Stainless steel pipe procurement involves navigating multiple specification systems — nominal pipe size (NPS), wall thickness schedule, material grade, and manufacturing method. Ordering errors in any of these parameters can result in non-conforming material that cannot be used. This guide explains the key specification elements buyers must understand to purchase stainless steel pipe with confidence.
Stainless steel pipe wall thickness is most commonly specified by Schedule number under ASME B36.19 (stainless steel pipe) and ASME B36.10 (welded and seamless wrought steel pipe). The Schedule number is a dimensionless designation that corresponds to a specific wall thickness for each nominal pipe size:
| NPS | OD (mm) | SCH 10S (mm) | SCH 40S (mm) | SCH 80S (mm) | |-----|---------|--------------|--------------|--------------| | 1/2" | 21.3 | 2.11 | 2.77 | 3.73 | | 1" | 33.4 | 2.77 | 3.38 | 4.55 | | 2" | 60.3 | 2.77 | 3.91 | 5.54 | | 3" | 88.9 | 3.05 | 5.49 | 7.62 | | 4" | 114.3 | 3.05 | 6.02 | 8.56 | | 6" | 168.3 | 3.40 | 7.11 | 10.97 | | 8" | 219.1 | 3.76 | 8.18 | 12.70 |
The "S" suffix (e.g., SCH 40S) denotes stainless steel pipe dimensions per ASME B36.19, which in many sizes are identical to carbon steel schedules. SCH 10S is the most common thin-wall specification for process piping where internal pressure is low and weight savings matter. SCH 40S is the general-purpose standard for medium-pressure systems. SCH 80S provides additional wall thickness for higher pressure ratings and threaded connections where thread depth reduces the remaining wall.
For high-pressure hydraulic and instrumentation systems, wall thickness may be specified directly in millimeters or by the pressure rating class (e.g., Class 3000, Class 6000) for socket-weld and threaded fittings rather than by schedule number.
The governing material standard for seamless and welded austenitic stainless steel pipe is ASTM A312. The most frequently ordered grades under this standard are:
TP304/304L — General-purpose austenitic stainless steel. Suitable for food processing, architectural, and general industrial service. 304L (low carbon, 0.03% C max) eliminates the need for post-weld annealing in most applications.
TP316/316L — Molybdenum-bearing grade with enhanced pitting and crevice corrosion resistance. Specified for chemical processing, marine environments, pharmaceutical, and any service involving chlorides. 316L is the standard for welded fabrications in corrosive service.
TP321/321H — Titanium-stabilized grade for high-temperature service to 815°C. Prevents intergranular corrosion in the heat-affected zone without requiring 304L/316L chemistry. Common in refinery heater tubes and boiler superheater tubes.
Duplex 2205 (UNS S32205) — Covered by ASTM A790 for seamless and welded duplex pipe. Specified where high strength plus chloride SCC resistance is required, particularly in offshore and chemical process service.
Seamless pipe is manufactured by piercing a solid billet and drawing or extruding to final dimensions. It offers consistent wall thickness, no longitudinal weld seam, and is available in heavier schedules (SCH 80S, SCH 160) that welded pipe may not cover in all sizes. Seamless pipe is typically specified for high-pressure, high-temperature, and critical-service applications.
Welded pipe is produced from stainless steel coil or plate, formed into a cylinder, and welded along the longitudinal seam, followed by heat treatment and sizing. Modern ERW (electric resistance welded) and laser-welded pipe from reputable mills achieves weld integrity indistinguishable from the parent metal in corrosion testing. Welded pipe offers cost savings of 15–25% versus seamless in common sizes and schedules.
At Zunkai Stainless Steel, we stock both seamless and welded pipe in 304/304L and 316/316L grades, from 1/8" to 12" NPS, with duplex pipe available on order. Standard pipe length is 6 meters with plain or bevelled ends.
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