ISO 5251:1981

Overview

ISO 5251:1981 - Stainless steel butt-welding fittings - is an International Standard that specifies the dimensions, tolerances, material grades and quality requirements for common stainless-steel butt-welding pipe fittings used in piping work. The standard covers bends (type 3D and 5D), concentric and eccentric reducers, tees (equal and reducing), caps and stub ends - including variants with and without straight ends.

Key topics and technical requirements

• Fitting types covered
  • Butt-welding bends: type 3D (90° and 180°), with and without straight ends; type 5D (90°).
  • Concentric and eccentric reducers, tees (equal and reducing), caps and stub ends.
• Dimensions & tolerances
  • Detailed dimension tables for each fitting type (DN, outside/inside diameters D/D1, wall thickness T/T1, radii R, centreline dimensions C, face dimensions F/B/L, etc.).
  • Angular and face-alignment tolerances (symbols such as Q, u, P are defined).
• Materials and grades
  • Specifies commonly used stainless-steel grades (examples: TS46, TS47, TS53, TS57, TS58, TS60, TS61 per ISO 2604/2; P- and TW- grades per ISO 2604 parts).
  • Fittings may be manufactured from seamless tubes, welded tubes or welded plate material; welds on welded material shall be previously tested by non‑destructive methods (NDT) agreed between purchaser and manufacturer.
  • Optional intergranular corrosion testing (e.g., ISO 3651-2) may be performed by agreement.
• Designation & marking
  • Standardized designation format (example): Butt-welding bend 3D-90-60,3 x 2,9-TS 47 ISO 5251 - including type, angle, outside diameter, thickness and grade with reference to ISO 5251.
• Quality references
  • The standard cross-references other ISO documents for material and dimensional conformity (e.g., ISO 2604, ISO 1127, ISO 404, ISO 6700, ISO 3545).

Applications and users

• ISO 5251 is intended for engineers, specifiers, manufacturers, fabricators and procurement teams involved in stainless-steel piping systems and piping components.
• Practical use cases include specifying and verifying butt-welding fittings for industrial piping applications where standardized dimensions, mechanical quality and corrosion resistance are required.
• Useful in procurement documentation, manufacturing drawings, inspection and acceptance criteria, and supplier quality agreements.

Related standards

• ISO 2604 (steel products for pressure purposes - various parts)
• ISO 1127 (stainless-steel tubes - dimensions)
• ISO 3651-2 (intergranular corrosion test)
• ISO 404, ISO 3545, ISO 6700 (symbols, nominal sizes, general requirements)

Keywords: ISO 5251, stainless steel butt-welding fittings, butt-welding bends, reducers, tees, stub ends, dimensions and tolerances, stainless steel grades, piping work, NDT, intergranular corrosion test.