Piping Materials Interview Questions and Answers (2026): ASTM Grades, Steels and Selection

Q1What is a pipe material, and what does it control?

Short answer: The pipe material is the metal or non-metal from which the pipe is made. It sets the pipe’s strength, toughness, corrosion resistance, temperature range, weldability and cost — and therefore which services, pressures and temperatures the line can safely handle.

On a project the “material” is never just a family name such as carbon steel. It is a full designation — specification, grade, class and product form, for example ASTM A106 Gr B seamless or ASTM A312 TP316L welded. That designation fixes:

• Allowable stress at temperature (from ASME B31.3 Table A-1 or ASME BPVC Section II Part D)
• Minimum design metal temperature and any impact-test requirement
• Resistance to general corrosion, pitting, cracking and erosion
• Welding procedure group (ASME Section IX P-Number), preheat and PWHT
• Inspection and certification requirements
• Total installed and life-cycle cost

Q2Why is material selection so important in piping engineering?

Short answer: Because a wrong material can fail in service even when the wall thickness is correct. Material selection keeps the pressure boundary intact for the full design life under the actual fluid, temperature and fabrication conditions.

Most piping failures traced to material choice are not overstress — they are corrosion, cracking or brittle fracture that the designer did not anticipate. Selection therefore weighs:

• Design pressure, design temperature and minimum design temperature, including start-up, shutdown and upset cases
• Fluid chemistry: water, chlorides, H₂S, CO₂, oxygen, hydrogen, acids, caustic, solids
• Credible damage mechanisms (see API RP 571)
• Fabrication: weldability, PWHT, forming, availability of fittings and flanges
• Inspection access, design life and life-cycle cost

Example: cooling water, seawater, superheated steam, wet sour gas and LNG can all run at moderate pressure, yet each needs a completely different material.

Q3What are the most common pipe materials used in industry?

Short answer: Carbon steel, low-temperature carbon steel, Cr-Mo alloy steel, austenitic stainless steel, duplex and super duplex, nickel alloys, copper-nickel and titanium among metals; and PVC, CPVC, HDPE, PP, PVDF and GRP/GRE among non-metallics.

Carbon steel covers the majority of process and utility piping by length on most oil, gas, power and fertilizer plants. Alloys are used where carbon steel cannot meet temperature, toughness or corrosion requirements, and non-metallics where corrosion resistance and low weight matter more than temperature capability. Cast iron and ductile iron (ISO 2531, AWWA C151) remain common for buried water and drainage.

Q4What is carbon steel?

Short answer: Carbon steel is iron alloyed mainly with carbon (up to about 0.30 % for common pipe grades) plus manganese and silicon. It is the default piping material because it is strong, weldable, widely available and inexpensive.

Pipe grades keep carbon low so that they weld without preheat in most thicknesses. Manganese adds strength and toughness; silicon is added for deoxidation in killed steels such as A106.

Advantages: low cost, good strength, easy welding (ASME Section IX P-No. 1), a huge range of sizes, fittings and flanges, and decades of service experience.

Limitations: general corrosion in water, wet CO₂ and oxygenated services; brittle fracture below its minimum temperature; graphitization and creep above about 425 °C; and susceptibility to wet H₂S cracking unless specified for sour service.

Q5Where is carbon steel piping commonly used?

Short answer: In dry or inhibited hydrocarbons, steam and condensate, boiler feed water, compressed air, nitrogen, lube and seal oil, firewater (lined or galvanised where specified) and general utilities across refineries, petrochemical, fertilizer, cement and power plants.

It is usually not the first choice for seawater, demineralised water, hot chloride service, strong acids, cryogenic temperatures or high-temperature hydrogen without an engineering evaluation.

Q6What is low-temperature carbon steel (LTCS)?

Short answer: LTCS is fine-grained, killed carbon steel that is Charpy impact-tested at a low temperature so it stays tough instead of fracturing in a brittle way. The common pipe grade is ASTM A333 Gr 6, impact-tested at −45 °C (−50 °F).

