weld profile — convexity must not exceed values in AWS D1.1 Table 6.1 (0.0625 inch for 5/16 inch fillet). Excessive convexity creates stress concentration at the weld toe. (7) Crater cracks — any shrinkage cavity or star crack at the weld termination is rejectable. (8) Arc strikes — unintentional arc strikes outside the weld zone must be ground smooth and checked for cracking with MT or PT. (9) Spatter — evaluated per project specification; excessive spatter prevents proper coating adhesion. (10) Underfill — groove welds must not be underfilled below the base metal surface.

What are the NDE requirements for seismic moment connections per AISC 341?

AISC 341 (Seismic Provisions for Structural Steel Buildings) imposes enhanced NDE requirements for connections in the Seismic Load Resisting System (SLRS): (1) All CJP groove welds in demand-critical connections require 100% UT inspection. This includes beam flange-to-column flange welds, column splice welds, and brace-to-gusset welds in special concentrically braced frames (SCBF). (2) All CJP groove welds in the SLRS require MT or PT of the completed weld surface, including after backgouging and backwelding, to verify no surface defects exist before UT is performed. (3) The acceptance criteria for UT is typically AWS D1.1 Class A (most stringent), not Class B. Class A uses a smaller reference reflector and therefore rejects smaller indications. (4) The weld root pass, after backgouging and before backwelding, must be inspected with MT or PT to confirm complete removal of the root pass defects (lack of fusion, porosity, cracking). (5) Tack welds incorporated into the final weld must meet the same NDE requirements as the primary weld. (6) Run-off tabs must be removed and the ends of the weld ground smooth, then inspected with MT or PT to confirm no end cracking. (7) For welded unreinforced flange (WUF-W) moment connections, the backing bar at the beam bottom flange must be removed, the root ground smooth, and inspected with MT or PT before the reinforcing fillet weld is applied. (8) Repair welds must be reinspected to the same standard as the original weld. (9) The Engineer of Record must review and accept all NDE reports before the connection is approved. These enhanced requirements exist because post-Northridge (1994) research showed that undetected weld defects in moment connections were the primary cause of brittle fracture during the earthquake.

What qualification does a weld inspector need?

Weld inspector qualifications are specified in AWS D1.1 Section 6.1 and are tiered by inspection method: (1) Visual Inspector — must be an AWS Certified Welding Inspector (CWI) or, for non-code work, qualified by the Engineer of Record. CWI certification requires passing a 3-part exam (Part A: Fundamentals, Part B: Practical, Part C: Code Book — typically AWS D1.1) administered by AWS. CWI certification is valid for 3 years and requires renewal by continuing education (80 PDH) or re-examination. The CWI is the gatekeeper who makes accept/reject decisions. (2) MT/PT Technician — must be qualified to ASNT SNT-TC-1A Level I (perform tests under Level II supervision) or Level II (set up, calibrate, perform tests, and interpret results independently). Certification includes classroom training (16-32 hours depending on method), on-the-job training hours (130-400 hours), and written practical examinations. (3) UT Technician — must be qualified to ASNT SNT-TC-1A Level II minimum for building structural UT. AWS D1.1 Annex K defines additional requirements specific to structural weld UT, including the need for a written UT procedure qualified on a calibration block. The Level II UT technician must demonstrate proficiency in angle-beam UT, flaw sizing, and AWS D1.1 acceptance criteria. (4) UT Procedure — must be written by a Level III (or Level II with Engineer approval) and qualified per AWS D1.1 Annex K. (5) RT Film Interpreter — qualified to ASNT SNT-TC-1A Level II minimum for radiographic interpretation, though RT is rarely used for building structures. The Engineer of Record may accept or reject the inspector's qualifications, and may require the inspector to demonstrate proficiency on project-specific mockups or calibration blocks before performing production inspection.

What are the main NDT methods for weld inspection?

The main non-destructive testing (NDT) methods are: Visual Testing (VT) per AWS D1.1 Clause 8, Ultrasonic Testing (UT) per AWS D1.1 Clause 6, Magnetic Particle Testing (MT) per AWS D1.1 Clause 7, Radiographic Testing (RT), and Dye Penetrant Testing (PT). VT is required on all welds; UT is typical for CJP groove welds.

