A2-70 vs A4-80 Stainless Fasteners: When Does the Application Require A4?

Does chloride exposure force A4/316?

Chloride ions raise electrolyte conductivity and break down the protective oxide layer on stainless, which is what drives pitting and crevice corrosion ¹⁵. Molybdenum in 316 resists that attack, which is why 316 is the standard recommendation for marine fasteners and 304 is rated moderate-good generally but not recommended for direct saltwater exposure ¹⁷. Saltwater is one of the most aggressive electrolytes, so galvanic compatibility matters most in marine, coastal and road-salt-exposed work ¹⁵. For stainless-to-stainless joints the grade choice matters because not all stainless alloys behave the same way in chloride-rich environments ²⁷. Docks and deck hardware exposed to direct saltwater are the named condition under which 316/A4 is the standard recommendation ¹⁷. The same source rates 316 excellent for direct saltwater and puts duplex 2205 above it for offshore and splash-zone structural work ¹⁷. Stainless is also often preferred for coastal environments, marine applications, food processing, chemical processing and highly corrosive conditions ¹⁶. The sources do not establish a specific chloride concentration or exposure duration at which A4 becomes mandatory over A2 ¹⁷. If the joint sits in a chloride-bearing environment but not in direct saltwater, ask the supplier to state the grade they would supply and why, rather than assuming a threshold.

Does the design load actually need the 800 MPa class?

The letter in the property class names the steel group and the number is the minimum tensile strength in tens of MPa, so A4-80 means 316 stainless at 800 MPa minimum tensile ²³. That 800 MPa is reached by cold working rather than by heat treatment ¹¹. Stainless bolts cannot be heat-treated to grades like 10.9 or 12.9, so when a joint needs both high strength and corrosion resistance, engineers typically move to coated carbon or alloy steel bolts instead ²³. That makes the strength question a routing decision, not just a grade upgrade. If the joint's load demand sits within what cold-worked 316 can carry, A4-80 keeps the corrosion resistance in the same part. If the demand is higher than stainless can reach, the corrosion resistance has to come from a coating on a carbon or alloy steel bolt ²³. The sources do not state the A2-70 minimum tensile value, so the numeric gap between A2-70 and A4-80 cannot be quantified from this evidence ²³. Ask the supplier for the proof stress and hardness values of the specific class before sizing the joint.

Does the aggressive-service caution on A4-80 change the grade call?

Cold working to reach 800 MPa is the same process that makes A4-80 worth cautioning in aggressive environments ¹¹. Severe cold deformation can transform some austenite into strain-induced martensite, which is the susceptible phase, so a high-strength cold-worked stainless is not simply a stronger version of the annealed grade ¹¹. The sources flag the caution but do not quantify the service limit, so the buyer should ask the supplier what environment the A4-80 they are quoting has been qualified for. Galling is the second assembly risk. It applies when any component in the joint is stainless steel or titanium, and the response is to specify anti-seize or grade differentiation ²¹. That is a joint-level decision: a stainless bolt into a stainless nut or a stainless parent thread is the condition to check, and the specification should say which of the two controls is required ²¹.

What documentation rules out a quote or lot?

A finish or grade claim is only worth what the paperwork behind it proves, so the certificate is the evidence that the material paid for is the material received ³⁵. ISO 3506 covers corrosion-resistant stainless steel grades, while ISO 898-1 supports mechanical property verification for many steel fasteners ²⁵. Make the following conditions of acceptance rather than follow-up requests.

• Material certificate naming the stainless grade and property class ³⁵
• Chemical composition report for the delivered lot ³⁵
• Mechanical test results for the specified property class ³⁵
• Hardness testing records where the standard or inspection plan requires them ³⁵
• Dimensional and thread inspection records ³⁵
• Coating or finish verification if a finish is specified ³⁵
• Lot traceability linking the certificate to the delivered goods ³⁵

How should the purchase specification be written?

The standard number alone is not a complete purchasing specification ¹³. A dimensional standard such as ISO 4014 or ISO 4017 defines the hex fastener form; the property class or stainless grade has to be specified separately, because changing from 8.8 to 10.9 is a mechanical-property change while changing from ISO 4014 to ISO 4017 is a dimensional and thread-length change ³¹. A complete inquiry separates the items the supplier has to quote against. State the dimensional standard and edition, nominal diameter and length, thread designation, the stainless grade and property class, the coating or finish, the quantity, and the inspection and documentation scope ²⁶. Attach the drawing whenever the geometry is non-standard ²⁶. That structure lets the supplier distinguish a standard product from a drawing-based special part and price the documentation scope rather than assuming it ²⁶.

What to ask on price, lead time and MOQ

Stainless is often preferred for coastal, marine, food processing, chemical processing and highly corrosive conditions, and the higher initial cost is offset by long-term corrosion resistance in those applications ¹⁶. That is the commercial case for the grade, but it is not a price figure. No source in this set gives an A2-70 versus A4-80 price differential, a lead time, or a minimum order quantity for specific sizes, so those have to be quoted per size and volume. One supplier states that minimum order quantities may apply to general fixings, which is the shape of the constraint to confirm rather than a number to plan against ²⁸. Ask each supplier for the price, lead time and MOQ at the actual order quantity, and ask whether the documentation scope in the previous section changes either the price or the lead time.

Which figures the sources state from only one side

The table separates what is corroborated from what rests on a single source, so the buyer knows which values to verify before they enter the specification.

FAQ

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