The designation system for stainless steel types and property classes of screws and nuts is described in the following overview. The designation of the material consists of two blocks that are separated by a hyphen.
The first block designates the steel type as follows:
A for austenitic nickel chromium steel with an alloying constituent of 15-20% chromium and 5-15% nickel. It cannot be hardened with heat treatment and is generally not magnetizable.
C for martensitic steel, which can be strengthened by hardening and is magnetizable. It is less resistant to corrosion than austenitic steels.
F for ferritic steel, which cannot normally be hardened. It is magnetizable and the environments in which it can be used include those with higher chloride content.
The letter is supplemented with a number that indicates the chemical composition field within this steel group.
A 1 The steel type A 1 is intended especially for processing by cutting (turning parts). Due to their high sulphur content, steels of this type are less resistant to corrosion than the other steel types.
A 2 Steels of the type A 2 are the most frequently used. They are, however, unsuitable for use in non-oxidizing acids and media containing chloride (e.g. swimming pools, salt water). Suitable for temperatures down to -200°C.
A 3 Same properties as A 2 steels, but stabilised with titanium, niobium or tantalum. These improve its resistance to corrosion in high temperatures.
A 4 Same properties as A 2 steels, but alloyed with 2-3% molybdenum. This makes it substantially more resistant to corrosion and acids. Suitable for temperatures down to -60°C.
A 5 Same properties as A 4 steels, but stabilised with titanium, niobium or tantalum. This also makes it resistant to high temperatures.
The second block denotes the property class, with the numbers indicating 1/10 of the minimum tensile strength of the fasteners (in N/mm²).
A 2-70 = austenitic steel, strain-hardened, tensile strength at least 700 N/mm²
Notwithstanding the aforementioned regulation, the property class of thin nuts (height = 0.5 – 0.8d, e.g. DIN 439, ISO 4035) is indicated with three digits, with the 0 in front referring to the lower stability.
A 2-035 = austenitic steel, strain-hardened, testing stress up to at least 350 N/mm²
Stainless steel with a particularly low carbon content of no more than 0,03% may be designated additionally with the letter L (e.g. A 4L-80).