Stainless Steel Pipe Specifications and Wall Thickness Guide

Product Forms of Stainless Steel Pipe

Stainless steel pipe is produced in two basic forms. Seamless pipe is pierced or extruded from solid round billet and has no longitudinal weld, which makes it the preferred choice for high-pressure, high-temperature and corrosive service where weld integrity matters. Welded pipe is formed from strip or plate and closed by a longitudinal weld; modern welded pipe with full penetration welds and, where specified, full heat treatment is accepted for the majority of process and architectural duties. A third family, welded mechanical tubing in round, square and rectangular section, is used for structural and architectural applications such as handrails and frames. Product form is selected on pressure, temperature, corrosion, cost and availability.

Material Grades and Applicable Standards

The most widely specified austenitic grades are 304/304L, UNS S30400/S30403, and 316/316L, UNS S31600/S31603, with 316 carrying 2.00 to 3.00 percent molybdenum for chloride resistance. Other grades include 321 for stabilised high-temperature service, 310S for furnace duty and 317L for aggressive chemicals. Standards governing pipe and tube are ASTM A312 for seamless and welded pipe, A358 for large-diameter welded pipe, A213 for seamless boiler and heat-exchanger tubes, A269 for small-diameter tubing, A554 for welded mechanical tubing, and A999 for dimensional tolerances. European equivalents follow EN 10216-5 for seamless tube and EN 10217-7 for welded tube.

Wall Thickness Systems: Schedule Numbers

For pipe, wall thickness is expressed by schedule number. The most common stainless series is defined in ASME B36.19 with the suffix S: 5S, 10S, 40S and 80S, where a higher number indicates a thicker wall for the same nominal size. The table gives representative outside diameters and schedule wall thicknesses in millimetres.

NPS	OD, mm	5S	10S	40S	80S
1/2	21.3	1.65	1.65	2.77	3.73
1	33.4	1.65	2.11	3.73	5.49
2	60.3	1.65	2.77	3.91	5.54
4	114.3	2.11	3.05	6.02	8.56

For metric tube, wall thickness is specified directly in millimetres, for example 21 x 4, 38 x 10 or 65 x 6, meaning outside diameter x wall thickness. The selection of wall thickness depends on design pressure, temperature, allowable stress and corrosion allowance, calculated in accordance with the applicable design code.

Common Size Ranges

Seamless pipe and tube are commonly supplied from 6 to 650 mm outside diameter with walls from 0.5 to 50 mm. Welded pipe is available from 8 to about 3000 mm outside diameter with walls from 1 to 150 mm, and is the only practical form at very large diameters. Square and rectangular mechanical tubing per ASTM A554 is typically produced from 10 x 10 mm up to 500 x 500 mm or equivalent sections, with walls from 1 to 12 mm, for architectural and structural use. Small-diameter tubing per ASTM A269 is supplied from about 1.6 to 50 mm outside diameter for instrumentation and heat-exchanger service.

Dimensional Tolerances and Verification

Dimensional tolerances for stainless pipe are verified to ASTM A999, covering outside diameter, wall thickness, length, straightness, ovality and end squareness. Tolerances vary with diameter, wall ratio and manufacturing method; for example, outside diameter tolerance for seamless pipe is commonly plus or minus about 0.8 percent of nominal diameter, and wall thickness is normally plus or minus 12.5 percent. At receiving, verify the mill certificate against the order, measure wall thickness with an ultrasonic gauge at several points, and check straightness on long lengths. For critical systems, PMI testing confirms grade chemistry, and pressure ratings are confirmed by calculation from actual dimensions.

Frequently Asked Questions

Q1. What does schedule 40S mean?
It is a wall-thickness series per ASME B36.19 for stainless pipe; the S distinguishes the stainless series from the carbon-steel schedules in ASME B36.10.

Q2. Which should I choose, seamless or welded pipe?
Seamless pipe suits high pressure, high temperature and small diameters; welded pipe is economical at large diameters and is fully accepted by ASTM A312 when manufactured to its requirements.

Q3. How is pipe wall thickness measured?
With a calibrated ultrasonic thickness gauge, or by micrometer at the ends where accessible; results are compared with the tolerance in ASTM A999.

Q4. What is the difference between pipe and tube?
Pipe is specified by nominal size and schedule and used mainly for pressure transmission; tube is specified by outside diameter and wall and used for heat transfer, instrumentation and structural purposes.

Q5. Which grade should I choose for seawater?
316L resists chloride attack far better than 304L; for permanent immersion or warm seawater, duplex or super-austenitic grades are recommended.

Q6. Are large-diameter welded pipes as strong as seamless?
Welded pipe made to ASTM A358 or A312 with full penetration welds and proper heat treatment meets the same minimum mechanical properties; weld joint efficiency factors apply in design codes.