Ordinary carbon steel loses toughness as temperature falls through its ductile-to-brittle transition. Under ASME B31.3 (para. 323.2.2 and Figure 323.2.2A) many unlisted-toughness carbon steels need an impact test or a thickness/stress-ratio exemption below about −29 °C (−20 °F). LTCS is specified for:

• Cold hydrocarbon and refrigerant service
• Flare and blowdown headers that chill during depressurisation (auto-refrigeration)
• LPG and propane systems
• Outdoor piping in very cold climates

The matching LTCS components are A420 WPL6 butt-weld fittings, A350 LF2 Cl.1 forgings and flanges, and A320 L7 bolting.

Q7What are the ASTM A333 grades and their impact test temperatures?

Short answer: ASTM A333 covers seamless and welded steel pipe for low-temperature service. Grades 1 and 6 are carbon steel tested at −45 °C, Grade 3 (3.5 % Ni) at −100 °C, Grade 7 (2.5 % Ni) at −73 °C and Grade 8 (9 % Ni) at −195 °C.

Q8What is alloy steel?

Short answer: Alloy steel is steel with deliberate additions of chromium, molybdenum, nickel, vanadium or other elements to improve high-temperature strength, creep resistance, oxidation resistance, hardenability or toughness beyond what carbon steel can offer.

In piping, “alloy steel” usually means low-alloy ferritic steel with up to about 9–10 % chromium, such as the ASTM A335 Cr-Mo grades. Higher-chromium alloys (≥ 10.5 % Cr) become stainless steels. Low-alloy steels are the backbone of power-plant main steam and hot reheat lines, refinery heaters, hydrotreaters, reformers and boilers.

Q9What is chromium-molybdenum (Cr-Mo) steel and why is it used?

Short answer: Cr-Mo steel is a low-alloy steel in which chromium improves oxidation, sulfidation and hydrogen resistance, and molybdenum improves creep strength. It is used for high-temperature steam, hot hydrocarbon and hydrogen services where carbon steel would creep, graphitize or suffer high-temperature hydrogen attack.

Resistance to high-temperature hydrogen attack (HTHA) is assessed with the Nelson curves in API RP 941. Cr-Mo welding needs tighter control than carbon steel:

• Qualified procedures with matching consumables
• Preheat and maximum interpass temperature
• Hydrogen control (low-hydrogen electrodes, dry storage)
• Post-weld heat treatment (PWHT) where required by ASME B31.3 Table 331.1.1 or B31.1
• Hardness testing after PWHT and PMI of every component

Q10What are the common ASTM A335 grades?

Short answer: ASTM A335 covers seamless ferritic alloy-steel pipe for high-temperature service. The grades you must know are P11 (1¼Cr-½Mo), P22 (2¼Cr-1Mo), P5 (5Cr-½Mo), P9 (9Cr-1Mo) and P91 (9Cr-1Mo-V), a creep-strength-enhanced ferritic (CSEF) steel.

Q11What is the difference between ASTM A106 Grade A, B and C?

Short answer: All three are seamless carbon steel pipe for high-temperature service. They differ mainly in maximum carbon (0.25 %, 0.30 % and 0.35 %) and therefore strength: minimum yield is 205, 240 and 275 MPa respectively. Grade B is by far the most used.

Q12What is the difference between ASTM A53, ASTM A106 and API 5L pipe?

Short answer: A106 is always seamless and intended for high-temperature service; A53 may be seamless (Type S), electric-resistance welded (Type E) or furnace butt-welded (Type F) and is used mainly for utilities and structural purposes; API 5L is line pipe for pipelines, specified as PSL1 or the stricter PSL2.

Many mills supply pipe triple-certified to A106 Gr B / A53 Gr B / API 5L Gr B, which is why the three are often confused. In process plants, A106 Gr B is the normal choice; API 5L grades (B, X42, X52, X60, X65, X70) are used for cross-country and plant pipelines designed to ASME B31.4 or B31.8. Type F furnace butt-welded A53 pipe is not permitted for flammable or toxic process fluids under most project specifications.

Q13Up to what temperature can carbon steel pipe be used?

Short answer: For long-term service, carbon steel is normally limited to about 425 °C (800 °F). Above that, graphitization and creep reduce its strength, so code tables carry cautionary notes and most project specifications switch to Cr-Mo alloy steel.

