Overview
ISO 21670:2014 specifies the characteristics of hexagon weld nuts with flange of product grade A. The standard covers sizes M5 to M16 (coarse thread) and fine-pitch variants (approximately 8–16 mm), and defines dimensional, material, mechanical and delivery requirements. Nuts conforming to ISO 21670 are suitable for use with bolts up to property class 10.9 (per ISO 898-1).
Key Topics
- Dimensions and drawings: Nut dimensions, connecting dimensions and mass per 1 000 pieces are given in the figures and tables of the standard. Threads follow ISO 724 with tolerance class 6G (ISO 965-3).
- Material requirements: Steel with maximum carbon 0.25 % and a maximum carbon equivalent (CEV) of 0.53 % is required. The CEV is calculated as:
CEV = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15
Free-cutting steels are not permitted. If quenched and tempered, hardness must not exceed 300 HV.
- Mechanical properties and proof load: Proof load values for each thread size are specified and proof load testing is to be carried out in accordance with ISO 898-2. In case of dispute, welding projections shall be removed before testing.
- Surface and finish: Hexagon weld nuts with flange are delivered uncoated; manufacturers must demonstrate corrosion protection that does not impair weldability. Surface discontinuities and limits are referenced to ISO 6157-2.
- Tolerances and product grade: Product grade A requirements follow ISO 4759-1; general fastener requirements per ISO 8992.
- Marking and designation: Nuts size M5 and larger shall carry the manufacturer’s mark on the face opposite the flange. Example designation:
Weld nut [ISO 21670](/search?q=ISO%2021670) - M10 – St (add - QT if quenched and tempered).
- Acceptance inspection: Inspection criteria reference ISO 3269.
Applications
ISO 21670 weld nuts with flange are specified where a welded, captive nut is required for reliable threaded connections. Practical benefits include:
- Secure, repeatable mating with bolts up to property class 10.9
- Standardized dimensions and tolerances for interchangeability
- Clear material and inspection requirements supporting weldability and mechanical performance
Typical use cases include welded assemblies and fixtures where a permanent, load-bearing nut is needed and standardized mechanical performance is required.
Related Standards
Key normative references to consult alongside ISO 21670:2014:
For specification, procurement or design work, reference the full ISO 21670:2014 document for detailed tables, figures and test procedures.
