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IS 6623:1972 is the Indian Standard (BIS) for high strength structural nuts - specification. This standard specifies the requirements for high strength structural nuts of Property Class 8, from sizes M12 to M36. It covers material, chemical composition, mechanical properties, dimensions, testing, and marking for nuts intended for use in high-strength friction-grip and bearing-type bolted connections in steel structures.
Specifies requirements for hexagon high strength structural nuts, complementing high strength structural bolts.
Overview
Status
Current
Usage level
Frequently Used
Domain
Structural Engineering — Bridges and Bridge Engineering
BIM-relevant code. See the BIM Hub for ISO 19650, IFC, and LOD/LOIN frameworks used alongside it.
Practical Notes
! Always pair these nuts with high strength bolts as per IS 3757 and hardened washers as per IS 6649 to ensure the integrity of the bolted connection.
! The '8' or '8S' marking on the nut face is crucial for on-site verification of the property class. Using a lower-grade nut can lead to catastrophic thread stripping failure under load.
! The proof load capacity of the nut is designed to be greater than the ultimate tensile strength of the corresponding bolt, ensuring that in case of overload, the bolt will fail in tension (a more ductile and predictable failure) rather than the nut stripping its threads.
Consolidated list per BIS. For the text of each amendment, refer to the BIS portal link above.
steelcarbon steelalloy steelfasteners
International Equivalents
Similar International Standards
EN 14399-4:2015CEN (European Committee for Standardization), Europe
HighCurrent
High-strength structural bolting assemblies for preloading - Part 4: System HV - Hexagon nuts
Specifies high-strength hexagon nuts (System HV) as part of a matched bolting assembly for preloaded joints.
ASTM A563/A563M-21ASTM International, USA
HighCurrent
Standard Specification for Carbon and Alloy Steel Nuts
Covers chemical and mechanical requirements for carbon and alloy steel nuts, including heavy hex nuts for structural use.
ISO 898-2:2022ISO (International Organization for Standardization), International
MediumCurrent
Mechanical properties of fasteners made of carbon steel and alloy steel - Part 2: Nuts with specified property classes - Coarse thread and fine pitch thread
Defines the mechanical properties (like proof load and hardness) for property classes of nuts, which IS 6623 references.
JIS B 1186:2022JSA (Japanese Standards Association), Japan
HighCurrent
Sets of high strength hexagon bolt, hexagon nut and plain washers for friction grip joints
Specifies requirements for sets of high-strength bolts, nuts, and washers intended for friction-grip structural joints.
Key Differences
≠IS 6623:1972 is withdrawn and superseded by IS 1367 (Part 6):2018, whereas its international counterparts like EN 14399-4 and ASTM A563 are current and regularly updated.
≠IS 6623 specifies a single property class '8.8' for the nut, which is unconventional. International standards like ISO 898-2 use single-digit classes (e.g., 8, 10) to denote nut strength, which is designed to match a two-digit bolt class (e.g., 8.8, 10.9).
≠Modern European standards like EN 14399-4 treat the nut as part of a certified bolt-nut-washer 'assembly' (System HV) to guarantee performance, whereas IS 6623 focuses on the nut as a standalone component.
≠The specified hardness ranges can differ. For example, the hardness range for a Class 8 nut under EN 14399-4 is narrower and higher than the range specified in the superseding Indian standard IS 1367-6.
Key Similarities
≈All standards specify nuts intended for high-strength structural applications, designed for use with high-strength bolts (e.g., Class 8.8 or 10.9).
≈The primary mechanical performance test required by IS 6623 and its international equivalents is the Proof Load test, ensuring the nut's threads can sustain a high tensile load without stripping.
≈The specified material for the nuts in all standards is typically medium carbon or alloy steel that is quenched and tempered to achieve the desired high strength and toughness.
≈All standards mandate clear marking on the nut to identify the manufacturer and the property class/grade, ensuring traceability and correct application in the field.
≈The dimensional form is a 'heavy hexagon' style, which is thicker and wider across flats than a standard nut, providing a larger bearing area and greater resistance to thread stripping under preload.
Parameter Comparison
Parameter
IS Value
International
Source
Standard Status
Withdrawn — superseded by IS 1367 (Part 6):2018
Current
EN 14399-4:2015
Nut Property Class Designation
Class 8.8 (for use with 8.8 bolts)
Class 8 or Class 10
ISO 898-2:2022
Proof Load Stress (for Class 8 equivalent)
800 MPa
800 MPa
ISO 898-2:2022
Hardness, Vickers (for Class 8 equivalent)
180-302 HV (from superseding IS 1367-6)
242-353 HV
EN 14399-4:2015
Width Across Flats (Size M24)
41 mm
41 mm
EN 14399-4:2015
Nut Height (Size M24)
24 mm
22.7 mm (min)
ASTM A563M-21
⚠ Verify details from original standards before use
Key Values6
Quick Reference Values
Specified Property Class8
Proof Load Stress (Size > M16 to M36)860 MPa
Hardness Range (HB)188 to 302 HB
Phosphorus Content (Max)0.050 %
Sulphur Content (Max)0.050 %
Chamfering Angle30° to the end face of nut
Tables & Referenced Sections
Key Tables
Table 1 - Dimensions for High Strength Structural Nuts
Table 2 - Mechanical Properties of High Strength Structural Nuts