ISO 3506 Compliant Fastener Forgings — Stainless Steel Property Classes A2-70, A4-70 & A4-80 for Corrosive-Environment Bolting
Shivam Forge manufactures forged stainless steel bolt, stud, and nut blanks to ISO 3506 (Mechanical properties of corrosion-resistant stainless steel fasteners) — the international standard governing property classes such as A2-70, A4-70, and A4-80 for austenitic stainless fasteners used where corrosion resistance, not just strength, is the design driver. Rajkot, India. Call +91-9265772827.
ISO 3506 exists as a separate standard from ISO 898 because stainless steel fasteners fail for fundamentally different reasons than carbon and alloy steel fasteners do, and a single property class system couldn't sensibly cover both. ISO 898 governs carbon and alloy steel fasteners like classes 8.8, 10.9, and 12.9, where the marking encodes tensile and yield strength derived from quench-and-temper heat treatment — the failure mode being designed against is primarily mechanical overload or fatigue. ISO 3506 governs austenitic, martensitic, and ferritic stainless fasteners, where corrosion resistance is frequently the primary specification driver and the material's strength comes from cold working and alloy content rather than heat treatment, since austenitic stainless steels like A2 (304-type) and A4 (316-type) are not hardenable by quench-and-temper. The marking convention reflects this: A2 or A4 identifies the steel grade family — A4 contains molybdenum for superior chloride and pitting resistance, parallel to how 316 outperforms 304 in the equivalent bar and forging grades — while the following number (50, 70, or 80) indicates the strength class achieved through controlled cold working, with higher numbers indicating higher strength but also higher cold-work reduction. This means an A4-80 fastener is stronger than an A4-70 of the identical base alloy, achieved purely through work-hardening rather than any change in chemistry, which is a genuinely different mechanism from how ISO 898's carbon steel classes reach their strength tiers. For marine hardware, chemical processing equipment, food and pharmaceutical fastening, and any bolted joint where the fastener itself must resist the same corrosive environment as the structure it's holding together, ISO 3506 is the standard that actually addresses the relevant failure mode.
Forged A2-70 stainless fastener blanks in 304-type austenitic chemistry for general corrosion-resistant bolting where chloride exposure is limited — food processing equipment, architectural fastening, and indoor industrial applications.
Forged A4-70 and A4-80 fastener blanks in 316-type molybdenum-bearing chemistry for marine, chemical processing, and coastal environments where chloride-induced pitting resistance is the specification driver, with A4-80 delivering higher strength through increased cold-work reduction.
Forged stainless nut blanks in property classes matched per ISO 3506-2 to corresponding bolt property classes, ensuring proof load compatibility across the complete corrosion-resistant bolted joint.
Forged stainless bolt and stud blanks across custom diameter and length ranges for OEM and project-specific bolting requirements outside standard catalog fastener dimensions.
Strength class (50, 70, or 80) developed through controlled cold-work reduction rather than heat treatment, since austenitic A2 and A4 stainless grades are not hardenable by quench-and-temper — process control on reduction ratio directly determines the achieved property class.
Positive material identification confirming A2 versus A4 grade family before forging, verifying molybdenum content for A4 material — critical because A2 and A4 fasteners can look visually identical but perform very differently in chloride environments.
Solution annealing available where application requirements call for it to restore maximum corrosion resistance after cold working, balanced against the strength class requirement since annealing softens the cold-worked structure.
Full chemical composition and mechanical property certification per EN 10204 3.1, with 3.2 third-party witnessed certification available for marine classification society and pharmaceutical project documentation requirements.
Fastener standards split into two families for a genuine engineering reason, not administrative convenience: carbon and alloy steel fasteners covered by ISO 898 fail primarily through mechanical overload or fatigue, and their strength comes from quench-and-temper heat treatment, while stainless steel fasteners covered by ISO 3506 are specified primarily to resist corrosion, and their strength comes from a completely different mechanism — controlled cold working of an austenitic microstructure that cannot be hardened by heat treatment in the first place. Trying to force stainless fasteners into the ISO 898 property class framework would misrepresent both how their strength is achieved and what property actually matters most in the applications where stainless fasteners get specified.
