12.9 - Black Oxide - Hexagon socket knurled heads cap screw

Technical Superiority of Grade 12.9 Knurled Screws

When engineers specify a "black socket screw," they are usually referring to this exact product: Grade 12.9 with a knurled head. The knurling acts as a visual and tactile indicator of quality, distinguishing it from lower-grade plain head screws. More importantly, the 12.9 classification means this bolt can withstand immense forces that would snap a standard 8.8 bolt.

Mechanical Properties Specification

The "12.9" marking on the head is a guarantee of extreme mechanical performance under ISO 898-1 standards:

Property	Value	Engineering Implication
Tensile Strength	1200 N/mm² (Min)	Offers 50% more load-bearing capacity than Grade 8.8 screws.
Yield Strength	1080 N/mm² (Min)	Allows for extremely high clamping forces in compact spaces.
Hardness	39-44 HRC	Extremely hard surface, resistant to deformation and wear.

Quality Assurance

For critical applications, material certainty is key. We can supply an EN 10204 3.1 Material Certificate verifying the alloy composition and mechanical tests upon request. Please note that third-party inspection services require an additional fee.

Key Industrial Applications

Grade 12.9 knurled socket screws are essential in sectors requiring compact, high-force connections:

• Plastic Injection Molding: Fastening mold plates where high pressure and tight tolerances are constant.
• Hydraulic Systems: Securing pumps and manifolds under extreme internal pressure.
• High-Performance Automotive: Connecting rods, flywheels, and cylinder heads.
• Heavy Earthmoving: Critical pivot pins and structural joints on excavators.

Frequently Asked Questions

Why is Black Oxide standard for Grade 12.9?

High-strength steels (like Grade 12.9) are very susceptible to "Hydrogen Embrittlement," which can occur during acid pickling in zinc plating processes. Black oxide is a chemical conversion process that carries a much lower risk of inducing hydrogen embrittlement, making it the safest standard finish for ultra-high-strength fasteners.

What is the purpose of the knurling on the head?

The diamond-pattern knurling provides a high-friction surface. This allows mechanics and assemblers to grip the screw head with their fingers and spin it into the hole quickly during the initial threading stage, saving time before grabbing the Allen key for final tightening.

Can I use Grade 12.9 screws in vibrating machinery?

Yes, but with caution. Grade 12.9 bolts allow for high clamping force, which inherently resists loosening. However, because they are harder and less ductile than Grade 8.8 bolts, they have less capacity to absorb severe impact shock. In applications with extreme, sudden shock loads, a tough Grade 8.8 bolt might sometimes be safer to avoid sudden brittle fracture.