A2-70 vs A4-70 Stainless Steel Fasteners: ISO 3506 Grade Comparison & Selection Guide

1. Introduction: Why Choosing Between A2-70 and A4-70 Matters

When specifying austenitic stainless steel bolts, screws, nuts and washers for solar mounting, structural steel and outdoor construction, A2-70 and A4-70 are two of the most common grades defined by ISO 3506.

Many project buyers and engineers make a costly mistake: they assume A2-70 and A4-70 can be swapped freely because both carry the property class "70". While their mechanical strength is identical, their resistance to chloride and pitting corrosion is not. Using A2-70 in coastal salt-spray environments can lead to premature pitting, crevice corrosion, bracket loosening and expensive rework.

This guide breaks down the difference between A2-70 and A4-70 fasteners, covers material chemistry, mechanical values, typical use cases for solar PV systems, and helps you pick the correct grade for your site conditions.

2. What Do A2-70 and A4-70 Designations Mean in ISO 3506?

ISO 3506 sets the standard for corrosion-resistant austenitic stainless steel fasteners. Each letter and number carries a fixed definition:

· A: Austenitic stainless steel. These fasteners gain strength through cold working, not heat treatment.

· A2: Equivalent to 1.4301 / AISI 304 stainless steel.

· A4: Equivalent to 1.4401 / AISI 316 stainless steel, widely called marine grade.

· 70: Property class. Minimum tensile strength 700 MPa, minimum 0.2% yield strength 450 MPa.

Key takeaway: The "70" rating applies equally to A2-70 and A4-70 fasteners. Strength will not be your deciding factor. The critical variable is corrosion resistance.

3. Material Chemistry: The Molybdenum Gap Between A2-70 and A4-70

3.1 A2-70 (1.4301 / 304) Composition

A2-70 stainless steel fastener contains roughly 18% chromium and 8% nickel, with no intentional molybdenum addition. Chromium creates the passive oxide layer that prevents general rust in normal atmospheric conditions.

Suitable for low-chloride environments, but vulnerable to pitting when exposed to salt spray, brine or persistent damp chemical cleaning.

3.2 A4-70 (1.4401 / 316) Composition

A4-70 also contains ~18% chromium and ~10% nickel, plus 2.0–3.0% molybdenum. Molybdenum is the alloying element that improves resistance to chloride-induced pitting and crevice corrosion.

This is the reason A4-70 is specified for coastal, marine and high-chloride sites.

4. Mechanical Properties of A2-70 and A4-70 Fasteners

Both grades are cold-worked to reach property class 70. The mechanical performance matches as below:

Important engineering note: Weak magnetism on A2-70 or A4-70 bolts does not indicate material substitution or poor quality. Cold working distorts the grain structure and introduces mild magnetism.

5. Core Performance Difference: Corrosion Resistance

5.1 A2-70: Good for Normal Atmospheric Exposure

A2-70 works reliably in inland outdoor sites, dry indoor equipment, handrails, cabinets and inland solar mounting systems where chloride levels are low.

Limitations: It cannot stand continuous salt mist, seawater splash, swimming pool environments or regular caustic washdown. Pitting corrosion will develop once the passive film breaks down under chloride attack.

5.2 A4-70: Superior Chloride and Pitting Resistance

Thanks to molybdenum, A4-70 marine grade fasteners offer much better tolerance for chlorides. It is the preferred selection for coastal solar farms, seaside structural brackets, water treatment equipment and pool facilities.

It is not completely immune to corrosion in extreme offshore splash zones, but it delivers a far longer service life than A2-70 under the same salty conditions.

6. Recommended Applications: A2-70 vs A4-70 for Solar & Construction

6.1 When to Specify A2-70 Fasteners

· Inland solar PV mounting systems without coastal salt exposure

· Indoor machinery, electrical cabinets, railings and general fabrication

· HVAC and dry food processing equipment

· Projects requiring stainless fasteners with controlled material cost

· Low humidity, low-chloride atmospheric environments

6.2 When to Specify A4-70 Fasteners

· Coastal solar farms and seaside PV bracket installations

· Marine hardware, dock and seawater splash zone structures

· Swimming pool, spa and water treatment plant equipment

· Food and pharmaceutical lines with frequent chemical sanitising

· Projects where long design life and low maintenance are mandatory

7. Cost, Machinability & Installation Considerations

A2-70 raw material and finished fastener pricing is generally lower than A4-70. If corrosion risk is minimal, A2-70 provides a cost-effective solution.

A4-70 alloy is more abrasive to cutting tools. Threading and machining take more effort, which contributes to higher production cost.

For both grades, stainless steel threads are prone to galling during tightening. We recommend applying anti-seize compound and controlling torque values to avoid thread seizure, especially on high-volume solar mounting installation.

8. Quick Selection Checklist: A2-70 or A4-70?

Pick A2-70 if:

· Site is inland, away from sea salt spray

· No regular exposure to brine, pool water or aggressive cleaners

· General atmospheric rust protection meets design requirements

Pick A4-70 if:

· Project is coastal, near ocean or subject to salt mist

· High chloride, wet cycling or repeated chemical washdown

· Engineer specifies marine-grade stainless for long-term reliability

9. Frequently Asked Questions (FAQ)

Q: Can I replace A4-70 with A2-70 to cut project cost?

A: No, this substitution is risky in chloride-rich environments. A2-70 and A4-70 share the same mechanical class 70, but corrosion performance is not equivalent. Upfront savings can lead to bolt pitting, bracket failure and costly field replacement. Always follow the project specification or site corrosion data.

Q: Are A2-70 and A4-70 fasteners non-magnetic?

A: Pure annealed austenitic stainless steel is non-magnetic. However, A2-70 and A4-70 fasteners are strengthened by cold drawing and rolling. This cold work creates slight magnetism. Mild magnetism is normal and does not mean wrong material. Request material test certificates (MTC/COA) for verification.

Q: Does A4-70 always outperform A2-70 in every environment?

A: Not necessary. In dry inland conditions, A2-70 will perform adequately. A4-70’s extra corrosion capacity is only an advantage when chlorides are present. Using A4-70 where it is not needed adds unnecessary material expense.

Q: Do you supply A2-70 and A4-70 fasteners for solar mounting projects?

A: Yes. We manufacture hex bolts, socket cap screws, nuts and flat/spring washers in A2-70 and A4-70 stainless steel, compliant with ISO 3506. We can provide material test certificates, batch traceability and custom packaging for solar PV bracket orders.

Q: What standard controls A2-70 and A4-70 bolt marking?

A: ISO 3506 covers marking, mechanical testing and material requirements for these austenitic stainless fasteners. Authentic products will be marked A2-70 or A4-70 on the head.

10. Get A2-70 & A4-70 Stainless Fasteners With Full Test Certificates

Selecting the correct stainless steel grade is critical to avoid premature failure on solar mounting and structural projects.

If you need A2-70 304 fasteners for inland PV systems or A4-70 marine-grade bolts for coastal solar farms, our factory can supply a full size range with MTC/COA.

Share your project details, drawings, size list or technical specification, and we will provide a competitive quotation and supporting material documentation.

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