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Stainless steel fasteners

Introduction

Stainless steel fasteners are specified to BS EN ISO 3506. Part 1 covers bolts, screws and studs. Part 2 covers nuts. These specifications replaced BS 6105. The latest edition was issued in 2020 and now includes 6% Mo Austenitic stainless steels and Duplex stainless steels, along with a new ‘100’ property class for these steels, (and for the previous ‘A4’ and ‘A5’ fastener classifications).

Chemical composition for austenitic stainless steel fasteners

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 stainless steel fasteners

Grade Chemical Composition1 (% maxima unless stated) Notes Alternative names
C Si Mn P S Cr Mo Ni Cu N
A1 0.12 1.00 6.5 0.020 0.15/0.35 16.0-19.0 0.70  5.0-10.0 1.75-2.25 – 2 3 4 303S31, 303S42, 1.4305
A2 0.10 1.00 2.00 0.050 0.030 15.0-20.0 –5 8.0-19.0 4.0 – 6 7 304, 394S17 (BS 3111), 1.4301, 1.4567
A3 0.08 1.00 2.00 0.045 0.030 17.0-19.0 –5 9.0-12.0 1.00 – 8 321, 1.4541, 347, 1.4550
A4 0.08 1.00 2.00 0.045 0.030 16.0-18.5 2.00-3.00 10.0-15.0 4.0 – 7 9 316, 1.4401, 1.4578
A5 0.08 1.00 2.00 0.045 0.030 16.0-18.5 2.00-3.00 10.5-14.00 1.00 – 8 9 316Ti, 1.4571, 316Cb, 1.4580
A8 0.030 1.00 2.00 0.040 0.030 19.0-22.0 6.0-7.0 17.5-26.0 1.50 – – 6% Mo, 1.4547, 1.4529
C1 0.09-0.15 1.00 1.00 0.050 0.030 11.5-14.0 – 1.00  – – 9 410, 1.4006
C3 0.17-0.25 1.00 1.00 0.040 0.030 16.0-18.0 – 1.50-2.50 – – 431, 1.4057
C4 0.08-0.15 1.00 1.50 0.060 0.15-0.35 12.0-14.0 0.60 1.00 – – 2 9 416, 1.4005
F1 0.08 1.00 1.00 0.040 0.030 15.0-18.0 –10 1.00 – – 11 12 430, 1.4016, 430Ti, 1.4520 430Cb, 1.4511
D2 0.040 1.00 6.00 0.040 0.030 19.0-24.0 0.10-1.00 1.50-5.5 3.00 0.05-0.20 13 Lean Duplex, DX 2202 1.4062, 1.4162 LDX 2101
D4 0.040 1.00 6.00 0.040 0.030 21.0-25.0 0.10-2.00 1.00-5.5 3.00 0.05-0.30 14 2304 1.4362
D6 0.030 1.00 2.00 0.040 0.015 21.0-23.0 2.50-3.5 4.5-6.5 – 0.08-0.35 – 2205 1.4462
D8 0.030 1.00 2.00 0.035 0.015 24.0-26.0 3.00-4.5 6.0-8.0 2.50 0.20-0.35 15 Zeron 100 1.4501, Ferrinox 255 1.4507

Notes

  1. Values are maximum unless otherwise indicated.
  2. Sulphur may be replaced by selenium.
  3. If the nickel content is below 8 %, the minimum manganese content shall be 5 %.
  4. There is no minimum limit to the copper content provided that the nickel content is greater than 8 %.
  5. Molybdenum may be present at the discretion of the manufacturer. However, if for some applications limiting of the molybdenum content is essential, this shall be stated at the time of ordering by the purchaser.
  6. If the chromium content is below 17 %, the minimum nickel content should be 12 %.
  7. For austenitic stainless steels having a maximum carbon content of 0,03 %, nitrogen may be present to a maximum of 0,22 %.
  8. Shall contain titanium ≥ 5 × C up to 0,8 % maximum for stabilisation and be marked appropriately in accordance with this table, or shall contain niobium, (columbium), and/or tantalum ≥ 10 × C up to 1,0 % maximum for stabilisation and be marked appropriately in accordance with this table
  9. At the discretion of the manufacturer the carbon content may be higher, where required in order to obtain the specified mechanical properties at larger diameters, but shall not exceed 0,12 % for austenitic steels.
  10. Molybdenum may be present at the discretion of the manufacturer.
  11. May contain titanium ≥ 5 × C up to 0,8 % maximum.
  12. May contain niobium, (columbium), and/or tantalum ≥ 10 × C up to 1 % maximum.
  13. Cr + 3.3Mo + 16N < or = 24.0 . This formula is used solely for the purpose of classifying duplex steels, it is not intended to be used as a selection criterion for corrosion resistance.
  14. 24.0 < Cr + 3.3Mo +16N . This formula is used solely for the purpose of classifying duplex steels, it is not intended to be used as a selection criterion for corrosion resistance.
  15. W < or = 1.00

Mechanical properties for stainless steel fasteners

There are now four ‘property classes’ which are assigned selectively across the various austenitic stainless steel fastener grades now included in the standard. 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

A8

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 50 500 210 0.6d
70 700 450 0.4d
80 800 600 0.3d
A4,A5,
70 700 450 0.4d
80 800 600 0.3d
100 1000 800 0.2d
70 700 450 0.4d
80 800 600 0.3d
100 1000 800 0.2d

Notes

  1. The tensile stress is calculated on the stress area
  2. To be determined on the actual screw length and not on a prepared test piece.

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.

Property class 100 condition, i.e. with a minimum tensile strength of 1000 MPa. is now part of the official standard.

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

There are three ‘property classes’ which are assigned to the Duplex stainless steel fastener grades now included in the standard. Table 3 shows the 0.2% proof stress, tensile strength and elongation values for each of these property classes.

Table 3 Mechanical properties for Duplex 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
Duplex D2, D4, D6, D8 70 700 450 0.4d
80 800 600 0.3d
100 1000 800 0.2d

Notes

  1. The tensile stress is calculated on the stress area
  2. To be determined on the actual screw length and not on a prepared test piece.

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.

Duplex stainless steels are intrinsically stronger than standard austenitic stainless steels, so higher strengths can be achieved at lower levels of cold work, and hence there is no class ’50’, for these steels.

Property class 80 is based on cold drawn bar.

Property class 100 condition, i.e. with a minimum tensile strength of 1000 MPa. is now part of the official standard.

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 4 — Mechanical properties for bolts, screws and studs – Martensitic and ferritic steel grades

Steel group Steel
grade		 Property
class		 Tensile Strength Rm1 min MPa 0.2% Proof Stress Rp0.21 min MPa Elongation A2 min mm Hardness
HB HRC HV
Martensitic C1 50 500 250 0.2 d 147/209 155/220
70 700 410 0.2 d 209/314 20/34 220/330
1103 1100 820 0.2 d  – 36/45 350/440
C3 80 800 640 0.2 d 228/323 21/35 240/340
C4 50 500 250 0.2 d 147/209  – 155/220
70 700 410 0.2 d 209/314 20/34 220/330
Ferritic F14 45 450 250 0.2 d 128/209  – 135/220
60 600 410 0.2 d 171/271  – 180/285
  1. The tensile stress is calculated on the stress area
  2. To be determined on the actual screw length and not on a prepared test piece
  3. Hardened and tempered at a minimum tempering temperature of 275 °C
  4. Nominal thread diameter d ≤ 24 mm

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