316L Hex Bolts vs 304 vs 316: Which Stainless Grade Should You Specify?

316L hex bolts are hexagon-head fasteners made from low-carbon 316L austenitic stainless steel, the molybdenum-bearing grade that resists chloride pitting and survives welded assembly without intergranular corrosion. When a project sits in saltwater, coastal, chemical, or food-contact service, the grade you put on the drawing matters more than the bolt dimensions. This review compares 316L against 304 and 316 across corrosion resistance, strength, welding behavior, and procurement criteria so you can specify the right hex bolt the first time.

316L hex bolts are made from low-carbon 316 stainless (UNS S31603 / A4) with 2–3% molybdenum, which gives far better resistance to chloride pitting than 304. The low carbon (≤0.03%) prevents sensitization, so 316L stays corrosion-resistant after welding, unlike standard 316. Specify them by ISO 3506 property class A4-70 or A4-80 and DIN 933/931 or ISO 4017 dimensions.

Quick Takeaways

• 316L is the low-carbon version of 316; the "L" caps carbon at 0.03% to prevent weld sensitization and intergranular corrosion.
• Molybdenum (2–3%) is what separates 316/316L from 304 in saltwater and chemical service.
• In ISO 3506 fastener terms, 316/316L is grade A4; 304 is grade A2.
• A4-70 (700 MPa tensile) suits general use; A4-80 (800 MPa) is the high-strength option for load-critical joints.
• 316L still needs passivation, and in hot chloride service it can suffer stress-corrosion cracking above roughly 60 °C.
• Standard dimensions come from DIN 933 (fully threaded), DIN 931 (partially threaded), ISO 4017, and ASME B18.2.1.

What Is a 316L Hex Bolt?

A 316L hex bolt is a six-sided-head fastener machined or cold-headed from 316L stainless steel, an austenitic grade that ranks as the second most common stainless steel after 304. According to the reference description of SAE 316L stainless steel, its primary alloying elements after iron are chromium (16–18%), nickel (10–12%), and molybdenum (2–3%), with up to 2% manganese and small quantities of silicon, phosphorus, and sulfur. The molybdenum is the load-bearing difference: it gives 316L substantially better resistance than 304 to localized corrosion from chlorides and to general corrosion from reducing acids such as sulfuric acid.

316L Stainless Steel: Composition and What the "L" Means

The "L" in 316L stands for low carbon. Standard 316 allows up to 0.08% carbon, while 316L caps it at 0.03%. That small delta changes how the steel behaves after welding. At elevated temperature, carbon in ordinary 316 can combine with chromium at grain boundaries to form chromium carbides, a process called sensitization. The chromium-depleted zones that result are vulnerable to intergranular corrosion. By keeping carbon low, 316L leaves too little carbon available to form a continuous carbide network, so it resists sensitization even in the as-welded condition.

Being austenitic, 316L is essentially non-magnetic in the annealed condition, although cold working during bolt manufacture introduces a slight magnetic response — a property that occasionally confuses buyers who expect stainless fasteners to be fully non-magnetic. Cold working is also how 316L reaches its higher strength classes: the deformation of thread rolling and cold heading raises yield strength without changing the alloy chemistry.

A4 vs 316L: How Fastener Grades Map to Steel Grades

Fastener standards name stainless steel differently from the steel mills. ISO 3506 classifies corrosion-resistant stainless fasteners into grades A1 through A5, where A2 corresponds to the 304 family and A4 corresponds to the 316/316L family. A4 is therefore the fastener-specification name for molybdenum-bearing, marine-capable stainless steel. In practice, "A4 hex bolt" and "316L hex bolt" describe the same corrosion-resistant hardware; the EN material number 1.4404 and the UNS designation S31603 are two more names for the same low-carbon molybdenum steel.

316L vs 316 vs 304: Head-to-Head Comparison

The three grades are frequently used interchangeably on drawings, but they are not interchangeable in service. Whether you are ordering stainless steel hex bolts in 304, 316, or 316L, the grade decision changes how the fastener performs in the field. The table below compares them across the dimensions that decide where each one belongs.

For a broader look at how stainless grades perform across fastening applications, see AOZHAN's stainless steel hex bolt applications guide.