ASME B31.3 Table A-1 lists allowable stresses for carbon steel above 425 °C, but with a note that prolonged exposure above about 427 °C can convert carbides to graphite and seriously weaken the steel, especially in heat-affected zones. The practical answer an interviewer expects is “about 425 °C for continuous service, with alloy steel above that.”

At the cold end, the limit is set by toughness, not strength: check the minimum design metal temperature against ASME B31.3 Figure 323.2.2A, wall thickness and stress ratio before using carbon steel below −29 °C (−20 °F).

Q14Which materials are used above about 425 °C?

Short answer: Cr-Mo low-alloy steels (A335 P11, P22, P5, P9, P91), austenitic stainless steels — preferably the H grades or stabilised 321/347 — and, for the most severe temperatures, nickel alloys such as Alloy 800H or 625.

The choice depends on temperature, pressure, design life (creep), oxidation or sulfidation, and hydrogen partial pressure. As a rough guide:

• 425–550 °C: P11 or P22
• 550–600 °C: P22, P91 or 304H/321H/347H
• 600–650 °C: P91, P92 or austenitic H grades
• above 650 °C: austenitic H grades and nickel alloys

A material that is strong at room temperature can fail by creep at high temperature, so time-dependent allowable stresses — not tensile strength — govern design in this range.

Q15What are the main families of stainless steel?

Short answer: Austenitic, ferritic, martensitic, duplex (austenitic-ferritic) and precipitation-hardening. All contain at least about 10.5 % chromium, which forms the protective passive film; they differ in microstructure, strength, toughness, weldability and corrosion resistance.

Q16What is austenitic stainless steel?

Short answer: Austenitic stainless steel contains roughly 16–20 % chromium and 8–14 % nickel, giving a face-centred cubic microstructure that is non-magnetic, very ductile, easy to weld and tough down to cryogenic temperatures. 304L and 316L are the common piping grades, supplied to ASTM A312.

Austenitics are the default choice for cryogenic service, many chemicals and high-purity systems. Their weak points are chloride stress corrosion cracking and pitting/crevice corrosion in warm chloride waters, plus a high thermal expansion (about 1.5 times carbon steel) that the stress engineer must accommodate.

Q17What is the difference between 304L and 316L stainless steel?

Short answer: Both are low-carbon austenitic grades, but 316L contains 2–3 % molybdenum. Molybdenum improves resistance to pitting and crevice corrosion in chloride-bearing water, so 316L is preferred for mildly chloride-contaminated services — at a higher cost.

Q18What does the “L” mean in 304L or 316L?

Short answer: “L” means low carbon. Under ASTM A312, TP304L and TP316L are limited to 0.035 % carbon against 0.08 % for TP304 and TP316. Less carbon means less chromium carbide forms at grain boundaries during welding, which protects the weld zone from intergranular corrosion (sensitization).

When austenitic stainless steel is held between roughly 425 and 815 °C (800–1500 °F) — as happens in a weld heat-affected zone — carbon combines with chromium to form carbides on grain boundaries. The adjacent metal is depleted of chromium and can corrode along the grain boundaries. Low carbon slows this reaction enough that normal welding does not sensitize the steel. Many mills supply dual-certified 304/304L and 316/316L pipe that meets the low-carbon limit and the higher strength of the straight grade.

Q19What are “H” grades and stabilised grades such as 321 and 347?

Short answer: H grades (304H, 316H, 321H, 347H) have a controlled minimum carbon of 0.04 % to give creep strength at high temperature. Stabilised grades add titanium (321) or niobium (347), which tie up carbon so chromium carbides cannot form — giving sensitization resistance during long high-temperature service.

Use L grades for welded, corrosion-critical piping operating below about 425 °C; use H or stabilised grades for heater outlets, hydroprocessing and high-temperature lines. In refinery hydroprocessing units, 321 and 347 also reduce the risk of polythionic acid SCC during shutdowns (see NACE SP0170).

Q20What is ferritic stainless steel?

Short answer: Ferritic stainless steel is a chromium stainless steel (typically 11–30 % Cr) with little or no nickel and a body-centred cubic structure. It is magnetic, cheaper than austenitic grades and resistant to chloride SCC, but has lower toughness and weldability.