What weld defects are unacceptable per AWS D1.1?

AWS D1.1 prohibits: cracks of any size or location, incomplete fusion, incomplete joint penetration (in CJP welds), porosity exceeding 3/8 inch diameter or 5% of weld surface area, undercut exceeding 1/32 inch for statically loaded structures, and slag inclusion exceeding 1/8 inch width.

How often should welded steel structures be inspected?

Per IBC 2021 Chapter 17 and AWS D1.1, visual inspection is required: continuously during initial welding, at completion of each structural frame, and periodically during the life of the structure. The Owner specifies inspection intervals in the Quality Assurance Program.

Steel Weld Inspection — Methods, Acceptance & Standards

Weld inspection ensures that structural welds meet the quality requirements of the project specifications and applicable codes. AWS D1.1 (Structural Welding Code - Steel) governs inspection for most building projects. This page covers inspection methods, acceptance criteria, and when each method is required.

Weld Inspection Methods

Method	Abbreviation	Detects	Typical Cost	Speed	Access Required
Visual	VT	Surface defects, size, profile	Low	Fast	Both sides
Magnetic Particle	MT	Surface and near-surface cracks	Medium	Medium	One side
Liquid Penetrant	PT	Surface cracks and porosity	Medium	Slow	One side
Ultrasonic	UT	Internal defects (cracks, slag, porosity)	High	Medium	One side
Radiographic	RT	Internal defects (through X-ray image)	Very High	Very Slow	Both sides

Visual Inspection (VT)

Visual inspection is required for ALL structural welds (AWS D1.1 Section 6.9). No weld is accepted without passing VT.

What VT Checks

Check	Acceptance Criterion
Weld size (fillet)	Measured by leg length or throat
Weld profile	Convexity ≤ 0.0625 in for 3/8 in welds, per Table 6.1
Undercut	Depth ≤ 1/32 in (building), ≤ 1/16 in (for static loads)
Overlap	Not permitted
Cracks	Not permitted (surface)
Porosity	Visible surface porosity per code
Crater cracks	Not permitted
Spatter	Per project specification
Arc strikes	Remove and grind smooth
Underfill	Not permitted on groove welds
Burn-through	Evaluated per code
Reinforcement	Groove weld reinforcement per Table 6.1

Weld Gauges

Gauge Type	Measures
Fillet weld gauge	Leg length and throat
V-WAC gauge	Undercut depth
Bridge cam gauge	Reinforcement height, undercut depth
Pocket bridge gauge	General purpose measurements

Magnetic Particle Testing (MT)

MT detects surface and near-surface discontinuities in ferromagnetic materials (all carbon steels). A magnetic field is applied, and iron particles (dry or wet, visible or fluorescent) accumulate at defects.

When MT Is Required

Application	Requirement
CJP groove welds (tension)	Per AWS D1.1 Table 6.2
Seismic connections (AISC 341)	MT or PT required
Fatigue-critical connections	Per project spec
Repair welds	Often required after repair
Tubular connections	Per AWS D1.1 Section 6

MT Acceptance (AWS D1.1 Table 6.1)

Indication	Acceptable	Rejectable
Crack	None	Any
Linear (L > 3x W)	L ≤ 1/8 in (3 mm)	L > 1/8 in
Rounded	≤ 1/8 in diameter, spaced per code	> 1/8 in or clustered

Liquid Penetrant Testing (PT)

PT is a surface-only method that uses a penetrating dye (visible or fluorescent) to detect surface-breaking discontinuities. Used on non-ferromagnetic materials (stainless steel, aluminum) or when MT equipment is not available.

PT Process

Clean the surface
Apply penetrant (dwell time 10-60 minutes)
Remove excess penetrant
Apply developer
Inspect for indications

PT Acceptance

Same criteria as MT (AWS D1.1 Table 6.1). PT cannot detect subsurface defects.

Ultrasonic Testing (UT)

UT uses high-frequency sound waves to detect internal weld defects. A transducer sends sound into the weld, and reflections from defects appear on a screen. UT is the primary method for inspecting CJP groove welds in buildings.

