A bolt head stamped A4-80 carries two separate pieces of information, and confusing them puts the wrong fastener into the wrong environment. A2 and A4 identify the stainless steel material group, which governs corrosion behavior. The numbers 70 and 80 identify the mechanical property class, which governs strength. Stainless steel fasteners carry both markings because neither describes the other, and specifying bolts, nuts, screws or studs correctly means reading the two halves together.<\/p>\n\n\n\n
A2 and A4 designate the two austenitic stainless steel groups used across most fastener production. A2 corresponds broadly to 304-type stainless steel and covers general-purpose fastening in machinery, structures and assembled equipment. A4 corresponds to 316-type stainless and carries molybdenum, which improves resistance to pitting and crevice corrosion in chloride-bearing conditions. Coastal construction, marine equipment, and chemical-processing installations may specify A4 where improved resistance to chloride-related corrosion is required. Chlorides attack 304-type steel at concentrations that A4 tolerates without visible pitting.<\/p>\n\n\n\n
70 and 80 are mechanical property classes under ISO 3506, and each number maps directly to a minimum tensile strength. Class 70 requires 700 MPa minimum tensile strength, with minimum stress at 0.2% permanent set of 450 MPa. Class 80 lifts those to 800 MPa and 600 MPa. Austenitic stainless will not harden through heat treatment, so manufacturers reach both classes by cold working during production. That constraint bites at larger diameters, where class 80 becomes difficult to achieve and ASTM A193 B8 or B8M Class 2 often takes over as the practical route from M20 upward.<\/p>\n\n\n\n
Combining the two halves of the marking produces four common specifications, each answering a different pair of demands.<\/p>\n\n\n\n *Values apply to relevant bolts, screws, and studs under the applicable standard.<\/p>\n\n\n\n A4-70 is not stronger than A2-70. Both meet the same 700 MPa minimum; the difference sits entirely in corrosion performance, since the material group and the property class describe unrelated characteristics. Moving from A2-70 to A4-70 buys chloride resistance, not load capacity. Moving from A2-70 to A2-80 buys strength, not durability in salt spray.<\/p>\n\n\n\n Exposure conditions, not habit, should decide which group goes into an assembly.<\/p>\n\n\n\n A2 fasteners hold together general machinery, indoor equipment, furniture and sheltered outdoor assemblies where moisture appears but chlorides do not accumulate. A4 fasteners go into marine equipment, coastal installations, chemical-processing systems and anywhere chloride-related corrosion threatens the joint.<\/p>\n\n\n\n A4 resists chlorides; it does not defeat them. Severe chloride exposure, stagnant crevices and elevated temperatures can still produce pitting or stress corrosion cracking in 316-type material, so marine service alone does not guarantee A4 will last the design life.<\/p>\n\n\n\n At Vision Alloy, we help you choose the right stainless steel fastener by working through six critical checks before releasing your specification. First, we determine your required tensile strength and evaluate actual load conditions, including shear and cyclic loading. Next, we assess environmental exposure to moisture, chlorides, process chemicals, and operating temperatures. We then confirm all bolt, nut, screw, or stud dimensions against applicable standards and check compatibility with mating materials to prevent galvanic corrosion. To avoid thread galling since austenitic stainless steel seizes readily under high contact pressure we precisely calculate installation torque. Finally, we verify all certification and material traceability requirements before finalizing your order.<\/p>\n\n\n\nFastener Class<\/strong><\/td> Material Group<\/strong><\/td> Minimum Tensile Strength*<\/strong><\/td> Typical Selection Consideration<\/strong><\/td><\/tr> A2-70<\/td> A2 stainless steel<\/td> 700 MPa<\/td> General-purpose corrosion resistance and strength<\/td><\/tr> A4-70<\/td> A4 stainless steel<\/td> 700 MPa<\/td> Improved resistance to many chloride-containing environments<\/td><\/tr> A2-80<\/td> A2 stainless steel<\/td> 800 MPa<\/td> Higher specified strength for suitable applications<\/td><\/tr> A4-80<\/td> A4 stainless steel<\/td> 800 MPa<\/td> Higher specified strength combined with A4 corrosion resistance<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n Where Are A2 and A4 Stainless Steel Fasteners Used?<\/strong><\/h2>\n\n\n\n
How to Choose the Right Stainless Steel Fastener<\/strong><\/h2>\n\n\n\n
Conclusion<\/strong><\/h2>\n\n\n\n