DIN 931 Hex Head Bolts

1. Standard & Designation

Standard: DIN 931 (Deutsches Institut für Normung - German Institute for Standardization).

Equivalent Standards: While DIN 931 is a European standard, it is largely superseded or harmonized with international standards for global compatibility.

ISO 4014 is the direct international equivalent.

ASTM A325 or SAE J429 (Grade 5) are rough US equivalents in terms of strength, though dimensions may differ slightly.

Designation Example: A bolt marked as DIN 931 - M12 x 60 - 8.8 means:

DIN 931: Standard for a partially threaded hex bolt.

M12: Metric thread with a 12mm nominal diameter.

60: Nominal length in millimeters (from under the head to the tip).

8.8: Property Class (strength grade).

2. Materials

DIN 931 bolts are typically manufactured from steel, but the specific material varies based on the required property class.

Property Classes 4.6, 5.6, 8.8: Made from low or medium carbon steel.

Property Classes 10.9 and 12.9: Made from medium carbon steel that is quenched and tempered (alloy steel may be used) to achieve higher strength and hardness.

Stainless Steel: Also available in corrosion-resistant grades, commonly:

A2 (AISI 304) - Good corrosion resistance for general use.

A4 (AISI 316) - Superior corrosion resistance, especially against chlorides and acids; ideal for marine and chemical environments.

Other Materials: Can also be found in brass, aluminum, or other specialized alloys for specific applications.

3. Common Surface Treatments

Surface treatments are applied to steel bolts to enhance corrosion resistance, appearance, or for lubrication.

Zinc Plating (Electroplating): The most common finish. Often with a clear (blue-bright) or yellow (chromate) passivation. Provides good corrosion resistance for indoor and general use. Thickness is typically a few microns.

Hot-Dip Galvanizing: A thick zinc coating applied by dipping the bolt in molten zinc. Provides excellent corrosion resistance for outdoor, agricultural, and structural applications. The coating is thick and can affect thread fit.

Geomet® (Dacromet): A zinc-flake coating offering excellent corrosion resistance without hydrogen embrittlement risks. It has a distinctive silver-grey appearance.

Black Oxide: A black finish that offers minimal corrosion resistance but is good for appearance and reduces light glare. Often used with an oil supplement.

Phosphating: A dark grey or black coating that provides a good base for holding oil or paint, improving anti-galling and break-in properties.

5. Size Range

DIN 931 bolts cover a wide range of sizes to suit various applications.

Thread Diameter (M): Commonly available from M5 up to M36.

Fine thread versions are covered under the DIN 6914 standard for high-strength structural bolts.

Length (L): The length range depends on the diameter. For example:

For an M8 bolt, typical lengths range from 16mm to 120mm.

For an M20 bolt, lengths can go from 40mm to 300mm or more.

Thread Length: According to the standard, the thread length is determined by the formula: For bolts with length ≤ 125mm: 2d + 6mm; for bolts with length > 125mm: 2d + 12mm (where d is the nominal diameter).

6. Application Range

Due to their versatility and strength, DIN 931 bolts are used in virtually every industry.

General Engineering & Machinery: Assembling machine frames, guards, and components.

Automotive Industry: Used in chassis, engines, and various non-critical assemblies (higher grades like 10.9 are used for critical parts).

Construction & Structural Steel: Used in building frameworks, bridges, and steel structures (often in higher grades like 8.8 or 10.9 and with heavy-duty corrosion protection).

Furniture Manufacturing: Especially in heavy-duty or industrial furniture.

Electronics & Appliances: For assembling robust casings and frames.

Maintenance, Repair, and Operations (MRO): The standard fastener for a vast range of repair and assembly tasks.