Chemical Composition and the Molybdenum Difference

Molybdenum is the element that earns 316 and 316L their marine reputation. Chlorides — the active ingredient in seawater, de-icing salt, and many process fluids — attack the passive oxide film that protects stainless steel. Molybdenum stabilizes that film and makes it harder for chloride ions to initiate localized attack. A widely used ranking, the pitting resistance equivalent number (PREN = %Cr + 3.3 %Mo + 16 %N), places the 316 family around 24–28 versus roughly 18–20 for 304. In practical terms, that gap is why 316L hex bolts are specified on docks, offshore platforms, and coastal railings while 304 is reserved for indoor or mildly corrosive duty.

Corrosion Resistance: Pitting, Crevice, and Chloride Attack

Stainless steel does not fail by uniform rusting; it fails by localized mechanisms, and 316L is designed to delay all of them. Pitting corrosion starts as tiny pits where chlorides breach the passive film, and it is the leading failure mode in seawater. Crevice corrosion attacks the oxygen-starved gaps under bolt heads and washers. Stress corrosion cracking is a brittle, delayed failure that develops when tensile stress, a hot chloride environment, and a susceptible grade combine — a real risk for austenitic stainless bolts in warm chloride service. Molybdenum raises the threshold for pitting and crevice attack, but it does not eliminate stress-corrosion cracking; above roughly 60 °C, even 316L can crack in chloride-rich environments, which is why hot-seawater duties may step up to duplex or super-austenitic grades.

One practical detail decides whether a 316L hex bolt actually delivers its rated corrosion resistance: passivation. Machining and cold heading can embed microscopic iron particles in the surface, and free iron rusts even when the surrounding steel will not. Passivation — a controlled nitric or citric acid treatment — removes surface iron and thickens the protective chromium oxide film. A marine-spec 316L bolt that is not passivated will underperform the same way a properly treated one will not, which is why reputable suppliers state passivation as part of the surface finish.

Why the "L" Matters: Welding and Intergranular Corrosion

Intergranular corrosion is the specific reason the "L" exists. When standard 316 is welded or heated into the 425–870 °C sensitization range, chromium carbide precipitates along grain boundaries, depleting the adjacent metal of chromium and leaving a corrosion-susceptible network. For a bolt, this matters whenever the fastener is welded in place, used near a weld heat-affected zone, or heat-treated after manufacture. 316L avoids the failure mode by design: with carbon held to 0.03% maximum, there is not enough carbon to form the damaging carbide network, even after slow cooling through the sensitization range. This is why pressure-vessel and piping codes permit 316L in the as-welded condition for many services where standard 316 would require post-weld solution annealing. For fasteners that will be welded, heat-treated, or placed beside a weld, 316L is the safer specification.

In fastener terms, intergranular corrosion shows up as a fine network of corrosion along grain boundaries that can cause a bolt to crumble or lose load capacity long before any visible rust appears. The risk is not limited to bolts that are themselves welded: any 316 hex bolt placed close to a weld, exposed to a heat-treating step, or subject to repeated hot-service thermal cycling can drift into the sensitization range. Specifying 316L removes this variable, which is why fabricators who weld brackets or frames to stainless fasteners, or who use bolts in hot process piping, default to the low-carbon grade.

Mechanical Properties: A4-70 vs A4-80 vs What 316L Delivers

Stainless steel fasteners are specified by property class rather than by a simple "grade 8" number. ISO 3506 defines the mechanical requirements, and the A4 classes are what 316/316L hex bolts are ordered to.

A4-70 is the workhorse for most marine and chemical bolting, while A4-80 is the cold-worked, higher-strength option for joints that must carry more load without yielding. Note that stainless fasteners are generally softer than alloy-steel bolts of the same size, so a 316L hex bolt reaches its strength through cold working and is not a drop-in strength substitute for a 10.9 or 12.9 steel bolt. For load-critical chloride service, AOZHAN's A4-80 stainless steel bolt corrosion resistance guide explains where the higher class is justified.

One more point trips up buyers moving from alloy steel to stainless. A4-80's 800 MPa tensile strength sounds impressive, but the 0.2% proof stress of 600 MPa is the number that governs permanent deformation, and it is still well below a grade 10.9 alloy-steel bolt. Stainless hex bolts also have a lower modulus and higher thermal expansion than carbon steel, which changes preload retention and relaxation in a joint. Engineers should size the bolt to the stainless property class, not simply swap a stainless grade into a carbon-steel joint design.

Standards to Specify: ISO 3506, ASTM F593, DIN 933/931

Putting the right standards on the drawing is what makes a 316L requirement enforceable and traceable. Four references cover nearly every 316L hex bolt order.