Grades such as 409, 430 and 444 are used for exhaust systems, heat-exchanger tubes and some water services. In process piping they are uncommon because of low impact toughness, grain growth when welded, and 475 °C (885 °F) embrittlement after long exposure to intermediate temperatures.

Q21What is martensitic stainless steel?

Short answer: Martensitic stainless steel (for example 410, 420 and 13Cr) contains about 11.5–18 % chromium with enough carbon to be hardened by quenching and tempering. It offers high strength and hardness but only moderate corrosion resistance and needs careful welding.

In piping systems you meet martensitic grades mostly in valve trim, pump shafts and seats, and in 13Cr / super-13Cr tubing for CO₂-containing wells. They are rarely used for process pipe because they need preheat and PWHT and must meet hardness limits in sour service (NACE MR0175/ISO 15156-3).

Q22What is duplex stainless steel?

Short answer: Duplex stainless steel has a roughly 50/50 mix of austenite and ferrite. The standard grade, 2205 (UNS S32205/S31803, about 22 % Cr, 5 % Ni, 3 % Mo, 0.17 % N), has about twice the yield strength of 316L and much better resistance to chloride pitting and stress corrosion cracking.

Pipe is supplied to ASTM A790 (seamless and welded) or A928 (welded with filler); fittings to A815; forgings to A182 F51/F60. Typical services include produced water, offshore process piping, desalination brine, cooling water and chemical plants. Its higher strength often allows a thinner wall, offsetting part of the material cost.

Q23What is super duplex stainless steel, and what is PREN?

Short answer: Super duplex (for example UNS S32750 and S32760, about 25 % Cr, 7 % Ni, 3.5–4 % Mo, 0.25 % N) is a more highly alloyed duplex with PREN of 40 or more. PREN — Pitting Resistance Equivalent Number = %Cr + 3.3 × %Mo + 16 × %N — ranks resistance to chloride pitting.

Super duplex is a standard choice for seawater, firewater and produced-water piping, usually within a maximum temperature set by the project (for example about 20–30 °C for crevice-free seawater service with chlorination, higher for unchlorinated systems — check the materials philosophy). It requires even stricter welding control than 2205.

Q24When are nickel alloys used for piping?

Short answer: When stainless steels and duplex cannot handle the corrosion or temperature: strong acids, wet chlorine, highly sour high-pressure gas, hot caustic and very high temperature. Common grades are Alloy 625, 825, C-276, Alloy 400 (Monel) and Alloy 800H.

• Alloy 825 (ASTM B423, UNS N08825): sulfuric and phosphoric acid, sour service
• Alloy 625 (ASTM B444, UNS N06625): seawater, severe sour gas, high-temperature strength; also used as weld overlay and cladding
• Alloy C-276 (ASTM B622, UNS N10276): wet chlorine, hypochlorite, mixed acids
• Alloy 400 (ASTM B165, UNS N04400): hydrofluoric acid, seawater
• Alloy 800H/HT (ASTM B407): reformer outlets and high-temperature furnace piping

Because nickel alloys cost many times more than carbon steel, engineers often use clad or weld-overlaid carbon steel instead of solid alloy pipe for large diameters.

Q25Where are titanium and copper-nickel used?

Short answer: Both are seawater materials. Titanium Grade 2 (ASTM B861/B862) is practically immune to seawater corrosion and is used in desalination, chlorination and offshore systems. 90/10 copper-nickel (UNS C70600) resists seawater and biofouling but is limited by flow velocity, typically to about 3–3.5 m/s.

Titanium needs clean, inert-gas-shielded welding to avoid embrittlement, and galvanic isolation from carbon steel. Copper-nickel must not be used in sulfide-polluted water or ammonia service, and needs careful flushing and commissioning to form its protective film.

Q26What is an ASTM material specification?

Short answer: An ASTM specification is a standard that defines a product form and its grades: chemical composition, mechanical properties, manufacturing method, heat treatment, tests, dimensions, tolerances, marking and certification. Examples are ASTM A106 for seamless carbon steel pipe and A312 for stainless pipe.

ASTM specifications are organised by product form: pipe (A53, A106, A312, A333, A335, A790), butt-weld fittings (A234, A403, A420, A815), forgings and flanges (A105, A182, A350), plate (A516, A240) and bolting (A193, A194, A320). Writing “carbon steel pipe” on a purchase order is not enough — the full specification, grade, class and supplementary requirements must be stated.

