Why changes were made to the international standard on nuts

Part 2 of BS EN ISO 3506 specifies the mechanical and physical properties of certain types of nut. This blog post looks at where Part 2 sits in the series and at who should be using this standard.

First published in 1998, the BS EN ISO 3506 series deals with the mechanical properties of corrosion-resistant stainless steel fasteners.

To date BS EN ISO 3506-2 has undergone two rounds of revisions: in 2009 and 2020. In 2020, we’ve published the third editions of Parts 1 and 2 of the series. These cover respectively: bolts, screws and studs; and nuts.

Part 3 specifies set screws and similar fasteners not under tensile stress and Part 4 is on tapping screws. Part 5 is currently under preparation and has yet to be published although the standard is based on special fasteners for high-temperature applications and also includes fasteners from nickel alloys. Part 6 is new for 2020. It was published at the same time as the revised Parts 1 and 2 and provides general rules and additional technical information on suitable stainless steels and their properties. Users of the revised Part 2 will generally need to also refer to Part 6. 

About Part 2

BS EN ISO 3506-2:2020 Fasteners — Mechanical properties of corrosion-resistant stainless steel fasteners — Part 2: Nuts with specified grades and property classes is the newly revised international standard on the mechanical and physical properties of nuts. It specifies nuts with coarse pitch thread and fine pitch thread, made of corrosion-resistant stainless steels, when tested at the ambient temperature range of 10 °C to 35 °C.

The standard covers designation systems for nuts, the design of bolt and nut assemblies, materials, mechanical and physical properties of nuts, inspections, test methods, and nut marking and labelling. The test methods section covers proof load testing and hardness testing.

BS EN ISO 3506-2 applies to nuts of any shape with ISO metric thread in accordance with ISO 68-1; diameter/pitch combinations in accordance with ISO 261 and ISO 262; coarse pitch thread M5 to M39, and fine pitch thread M8×1 to M39×3; thread tolerances in accordance with ISO 965-1 and ISO 965-2, specified property classes, including proof load; different nut styles: thin nuts, regular nuts and high nuts; a minimum nut height m ≥ 0,45D; a minimum outside diameter or width across flats s ≥ 1,45D; and able to mate with bolts, screws and studs with property classes in accordance with ISO 3506-1.

This standard is written for use by manufacturers and distributors of stainless steel fasteners as well as structural and consulting engineers. It will also be used by construction and civil engineering contractors and larger sub-contractors as well as test houses and laboratories. It will be of particular interest to industries associated with oil and gas production and distribution.

How the standard has changed

BS EN ISO 3506-2 has undergone a complete revision to reflect the fact that nuts made of duplex stainless steels (austenitic-ferritic) are now well established in a range of applications. As a result, the revised standard now includes duplex stainless steels for property classes 70, 80 and 100

Several other additions have been made to the standard, including information on nut styles; information on the design of stainless steel bolt and nut assemblies; and design principles of stainless steel nuts. Calculated proof load values and rounding rules have been added, as have requirements and guidance for inspection procedures; thread dimensions of the test mandrel for proof load; and finally nut marking and labelling especially for thin nuts with reduced loadability.

The sections on finish and on test methods for proof load and hardness have also been improved while the section on operational temperature ranges has been clarified. The structure and content of the document have been brought into line with ISO 898-2. Finally, annexes that are common to several parts of the ISO 3506 series have been withdrawn from Part 2 and are now included in the new Part 6.

Learn more about BS EN ISO 3506-2:2020 or buy now