Schematic diagram of the manufacturing process of cold-headed screws.

The Evolution of Fastener Integrity

In modern industrial assembly, the humble screw is an engineering marvel. The transition from a simple metal wire to a high-precision fastener capable of withstanding tons of force involves complex metallurgical and mechanical stages. At Ever Power, we utilize advanced cold heading technology to ensure that the grain structure of our stainless steel and carbon steel fasteners remains continuous, maximizing structural reliability and performance.

Phase 1: Raw Material Preparation (Annealing & Drawing)

Before a screw is “born,” the raw wire rod (盤元) must be meticulously prepared to ensure it can survive the extreme pressures of the cold heading machine.

Spheroidizing Annealing

The goal of annealing is to adjust the crystalline structure and reduce hardness. For materials like 1018 or 1022 low-carbon steel, we heat the wire to 680°C – 715°C for 4 to 6 hours. For high-performance alloys like 10B21 or 1039, the temperature is increased to 740°C – 760°C for 5.5 to 7.5 hours. After a controlled slow cooling to below 550°C, the wire achieves a target hardness of HV120 – HV180, providing the ideal ductility for room-temperature shaping.

Pickling and Lubrication

Wire rods often carry oxide scales that can damage precision dies. We remove this through a 20-25% hydrochloric acid bath. Following this, a phosphate film (such as Zinc Iron Phosphate) is applied to the metal surface. This film, combined with a metallic soap lubricant, acts as a sacrificial barrier, significantly reducing friction during the high-speed forming process.

Phase 2: The Core of Shape (Cold Heading & Trimming)

Cold heading is the process of using high-pressure punches to displace metal without cutting it. Unlike machining, cold heading creates a continuous internal “flow line” that follows the shape of the screw, resulting in superior shear strength.

The Mechanics of Forming

Standard screws are typically produced using the 1-Die 2-Blow method, where the first blow gathers the metal into a preliminary shape and the second blow completes the head profile (such as Pan, Flat, or Truss). For complex Hexagon Bolts, we utilize multi-station machines (3-Die 3-Blow or 4-Die 4-Blow) to cut the wire, upset the head, trim the hexagonal flats, and reduce the shank diameter in a single, high-speed sequence.

“Red Heading” (Hot Forging)

For large-diameter structural bolts (typically above 1 inch), room-temperature deformation is impossible. We heat the end of the blank to a “white-hot” state (heating for 7-15 seconds depending on size) before forging. This “Hot Heading” or “Red Heading” allows for the manufacture of massive fasteners used in bridge and ship construction.

Phase 3: Precision Threading (Rolling vs. Tapping)

Threads are rarely “cut” in the modern fastener industry; they are “rolled.”

Thread Rolling involves squeezing the screw blank between two reciprocating dies. This plastic deformation moves the metal from the valley to the crest. This process doesn’t just create the thread; it work-hardens the surface, making the threads incredibly resistant to stripping. In contrast, nuts are tapped using specialized high-speed taps to ensure internal thread precision.

Phase 4: Enhancing Performance (Thermal Hardening)

To reach grades such as 8.8, 10.9, or 12.9, fasteners must undergo heat treatment. At Ever Power, our continuous mesh-belt furnaces provide computerized control over atmosphere and temperature to prevent surface decarburization.

• Through-Hardening: Quenching at 850°C followed by tempering at 500-650°C to achieve a balance of strength and toughness.
• Case Hardening (Carburizing): For self-tapping screws, we diffuse carbon into the surface layer at 890°C, then quench. This creates a hard “skin” (to cut through steel) and a tough “core” (to resist snapping).
• Hydrogen Embrittlement Mitigation: High-strength fasteners (over HRC 35) must be baked immediately after plating at 176°C – 190°C for 3 to 24 hours. This allows trapped hydrogen atoms to escape, preventing delayed brittle failure under load.

Phase 5: Protection (Surface Treatments)

The lifespan of a fastener is often determined by its coating. We offer three primary solutions:

1. Electroplating (Zinc/Nickel/Chrome): Provides aesthetic appeal and moderate corrosion resistance (4–12 μm thickness).
2. Hot-Dip Galvanizing: The bolt is dipped in 510°C molten zinc, creating a thick (43–54 μm) alloy layer. This is the gold standard for outdoor infrastructure.
3. Mechanical Plating: Utilizing impact energy to cold-weld metal powder to the surface, effectively eliminating the risk of hydrogen embrittlement.