Interview Question & Answer Set 4

Piping Materials Interview Questions & Answers (Set 4) – ASTM Codes, Grades, Selection & Real-World Tips

If you’re preparing for a piping engineer / QA-QC / stress / design interview, “materials” is one topic that almost always appears. Interviewers don’t just test memory—they check whether you understand why a grade is used, what changes between grades, and what mistakes can cause wrong procurement or failures.

This is Interview Question & Answer Set 4 focused on ASTM material codes for pipes/tubes/fittings, carbon vs alloy vs stainless vs nickel, grade differences (A106, A312, etc.), and practical selection logic. Explanations are written in easy language with real plant examples.

1) What are the common ASTM codes used for Pipes (Carbon, Alloy, Stainless, Nickel)?

Answer (with explanation):

• Carbon Steel Pipes: ASTM A53 (general), ASTM A106 (high-temperature service), ASTM A333 (low-temperature service)
• Alloy Steel Pipes: ASTM A335 (high-temperature alloy steel seamless pipes like P11, P22, P91 etc.)
• Stainless Steel Pipes: ASTM A312 (TP304/304L/316/316L/321 etc.)
• Nickel Alloy / Low-Temp / Special: In many project notes you may see ASTM A333 for low-temp carbon steel and nickel alloy families may come under different ASTM/ASME specs depending on alloy type (Inconel, Monel etc.).

Interview tip: Don’t only say the code. Add one line about where it is used:

• A106 → common for steam/hot services.
• A333 → common for cold services (chilled water, cryogenic, low ambient).
• A335 → hot high-pressure lines in power/refinery.
• A312 → stainless process lines (corrosion resistance).

2) What are the common ASTM codes used for Tubes (Carbon, Alloy, Stainless, Nickel)?

Answer (with explanation): Tubes are often used in heat exchangers, boilers, condensers etc.

• Carbon Steel Tubes: ASTM A178, ASTM A179, ASTM A192, ASTM A334 (low-temp)
• Alloy Steel Tubes: ASTM A213 (T11/T22/T91 etc.), ASTM A161 (older references may appear in notes)
• Stainless Steel Tubes: ASTM A213 (stainless tube grades), and stainless families in A213/A269 depending on application
• Nickel Alloy Tubes: Depends on alloy family and service; often special specs and vendor requirements apply

Practical point: In interviews, explain “pipe vs tube”: pipes are usually specified by NPS and schedule, while tubes are often specified by OD and wall thickness and are common in exchangers.

3) What are ASTM codes for Wrought Butt-Weld Fittings (Carbon/Alloy/Stainless)?

Answer:

• Carbon Steel BW Fittings: ASTM A234 (WPB/WPC), ASTM A420 (WPL6 for low temperature)
• Alloy Steel BW Fittings: ASTM A234 (WP1/WP5/WP9/WP11/WP22 etc.)
• Stainless Steel BW Fittings: ASTM A403 (WP304/WP304L/WP316/WP316L/WP321 etc.)

Simple explanation: “Wrought fittings” are made from wrought material (rolled/forged form) and then formed into elbows, tees, reducers. Most plant BW fittings are in A234 (CS) and A403 (SS).

4) What are ASTM codes for Forged Fittings (socket weld / threaded) and Flanges?

Answer:

• Carbon Steel Forged Fittings: ASTM A105 (very common), ASTM A181 (less common), ASTM A350 LF2 (low-temp)
• Alloy Steel Forged: ASTM A182 (F11/F22/F91 etc.)
• Stainless Steel Forged: ASTM A182 (F304/F304L/F316/F316L/F321 etc.)

Interview trap: Many candidates say “A105 for everything.” Correct statement is:

• A105 = carbon steel forged components (flanges, forged fittings) for normal temperature.
• A350 LF2 = low-temperature carbon steel forged components.
• A182 = stainless/alloy forged components.

5) What are ASTM codes for Cast Fittings / Valves / Cast Components?

