Materials
Stainless Steel: ISO 3506-3 A2-12H / 21H and A4-12H / 21H
Stainless Steel ISO 3506-3
(A2 & A4 Austenitic Cold-Worked Fasteners – High Hardness Classes)
Material:
ISO 3506-3 defines stainless steel fasteners produced by cold heading or machining into high-strength, high-hardness conditions.
Based on:
A2 → 304 / 304L stainless steel (general corrosion resistance)
A4 → 316 / 316L stainless steel (molybdenum-enhanced, chloride resistant)
The suffixes 12H and 21H define high hardness / high strength cold-worked conditions, beyond standard ISO 3506-1 classes.
A2-12H / A2-21H
Material
Austenitic stainless steel (304 / 304L equivalent)
Strength achieved through severe cold working
Mechanical Properties
Higher strength than A2-70 / A2-80 classes
Approximate behavior:
A2-12H → moderate-high hardness, mid-strength high-hardness fasteners
A2-21H → higher hardness, near upper limit of cold-worked 304 fasteners
Excellent toughness but reduced ductility compared to A2-70/80
Not heat-treatable; properties are entirely strain-hardened
Corrosion Resistance
Comparable to standard 304 stainless steel
Good resistance to:
Atmospheric environments
Freshwater
Mild chemicals
Limited resistance in:
Chloride-rich or marine environments
Corrosion resistance is not improved by hardening (only strength increases)
Manufacturing Considerations
Produced via severe cold heading or machining + cold work
Increased tool wear due to work-hardening behavior
More sensitive to cracking if over-stressed during installation
Requires careful control of:
Thread rolling
Forming loads
Torque application during assembly
Applications in Fasteners
High-strength stainless screws and small bolts
Precision mechanical assemblies
Automotive and appliance components
Equipment requiring compact, high-strength stainless fasteners
A4-12H / A4-21H
Material
Austenitic stainless steel (316 / 316L equivalent)
Includes molybdenum for chloride and marine resistance
Strength achieved via cold working
Mechanical Properties
Higher strength than A4-70 / A4-80 in many geometries (depending on processing)
General behavior:
A4-12H → high hardness, balanced ductility
A4-21H → very high hardness / maximum cold-worked strength range
Excellent fatigue resistance for stainless fasteners
Reduced ductility compared to standard ISO 3506-1 classes
Corrosion Resistance
Superior to A2 due to molybdenum content (2–3%)
Excellent resistance to:
Seawater and marine atmospheres
Chloride-induced pitting and crevice corrosion
Chemical processing environments
Preferred for offshore and coastal applications
Manufacturing Considerations
Requires heavy cold working to achieve H-class strength
More difficult machining and forming than A4-70 / A4-80
Tight control needed to prevent:
Work hardening cracking
Thread damage during installation
Often used where space constraints require maximum strength in a small fastener
Applications in Fasteners
High-strength marine fasteners
Offshore oil & gas equipment
Chemical processing and desalination systems
Compact structural assemblies requiring stainless corrosion resistance + higher load capacity
Key Takeaway for Engineered Fasteners
SO 3506-3 H-grade fasteners (12H and 21H) are designed for maximum cold-worked strength stainless applications, where:
No heat treatment is used
Strength comes entirely from severe strain hardening
Corrosion resistance depends on base alloy (A2 vs A4), not hardness level
Engineering Context for Engineered Fasteners:
12H grades = high-strength alternative to standard ISO 3506-70/80 classes
21H grades = extreme cold-worked stainless fasteners used when maximum strength is required without switching to precipitation-hardening alloys
They sit in a niche between:
Standard stainless (A2-70 / A4-70)
Precipitation-hardened stainless (15-5PH, 17-4PH, A286)
Materials
BRASS
BRONZE
COPPER
PH STAINLESS STEEL
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