Stainless Steel Bolt Grades: Alloys, ISO 3506, ASTM Standards Explained

Selecting the wrong stainless steel bolt grade is one of the most common — and most costly — mistakes in fastener specification. A bolt that looks identical to the correct one may corrode within months in a marine environment, seize permanently during installation, or fail under load in a high-temperature application. The reason is that "stainless steel bolt grade" actually refers to two distinct dimensions that must both be matched to the application: the alloy grade (the chemical composition of the steel, which determines corrosion resistance and temperature performance) and the strength class (the mechanical properties after manufacturing, which determines load-bearing capability). Understanding both — and how international standards define them — is the foundation of reliable fastener specification.

Why Stainless Steel Grade Matters for Bolts

Stainless steel fasteners are chosen primarily for their corrosion resistance, not their raw strength. Carbon steel bolts of the same diameter will typically outperform stainless in tensile strength, but they corrode rapidly in wet, chemical, or marine environments. Stainless steel derives its corrosion resistance from chromium: a minimum of 10.5% chromium content causes a thin, self-repairing chromium oxide layer to form on the surface, passivating the metal against oxidation. Unlike a zinc coating or paint, this passive layer is intrinsic to the material — scratching the bolt during installation does not remove it.

However, not all stainless steel performs equally in all environments. A bolt specified as "stainless steel" without a defined grade is an incomplete specification. The difference between a 304 bolt and a 316 bolt may be invisible to the eye, but in a chloride-rich coastal or offshore environment, one will last decades while the other corrodes within years. Equally, two bolts of the same alloy grade — say, both 316 — may have tensile strengths differing by 30% or more depending on their strength class designation.

The Three Families of Stainless Steel

Stainless steel is not a single material but a family of iron-chromium alloys whose microstructural characteristics — determined by alloying elements and heat treatment — define their mechanical and corrosion properties. For bolts, three families are relevant.

For the vast majority of industrial, marine, construction, and chemical fastening applications, austenitic stainless steel is the standard choice. Its combination of corrosion resistance, weldability, and fabrication characteristics makes it the dominant family in global fastener production. Martensitic grades are used where high hardness is required and corrosion demands are moderate — such as cutlery screws or certain tooling applications. Ferritic grades are rarely specified for structural bolts due to their lower corrosion resistance and limited strength.

Common Stainless Steel Bolt Alloy Grades

Within the austenitic family, several specific alloy grades dominate fastener production. Each represents a distinct balance of corrosion resistance, strength, cost, and machinability.

Grade 304 / 18-8

Grade 304 is the most widely used stainless steel in the world and the standard alloy for general-purpose fasteners. Its composition — approximately 18% chromium and 8% nickel — is the origin of the "18-8" designation still widely used in the North American market. Grade 304 offers good corrosion resistance in most atmospheric, freshwater, and mild chemical environments. It is used extensively in construction, food processing equipment, architectural hardware, and automotive trim. Its limitation is sensitivity to chloride-rich environments: in marine atmospheres, saltwater, or de-icing salt exposure, pitting and crevice corrosion can develop over time. The metric ISO equivalent is A2.

Grade 316 / 316L

Grade 316 adds 2–3% molybdenum to the 304 composition, which significantly increases resistance to chloride pitting and crevice corrosion. This is the defining upgrade that makes 316 the standard "marine grade" stainless steel. It is specified for offshore platforms, coastal structures, chemical processing, pharmaceutical equipment, and any environment involving chlorides, acids, or reducing chemicals. Grade 316L is a low-carbon variant (maximum 0.03% carbon versus 0.08% in standard 316) that reduces the risk of sensitization and intergranular corrosion in welded assemblies. The metric ISO equivalent is A4.

Grade 303

Grade 303 is a free-machining variant of 304, with sulfur or selenium added to improve machinability and chip breakage during high-speed CNC turning. This makes it popular for precision-machined fasteners and screw machine products. The trade-off is slightly reduced corrosion resistance compared to 304, particularly in weld zones. Grade 303 should not be used where continuous immersion or aggressive chemical exposure is expected.

Grade 310S

Grade 310S contains approximately 25% chromium and 20% nickel, giving it exceptional oxidation resistance at elevated temperatures — up to 1,100°C for intermittent service. It is specified for high-temperature bolting in furnaces, kilns, heat treatment equipment, and exhaust systems where standard austenitic grades would rapidly oxidize or lose strength.

Duplex Grade 2205

Duplex stainless steel — so named because its microstructure contains approximately equal proportions of austenite and ferrite — offers a tensile strength nearly double that of standard austenitic grades, combined with excellent resistance to chloride stress corrosion cracking. Grade 2205 (UNS S32205) is the most widely used duplex grade and is increasingly specified in oil and gas, offshore, subsea, and desalination applications where standard 316 has proven insufficient. Its higher strength allows downsizing of fastener diameter for weight-critical designs, and its chloride resistance makes it suitable for permanent immersion in seawater — applications where 316 would eventually pit and fail.

ISO 3506 Strength Classes: A2-50, A2-70, A4-70, A4-80

ISO 3506 is the international standard that defines both the alloy group and the mechanical strength class of stainless steel fasteners in a single compact designation. Understanding the code structure is essential for interpreting fastener certificates, drawings, and procurement documents.

