High Strength Black Oxide Flange Bolts

A flange bolt is a bolt with a built-in washer-like flange under the head instead of a separate washer. The flange spreads the clamping load over a larger bearing area, which means higher clamp force without crushing soft materials. And it eliminates the need to handle a loose washer during assembly. Black oxide is a chemical conversion coating that gives the bolt its dark finish while adding mild corrosion resistance and reducing thread friction.

Why does this combination matter in the field? Because the flange does two jobs at once: it distributes load and it resists loosening. Engineers specify flange bolts when they want a cleaner assembly with fewer loose parts — automotive chassis, machinery guards, and structural connections.

Product Specifications

Grades and Finishes

Grade 8.8: general high-strength duty, 800 MPa tensile — machinery and equipment

Grade 10.9: automotive and structural applications, 1,040 MPa tensile

Grade 12.9: maximum strength for critical connections, 1,220 MPa tensile

Materials: 40Cr/SCM440, 35CrMo, 42CrMo (AISI 4140 family), 1045 for 8.8 grade

Finishes: black oxide (standard), zinc plating, zinc-nickel, Dacromet, phosphate+oil

Threads: metric ISO 6g, UNC/UNF, full or partial thread per drawing

Serrated flange option: teeth under the head bite into the mating surface for anti-vibration

The black oxide finish is more than cosmetic. It forms a thin magnetite layer that holds a light oil film, giving the bolt corrosion resistance in indoor environments plus a uniform, professional appearance. For outdoor or corrosive service, we recommend zinc-nickel or Dacromet — we will advise based on your environment.

High-Strength Black Oxide Flange Bolts Real Case

Industry: Heavy Machinery & Automotive Assembly

Material: Alloy Steel (Grade 10.9 / 12.9)

Finish: Black Oxide (Corrosion & Wear Resistance)

A European industrial machinery manufacturer faced bolt loosening and surface oxidation issues on heavy-duty engine mounts under high vibration and high torque conditions. They required custom high-strength flange bolts with precise thread engagement and superior clamping force.

We produced custom Grade 10.9 hex flange bolts with integrated serrated flanges to evenly distribute load and eliminate the need for separate washers. Applied a high-grade black oxide finish for enhanced corrosion resistance and consistent torque performance.

Key Features & Benefits

Grade 8.8 to 12.9 Strength, Certifiable Alloy steel flange bolts are heat-treated and quenched-tempered to grade 8.8, 10.9, or 12.9, verified by tensile testing and hardness checks on every batch. You get certified strength, not a guess — for critical connections, that certification is the whole point.

Integrated Flange, Fewer Loose Parts The flange replaces the separate washer, cutting part count and assembly time. On a production line assembling 1,000 joints per shift, eliminating a washer per joint saves real minutes — and removes a whole class of "missing washer" quality issues.

Vibration Resistance by Design The wide flange bearing area raises the friction under the head, resisting self-loosening under vibration. Add serrations under the flange and the bolt grips the surface like a lock washer — without the washer.

Consistent Torque-Tension Behavior Black oxide plus controlled lubrication gives predictable friction, so torque wrenches produce consistent clamp loads. No more bolts that "feel tight" at wildly different torques — your QC torque spec actually means something.

CNC Precision, Real Quality Control Threads are rolled or cut to 6g tolerance with full gauging, and heads are formed or machined with concentric flange faces. 100% optical sorting catches dimensional defects before they reach your line.

Production Process

Flange bolts start as alloy steel bar or wire, processed by cold heading or CNC machining. The flange head is formed, then threads are rolled (for strength) or cut (for precision) to 6g tolerance. Heat treatment quench-and-temper achieves the target grade, followed by surface finishing — black oxide, plating, or coating per specification.

Quality control: tensile testing, hardness testing, thread gauging, and dimensional checks on every batch. 100% optical sorting screens for thread defects, damaged flanges, and missing coating. Batch certificates with actual test values accompany each shipment.

Packaging & Shipping

Bolts are packed in bulk cartons, poly bags, or custom kitting per your assembly needs, with rust protection for sea freight. Express for small orders, sea freight from Qingdao Port for bulk. Documentation: Commercial Invoice, Packing List, Bill of Lading, Certificate of Origin, Material Test Certificates. For high strength black oxide flange bolts prices, bulk orders, or a quotation, contact our export team.

FAQ

Q: What is the difference between a flange bolt and a regular bolt with a washer? 

A: The flange is integral — it cannot be lost, dropped, or mis-positioned during assembly, and it provides a consistent bearing surface every time. Flange bolts also typically offer better vibration resistance than bolt-plus-washer assemblies.

Q: Which grade should I choose? 

A: Grade 8.8 for general machinery, 10.9 for automotive and structural, 12.9 for critical high-load connections. If you tell us the application and clamping load, we will specify the grade — and provide the test certificate.

Q: Is black oxide enough protection for outdoor use? 

A: Black oxide suits indoor and mildly humid environments. For outdoor or corrosive service, we recommend zinc-nickel plating or Dacromet coating. We will match the finish to your environment.

Q: Can you provide serrated flange bolts? 

A: Yes — serrations under the flange bite into the mating surface for maximum anti-vibration performance. Specify "serrated flange" in your order.

Q: Do you provide test certificates with the bolts? 

A: Yes. Each batch ships with a material certificate and test report including tensile and hardness values. Third-party inspection (SGS）available on request.