Stainless Steel Fasteners

ISO-designated steel groups

1) This grade is protected against intergranular corrosion by adding stabilizing elements such as titanium, niobium or tantalum.

2) Austenitic stainless steels with a very low carbon level, typically not more than 0.03%, are identified by the letter “L”, for e.g. A4L-80.

Understanding Stainless Steel Fastener Designations

In the fastener industry, screws that have a lower load-bearing capacity because of their head or shaft design (but can still undergo a tensile test) are identified with a strength class followed by the number “0”.

For example: 050, 070, 080, 0100.

Steel Grade Identification

The first letter in the designation indicates the type of stainless steel used:

A – Austenitic steel

C – Martensitic steel

F – Ferritic steel

D – Duplex steel

Examples of Common Designations

A2-70 - Austenitic steel, cold worked, with a minimum tensile strength of 700 N/mm².

A8-100 - Austenitic steel, cold worked, with a minimum tensile strength of 1000 N/mm².

C4-70 - Martensitic steel, hardened and tempered, with a minimum tensile strength of 700 N/mm².

The two digits after the dash (e.g., 70, 80, 100) represent 1/10 of the minimum tensile strength (for bolts and screws) or 1/10 of the proof load (for nuts).

Hardness Classification

Some fasteners are identified based on hardness instead of tensile strength.

In such cases, the hardness class is shown by two numbers that indicate 1/10 of the minimum Vickers hardness (HV) value. The letter H stands for hardness.

For example:

A4 25 H - Austenitic steel, work-hardened, with a minimum hardness of 250 HV.

This system helps users and manufacturers easily identify the material type, strength level, and mechanical properties of stainless steel fasteners, ensuring they meet the correct standards for each application.

Table 1 – Stainless steel grades — Chemical composition:

a According to material standards. values are maximum unless the number of digits shown is in accordance with usual rules.

b In case of disput, product analysis applies.

c Selenium can be used to replace sulphur, however restrictions may apply to its use.

d if the nickel content is below 8.0%, the minimum manganese content shall be 5.0% .

e There is no minimum limit to the copper content provided that the nickel content is greater than 8.0%.

f Molybdenum is optional; if limits are required, the purchaser must specify when ordering.

g if the chromium content is below 17.0%, the minimum nickel content should be 12.0%.

h For austenitic stainless steels having a maximum carbon content of 0.030%, nitrogen may be present but shall not exceed 0.22%.

i Manufacturer may increase carbon for larger diameters, but austenitic steel must not exceed 0.12%.

j Titanium and/or niobium may be included to improve corrosion resistance.

k This formula classifies duplex steels only and is not for selecting corrosion resistance.

Table 2 – Mechanical properties for bolts, screws and studs — Austenitic and duplex grades

a Minimum ultimate tensile loads (Fmf) are specified in Table 4 for coarse pitch thread, and in Table 6 for fine pitch thread..

b Minimum loads at Rpf (Fpf) are specified in Table 5 for coarse pitch thread, and in Table 7 for fine pitch thread.

Table 3 – Mechanical properties for bolts, screws and studs — Martensitic and ferritic grades

a Minimum ultimate tensile loads (Fmf) are specified in Table 4 for coarse pitch thread, and in Table 6 for fine pitch thread.
b Minimum loads at Rpf (Fpf) are specified in Table 5 for coarse pitch thread, and in Table 7 for fine pitch thread.
c Hardened and tempered at a minimum tempering temperature of 275 °C.
d Only for nominal thread diameters d = 24 mm.

Table 4 — Minimum ultimate tensile loads — Coarse pitch thread

a Fmf values are calculated from exact As,nom figures as per 9.1.5, rounded to the nearest 10 N or 100 N above..
b Property class 50 refers to the austenitic grades A1 to A5 only.

Table 5 — Minimum loads at Rpf — Coarse pitch thread

a Fpf values are calculated from exact As,nom figures as per 9.1.5, rounded to the nearest 10 N or 100 N above..
b Property class 50 refers to the austenitic grades A1 to A5 only.