Stainless Steel Hexagon Head Bolts

Give thread, length and quantity, then the grade and property class to ISO 3506-1, for example "M12 × 50, A4-80". The letter is the steel group: A austenitic, C martensitic, F ferritic. The following digit, 1 to 5, rates corrosion resistance. The last two digits are the property class: 70 means 700 N/mm² tensile strength and 80 means 800 N/mm².^[2]

A2 has the corrosion resistance of 304 (1.4301). A4 contains molybdenum and matches 316 (1.4401). For bolts exposed to seawater or salt spray, ask for A4. Atlas Steels puts 304 as susceptible to pitting in seawater above about 10 °C.^[2][3]

An M size without a pitch means coarse thread (for example 1.75 mm for M12). Give the pitch only for a fine-thread bolt.^[4]

ISO 3506-1:2020 sets the mechanical and physical properties of stainless steel bolts, screws and studs with coarse threads M1.6 to M39 and fine threads M8×1 to M39×3. It covers austenitic, martensitic, ferritic and duplex grades. Bolts to it are used without restriction from −20 °C to +150 °C. Outside that range the choice should be made with a fastener metallurgist.^[1]

Class 70 bolts are made from cold-drawn bar and class 80 from severely cold-drawn bar, close in strength to an 8.8 carbon steel bolt. Class 50 is the annealed condition. With machined threads it is more prone to galling.^[2]

To avoid bimetallic corrosion, use stainless bolts when joining stainless steel parts. They are also suitable for galvanised steel and aluminium parts.^[2]