Q27What is the difference between ASME and ASTM material specifications, and what does “SA” mean?

Short answer: ASTM writes the material specifications; ASME adopts many of them into BPVC Section II with an “SA” prefix (or “SB” for non-ferrous). SA-106 is ASME’s version of ASTM A106, sometimes with edition differences or additional requirements. Material for ASME code construction must meet the SA/SB specification and the code’s acceptance rules.

Section II is split into Part A (ferrous), Part B (non-ferrous), Part C (welding consumables) and Part D (properties, including allowable stresses). ASME B31.3 accepts listed ASTM materials directly, while boilers and pressure vessels require SA/SB material — often satisfied by mill certificates that state dual compliance, such as “ASTM A106/ASME SA-106 Gr B”. Never assume two editions are identical without checking.

Q28What is a pipe grade?

Short answer: A grade identifies one material within a specification, each with its own chemistry and minimum mechanical properties. For example, ASTM A106 contains Grades A, B and C, and ASTM A312 contains TP304, TP304L, TP316L and many more.

A complete material call-out therefore looks like ASTM A106 Gr B, seamless, NPS 6, Sch 40, ASME B36.10M — specification, grade, manufacturing type, size, schedule and dimensional standard. The project piping class adds supplementary requirements such as maximum carbon equivalent, hardness limits or impact testing.

Q29Which fittings, flanges and bolts match each pipe material?

Short answer: Each pipe specification has matching component specifications of the same chemistry: for A106 Gr B pipe use A234 WPB fittings, A105 flanges and A193 B7/A194 2H bolting; for A312 TP316L use A403 WP316L fittings and A182 F316L flanges; for A333 Gr 6 use A420 WPL6 and A350 LF2.

Q30What is a Material Test Certificate (MTC), and what are EN 10204 3.1 and 3.2?

Short answer: An MTC (also called a mill test report) is the manufacturer’s document proving the product meets its specification. EN 10204 defines its level: a type 3.1 certificate is validated by the manufacturer’s independent inspection department; a type 3.2 is additionally validated by the purchaser’s representative or a third-party inspector.

A compliant MTC typically shows the specification and grade, heat (and lot) number, size and quantity, chemical analysis, tensile and yield results, impact results where required, hardness, heat-treatment condition, hydrostatic or NDE test results, and a statement of compliance. Check every value against the specification during receiving inspection — not just that the certificate exists.

Q31What is a heat number?

Short answer: A heat number is the unique identifier of one batch (heat) of steel melted together. It is marked on the pipe and printed on the MTC, linking the physical product to its chemistry and test results.

When pipe is cut during fabrication, the heat number (or a project code that maps to it) must be transferred to each cut piece before cutting, using methods allowed by the project — low-stress stamping, paint marker or tags. Chloride- and sulfur-free markers are required on stainless steel and nickel alloys.

Q32What is material traceability?

Short answer: Material traceability is the ability to link every installed pipe, fitting and flange back to its heat number, MTC and specification — through receiving, storage, cutting, fabrication and installation. It proves that the right material is in the right place.

Traceability prevents material mix-ups, unsafe substitutions, loss of certification and disputes at handover. On alloy and critical-service systems, projects usually require 100 % traceability recorded on weld maps or isometrics; for general carbon steel utilities, traceability may be by lot or material type only — the project quality plan decides.

Q33What is Positive Material Identification (PMI)?

Short answer: PMI is a non-destructive check that measures the alloying elements of a component to confirm it is the specified alloy. The two main methods are portable X-ray fluorescence (XRF) and optical emission spectroscopy (OES).

XRF is fast and leaves no mark, but cannot measure carbon and is weak on light elements. OES (spark or arc) can measure carbon — needed to tell 316 from 316L or to check carbon steel residuals — but it leaves a small burn mark and requires surface preparation. Laser-induced breakdown spectroscopy (LIBS) is increasingly used as a portable option that can also detect carbon. API RP 578 is the reference for setting up a PMI programme.

Q34Why is PMI important, and where is it applied?

Short answer: Because one wrong component — such as a carbon steel elbow in a Cr-Mo or stainless line — can corrode or crack rapidly and cause a leak or fire. PMI catches such mix-ups before start-up.

