Home » DIN 916 / ISO 4029 Socket Set Screw Cup Point A4 Stainless

DIN 916 / ISO 4029 Socket Set Screw Cup Point A4 Stainless

DIN 916 / ISO 4029 — Socket Set Screw with Cup Point | A4 Stainless Steel (Marine Grade)

Brand: FULLERKREG  |  Material: A4 Stainless Steel (SUS316 / 1.4401)  |  Surface: Passivated (Bright Finish)

Product Overview

FULLERKREG DIN 916 / ISO 4029 Socket Set Screws with Cup Point in A4 Stainless Steel (SUS316) represent the premium stainless option for the most demanding corrosive environments. The addition of 2–3% molybdenum to the SUS316 alloy delivers significantly superior resistance to chloride-induced pitting and crevice corrosion compared to standard A2 (SUS304) — making A4 the material of choice for marine, offshore, chemical processing, and high-salinity industrial applications.

The cup point geometry — a concave spherical recess at the tip — provides reliable shaft locking, anti-rotation, and positioning performance. Combined with a bright passivated finish, these set screws deliver long-term reliability in environments where standard stainless or carbon steel fasteners would rapidly corrode.

Standards & Specifications

  • Standards: DIN 916, ISO 4029
  • Thread Form: ISO Metric Coarse Thread (per ISO 68-1)
  • Thread Tolerance: 6g (per ISO 965-1)
  • Drive Type: Hexagon Socket (Allen / Hex Key)
  • Point Type: Cup Point — concave spherical recess at tip
  • Material: A4 Stainless Steel (SUS316 / 1.4401 / 316 SS)
  • Material Grade: A4 per ISO 3506-3
  • Surface Treatment: Passivated — bright, clean finish with enhanced corrosion resistance
  • Applicable Standard for Mechanical Properties: ISO 3506-3

Material & Mechanical Properties

Property Value Standard
Material A4 Stainless Steel (SUS316 / 1.4401) ISO 3506-3
Chemical Composition (Cr) 16.5 – 18.5% —
Chemical Composition (Ni) 10.0 – 13.0% —
Chemical Composition (Mo) 2.0 – 2.5% —
Minimum Tensile Strength 700 MPa (A4-70) ISO 3506-3
Hardness (typical) HV 200 – 300 ISO 3506-3
Thread Tolerance 6g ISO 965-1
Surface Treatment Passivated (Bright / 洗白钝化) —
Magnetic Properties Slightly magnetic (work-hardened) —
Operating Temperature -196°C to +800°C (intermittent) —

Note: A4 stainless steel set screws are not hardened to the same level as alloy steel 45H/12.9 grade. A4 is optimized for superior corrosion resistance, particularly in chloride-rich and marine environments. For maximum mechanical holding force on hardened shafts, alloy steel grade is recommended.

Why A4 (SUS316)? — The Role of Molybdenum

The key differentiator of A4 (SUS316) over A2 (SUS304) is the addition of 2.0–2.5% Molybdenum (Mo):

  • Pitting resistance: Molybdenum dramatically increases resistance to chloride-induced pitting corrosion — the primary failure mode of A2 in marine and coastal environments.
  • Crevice corrosion resistance: Superior performance in tight joints, threaded connections, and under-head contact areas where oxygen depletion creates aggressive local chemistry.
  • Higher PREN value: Pitting Resistance Equivalent Number (PREN) for SUS316 is approximately 24–26, vs. 18–20 for SUS304 — a significant improvement in chloride tolerance.
  • Acid resistance: Better resistance to dilute sulfuric acid, phosphoric acid, and organic acids compared to A2.

Surface Treatment: Passivation (洗白钝化)

All FULLERKREG A4 stainless set screws undergo passivation treatment after manufacturing:

  • Process: Acid passivation (citric acid or nitric acid bath) removes free iron, machining residues, and surface contaminants.
  • Result: A bright, clean, silver-white finish with a reinforced chromium-molybdenum oxide passive layer.
  • Benefit: Maximizes the inherent corrosion resistance of SUS316, especially critical in chloride environments where surface contamination can initiate pitting.
  • Appearance: Uniform bright metallic finish — no discoloration, no scale, no heat tint.

