Defined terms

Steel, certification and fabrication glossary

The terms that appear on mill certificates, datasheets and project specifications, defined in plain language with the numbers that matter.

Austenitic stainless steel: The face-centred cubic chromium-nickel stainless family (304, 316, 321, 310S, 904L). Non-magnetic as annealed, tough at cryogenic temperature, readily welded, and not hardenable by heat treatment — only by cold work.
Average wall vs minimum wall: Two ways of specifying exchanger and boiler tube thickness. Average wall allows a tolerance either side of nominal; minimum wall permits no thinning below the stated figure, so the tube is effectively heavier and dearer. Pressure design uses minimum wall.
Charpy V-notch: An impact test measuring the energy in joules absorbed by a notched specimen at a stated temperature. It is how resistance to brittle fracture is specified: '27 J at -20 C' means three specimens averaging 27 J at that temperature.
Cold work: Deformation below the recrystallisation temperature. It raises strength and hardness while cutting ductility, and because it also raises susceptibility to cracking it is limited by NACE for sour service — cold formed items often need re-solution annealing to stay compliant.
Creep rupture strength: The stress a material can carry for a defined time at temperature before rupturing, typically quoted for 100,000 hours. The property that governs steam and heater design above about 400 C, and the reason Grade 91 replaces Grade 22 on supercritical lines.
Duplex stainless steel: A roughly 50:50 austenite and ferrite structure (2205, 2507) giving about twice the yield strength of 316L with much higher chloride pitting resistance. Phase balance must be controlled in welding, and service above about 300 C is ruled out by embrittlement.
EN 10204 3.1: A mill test certificate reporting specific test results for the actual heat supplied, validated by the manufacturer's inspection department, which must be independent of production. Traceable to a heat number marked on the material. Type 3.2 is the same document countersigned by an independent third party.
Ex-stock vs mill indent: Ex-stock material is physically held in the Dubai warehouse with its certificate on file and ships in days. Mill indent material is rolled to order, typically 4 to 8 weeks plus freight and subject to a minimum rolling quantity.
HBW (Brinell hardness): Brinell hardness measured with a tungsten carbide ball. The standard scale for abrasion resistant plate, where the class name is the nominal hardness — a 450 class plate runs roughly 425 to 475 HBW through the thickness.
Heat number: The identifier of a single melt of steel. Stamped or painted on the plate, pipe or bar and quoted on the mill certificate, it is the link that proves the certificate in your file belongs to the metal in your hand.
HIC testing: Hydrogen Induced Cracking testing to NACE TM0284: specimens are exposed to a sour solution and sectioned to measure crack length, thickness and sensitivity ratios. Routinely required on line pipe and pressure vessel plate for wet H2S service.
NACE MR0175 / ISO 15156: The materials standard for H2S-containing oil and gas service. Compliance is conditional on the specific environment and sets hardness caps, heat treatment conditions and cold work limits — it is a service qualification, not a grade label.
NB / NPS: Nominal bore or nominal pipe size: a designation, not a measurement. A 2" NB pipe has an outside diameter of 60.3 mm regardless of wall thickness, which is what lets fittings and flanges interchange across schedules.
Nelson curve: API 941 curves defining the temperature and hydrogen partial pressure limits at which steels resist high temperature hydrogen attack. They are why Cr-Mo grades (SA387 Gr 11, 12, 22) rather than carbon steel are used in hydroprocessing service.
Normalised and tempered (N+T): Heating above the transformation range, cooling in air to refine the grain, then tempering to set final properties. The standard delivery condition for pressure vessel and Cr-Mo plate, and a condition that codes and NACE qualification both reference explicitly.
OCTG: Oil Country Tubular Goods: the drilling and completion tubulars — casing, tubing and drill pipe — specified to API 5CT and API 5DP, as distinct from line pipe to API 5L used for transmission.
Plasma profiling: Thermal cutting with an ionised gas jet, used to cut plate to a drawing — wear liners, chute plates, flange rings, gussets. Faster than oxy-fuel on stainless and abrasion resistant plate, with a narrower heat affected edge.
PMI: Positive Material Identification: on-site verification of alloy composition, usually by handheld XRF or optical emission. The practical defence against material mix-up, and routinely required on alloy and stainless items before installation.
PREN: Pitting Resistance Equivalent Number, calculated as %Cr + 3.3 x %Mo + 16 x %N. A screening figure for chloride pitting resistance: 316L is about 24, duplex 2205 about 35, super duplex 2507 about 42.
PSL2: The higher product specification level of API 5L. Adds carbon equivalent limits, maximum as well as minimum yield, a 0.93 yield/tensile cap, mandatory Charpy testing and tighter sulphur and phosphorus. Standard for transmission and offshore pipelines.
PWHT: Post weld heat treatment: a controlled soak after welding to relieve residual stress and temper the heat affected zone. Mandatory on many pressure boundaries, and strictly controlled on Grade 91 material, where hardness after PWHT must be measured and reported.
Quenched and tempered (Q+T): Rapid cooling from austenitising temperature to form martensite, then tempering back to the required strength and toughness. How abrasion resistant plate, armour plate and high strength structural steel get their properties — which is why none of them may be stress relieved afterwards.
Schedule (SCH): The wall thickness designation for pipe at a given nominal bore — SCH 40, 80, 160, STD, XS, XXS. For the same schedule the wall increases with diameter, so a schedule alone is not a thickness until the bore is stated.
Sensitisation (weld decay): Precipitation of chromium carbides at grain boundaries in stainless steel held in the 450 to 850 C range, which strips those boundaries of chromium and leaves them open to corrosion. Avoided with low carbon (L) grades or stabilised grades such as 321 and 347.
Sulphidic corrosion: High temperature attack by sulphur compounds in crude and vacuum unit hot sections, modelled by the modified McConomy curves. Resistance rises with chromium, which is why 5Cr (Gr 5 / P5) and 9Cr (Gr 9 / P9) are specified for heater passes and transfer lines.
TEMA: Standards of the Tubular Exchanger Manufacturers Association, which classify shell and tube heat exchangers (classes R, C and B) and fix the tube, tubesheet and baffle conventions that exchanger tube enquiries are quoted against.
UHMW-PE: Ultra high molecular weight polyethylene: a self-lubricating engineering plastic used as a liner where material sticks or bridges. About one eighth the weight of steel with roughly a third of its surface friction, but limited to around 80 C continuous service.
Weathering steel (Corten): Low alloy structural steel with copper, chromium and nickel that forms a stable, adherent rust patina, allowing unpainted exposed structures. Supplied to S355J2W+N or ASTM A588; not suitable where chlorides or constant wetting prevent the patina stabilising.

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