When UT Is Required

Weld Type	Application	Requirement
CJP groove weld (tension)	Buildings	UT per Table 6.3
CJP groove weld (compression)	Buildings	UT per Table 6.3
CJP groove weld (transverse)	Connections	UT required
Fillet welds	—	UT not typically required
PJP groove welds	—	UT per project specification

UT Acceptance (AWS D1.1 Table 6.2)

Welds are evaluated based on the amplitude of reflected signals relative to a reference calibration:

Class	Description	Typical Application
Class A	Most stringent	CJP groove welds subject to tension
Class B	Moderate	CJP groove welds subject to compression
Class C	Least stringent	PJP groove welds, non-critical

Indications are evaluated based on amplitude, length, and proximity to other indications.

Radiographic Testing (RT)

RT uses X-rays or gamma rays to produce an image of the weld on film or a digital detector. Defects appear as dark or light areas on the radiograph.

When RT Is Used

Application	Notes
Pressure vessels	ASME BPVC requires RT
Pipe welding	API 1104, ASME B31
Building structural	Rarely used (UT preferred)
Bridge welding	Sometimes specified for critical joints

RT is less common for building structural welds because:

Both sides of the weld must be accessible
Radiation safety requirements
Slower and more expensive than UT
Cannot determine defect depth (only presence)

Inspector Qualifications (AWS D1.1 Section 6.1)

Level	Qualification	Responsibilities
Welder	Performance qualification	Executing welds
Welding Operator	Performance qualification	Operating semi-automatic/automatic equipment
Visual Inspector (CWI)	AWS Certified Welding Inspector	VT, acceptance decisions
NDE Technician (MT/PT)	Per SNT-TC-1A or CP-189	Performing and interpreting MT/PT
UT Technician	Per SNT-TC-1A Level II minimum	Performing and interpreting UT
RT Interpreter	Per SNT-TC-1A Level II minimum	Interpreting radiographs

Inspection Requirements by Connection Type

Connection Type	VT	MT/PT	UT	Notes
Fillet welds (all)	Required	Not required	Not required	Size and profile check
PJP groove welds	Required	Per spec	Per spec	Depth of fusion
CJP groove weld (tension)	Required	Required	Required	Full inspection
CJP groove weld (compression)	Required	Required	Required	Full inspection
CJP groove weld (transverse)	Required	Required	Required	Full inspection
Seismic (AISC 341) connections	Required	Required (MT or PT)	Required (CJP)	Enhanced inspection
Tack welds	Required	Not required	Not required	Size and location

Frequently Asked Questions

What is the difference between MT and PT? MT (magnetic particle) uses magnetic fields and iron particles to detect surface and near-surface defects. PT (penetrant testing) uses a dye to detect only surface-breaking defects. MT is preferred for carbon steel (faster, detects subsurface). PT is used for non-magnetic materials (stainless steel, aluminum).

When is UT required for structural welds? UT is required for all CJP (complete joint penetration) groove welds subject to tension or transverse loading in building structures, per AWS D1.1. Fillet welds and PJP groove welds typically require only visual inspection.

What does a CWI do? A Certified Welding Inspector (CWI) performs visual inspection, reviews welding procedures (WPS), verifies welder qualifications, and makes accept/reject decisions. CWI certification is through AWS.

Can a weld be too big? Yes. Oversized fillet welds increase distortion, cost, and may violate the maximum weld profile requirements. AWS D1.1 limits convexity on fillet welds. Oversized welds also increase the heat-affected zone, potentially affecting base metal properties.

What is the most common weld defect? Porosity (gas trapped in the solidifying weld metal) is the most common defect. It is usually visible and evaluated against AWS D1.1 Table 6.1. Crack indications (hot cracks, cold cracks, crater cracks) are the most critical and are always rejectable.

Welding Procedure — WPS requirements
Weld Symbols — Reading weld symbols
Min Weld Size — Minimum fillet weld sizes
Welded Connections — Weld capacity calculator
Weld Joint Types — Joint preparation

Disclaimer

This is a calculation tool, not a substitute for professional engineering certification. All results must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in construction, fabrication, or permit documents. The user is responsible for the accuracy of all inputs and the verification of all outputs.