A complete specification states the material, the property class, and the dimensional standard together — for example, "hex bolt, 316L (A4-80), ISO 4017, M12 × 40." ASTM F593 is the North American counterpart for cold-worked 316L hex cap screws and hex bolts, and it is the reference to cite when a material test certificate or EN 10204 inspection document is required.

Where 316L Hex Bolts Are Used: Marine, Chemical, Food & Pharma

316L earns its specification in environments that defeat ordinary stainless. In marine and offshore service, marine grade stainless fasteners must resist decades of salt spray, and 316L hex bolts are the standard choice for deck hardware, dockside structures, and coastal installations where 304 would pit within a few seasons. Chemical and petrochemical plants specify 316L for its resistance to reducing acids and chloride-bearing process streams. Food, pharmaceutical, and potable-water applications favor the low-carbon grade because it resists sensitization during the frequent weld repairs and hot cleaning those industries demand.

AOZHAN manufactures 316L hex bolts in-house: wire rod is drawn to size, cold-headed into the hexagon form, and thread-rolled to the required pitch, after which the finished fasteners are passivated to restore the protective oxide film. Each production lot is traceable to its heat number, and material test certificates are issued to confirm chemistry and mechanical properties. This control is documented in AOZHAN's production and inspection workflow, and representative installations appear in AOZHAN's customer cases.

Across AOZHAN's supply record, 316L hex bolts have gone into coastal and process-industry assemblies where both salt spray resistance and weldability were written into the specification: stainless railing and cladding fixings near the seafront, chemical plant flanges, and food-processing equipment frames that are hot-washed and frequently repaired by welding. In each case the low-carbon grade was chosen specifically so that field welding would not create a corrosion-weak grain-boundary network in the heat-affected zone.

How to Choose the Right 316L Hex Bolt for Your Project

Selection should follow the environment first, the mechanical requirement second, and the specification third. Working in that order prevents the common mistake of fixing dimensions before the corrosion and load cases are known.

A 4-Step Selection Path

1. Define the environment. If chlorides, seawater, or corrosive chemicals are present, choose 316L over 304. If the fastener will be welded or heat-treated, choose 316L over 316.
2. Fix the property class. Use A4-70 for general corrosion-resistant duty; move to A4-80 only where higher tensile and proof strength are required.
3. State the standards. Specify the material (316L / A4), the dimensional standard (ISO 4017, DIN 933/931, or ASME B18.2.1), and the material standard (ISO 3506 or ASTM F593).
4. Confirm surface finish and documentation. Require passivation for optimal corrosion resistance and a material test certificate for traceability.

Pre-Order Inspection Checklist

• Material grade stated as 316L (or A4, UNS S31603, 1.4404) on the purchase order
• Property class named (A4-70 or A4-80) with the governing ISO 3506 or ASTM F593 standard
• Dimensional standard and thread pitch specified (ISO 4017, DIN 933/931, ASME B18.2.1)
• Passivation finish required for marine or food-contact service
• Material test certificate (EN 10204 3.1) requested with the shipment
• Heat-number traceability confirmed for critical applications

For a quotation or to confirm a grade and property class against your project spec, contact AOZHAN's engineering team.

316L Hex Bolts FAQ

Will 316L bolts rust?

316L is highly corrosion-resistant, but it is not rust-proof. It can still pit in hot or stagnant chloride environments and can suffer stress-corrosion cracking in hot chloride service above roughly 60 °C. Proper passivation and appropriate grade selection minimize these risks.

Are A4 and 316L the same thing?

Yes, in fastener terminology A4 is the ISO 3506 grade for the 316/316L stainless family. "A4 hex bolt" and "316L hex bolt" refer to the same molybdenum-bearing, marine-capable hardware.

What is the difference between 316 and 316L?

Both contain 2–3% molybdenum. 316L is the low-carbon version (≤0.03% C versus ≤0.08%), which resists weld sensitization and intergranular corrosion and is the better choice for welded or heat-treated fasteners.

Can 316L bolts be used in seawater?

Yes — 316L is a standard marine-grade fastener material for seawater and coastal exposure. In continuously submerged or hot-chloride service, higher grades such as duplex stainless may be warranted.

How strong is a 316L bolt?

Depends on the property class. A4-70 offers 700 MPa minimum tensile strength, and A4-80 offers 800 MPa with 600 MPa proof stress. Both are softer than high-strength alloy-steel bolts of the same size.

What standards cover 316L hex bolts?

Mechanical properties come from ISO 3506 (grade A4) or ASTM F593 (Type 316L), and dimensions from DIN 933, DIN 931, ISO 4017, or ASME B18.2.1.