What Is Visual Testing (VT)?

Visual testing (VT) is the most basic and most frequently performed NDT method. Every weld on a piping system undergoes visual inspection—before, during, and after welding. VT detects surface discontinuities such as cracks, undercut, incomplete penetration, porosity, misalignment, spatter, and dimensional deviations using direct eyesight, aided by simple tools or remote viewing equipment. ASME Section V Article 9 and AWS D1.1 govern VT procedures. ASME B31.3 requires 100% visual examination of all piping welds.

VT Types

Type	Method	Access	Equipment
Direct VT	Naked eye or low-power magnifier	Inspector within 600 mm, angle >30 degrees to surface	Flashlight, mirror, magnifying lens (up to 10x)
Remote VT	Camera, borescope, videoscope	Internal surfaces, inaccessible areas	Borescope, videoscope, drone-mounted camera

ASME V Article 9 requires a minimum light intensity of 1,000 lux (100 ft-candles) at the examination surface. For critical welds, 1,500-2,000 lux is standard practice.

VT Inspection Stages

Stage	Timing	What to Check
Before welding	Fit-up, before tacking	Joint preparation (bevel angle, root face, root gap), alignment/hi-lo, cleanliness, material traceability
During welding	Between passes	Root pass penetration, interpass cleaning, interpass temperature, visible defects
After welding	Final cap complete	Weld profile, undercut, porosity, cracks, spatter, reinforcement height, weld width
After PWHT	Post heat treatment	Surface condition, distortion, new indications from stress relief

Common VT Defects and Criteria

Defect	Description	ASME B31.3 Limit
Undercut	Groove melted into base metal at weld toe	Max 0.8 mm depth (1/32 in.)
Reinforcement (excess weld metal)	Cap height above base metal	Max 1.5 mm for t<6 mm; 3 mm for t>13 mm
Lack of fill	Weld face below base metal surface	Not permitted
Surface porosity	Gas pockets on weld surface	Per Table 341.3.2 acceptance limits
Cracks	Any surface crack	Not permitted (any size)
Arc strikes	Stray arc marks on base metal	Must be ground and examined by MT/PT
Hi-lo (misalignment)	Offset between pipe ends at joint	Per WPS and code limits (typically <1.5 mm)
Spatter	Weld droplets on base metal	Must be removed

VT Tools

Tool	Purpose
Weld gauge (Cambridge type)	Measure reinforcement height, undercut depth, fillet weld leg/throat
Hi-lo gauge	Measure internal misalignment at butt welds
Pit gauge	Measure corrosion pit depth
Magnifying lens (5-10x)	Examine suspect indications
Flashlight/torch	Illuminate weld surfaces and root areas
Mirror (dental or telescoping)	View root side of welds without crawling
Borescope/videoscope	Internal pipe inspection, small-bore piping

VT Qualification

VT inspectors require certification per ASNT SNT-TC-1A or ISO 9712. Level II certification is standard for piping weld VT. The inspector must demonstrate near-distance visual acuity (Jaeger J1 or equivalent) annually. Color perception testing is required where color-coded indications are evaluated (e.g., liquid penetrant testing performed alongside VT).

Common Mistake

Treating VT as a formality. Visual inspection catches the majority of weld defects in piping fabrication. A qualified VT inspector at fit-up prevents misalignment and wrong materials from entering the weld. Skipping fit-up VT is the single largest source of avoidable rework in piping construction.

VT is the first hold point in any inspection and test plan and is performed on 100% of welds regardless of whether further NDT (RT, UT, MT) is specified. Results are documented in daily weld inspection reports reviewed during pipe inspections.

Read the full guide to non-destructive testing