Stainless steel screws

The screws marked A2 and A4 are the most commonly used stainless steel fasteners. The marking A2 means a commonly known and valued “non-rusting bolt” (Polish: “nierdzewka”), while the marking A4 means that it is a bolt adapted to the use in sulfuric acid. These screws are of course made of different types of steel and have different mechanical properties.

For the production of corrosion-resistant screws, the following steel types are used:

• Austenitic, where there is a division into groups from A1 to A5. This type of steel is usually non-magnetic and hardened.
• Martensitic, marked with the symbols C4, C3, C1. This type of steel is strongly hardened and has magnetic properties.
• Ferritic, marked with the symbol F1. These steels are not hardened and do not have magnetic properties.

The most commonly used type of steel are austenitic steels. The screws of the other two types of steel are rather rare.

The class of the mechanical properties is defined by two digits which denote the tensile stress ratio in Mpa. For ferritic steels these will be grades 45 and 60, for martensitic steels 50, 70, 80 and 110, and for austenitic steels 50, 70 and 80.

For example, the number 60 means 600Mpa and 110 means 1100Mpa etc.

Application:

The most popular fasteners are marked with the symbol A2. Popular “non-rusting bolts” are often used in the production of kitchen furniture, or various types of chemical equipment. However, they are not used where they are exposed to non-oxidizing acids and (common, sea) salt. In this case, A4s, which can be used in an environment where chlorine compounds (the aforementioned salt), show much greater resistance. Because of that, they are most often used in shipbuilding and in the food industry.