Mechanical NDT Tech Engineering
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Mechanical engineering study materials, maintenance, production, piping, QA/QC, fabrication, HVAC, oil & gas, interview questions and technical updates.
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Channel Posts
Carbon steel pipes — ASTM A106 Gr. B, API 5L Gr. B/X42/X52
Low-temperature carbon steel — ASTM A333 Gr. 6
Stainless steel pipes — ASTM A312 TP304/304L, TP316/316L
Carbon steel fittings — ASTM A234 WPB
Stainless steel fittings — ASTM A403 WP304/304L, WP316/316L
Carbon steel flanges — ASTM A105
Stainless steel flanges — ASTM A182 F304/304L, F316/316L
Bolts & nuts — ASTM A193 B7 / A194 2H, ASTM A320/A194
Valves — applicable ASTM/API specifications
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| 3 | 👇 | 207 |
| 4 | SMAW — Shielded Metal Arc Welding
SMAW: THE CLASSIC FIELD WELDING PROCESS
SMAW is one of the most widely used welding processes, especially for field fabrication, maintenance, construction, and repair work.
How it works:
An electric arc forms between the consumable electrode and the workpiece.
The electrode coating produces shielding gases and slag that protect the molten weld pool.
Advantages:
• Portable equipment
• Excellent for field work
• Works on many steels
• Suitable for outdoor applications
• No external shielding gas required
Important variables:
• Electrode type
• Electrode diameter
• Welding current
• Arc length
• Travel speed
• Electrode angle
📌 Remember:
Poor electrode storage can introduce moisture and contribute to weld quality problems | 292 |
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| 7 | Piping Materials – Codes, Grades & Applications 🔧📚
Choosing the right piping material is more than just selecting a grade.
A quick reference for commonly used materials:
🔹 Carbon Steel: A106 Gr. B, A53, A234 WPB, API 5L
➡️ Process, oil & gas, steam & general piping
🔹 Stainless Steel: 304/304L, 316/316L, 321, 347
➡️ Corrosive & chemical services
🔹 Alloy Steel: P5, P9, P11, P22, P91, P92
➡️ High-temperature & high-pressure services
🔹 Cu-Ni / Copper: 90/10, 70/30
➡️ Seawater, condensers & heat exchangers
🔹 Nickel Alloys: Inconel 600/625, Monel 400
➡️ Severe corrosion & high-temperature environments
🔹 Non-Metallic: FRP, PVC/CPVC, HDPE, PP
➡️ Water treatment & selected chemical services
🔍 Before selecting a material, consider:
Pressure + Temperature + Fluid + Corrosion + Mechanical Properties + Weldability + Applicable Codes + Project Specifications
📚 Key codes: ASME B31.1 | B31.3 | B16.5 | B16.9 | 310 |
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| 10 | 𝗣-𝗡𝗼., 𝗙-𝗡𝗼. & 𝗔-𝗡𝗼. — 𝗪𝗵𝗮𝘁’𝘀 𝘁𝗵𝗲 𝗗𝗶𝗳𝗳𝗲𝗿𝗲𝗻𝗰𝗲?
In welding procedure qualification, especially when working with ASME Section IX, three important grouping systems are frequently encountered:
🔹 𝗣-𝗡𝗼. — 𝗕𝗮𝘀𝗲 𝗠𝗲𝘁𝗮𝗹 𝗚𝗿𝗼𝘂𝗽𝗶𝗻𝗴
P-Numbers are assigned to base metals with similar characteristics for welding procedure qualification.
👉 Think: “What material am I welding?”
🔹 𝗙-𝗡𝗼. — 𝗙𝗶𝗹𝗹𝗲𝗿 𝗠𝗲𝘁𝗮𝗹 𝗚𝗿𝗼𝘂𝗽𝗶𝗻𝗴
F-Numbers group filler metals/electrodes primarily according to their usability characteristics.
👉 Think: “What electrode/filler metal am I using?”
🔹 𝗔-𝗡𝗼. — 𝗗𝗲𝗽𝗼𝘀𝗶𝘁𝗲𝗱 𝗪𝗲𝗹𝗱 𝗠𝗲𝘁𝗮𝗹 𝗖𝗵𝗲𝗺𝗶𝘀𝘁𝗿𝘆
A-Numbers classify the deposited weld metal based primarily on its chemical composition.
👉 Think: “What weld metal chemistry was deposited?”
𝗦𝗶𝗺𝗽𝗹𝗲 𝗪𝗮𝘆 𝘁𝗼 𝗥𝗲𝗺𝗲𝗺𝗯𝗲𝗿:
P-No. → Base Metal
F-No. → Filler Metal
A-No. → Deposited Weld Metal
⚠️ 𝗜𝗺𝗽𝗼𝗿𝘁𝗮𝗻𝘁:
P-No., F-No., and A-No. are not interchangeable. Each serves a different purpose in welding procedure qualification, and the applicable essential variables must always | 391 |
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| 13 | AWS & ASME Welding Procedures.pdf | 263 |
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| 15 | Heat Exchangers Working Principle and It's Types ?
Heat Exchangers: Working Principle and Types
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🔧 Working Principle of Heat Exchangers
A heat exchanger is a device that transfers heat between two or more fluids (liquid or gas) without mixing them. The main principle behind heat exchangers is thermal conduction, where heat flows from a hot fluid to a cooler one through a solid barrier (usually metal) or directly when fluids are in contact.
🔁 Key Concepts:
No direct mixing: Fluids are usually separated by a solid wall or flow in separate channels.
Heat Transfer Direction: Heat always moves from the hotter to the colder fluid.
Types of Flow:
Parallel Flow – both fluids move in the same direction.
Counter Flow – fluids move in opposite directions (more efficient).