The grade family designation — A2 or A4 — is where the real engineering decision lives. A2 corresponds to 304-type chemistry, adequate for general corrosion resistance in indoor, low-chloride environments. A4 corresponds to 316-type chemistry with 2–3% molybdenum, the same alloying addition that separates 316 from 304 in bar stock and forgings generally, and it's this molybdenum that gives A4 fasteners meaningfully better resistance to pitting and crevice corrosion in seawater, de-icing salt spray, and chloride-bearing process chemicals. Specifying A2 where A4 is actually needed is a common and costly mistake precisely because the two grades are visually indistinguishable — a fastener that looks perfectly correct on receiving inspection can be the wrong chemistry entirely, which is why PMI verification on incoming stainless fastener material matters more than it might for carbon steel equivalents.
The strength class suffix — 50, 70, or 80 — layers a second dimension onto the grade family choice. Because austenitic stainless can't be strengthened by heat treatment, ISO 3506 achieves its strength tiers through controlled cold-work reduction during manufacture: more reduction produces higher strength but also somewhat reduced ductility. A4-70 covers the large majority of marine and chemical-processing bolting requirements; A4-80 is reserved for applications genuinely needing the additional strength margin within a corrosion-resistant chemistry. Correctly matching grade family to the corrosive environment and strength class to the mechanical load is the core of specifying ISO 3506 fasteners correctly, and it's a different specification exercise from selecting an ISO 898 carbon steel class.
For marine hardware fabricators, chemical processing equipment manufacturers, and industrial customers sourcing forged stainless fasteners to ISO 3506 property class, Shivam Forge manufactures A2 and A4 grade families across strength classes 50, 70, and 80 with PMI-verified chemistry on every heat. Contact our engineering team at +91-9265772827 or sales@shivamforge.com with your drawing and grade requirement for a manufacturability review and quotation.
A2 is a 304-type austenitic stainless chemistry without molybdenum. A4 is a 316-type chemistry containing 2–3% molybdenum, which significantly improves resistance to pitting and crevice corrosion in chloride environments such as seawater and de-icing salt exposure. A4 is specified wherever marine or aggressive chloride exposure is expected; A2 is adequate for general indoor or mild-exposure corrosion resistance.
ISO 898 classes like 8.8 and 10.9 develop their strength through quench-and-temper heat treatment of carbon or alloy steel. ISO 3506 classes like A4-70 and A4-80 develop strength through controlled cold working of austenitic stainless steel, which isn't hardenable by heat treatment. The numbers in ISO 3506 (50, 70, 80) indicate strength tier achieved by cold-work reduction, not a direct tensile-strength-times-100 reading the way ISO 898's first digit works.
Not necessarily. A4-80 requires more cold-work reduction than A4-70, which can slightly reduce ductility and ease of installation. A4-70 is adequate and more commonly specified for the majority of corrosion-resistant bolting applications; A4-80 is reserved for applications where the higher strength is genuinely needed within the corrosion-resistant chemistry, since specifying more strength than required adds cost without benefit.
Not reliably by eye — both are austenitic stainless steels with similar appearance. This is exactly why PMI (positive material identification) verification matters: an A2 fastener substituted where A4 was specified will look correct on visual inspection but will underperform in chloride service, so we verify molybdenum content on incoming material before forging rather than relying on mill certificates alone.
Yes. Nut blanks in property classes matched per ISO 3506-2 to corresponding bolt property classes are available, ensuring the nut's proof load capacity is compatible with the bolt it's paired with in the finished corrosion-resistant joint.
Why Choose Shivam Forge
Shivam Forge delivers precision hot-forged components from our integrated Shapar, Rajkot facility — covering forging, CNC machining, heat treatment, and quality inspection under one roof.
