Buyers often say "tube" and "pipe" interchangeably, but the specifications differ.

Pipe: • Sized by nominal bore (NPS/DN) and schedule (wall thickness class) • Used mainly for conveying fluids; pressure rating and code compliance are central • Standards: ASTM A312 (seamless/welded austenitic), EN 10216-5, GB/T 14976

Tube: • Sized by outside diameter × wall thickness (e.g. 25.4 × 1.5 mm) • Used for heat exchangers, instrumentation, structural and decorative purposes; tighter tolerances and surface finish • Standards: ASTM A269 (general), A213 (heat exchanger), A249 (welded heat exchanger), EN 10216-5 / 10217-7

How to specify on an inquiry:

1) Product type: seamless or welded; tube or pipe. 2) Standard and grade: e.g. ASTM A312 TP316L, or EN 10216-5 1.4404. 3) Dimensions: OD × wall or NPS × schedule; length; tolerances. 4) Test requirements: hydrostatic, eddy current, ultrasonic, intergranular corrosion. 5) Delivery condition: annealed/pickled, bright annealed; end finish (plain, beveled, threaded). 6) Certification: EN 10204 3.1 or 3.2.

Substituting tube for pipe or vice versa can fail pressure or dimensional requirements — specify the product type explicitly.