Answer:

• Carbon Steel Castings: ASTM A216 (WCB/WCC), ASTM A352 (LCB/LCC for low-temp)
• Stainless Steel Castings: ASTM A351 (CF8/CF8M/CF3/CF3M etc.)

Simple explanation: If you see WCB it is cast carbon steel (A216). If you see CF8M it is cast stainless (A351) similar to 316 family.

6) What is the basic difference between ASTM A106 Gr A / Gr B / Gr C?

Answer: The difference is mainly in mechanical properties (strength) and chemical limits. Many interview notes mention carbon percentage trends, but in real practice, the biggest field difference is:

• Gr B is the most commonly used grade in plants.
• Gr C generally offers higher strength than Gr B.

Easy explanation: Think of A106 as “high-temp seamless CS pipe.” Within it, Grade changes strength level. Most projects standardize on A106 Gr.B because it is widely available, economical, and accepted by clients/vendors.

7) What is the difference between ASTM A312 TP304 vs TP304L, and TP316 vs TP316L?

Answer: The letter “L” means Low Carbon.

• 304L / 316L have lower carbon compared to 304 / 316.
• Lower carbon reduces risk of sensitization and intergranular corrosion after welding.

Real-world example: If a stainless line has lot of field welding (fabrication heavy), engineers prefer 304L/316L because weld zones remain safer from corrosion issues.

8) Why do we use ASTM A333 Gr.6 in some projects instead of A106?

Answer: ASTM A333 is designed for low-temperature service.

• If minimum design metal temperature (MDMT) is low, impact toughness becomes critical.
• A333 grades are tested to ensure toughness at low temperatures (as per spec requirements).

Simple example: Chilled water, cold ambient areas, outdoor winter conditions, cryogenic lines, or any case where brittle fracture risk increases.

9) Explain “WPL6” and where it is used.

Answer: WPL6 comes under ASTM A420 (butt-weld fittings for low-temperature service).

• Used for elbows, tees, reducers in low temp services.
• Pairs commonly with low-temp pipes like A333.

Interview line: “WPL6 is low-temperature BW fitting material; selection is linked to MDMT and impact requirement.”

10) What is the difference between A234 WPB and A105?

Answer: Both are carbon steel, but used for different product forms.

• A234 WPB → butt-weld fittings (elbows, tees, reducers) typically for piping.
• A105 → forged flanges and forged fittings (socket-weld / threaded).

Easy rule: BW fittings → A234. Forged flange/SW fittings → A105.

11) What is the meaning of “WP” in ASTM A234/A403?

Answer: “WP” generally indicates Wrought Pipe fittings grades. For example:

• A234 WPB → wrought carbon steel BW fittings grade B
• A403 WP316L → wrought stainless BW fittings 316L

Why interviewer asks this: They want to ensure you understand it’s not random letters—these relate to fitting material form and grade family.

12) How do you decide between Stainless Steel 304/316 in a project?

Answer: Mainly based on corrosion environment.

• 304/304L works well for general corrosion resistance.
• 316/316L has molybdenum (Mo) which improves resistance in chlorides and more aggressive environments.

Real plant example: Cooling water, seawater splash, coastal areas, chemical service—engineers prefer 316L due to better chloride resistance and pitting resistance.

13) What is “NACE” relevance in material selection?

Answer: NACE requirements apply mainly for sour service (H₂S environments) to prevent sulfide stress cracking (SSC) and hydrogen-induced cracking (HIC).

• Material hardness limits may apply.
• Heat treatment / chemistry control may be required.

Interview point: “If sour service is present, code compliance alone is not enough—NACE compliance becomes an additional requirement.”

14) What is “killed steel” and why is it specified?

Answer: Killed steel is fully deoxidized steel (often using aluminum/silicon).

• More uniform properties.
• Better for pressure piping and welding.

Why it matters: Some specs require killed steel for reliability in pressure service and to reduce quality issues like porosity.

15) What is “impact test” and when is it required?