Cross Flow – fluids move at right angles to each other.
---
🧰 Types of Heat Exchangers
1. Shell and Tube Heat Exchanger
Structure: Consists of a series of tubes inside a cylindrical shell.
Use: Common in oil refineries and power plants.
Advantage: Handles high pressure and temperature.
2. Plate Heat Exchanger
Structure: Thin corrugated plates stacked together, fluids flow between alternate plates.
Use: HVAC systems, dairy and food processing.
Advantage: High efficiency, compact design.
3. Finned Tube Heat Exchanger
Structure: Tubes with external fins to increase surface area.
Use: Air conditioning, car radiators.
Advantage: Improved heat transfer to air.
4. Double Pipe Heat Exchanger
Structure: One pipe inside another; fluids flow in inner and outer pipes.
Use: Small industries and labs.
Advantage: Simple design, low cost.
5. Air Cooled Heat Exchanger
Structure: Uses ambient air to cool process fluids.
Use: Oil refineries, chemical plants.
Advantage: No water required for cooling.
6. Regenerative Heat Exchanger
Structure: Uses a temporary heat storage medium (like a ceramic matrix) that gets alternately heated and cooled.
Use: Gas turbines, air preheaters.
Advantage: Reuses heat efficiently.
7. Condensers and Evaporators (in HVAC)
Condensers: Remove heat from refrigerant gas, turning it into liquid.
Evaporators: Absorb heat into the refrigerant, turning liquid into vapor.
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📌 Summary
Heat exchangers are critical in power generation, chemical processing, automotive, and HVAC systems. Choosing the right type depends on:
* Fluid type
* Operating pressure & temperature
* Heat transfer efficiency needed
* Space and cost constraints | 297 |
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| 18 | 7 WELDING DEFECTS EVERY WELDING INSPECTOR MUST KNOW 🔍🔥
Welding defects can reduce the strength, reliability, and service life of a welded joint. A professional Welding Inspector must be able to identify defects, understand their causes, and verify whether the weld meets the applicable acceptance criteria.
Here are 7 important welding defects every inspector should know:
1️⃣ POROSITY
Porosity is the presence of gas cavities or holes trapped inside or on the surface of the weld metal.
Common Causes:
• Moisture or contamination
• Poor shielding gas coverage
• Incorrect gas flow
• Dirty base material
• Excessive arc length
Prevention:
Keep the joint clean, control shielding gas, and use properly stored welding consumables.
---
2️⃣ UNDERCUT
Undercut is a groove melted into the base metal alongside the weld toe or weld edge that is not properly filled with weld metal.
Common Causes:
• Excessive welding current
• Excessive travel speed
• Incorrect electrode angle
• Excessive arc length
Inspection:
Visual inspection is commonly used to detect undercut. The allowable size depends on the applicable code or specification.
---
3️⃣ LACK OF FUSION
Lack of fusion occurs when the weld metal does not properly fuse with the base metal or with a previous weld pass.
Common Causes:
• Low heat input
• Incorrect electrode angle
• High travel speed
• Poor joint preparation
• Improper cleaning between passes
Why It Matters:
It can create a serious discontinuity and may significantly reduce the effective strength of the joint.
---
4️⃣ LACK OF PENETRATION
Lack of penetration occurs when the weld metal does not extend completely through the required joint thickness at the root.
Common Causes:
• Incorrect root gap
• Excessive root face
• Low welding current
• Incorrect electrode size
• Poor welding technique
Detection:
Depending on the joint and requirements, NDT methods such as RT or UT may be used.
---
5️⃣ SLAG INCLUSION
Slag inclusion occurs when non-metallic slag becomes trapped inside the weld metal.
Common Causes:
• Inadequate cleaning between passes
• Incorrect electrode angle
• Low welding current
• Poor manipulation technique
• Improper joint geometry
Prevention:
Remove slag completely between passes and maintain correct welding parameters.
---
6️⃣ CRACKS ⚠️
Cracks are one of the most serious welding discontinuities.
They may occur as:
• Hot cracks
• Cold cracks
• Crater cracks
• Longitudinal cracks
• Transverse cracks
• Hydrogen-assisted cracks
Possible Causes:
• Hydrogen contamination
• High residual stress
• Improper preheating
• Unsuitable welding procedure
• Poor termination technique
Important:
Crack acceptance is highly code/specification dependent, and many fabrication codes do not permit cracks.
---
7️⃣ OVERLAP
Overlap occurs when weld metal flows onto the base metal surface without proper fusion.
Common Causes:
• Excessive welding current
• Low travel speed
• Excessive weld metal
• Incorrect electrode angle
• Poor welding technique
Overlap can create a stress concentration and may require repair depending on the applicable acceptance criteria.
---
🔎 INSPECTOR'S KEY CHECKPOINTS
A Welding Inspector should evaluate:
✅ Joint preparation
✅ Welding parameters
✅ Preheat & interpass temperature
✅ Electrode/consumable condition
✅ Welder qualification
✅ WPS compliance
✅ Weld profile
✅ Surface condition
✅ Applicable acceptance criteria
✅ Required NDT
📌 IMPORTANT REMINDER
A welding discontinuity is not automatically a rejectable defect.
Whether a discontinuity is acceptable depends on the applicable welding code, project specification, drawing, and acceptance criteria.
Examples include AWS, ASME, API, ISO and project-specific requirements.
🧠 QUICK MEMORY
POROSITY → Gas
UNDERCUT → Groove at weld toe
LACK OF FUSION → No proper bonding
LACK OF PENETRATION → Root not fully penetrated
SLAG INCLUSION → Slag trapped
CRACK → Fracture-type discontinuity
OVERLAP → Weld metal flows over without fusion | 255 |
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