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Stainless steel fasteners are specified to BS EN ISO 3506. Part 1 covers bolts, screws and studs. Part 2 covers nuts. These specifications replace BS 6105. The latest edition was issued in 2020.
The chemical compositions of the various fastener grades are shown in Table 1. The approximate equivalent grades in other systems are shown for reference. The composition ranges are much wider in this standard than in the standards for the feedstock product, usually bar.
Table 1 Chemical composition for austenitic stainless steel fasteners
|Grade||Chemical Composition1 (% maxima unless stated)||Notes||Alternative names|
|A1||0.12||1||6.5||0.2||0.15/0.35||16-19||0.7||5-10||1.75-2.25||2 3 4||303S31, 303S42, 1.4305|
|A2||0.1||1||2||0.05||0.03||15-20||–5||8-19||4||6 7||304, 394S17 (BS 3111), 1.4301, 1.4567|
|A3||0.08||1||2||0.045||0.03||17-19||–5||9-12||1||8||321, 1.4541, 347, 1.4550|
|A4||0.08||1||2||0.045||0.03||16-18.5||2-3||10-15||4||7 9||316, 1.4401, 1.4578|
|A5||0.08||1||2||0.045||0.03||16-18.5||2-3||10.5-14||1||8 9||316Ti, 1.4571, 316Cb, 1.4580|
|C4||0.08-0.15||1||1.5||0.06||0.15-0.35||12-14||0.6||1||2 9||416, 1.4005|
|F1||0.12||1||1||0.04||0.03||15-18||–10||1||11 12||430, 1.4016, 430Ti, 1.4520 430Cb, 1.4511|
Three ‘property classes’ are assigned to each of the austenitic stainless steel fastener grades. Table 2 shows the 0.2% proof stress, tensile strength and elongation values for each of these property classes.
Table 2 Mechanical properties for austenitic stainless steel bolts, screws and studs
|Steel Group||Steel Grade||Property Class||Tensile Strength Rm1 min MPa||0.2% Proof Stress Rp0.21 min MPa||Elongation A2 min mm|
|Austenitic||A1, A2, A3, A4, A5||50||500||210||0.6d|
This latest revision has removed the restrictions on the size of fastener (M24) for which the properties in table 2 can be guaranteed. However, there is a general clause which says that the standard is only applicable up to 39 mm diameter. Above that, the standard can still be used provided all parties agree that the properties can be achieved. The effect of these changes is to increase the maximum diameter on which the class 70 and 80 properties can be specified without discussion from 24 to 39 mm. This reflects the conservative nature of the previous 24 mm limit. Indeed, class 70 and 80 properties are available above 39 mm but this must always be discussed with the supplier.
Property class 50 represents the steel in the annealed condition.
The most common and readily available supply condition is property class 70, which represents a ‘cold drawn’ for the bar stock from which the fasteners are made.
Property class 80 is based on severely hard cold drawn bar.
Some fasteners, depending on size and grade, can be supplied in an “unofficial” class A100 condition, i.e. with a minimum tensile strength of 1000 MPa.
All tensile stress values are calculated and reported in terms of the nominal tensile stress area of the thread.
The elongation measurement is determined on the actual bolt or screw length and not on a prepared test piece. It is expressed in millimetres (mm) of extension and not as percentage elongation, i.e. A = (L2 – L1)
(where L1 = original length and L2 = length after fracture)
d = nominal diameter of bolt, screw or stud
Table 3 — Mechanical properties for bolts, screws and studs – Martensitic and ferritic steel grades
|Tensile Strength Rm1 min MPa||0.2% Proof Stress Rp0.21 min MPa||Elongation A2 min mm||Hardness|