Chapter 9: Non-Destructive Testing (NDT) in Welding QA/QC

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📘 9.1 What is NDT?

Non-Destructive Testing (NDT) is a group of inspection techniques used to evaluate the properties of a material, component, or weld without causing any damage.

✅ Used to detect:

Cracks

Porosity
Incomplete fusion
Corrosion
Internal flaws

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🔧 9.2 Why NDT is Critical in Mechanical QA/QC

Ensures weld quality meets code and safety standards

Detects hidden/internal defects not visible during Visual Inspection (VT)
Helps avoid failure in service (e.g., pressure vessel explosion)

🔁 Common in:

Boilers, pressure vessels (ASME Sec VIII)

Pipelines (API 1104)
Storage tanks (API 650)
Structural welding (AWS D1.1)

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🧪 9.3 Types of NDT Methods (Welding Applications)

Method	Purpose	Used For
Radiographic Testing (RT)	Uses X-rays/gamma rays to detect internal flaws	Porosity, lack of fusion, inclusions
Ultrasonic Testing (UT)	Uses high-frequency sound waves	Thickness, cracks, fusion
Magnetic Particle Testing (MT/MPI)	Magnetic field + iron particles	Surface & near-surface cracks (ferromagnetic only)
Dye Penetrant Testing (PT/DPI)	Dye + developer	Surface cracks in non-ferrous and ferrous metals
Visual Testing (VT)	Direct visual inspection	Surface defects, weld size, profile

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🏭 9.4 Real-World Application

🔧 Example: In a power plant pipe weld joint:

VT done → Acceptable

RT performed → Found internal porosity
Weld rejected and repaired
Cost avoided: Possible future rupture under steam pressure

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🧰 9.5 Common Equipment Used

Method	Equipment
RT	X-ray machine, film, darkroom
UT	Ultrasonic flaw detector, probes
MT	Yoke, powder, UV light
PT	Cleaner, penetrant dye, developer
VT	Torch, weld gauge, mirror

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📜 9.6 Acceptance Criteria (as per standards)

ASME Section V & VIII for RT/UT/PT/MT

API 650/1104 for tanks and pipelines
AWS D1.1 for structural welds

📌 Defects are accepted or rejected based on size, length, location, and code limitations.

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🔄 9.7 Repair & Retesting

Rejected welds must follow documented weld repair procedure

Retested by same or higher-level NDT method
All repairs are logged and traceable

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🧑‍🔧 9.8 Who Performs NDT?

Certified inspectors: ASNT Level II / ISO 9712 Level II

Report signed, traceable to inspector and project

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🗂️ 9.9 NDT Report Must Include

Method used (RT, UT, etc.)

Equipment calibration details
Weld joint number
Result: Accepted / Rejected
Inspector name, certificate no.
Film/photo references (if RT)

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🔐 9.10 Summary

NDT ensures deep-level assurance of weld integrity

Different methods detect different defects
Compliance to ASME/API/AWS codes is mandatory
Reports are essential part of final QA documentation

Quiz: Non-Destructive Testing (NDT)

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Q1. Which NDT method uses X-rays or gamma rays to detect internal flaws?
A) Ultrasonic Testing
B) Radiographic Testing
C) Magnetic Particle Testing
D) Dye Penetrant Testing
🟩 Correct Answer: B

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Q2. Dye Penetrant Testing is suitable for detecting:
A) Internal cracks
B) Surface cracks
C) Sub-surface porosity
D) Thickness variations
🟩 Correct Answer: B

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Q3. Which NDT technique is applicable only for ferromagnetic materials?
A) UT
B) RT
C) MT
D) PT
🟩 Correct Answer: C

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Q4. In Ultrasonic Testing (UT), what is primarily used to detect flaws?
A) Magnetic waves
B) High-frequency sound waves
C) Gamma rays
D) Colored dyes
🟩 Correct Answer: B

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Q5. Which standard typically governs NDT requirements for pressure vessels?
A) AWS D1.1
B) ISO 14001
C) ASME Section V & VIII
D) API 500
🟩 Correct Answer: C

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Q6. A defect found during RT must be evaluated based on:
A) Welder’s preference
B) Acceptance criteria of governing code
C) Operator experience
D) Surface finish only
🟩 Correct Answer: B

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Q7. Which of the following is not a visual inspection tool?
A) Flashlight
B) Weld gauge
C) Dye developer
D) Inspection mirror
🟩 Correct Answer: C

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Q8. Which NDT method would you use to inspect a fully welded pipeline internally without cutting?
A) MT
B) PT
C) RT
D) VT
🟩 Correct Answer: C

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Q9. Who is allowed to issue certified NDT reports?
A) Fabricator’s helper
B) Welder
C) ASNT/ISO Level II Inspector
D) Site Engineer
🟩 Correct Answer: C

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Q10. After a weld is repaired, what is the next step?
A) Restart the project
B) Retest the weld using same or higher-level NDT
C) Ignore the repair
D) Scrap the component
🟩 Correct Answer: B