When a B2B buyer sources stainless steel bolts, nuts, and screws, the single most important line on the RFQ is the grade mark. Labels such as A2-70 and A4-80 are not interchangeable marketing terms—they define both the alloy chemistry and the mechanical strength. Choosing the wrong one causes either premature corrosion in service or paying a premium for a grade the application does not need.
How stainless steel fastener grades are coded
Stainless steel fasteners are specified under ISO 3506 (replacing earlier national rules such as DIN 267-13). A marking like A2-70 has two parts:
- Letter + first number identifies the austenitic steel group. A2 is the common 18/8 grade (roughly AISI 304); A4 is the molybdenum-bearing grade (roughly AISI 316).
- Second number is the property class, equal to the minimum tensile strength in units of 10 N/mm². Class 70 means at least 700 N/mm²; class 80 means at least 800 N/mm².
So A4-80 is a 316-family fastener that has been cold-worked to a minimum tensile strength of 800 N/mm². This marking should be stamped on the head of the bolt—if it is not, you have no independent proof of grade.
A2-70 (304) vs A4-80 (316): which to specify
| Attribute | A2-70 (approx. 304) | A4-80 (approx. 316) |
|---|---|---|
| Alloy family | 18% Cr / 8% Ni, no molybdenum | ~2% Mo added for pitting resistance |
| Best environment | Dry atmosphere, indoor use, freshwater, general fastening | Coastal/marine, chloride exposure, chemical, food & pharma |
| Minimum tensile | 700 N/mm² (class 70) | 800 N/mm² (class 80) |
| Relative cost | Lower | Higher |
The deciding factor is usually the environment, not the load. A2-70 is perfectly adequate for most dry, indoor structural and assembly work. A4-80 becomes the correct choice when chloride, salt spray, chemicals, or outdoor coastal exposure are present, because the molybdenum in 316 resists pitting that 304 cannot.
Common product standards to name on the PO
Grade alone is not enough—buyers should also specify the dimensional standard for the exact part:
- Hex head bolts: ISO 4014 (partial thread) and ISO 4017 (full thread); historically DIN 931 and DIN 933.
- Hex nuts: ISO 4032; historically DIN 934.
- Socket head cap screws: ISO 4762.
A complete line item reads like: “M12×40 hex bolt, ISO 4017, A4-70, passivated, with 3.1 certificate.”
Procurement / RFQ checklist
- State the grade explicitly. Do not accept “stainless steel” alone—require A2-70 or A4-80 on every line.
- Give the thread and size in metric (e.g. M10×30) with coarse or fine pitch.
- Name the dimensional standard (ISO/DIN) for the product type.
- Request material certification to EN 10204 3.1 with a traceable heat number. For how to review these documents, see our buyer checklist on the mill test certificate.
- Verify the head marking on random samples against the declared grade.
- Confirm surface condition (plain, passivated) and packing, so fasteners arrive clean and free of handling contamination.
Common buying mistakes
The most frequent error is confusing austenitic A2-70 with carbon-steel class 8.8—both carry a “70/8.8”-style strength number but are entirely different materials and cannot be used interchangeably. The second is specifying A2 where de-icing salt or coastal air is present, which leads to rust staining and failed assemblies. The third is mixing production lots without re-checking certification, which breaks heat-number traceability.
Why get the grade right with SOROYA
Specifying the correct fastener grade up front prevents corrosion failures, avoids overspending on unnecessary 316, and keeps every bolt traceable to a mill certificate. Whether your project needs general-purpose A2-70 or chloride-resistant A4-80, the procurement team at SOROYA can consolidate hex bolts, nuts, and screws to your required standard and deliver the supporting documentation.
SOROYA PTE. LTD.
Email：nancy@soroya.com.sg
WhatsApp：+65 8058 3416
