Home » DIN 7991 Flat Head Countersunk Socket Cap Screw A4 Stainless

DIN 7991 Flat Head Countersunk Socket Cap Screw A4 Stainless

FULLERKREG DIN 7991 / ISO 10642 A4 Marine Stainless Steel Flat Head Countersunk Socket Cap Screws

FULLERKREG DIN 7991 / ISO 10642 A4 Marine Stainless Steel Flat Head Countersunk Socket Cap Screws are premium corrosion-resistant fasteners featuring a 90° countersunk flat head with a hexagon socket drive. Manufactured from A4 (SUS316) austenitic stainless steel with a passivated natural finish, these screws are designed to sit flush with or below the surface of the mating material when installed in a countersunk hole. The addition of 2–3% molybdenum (Mo) in SUS316 provides dramatically better resistance to chlorides, saltwater, and acids compared to A2 (SUS304), making these the definitive choice for marine, coastal, chemical, and food processing environments where both a flush-fit appearance and maximum corrosion resistance are required.

Product Specifications

  • Standards: DIN 7991 / ISO 10642
  • Material: A4 Marine Stainless Steel (SUS316 / 316 SS)
  • Strength Grade: A4-070
  • Tensile Strength: Min. 700 MPa
  • Proof Load Stress (Rp0.2): Min. 450 MPa
  • Hardness: ≤95 HRB (max. 210 HV)
  • Head Type: Flat countersunk head, 90° included angle
  • Surface Treatment: Passivation — natural silver finish (pickling & passivation)
  • Thread Type: Metric coarse, full thread
  • Thread Tolerance: 6g
  • Drive Type: Hexagon socket (Allen key / hex key)
  • Sizes Available: M3 – M20

Why A4 (SUS316) vs A2 (SUS304)?

The molybdenum content in A4 is the critical differentiator for corrosive environments:

Property A2 (SUS304) A4 (SUS316) — This Page
Chromium (Cr) 18–20% 16–18%
Nickel (Ni) 8–10.5% 10–14%
Molybdenum (Mo) None 2–3% (key advantage)
Chloride resistance Good Excellent
Saltwater / marine Limited Superior — recommended grade
Acid resistance Moderate Superior
Pitting corrosion resistance Standard Significantly better
Price point Standard Premium

Countersunk Head Design — What Makes DIN 7991 Different

Property DIN 7991 Countersunk (This Page) ISO 7380-1 Button Head DIN 912 Socket Cap
Head profile 90° flat cone — flush with surface Low dome above surface Cylinder above surface
Installed appearance Completely flush / recessed Protrudes above surface Protrudes above surface
Countersunk hole required Yes — 90° countersink No No
Snag-free surface Yes — no protrusion Minimal protrusion Significant protrusion
Best for Flush-fit, aesthetic, safety, marine Low-profile aesthetic High-torque structural

Material — A4 Marine Stainless Steel (SUS316) Chemical Composition (%)

All values are maximum unless a range is indicated.

Grade C (max) Si (max) Mn (max) P (max) S (max) Cr Ni Mo
SUS316 (A4) ≤0.08 ≤1.00 ≤2.00 ≤0.045 ≤0.030 16.00–18.00 10.00–14.00 2.00–3.00

Mechanical Properties — A4-070 (ISO 3506-1)

“A4-070” is the property class designation per ISO 3506-1. “A4” refers to the austenitic stainless steel grade (SUS316), and “070” indicates a minimum tensile strength of 700 MPa. This is equivalent to the commonly used designation A4-70.

Property Requirement Unit
Tensile Strength (Rm) Min. 700 MPa
Proof Load Stress (Rp0.2) Min. 450 MPa
Hardness (max) ≤95 HRB / ≤210 HV HRB / HV
Elongation after fracture (A) Min. 25 %
Thread Tolerance Class 6g —
Head Angle 90° degrees
Surface Treatment Passivation (natural finish) —

Key Features

  • Flush-fit countersunk head — 90° flat head sits flush with or below the mating surface, eliminating protrusions for a clean, snag-free finish
  • A4 Marine Stainless Steel (SUS316) — the highest corrosion resistance among standard stainless steel fastener grades
  • Molybdenum-enhanced alloy — 2–3% Mo content provides outstanding pitting and crevice corrosion resistance in chloride environments
  • Passivated natural finish — clean silver appearance with enhanced oxide layer for maximum rust resistance
  • Hexagon socket drive — high torque transmission in a compact, recessed head profile
  • DIN 7991 / ISO 10642 dual standard — internationally recognized specification
  • Non-magnetic — suitable for applications sensitive to magnetic interference
  • Hygienic & food-safe — smooth flush surface compliant for food processing and medical equipment
  • RoHS & REACH compliant — no restricted hazardous substances