Typical PMI scope, defined in the project specification or inspection and test plan (ITP):

• All alloy components (Cr-Mo, stainless, duplex, nickel alloys) at receiving and after fabrication
• Alloy weld deposits, often on a percentage basis
• Carbon steel in services where residual elements matter, such as HF alkylation or sulfidation-prone lines
• Spare parts and replacement components during shutdowns and turnarounds

Q35What is impact testing of piping materials?

Short answer: Impact testing — usually the Charpy V-notch test — measures how much energy a notched specimen absorbs when broken at a set temperature. It proves the material and its welds are tough enough not to fail by brittle fracture at the minimum design metal temperature.

ASME B31.3 para. 323.2.2 decides whether impact testing is needed, based on the material’s curve in Figure 323.2.2A, wall thickness, design minimum temperature and stress ratio; para. 323.3 sets the test method and acceptance criteria. Tests apply to base metal, weld metal and heat-affected zone, and are part of welding procedure qualification for low-temperature service.

Q36What is the difference between an MTC and PMI?

Short answer: An MTC documents what the mill tested and certified for a heat; PMI independently verifies the alloy of the actual piece in your hands. The MTC covers chemistry and mechanical properties; PMI checks chemistry only. Neither replaces the other.

Q37What is heat treatment of piping materials?

Short answer: Heat treatment is controlled heating and cooling that changes a metal’s microstructure to obtain the required strength, toughness, hardness or corrosion resistance, or to relieve residual stress.

Common treatments and where you meet them:

• Normalizing: refines grain in carbon and low-alloy steel (e.g. A333 Gr 6, A106 when specified)
• Normalizing and tempering: Cr-Mo and P91 pipe
• Quenching and tempering: bolting such as A193 B7, martensitic stainless
• Solution annealing: austenitic and duplex stainless steels to dissolve carbides and restore corrosion resistance
• Stress relieving / PWHT: after welding or cold forming

Q38What is PWHT, and when is it required?

Short answer: Post-weld heat treatment heats a completed weld to a set temperature, holds it and cools it slowly to relieve residual stress and temper hard microstructures. Under ASME B31.3 Table 331.1.1, PWHT depends on P-Number and thickness — for P-No. 1 carbon steel it is required above 19 mm (¾ in) nominal thickness, at 595–650 °C.

PWHT may also be required by service regardless of thickness — for example caustic, amine, wet H₂S or HF service — under the owner’s specification or NACE SP0472. Key records are the heat-treatment procedure, thermocouple locations, calibrated time-temperature charts and post-PWHT hardness results. Always confirm requirements against the current B31.3 or B31.1 edition and project specification.

Q39Why are preheat and PWHT so critical for Cr-Mo steels?

Short answer: Cr-Mo steels are hardenable: if they cool quickly after welding, the heat-affected zone forms hard, brittle martensite that can crack from hydrogen or in service. Preheat slows cooling, and PWHT tempers the hard structure and relieves stress.

Poor control leads to delayed hydrogen cracking, reheat cracking, excessive hardness, low toughness and — in P91 — loss of creep strength. A typical Cr-Mo welding sequence is: preheat → weld with low-hydrogen consumables within interpass limits → maintain preheat or apply a hydrogen-bake (post-heat) → cool under control → PWHT → hardness test → PMI and NDE.

Q40What is preheating in welding, and how is the temperature decided?

Short answer: Preheating is heating the joint area before and during welding to slow the cooling rate, drive off moisture and reduce the risk of hydrogen cracking. The temperature depends on material (P-Number), thickness, carbon equivalent, hydrogen level and restraint.

ASME B31.3 Table 330.1.1 gives recommended minimum preheat temperatures; for carbon steel it can be as low as about 10 °C (50 °F) for thin wall, rising for thick or high-strength material, while Cr-Mo grades commonly require around 150–200 °C. The qualified WPS and project specification set the actual values, measured with temperature-indicating crayons or contact thermometers.

Q41What is hardness testing, and why are hardness limits applied?

Short answer: Hardness testing measures resistance to indentation (Brinell HBW, Vickers HV or Rockwell HRC). High hardness in welds and heat-affected zones signals brittle microstructures that are prone to cracking — especially in sour, caustic and hydrogen services — so codes and specifications set maximum values.