Answer: Impact test (Charpy V-notch) checks material toughness, especially at low temperatures.

• Required when MDMT is low.
• Important for preventing brittle fracture.

Simple way to say: “Low temperature + thick section + high stress = brittle fracture risk → impact test becomes important.”

16) What are common plate material specs used for fabrication (CS/SS)?

Answer:

• Carbon Steel Plate: ASTM A516 (pressure vessel quality), ASTM A36 (structural/general)
• Stainless Plate: ASTM A240 (SS plate/sheet)

Interview tip: Mention A516 is popular for pressure-retaining items; A36 is more structural; A240 for stainless plate.

17) What is PMI and why is it important?

Answer: PMI (Positive Material Identification) verifies alloy chemistry using portable instruments.

• Prevents wrong material mixing (e.g., 304 used in place of 316).
• Critical in alloy and stainless systems, especially after site handling.

Real-world failure example: Wrong alloy in chloride service can cause pitting leaks quickly; PMI prevents such costly mistakes.

18) What is “MTC / Mill Test Certificate” and what should you check in it?

Answer: MTC is manufacturer’s certificate showing chemical composition, mechanical properties, heat number, tests, and compliance.

Key checks:

• Material grade and specification (A106 Gr.B etc.)
• Heat number traceability
• Chemistry limits (C, Mn, Cr, Ni, Mo as applicable)
• Mechanical properties (YS/UTS/Elongation)
• Impact test results if required

19) Why are “seamless” pipes preferred in some services?

Answer: Seamless pipes have no weld seam from manufacturing, so they are often preferred for:

• High pressure/high temperature services
• Critical process lines where quality margin is needed

But interview smart point: “ERW pipes are also widely used and acceptable in many services if spec allows; selection depends on design and project standards.”

20) What is a simple “material selection checklist” you can explain in an interview?

Answer: You can confidently say:

• Step 1: Identify fluid, operating T & P, corrosion environment.
• Step 2: Check code + client spec + company spec.
• Step 3: Check low-temp toughness (MDMT) → choose A333/WPL6/LF2 if needed.
• Step 4: Check corrosion → choose 304L/316L/duplex/nickel alloys as needed.
• Step 5: Confirm availability, cost, welding requirements, NACE requirement.
• Step 6: Ensure documentation (MTC, PMI, traceability).

This answer looks “experienced” because it shows a real engineer’s thinking, not just memorized codes.

Quick Reference Table (Easy to Remember)

Item	Carbon Steel	Low Temp	Alloy Steel	Stainless Steel
Pipes	A106 / A53	A333	A335	A312
BW Fittings	A234 WPB	A420 WPL6	A234 WP11/WP22	A403 WP304L/WP316L
Forged (Flange/SW)	A105	A350 LF2	A182 F11/F22	A182 F304L/F316L
Castings	A216 WCB	A352 LCB/LCC	(project-dependent)	A351 CF8/CF8M

Common Interview Mistakes (Avoid These)

• Saying “A105 is pipe material” → Wrong (A105 is forged fittings/flanges, not pipe spec).
• Mixing BW fittings and forged fittings specs.
• Ignoring low-temperature requirement (MDMT) and impact test needs.
• Not mentioning weldability and “L” grades in stainless.
• Forgetting traceability (Heat no., MTC, PMI).

Bonus: Mini Practice Questions (Rapid Fire)

• Q: A105 is used for? A: Forged flanges and forged fittings (SW/THD).
• Q: A403 is used for? A: Stainless BW fittings.
• Q: WCB indicates? A: Cast carbon steel (A216).
• Q: Why 316L in cooling water? A: Better chloride resistance and safer welding.

Conclusion

Materials knowledge is not about memorizing hundreds of ASTM numbers. Interviewers want to see that you can connect service conditions → code/spec → correct material family → documentation. If you can explain A106 vs A333, A234 vs A105, and why 316L is chosen over 304 in chlorides—your materials round will become very strong.