Countersunk Hole Requirements

  • Countersink angle: 90° included angle (standard metric countersink)
  • Countersink diameter: Must match or slightly exceed the screw head diameter (dk) to allow flush seating
  • Surface finish: Smooth countersink surface ensures consistent seating and torque transfer
  • Material consideration: In soft materials (aluminum, plastic), the countersink may deform under high torque — consider reducing torque or using a larger head diameter

Corrosion Resistance Guide — A4 (SUS316)

Environment A4 (SUS316) Performance Recommendation
Indoor / dry environments Excellent ✅ Ideal
Outdoor / atmospheric exposure Excellent ✅ Ideal
Marine / saltwater (direct) Very good ✅ Recommended grade
Offshore / coastal structures Very good ✅ Recommended grade
Food & pharmaceutical processing Excellent ✅ Ideal
Chemical processing (mild acids) Very good ✅ Suitable
Chloride-rich environments Very good ✅ Recommended grade
Swimming pools / aquatic facilities Very good ✅ Recommended grade
Strong oxidizing acids Moderate ⚠️ Consult technical team

Applications

FULLERKREG DIN 7991 A4 marine stainless steel countersunk socket cap screws are widely used in demanding corrosive environments where a flush-fit fastener is required:

  • Marine vessels, boat deck fittings, and dock hardware (flush + corrosion resistant)
  • Offshore platforms and coastal architectural hardware
  • Food processing and pharmaceutical machinery (hygienic flush surface + SUS316)
  • Swimming pools, aquatic facilities, and spa equipment
  • Chemical processing equipment requiring flush fasteners
  • Medical devices and laboratory instruments
  • Outdoor architectural hardware and signage in coastal areas
  • Desalination and water treatment equipment
  • Any application requiring flush-fit fasteners with maximum corrosion resistance

Dimensional Specifications — DIN 7991 / ISO 10642 (Unit: mm)

Thread (d) Pitch (p) Head Dia. dk Max. Head Dia. dk Min. Head Height k Max. Head Height k Min. Key Size s Key Depth t Min. Body Dia. ds Max. Body Dia. ds Min.
M3 0.5 6.72 6.48 1.86 1.66 2.0 1.1 3.00 2.86
M4 0.7 8.96 8.62 2.48 2.18 2.5 1.5 4.00 3.82
M5 0.8 11.20 10.82 3.10 2.72 3.0 2.0 5.00 4.82
M6 1.0 13.44 13.02 3.72 3.32 4.0 2.5 6.00 5.82
M8 1.25 17.92 17.42 4.96 4.46 5.0 3.5 8.00 7.78
M10 1.5 22.40 21.82 6.20 5.52 6.0 4.4 10.00 9.78
M12 1.75 26.88 26.22 7.44 6.64 8.0 5.3 12.00 11.73
M16 2.0 33.60 32.82 8.80 7.90 10.0 6.8 16.00 15.73
M20 2.5 40.32 39.42 10.16 9.06 12.0 8.2 20.00 19.67

Frequently Asked Questions

Q: What is the difference between DIN 7991 and ISO 10642?
DIN 7991 is the German standard for flat head countersunk socket cap screws. ISO 10642 is the equivalent international standard. The two are dimensionally identical and fully interchangeable. FULLERKREG screws comply with both standards.

Q: What countersink angle is required for DIN 7991 screws?
DIN 7991 uses a 90° included angle countersink, which is the standard metric countersink angle. Ensure your countersink tool or pre-drilled hole uses a 90° angle for correct flush seating.

Q: What does A4-070 mean?
A4-070 is the property class designation per ISO 3506-1 for stainless steel fasteners. “A4” refers to the austenitic stainless steel grade (SUS316), and “070” indicates a minimum tensile strength of 700 MPa. This is equivalent to the commonly used designation A4-70.

Q: Why choose A4 over A2 for countersunk screws in marine applications?
In marine, coastal, or chloride-rich environments, A4 (SUS316) is the correct choice. The 2–3% molybdenum content provides significantly better pitting and crevice corrosion resistance than A2 (SUS304), which is critical for flush-fit fasteners that may trap moisture in the countersink recess.

Q: Can I get material certificates and test reports?
Yes. PPAP reports, material certificates (including Mo content verification), hardness test reports, and tensile test reports are available upon request for all FULLERKREG products.

Packaging & Availability

  • Standard sizes M3 – M20 available in stock
  • Bulk carton packaging; small box and custom packaging available
  • Material certificates (including Mo content verification) available on request
  • PPAP report available on request
  • Custom lengths accepted for large orders

Download Specification

Quality Assurance

All FULLERKREG DIN 7991 A4 marine stainless steel countersunk socket cap screws are manufactured under strict quality control procedures including:

  • Hardness test (≤95 HRB)
  • Tensile strength test (min. 700 MPa)
  • Head angle verification (90° countersink)
  • Dimensional inspection per DIN 7991 / ISO 10642
  • Chemical composition verification (SUS316, Mo content confirmed)
  • Salt spray corrosion test
  • PPAP documentation available upon request

For wholesale pricing, bulk orders, material certificates, or technical inquiries, please contact our sales team.

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