• Sour service (NACE MR0175/ISO 15156-2): carbon and low-alloy steel ≤ 22 HRC (≈ 250 HV) for base metal and welds
• Refinery carbon steel welds in corrosive service (NACE SP0472): commonly ≤ 200 HBW
• Cr-Mo welds after PWHT: limits set by the project specification, typically around 225–250 HBW depending on grade
• P91: both a maximum and a minimum hardness are normally specified

Always interpret a reading against the specification, test method and location (weld cap, HAZ, base metal).

Q42What is corrosion allowance, and how does it affect wall thickness?

Short answer: Corrosion allowance (CA) is extra wall thickness added to the pressure-design thickness to cover metal loss over the design life. Under ASME B31.3, the minimum required thickness is t_m = t + c, where c includes corrosion, erosion and thread or groove depth.

Worked example: pressure design needs t = 8.0 mm and the piping class specifies CA = 3.0 mm, so t_m = 11.0 mm. Seamless A106 pipe has a 12.5 % under-tolerance, so the nominal wall must be at least 11.0 / 0.875 = 12.6 mm. Select the next schedule at or above that value. Typical project values are 1.5 mm for mildly corrosive and 3 mm for corrosive carbon steel service, and often 0 mm for stainless steel — but CA is a design decision based on expected corrosion rate × design life.

Q43Is a corrosion allowance always required?

Short answer: No. It depends on the corrosion study: expected corrosion rate, design life, fluid chemistry, mitigation (inhibitors, coatings, cathodic protection) and material. Corrosion-resistant alloys in suitable service often have zero allowance, while carbon steel in wet or corrosive service may need 3 mm or more.

Remember that corrosion allowance only helps against general (uniform) corrosion. It does nothing against pitting, stress corrosion cracking, hydrogen cracking or brittle fracture — those need the right material, not a thicker wall.

Q44What is sour service, and how does it affect material selection?

Short answer: Sour service means exposure to wet hydrogen sulfide (H₂S) at levels that can cause sulfide stress cracking (SSC), hydrogen-induced cracking (HIC) or stress-oriented HIC. Materials must then comply with NACE MR0175/ISO 15156 (oil and gas production) or NACE MR0103/ISO 17945 (refining).

Typical sour-service requirements for carbon steel piping include hardness ≤ 22 HRC (≈ 250 HV) for base metal, weld and HAZ; controlled chemistry (low sulfur, limited carbon equivalent); HIC testing to NACE TM0284 for plate-based welded pipe; and SSC testing to NACE TM0177 where required. Severity depends on H₂S partial pressure and pH — “sour” is not one single condition, so always classify the actual environment.

Q45What is chloride stress corrosion cracking (Cl-SCC)?

Short answer: Cl-SCC is branched cracking of susceptible alloys — mainly 300-series austenitic stainless steels — when tensile stress, chlorides and elevated temperature (typically above about 60 °C) act together. It can occur inside the pipe or externally under wet, chloride-contaminated insulation.

Q46What is galvanic corrosion, and how do you prevent it in piping?

Short answer: Galvanic corrosion occurs when two dissimilar metals are electrically connected in a conductive electrolyte; the less noble metal (for example carbon steel joined to stainless, copper-nickel or titanium) corrodes faster. Prevent it with isolation kits, insulating spools, coatings on the noble side, or by avoiding the combination.

The risk rises with a large noble (cathode) area joined to a small active (anode) area — for example a small carbon steel nipple in a titanium or super duplex seawater line. Dissimilar-metal welds (for example carbon steel to stainless) also need the right filler metal (often 309L or nickel-based) and consideration of thermal-expansion mismatch at high temperature.

Q47What is material compatibility, and how do damage mechanisms drive selection?

Short answer: Material compatibility means the material can withstand every chemical, thermal and mechanical condition of the service. You prove it by identifying the credible damage mechanisms (API RP 571) and choosing a material, allowance or mitigation for each.

Q48What are non-metallic piping materials, and which code covers them?