The designation system uses a letter-number-number format. The letter-number prefix identifies the alloy group: A1 corresponds to free-machining grades (303 type); A2 corresponds to standard austenitic grades (304 type); A4 corresponds to molybdenum-bearing austenitic grades (316 type). The second number indicates the minimum tensile strength in units of 10 MPa — so A2-70 means a minimum tensile strength of 700 MPa, and A4-80 means 800 MPa.

The strength difference between the -50 and -70 suffix is achieved through cold working during manufacturing — not heat treatment, which is not applicable to austenitic stainless. This is an important distinction: a bolt marked A2-70 has been cold-worked to a higher strength level than A2-50 from the same base alloy. For most structural and mechanical applications, A2-70 and A4-70 are the baseline specifications. A4-80 is specified where higher clamping force is required in corrosive environments. For further guidance on applying these grades to hex bolt selection specifically, see our stainless steel hex bolts guide.

ASTM Standards: B8, B8M, and F593

In North American markets — and in projects governed by ASME pressure vessel or piping codes — stainless steel bolts are frequently specified under ASTM standards rather than ISO 3506. Three designations appear most commonly on engineering drawings and procurement specifications.

The key distinction between A193 and A320 lies in the service temperature range: A193 B8 is specified for elevated-temperature applications in pressure vessels and flanged piping systems; A320 B8 covers the same alloy but is qualified specifically for cryogenic and sub-zero service where notch toughness at low temperatures must be verified. Both require carbide solution annealing and carry specific head-marking requirements — forged hex bolts must be stamped with "B8" or "B8M" along with the manufacturer's identification mark. ASTM F593 applies to headed fasteners for structural applications and includes four condition classes that correspond to different cold-working levels and their associated strength levels.

Galling: The Hidden Risk of Stainless Fasteners

Galling is a form of adhesive wear that occurs when two stainless steel surfaces slide against each other under pressure — as happens during bolt tightening. The passive chromium oxide layer that gives stainless its corrosion resistance also has a tendency to break down under the high contact pressure and frictional heat of thread engagement. When this happens, the raw metal surfaces weld together momentarily, tearing metal from one surface and depositing it on the other. The result ranges from rough thread damage to complete seizure — the nut and bolt fused together and impossible to separate without destruction.

Galling is most common with austenitic grades (particularly 304 and 316) because their relatively low hardness and tendency to work-harden during sliding makes them susceptible. The risk is compounded when both the bolt and nut are the same grade. Practical prevention measures include:

• Applying an anti-galling lubricant — nickel-based anti-seize compound, molybdenum disulfide paste, or PTFE-based thread lubricant — to all threads before assembly.
• Tightening at a slow, steady speed rather than using high-speed pneumatic impact tools.
• Pairing fastener components with different hardness levels where possible — for example, using a higher-strength class nut with a standard class bolt increases the hardness differential between mating surfaces.
• Specifying rolled threads rather than cut threads, as rolled thread surfaces are smoother and work-hardened during the rolling process, reducing susceptibility.

Contrary to a common misconception, pairing a 316 bolt with a 304 nut (or vice versa) does not reliably prevent galling — the hardness difference between these two grades is insufficient. Genuine galling prevention requires either lubrication, a meaningful hardness differential, or the use of a different fastener material for one of the mating components.

How to Choose the Right Stainless Steel Bolt Grade

Grade selection requires evaluating the application against four key variables: environment, mechanical load, applicable standards, and total cost of ownership.

Step 1 — Define the corrosive environment

Indoor or sheltered atmospheric service with no chemical exposure: Grade 304 / A2 is sufficient. Outdoor exposure to rain, humidity, or mild industrial atmosphere: Grade 304 / A2-70. Coastal location, marine atmosphere, or intermittent saltwater contact: Grade 316 / A4. Direct immersion in seawater, concentrated chlorides, or aggressive acids: Grade 316L / A4, or duplex 2205 for long-term service. High-temperature oxidizing environments above 600°C: Grade 310S.

Step 2 — Determine the mechanical load requirement

Match the ISO 3506 strength class to the clamping force and tensile load calculated in your joint design. For most non-critical structural applications, A2-70 or A4-70 is the appropriate baseline. Where higher preload is needed in corrosive environments, specify A4-80. For ASME-governed pressure equipment or low-temperature service, specify ASTM A193 B8 / B8M or A320 B8 / B8M as applicable.

Step 3 — Confirm the applicable standard

Projects governed by European or international standards typically specify ISO 3506 designations. North American projects governed by ASME B31, ASME VIII (pressure vessels), or structural steel codes typically reference ASTM standards. Confirm which system applies before ordering — the alloy may be equivalent, but certification documents, head markings, and testing requirements differ. Jiangsu Jiajie produces fasteners compliant with GB, DIN, and ISO standards and can supply material test certificates upon request for export projects. For questions on matching specific application requirements to the right stainless steel hex nuts and bolts from our product range, our engineering team is available within 12 hours.

Step 4 — Account for galling risk and installation conditions

If the assembly will be tightened to high torque, or if disassembly will be required periodically, specify anti-seize lubrication as part of the installation procedure and consider specifying rolled-thread fasteners. For assemblies that must be disassembled regularly — such as maintenance flanges in chemical plants — the long-term cost of fasteners that gall and must be cut off is substantially higher than the initial cost of a premium lubricated or coated specification.