Dimensional Range

Thread Size Pitch (mm) Socket Size s (mm) Length Range (mm)
M3 0.5 1.5 3 – 16
M4 0.7 2.0 4 – 20
M5 0.8 2.5 5 – 25
M6 1.0 3.0 6 – 30
M8 1.25 4.0 8 – 40
M10 1.5 5.0 10 – 50
M12 1.75 6.0 12 – 60
M16 2.0 8.0 16 – 80
M20 2.5 10.0 20 – 100
M24 3.0 12.0 25 – 120

Custom lengths and special configurations available upon request. Contact FULLERKREG for non-standard requirements.

Cup Point Geometry

The cup point (concave spherical recess) is the defining feature of DIN 916 / ISO 4029 set screws:

  • Positive mechanical lock: The rim of the cup bites into the shaft surface, resisting rotation and axial movement.
  • Reduced shaft damage: The concave geometry distributes contact stress more evenly than cone or dog points, minimizing shaft marring.
  • Vibration resistance: Reliable performance under dynamic and cyclic loads.
  • Repositionable: Allows limited repositioning without severe shaft damage.
  • Corrosion-environment optimized: Ideal for stainless and non-ferrous shafts in marine, chemical, and wet environments.

Corrosion Resistance of A4 Stainless Steel

  • Excellent resistance to seawater, marine atmosphere, and coastal environments.
  • Excellent resistance to chloride solutions, brine, and de-icing salts.
  • Excellent resistance to dilute sulfuric acid, phosphoric acid, acetic acid, and many organic acids.
  • Good resistance to food acids, alkalis, and most industrial chemicals.
  • Superior to A2 (SUS304) in all chloride-containing environments due to molybdenum content.
  • Not recommended for strong oxidizing acids (e.g., concentrated nitric acid) or high-temperature halide environments without engineering review.

Typical Applications

  • Marine and offshore equipment — deck hardware, rigging, underwater assemblies
  • Coastal and harbor infrastructure exposed to salt spray
  • Chemical processing plants — pumps, valves, reactors, pipework
  • Desalination and water treatment equipment
  • Food and beverage processing — washdown and CIP (clean-in-place) environments
  • Pharmaceutical manufacturing — sterile and high-purity environments
  • Swimming pool and aquatic facility equipment
  • Oil & gas topside equipment and subsea assemblies
  • Shaft-to-hub locking in corrosive or high-humidity environments
  • Architectural hardware in coastal or high-pollution urban environments

Material Comparison — Selection Guide

Property A4 / SUS316 (This Product) A2 / SUS304 Alloy Steel 45H/12.9
Corrosion Resistance ✅ Superior (Mo-enhanced) ✅ Good ⚠️ Limited
Chloride / Marine Resistance ✅ Excellent ⚠️ Moderate ❌ Poor
Surface Hardness HV 200–300 HV 200–300 HRC 45–53
Tensile Strength 700 MPa (A4-70) 700 MPa (A2-70) 1220 MPa (12.9)
Holding Force on Hard Shafts ⚠️ Moderate ⚠️ Moderate ✅ Maximum
Food / Pharma Suitability ✅ Yes ✅ Yes ❌ No
Marine / Seawater Suitability ✅ Yes ⚠️ Limited ❌ No
Appearance Bright silver-white Bright silver-white Matte black
Cost Premium Standard Standard

Installation Guidelines

  • Use the correct hex key (Allen wrench) matching the socket size — refer to the dimensional table above.
  • Ensure the shaft and bore are clean and free of oil or debris before installation.
  • For vibration-critical applications, apply a stainless-compatible thread-locking compound.
  • Anti-galling precaution (critical for stainless): A4 stainless steel is susceptible to galling (cold welding / thread seizure) during installation. Always apply a molybdenum disulfide (MoS₂) or nickel-based anti-seize compound to the threads before tightening.
  • Tighten slowly and steadily using a calibrated torque wrench — avoid high-speed power tools which dramatically increase galling risk.
  • If a fastener begins to seize during installation, back it off immediately and re-apply anti-seize before continuing.
  • Do not exceed the rated tightening torque for the thread size and A4-70 grade.

