What’s new about the main corrosion resistant bolts, screws and studs standard?
Part 1 of BS EN ISO 3506 specifies the mechanical and physical properties of certain types of bolts, screws and studs. This blog post looks at what it covers and at how the 2020 version of the standard has changed.
BS EN ISO 3506-1:2020 Fasteners - Mechanical properties of corrosion-resistant stainless steel fasteners -- Part 1: Bolts, screws and studs with specified grades and property classes is the newly revised international standard on the mechanical and physical properties of bolts, screws and studs. It specifies property classes in relation to austenitic, martensitic, ferritic and duplex (austenitic-ferritic) stainless steel grades for bolts, screws and studs when tested at the ambient temperature range of 10 °C to 35 °C.
What the standard covers
BS EN ISO 3506-1 applies to bolts, screws and studs of any shape with ISO metric thread in accordance with ISO 68-1; with diameter/pitch combinations in accordance with ISO 261 and ISO 262; with coarse pitch thread M1,6 to M39, and fine pitch thread M8×1 to M39×3; with thread tolerances in accordance with ISO 965-1 and ISO 965-2; and with specified property classes.
This part doesn’t apply to set screws and similar threaded fasteners not under tensile stress (that’s covered in Part 3.) Nor does it specify requirements for functional properties such as torque/clamp force properties, shear strength, fatigue resistance or weldability.
Also, the properties of stainless steel fasteners are derived from the chemical composition of the material (especially corrosion resistance) and from the mechanical properties imbued by the manufacturing processes. Ferritic, austenitic and duplex (austenitic-ferritic) stainless steel fasteners are generally manufactured by cold working. As a result, they don’t have homogeneous local material properties when compared to quenched and tempered fasteners.
Austenitic-ferritic stainless steels referred to as duplex stainless steels were originally invented in the 1930s. Standard duplex grades used today have been developed since the 1980s. Fasteners made of duplex stainless steels have been long established in a range of applications. This document was revised to reflect their standardization.
How the standard has changed
The standard has undergone a complete revision. A lot of new content has been added. This includes: specifications for duplex (austenitic-ferritic) stainless steels for property classes 70, 80 and 100; property class 100 for austenitic stainless steel grades as well as grade A8; and specifications on finish.
The revision also adds in the matching of stainless steel bolt and nut grades; calculated minimum ultimate tensile loads and minimum loads at 0,2 % non-proportional elongation and rounding rules; reduced loadability for fasteners due to head or shank design; requirements and guidance for inspection procedures; and content on the applicability of test methods, also in relation to full or reduced loadability and application to fasteners with reduced loadability.
In addition, the tensile test procedure has been entirely amended; the wedge tensile test and hardness test have been improved; operational temperature ranges have been clarified; content on marking and labelling has been improved; and fasteners with reduced loadability have been included. The text on mechanical properties at elevated temperatures and application at low temperatures has been improved; and the structure and content of the document have been brought in line with ISO 898-1.
Finally, ISO has added another standard to the series – ISO 3506-6. It provides general rules and additional technical information on suitable stainless steels for fasteners and their properties. To that end, annexes that are common to several parts of the ISO 3506 series have been withdrawn from Part 1 and are now included in the new Part 6. Users of the revised Part 1 will generally need to also refer to Part 6.