Short answer: Non-metallic piping uses thermoplastics (PVC, CPVC, HDPE, PP, PVDF), thermosets (GRP/GRE/FRP) or linings instead of metal. In process plants it is designed to ASME B31.3 Chapter VII, together with product standards such as ISO 14692 for GRE.

Non-metallics offer excellent corrosion resistance, low weight and smooth bores, but their strength falls quickly with temperature and they creep under sustained load. Check pressure-temperature derating, UV exposure, fire performance, static electricity in hydrocarbon service, support spacing and joining qualifications.

Q49What is the difference between PVC and CPVC pipe?

Short answer: CPVC is PVC that has been further chlorinated. The extra chlorine raises its usable temperature from about 60 °C for PVC to about 93 °C (200 °F) for CPVC and improves chemical resistance, so CPVC is used for hot water and many acids while PVC is used for cold water and drainage.

Both are joined mainly by solvent cement, both become brittle at low temperature and both lose pressure rating as temperature rises. Neither should be used for compressed air or gas, because a brittle failure releases stored energy violently. CPVC fire-sprinkler pipe is only acceptable where the product is specifically listed for that use.

Q50What is HDPE piping, and where is it used?

Short answer: HDPE (high-density polyethylene) is a tough, flexible thermoplastic joined by heat fusion into a leak-free continuous line. PE100 has a minimum required strength (MRS) of 10 MPa at 20 °C for 50 years and is widely used for water mains, slurry, buried firewater, wastewater and gas distribution.

Pressure class is set by the SDR (outside diameter ÷ wall thickness) and derated for temperatures above 20 °C. HDPE has high thermal expansion (about 10 times carbon steel), so above-ground lines need restraint or expansion loops. Fusion welders and procedures must be qualified — for example to ISO 21307 or ASTM F2620.

Q51What is GRP/GRE (FRP) piping?

Short answer: GRP/GRE is glass-fibre reinforced thermoset pipe — polyester or vinyl-ester resin (GRP) or epoxy resin (GRE). It combines high corrosion resistance with good strength-to-weight and is widely used for large cooling-water, seawater and firewater lines.

Design and qualification follow ISO 14692 in oil and gas and AWWA C950 in water. Key checks include resin compatibility, maximum temperature, UV protection, fire endurance (for firewater), vacuum and buckling, support spacing, surge pressure, and bonder qualification for adhesive joints.

Q52What is the difference between lined pipe and clad pipe?

Short answer: Lined pipe has a separate liner — usually a polymer such as PTFE, PFA or PP, or rubber — held inside a steel pipe that carries the pressure. Clad pipe has a thin corrosion-resistant alloy layer metallurgically bonded (or tightly mechanically bonded) to the steel bore. Both combine steel strength with corrosion resistance at lower cost than solid alloy.

Q53Which materials are suitable for cryogenic piping?

Short answer: Materials that keep their toughness at very low temperature: austenitic stainless steels (304L, 316L) down to −196 °C, 9 % nickel steel (A333 Gr 8) to −195 °C, aluminium alloys, copper and some nickel alloys. Ordinary carbon steel and ferritic steels become brittle and are not used.

Design must also handle thermal contraction (about 3 mm per metre for stainless from ambient to −196 °C), cold insulation and vapour barriers, cold supports with insulated shoes, and impact testing of welds where required by ASME B31.3 para. 323.2.2 for the austenitic weld metal and filler used.

Q54A supplier proposes a material substitution. What do you do — and is a stronger material always better?

Short answer: Never accept a substitution just because the material is stronger or available sooner. Raise a formal technical query or deviation, review it against the design basis, and get client approval through the project’s management of change (MOC) process. Higher tensile strength alone proves nothing.

The review should compare chemistry, mechanical properties, allowable stress at temperature, toughness and MDMT, corrosion resistance and damage mechanisms, P-Number and weldability (existing WPS coverage), PWHT and hardness, NDE, code acceptability, wall-thickness and stress calculations, and matching fittings and flanges. A stronger steel can still crack in sour service, be harder to weld, or be incompatible with the existing pipe class.

Q55How would you select a piping material systematically?

Short answer: Define the service, establish design conditions (including minimum temperature and upsets), identify the damage mechanisms, shortlist candidate materials, check fabrication and code compliance, compare life-cycle cost, and document the decision in a materials selection report approved by the client.