Product Summary

Longze High-Strength Socket Countersunk Screws (DIN 7991 / ISO 10642) are designed for applications where a flush, flat-finish surface is required without sacrificing

clamping force. Manufactured in Yongnian, China, our flat head socket bolts are forged from high-grade alloy steel and precision-heat-treated to achieve Class 10.9 and 12.9

mechanical properties. Whether for precision machine tools, automotive interiors, or high-end electronics assembly, Longze countersunk screws provide the streamlined aesthetics and structural reliability required for modern engineering.

• ✔ Precision 90° Countersink: Perfectly formed conical heads ensure a flush fit with the mating surface, eliminating protrusions and potential snagging hazards.

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• ✔ High-Torque Hex Socket: Deep-seated, standardized hex drives are cold-swaged for maximum tool engagement, preventing "cam-out" and socket stripping during high-torque tightening.

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• ✔ Fatigue-Resistant Forging: Our drop-forged head-to-shank transition is optimized to reduce stress concentration, a critical factor for countersunk bolts under cyclic loading.

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• Standards: DIN 7991, ISO 10642, ASME B18.3.

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• Diameter Range: M3 to M24 | #4 to 1".

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• Length Range: 6mm to 150mm (Fully threaded or partial thread available).

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• Material Grade: Alloy Steel (Class 10.9, 12.9) | Stainless Steel 304 (A2) & 316 (A4).

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• Surface Finish: Black Oxide, Zinc Plated, Dacromet, Geomet, Oil Dipped.

Mechanical Properties & Material Standards

• Matching the steel grade to the structural requirements of the transmission line.

• Property Class	재료	Tensile Strength (min)	Yield Strength (min)	Hardness Range
  Class 10.9	Alloy Steel (SCM435)	1040 MPa	940 MPa	32 - 39 HRC
  Class 12.9	Alloy Steel (SCM440)	1220 MPa	1100 MPa	39 - 44 HRC
  Stainless 304	A2-70	700 MPa	450 MPa	200 - 280 HV
  Stainless 316	A4-80	800 MPa	600 MPa	240 - 320 HV

Standard Cross Arm Dimensions

• Common angle steel configurations for standard utility pole mounting.

• Thread Size (D)	Head Dia (dk)	Head Height (k)	Socket Size (s)	Max Torque (12.9)
  M6	12 mm	3.3 mm	4.0 mm	14.0 Nm
  M8	16 mm	4.4 mm	5.0 mm	34.0 Nm
  M10	20 mm	5.5 mm	6.0 mm	68.0 Nm
  M12	24 mm	6.6 mm	8.0 mm	118.0 Nm

Galvanizing & Corrosion Resistance

• Ensuring 30+ years of service life in corrosive environments.

• Surface Treatment	Coating Thickness	Salt Spray Test	Best Application	Appearance
  Black Oxide	N/A	24 - 48 Hours	Oiled Machinery	Matte Black
  Zinc Plated	8 - 12 μm	72 - 144 Hours	General Framing	Blue/White
  Dacromet	8 - 12 μm	500 - 1000 Hours	Automotive / Marine	Silver Grey
  Oil Dipped	N/A	< 12 Hours	Temporary / Tooling	Dark Oily

Avoid Pitfalls:

• Socket Depth vs. Head Strength: A socket that is too deep weakens the head, while one that is too shallow causes stripping. Longze strictly adheres to DIN 7991 tolerances to find the "Sweet Spot" between driving torque and structural integrity.

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• Conical Angle Deviation: Standard countersinks are 90°. We utilize precision gauges to ensure the angle is consistent across every batch, preventing "gap-fit" issues where the bolt sits too high or too low in the hole.

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• Hydrogen Embrittlement (12.9 Grade): For high-tensile 12.9 screws that require electroplating, we implement an immediate 8-hour de-embrittlement bake to maintain the molecular stability of the alloy steel.

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Perfect Fit Assembly:

• Correct Bit Selection: Always use high-quality metric hex bits for DIN 7991 screws. Worn bits are the primary cause of hex socket failure.

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• Chamfered Holes: For a truly flush finish, ensure the hole in the base material is pre-chamfered to the correct 90° angle using a professional countersink bit.

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Quality Control & Logistics:

• ISO 9001 Certified: 5-step QC including computerized optical sorting for head diameter, socket depth verification, and tensile testing of heat-treated batches.

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• Sea-Worthy Packaging: Screws are packed in double-walled 25kg cartons, plastic-lined for moisture protection, and palletized on reinforced crates for export from our Yongnian facility.

• Q: Why is Class 12.9 often supplied in Black Oxide rather than Zinc? A: Class 12.9 is highly sensitive to hydrogen embrittlement during the pickling process of zinc plating. Black oxide or Dacromet is often safer and more functional for these ultra-high-strength fasteners.

• Q: Can you manufacture stainless steel DIN 7991 in A4-80 (High Strength)? A: Yes, while standard stainless is A2-70, we can cold-forge A4-80 (316 High Strength) for specialized marine applications requiring both corrosion resistance and 800MPa tensile strength.