Quality Assurance

  • Dimensional Inspection: 100% thread gauge inspection per ISO 965-1 (6g tolerance)
  • Material Verification: PMI (Positive Material Identification) / spectrometer analysis confirming SUS316 composition including Mo content
  • Hardness Testing: Vickers hardness test per ISO 3506-3
  • Tensile / Proof Load Testing: Per ISO 3506-3 (A4-70)
  • Salt Spray Test: Extended corrosion resistance verification post-passivation
  • Socket Torque Test: Drive socket integrity verification
  • Lot Traceability: Full material and process traceability per batch, with mill certificates available on request

Packaging Options

  • Bulk (polybag or carton) — standard for wholesale orders
  • Small box (labeled, per size) — available for mixed or sample orders
  • Custom packaging — available for OEM and private-label customers

Ordering Information

  • Price: Unit price per piece (USD), not per thousand. Contact us for volume pricing.
  • Customer Base: B2B wholesale only. We do not serve retail / B2C customers.
  • Order Method: All orders are placed offline. Online prices are for reference and inquiry only. Please contact us via email to place orders.
  • MOQ: Varies by size — contact FULLERKREG for minimum order quantities.
  • Lead Time (Small Batch): Approx. 30 days, delivered DDP.
  • Lead Time (Large Volume): FOB Shanghai; other Incoterms negotiable.
  • Mill Certificates: Available upon request for material traceability.
  • Custom Orders: Non-standard lengths, special configurations, or OEM requirements available upon request.

Frequently Asked Questions

Q: What is the difference between DIN 916 and ISO 4029?
A: DIN 916 is the German standard; ISO 4029 is the international equivalent. Both specify the same cup point set screw geometry and are dimensionally interchangeable. FULLERKREG products comply with both standards.

Q: What does A4 stainless steel mean?
A: A4 is the designation for molybdenum-bearing austenitic stainless steel per ISO 3506-3, corresponding to SUS316 / 1.4401 (16–18% Chromium, 10–13% Nickel, 2–2.5% Molybdenum). The molybdenum addition provides significantly superior resistance to chloride pitting and crevice corrosion compared to standard A2 (SUS304).

Q: What is the difference between A4 and A2 stainless steel?
A: The key difference is the addition of 2.0–2.5% Molybdenum in A4 (SUS316). This dramatically improves resistance to chloride-induced pitting and crevice corrosion. A4 is the preferred choice for marine, seawater, chemical, and high-salinity environments where A2 may suffer pitting failure.

Q: What is passivation (洗白钝化)?
A: Passivation is an acid treatment that removes free iron and surface contaminants, restoring and enhancing the natural chromium-molybdenum oxide passive layer on the stainless steel surface. The result is a bright, clean finish with maximized corrosion resistance.

Q: Are A4 stainless set screws suitable for seawater immersion?
A: A4 (SUS316) provides excellent resistance to seawater splash and marine atmosphere, and good resistance to intermittent seawater immersion. For continuous full seawater immersion under high stress, super-duplex or higher-alloy grades may be required. Contact FULLERKREG for engineering guidance on extreme marine applications.

Q: Why do stainless steel set screws sometimes seize during installation?
A: Stainless steel is prone to galling (cold welding) due to its tendency to work-harden under friction. Always apply a molybdenum disulfide (MoS₂) or nickel-based anti-seize compound to the threads and tighten slowly with a calibrated torque wrench to prevent seizure.

Q: Can I get mill certificates for A4 material?
A: Yes. FULLERKREG provides mill certificates confirming SUS316 chemical composition including molybdenum content upon request. Please specify this requirement when placing your order.

Q: Do you offer custom lengths or thread pitches?
A: Yes. FULLERKREG can produce non-standard configurations. Please contact us with your technical requirements.

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