The manufacturing of high-quality industrial fasteners—such as bolts, screws, and nuts—begins long before the cold-heading or hot-forging stages. The raw steel, typically supplied as hot-rolled wire rod or bar stock, must undergo extensive preparation to ensure the correct mechanical properties, surface quality, and dimensional consistency required for high-speed, precision forming. This preparation is crucial for preventing defects, extending tool life, and achieving the final product’s strength specifications.

1. Raw Material Selection and Testing

The first step is selecting the appropriate grade of steel, which dictates the final fastener’s properties (strength, corrosion resistance, and heat-treatability). Materials chosen are usually various grades of carbon, alloy, or stainless steel.

• Quality Assessment: The incoming wire rod or bar stock is subjected to rigorous testing, including chemical analysis, hardness assessment, and tensile testing to verify that it meets the required standards (e.g., ISO, EN, or IATF for automotive parts). Any defects like seams, voids, or incorrect microstructure must be identified.
• Grades for Fasteners: Manufacturers primarily use “cold upsetting steel” grades, which are specifically designed for the high deformation involved in cold forming. These steels have low impurity levels and controlled grain structures.

2. Cleaning and Descaling (Surface Preparation)

Raw steel that is hot-rolled inevitably has a layer of iron oxide, known as mill scale, on its surface. This scale must be completely removed, as it is abrasive and would cause excessive tool wear, poor surface finish, and potential defects during the drawing or heading processes.

• Mechanical Descaling: The wire rod is bent or flexed as it passes through a series of pullies or rolls, which physically breaks the brittle scale off the surface. This is often followed by shot-blasting, where steel particles are fired at high speed onto the material.
• Chemical Descaling (Pickling): The material is immersed in an acid solution (e.g., sulfuric or hydrochloric acid) to dissolve the mill scale. This process must be carefully controlled to prevent hydrogen embrittlement.
• Rinsing and Neutralization: After acid pickling, the material is thoroughly rinsed and often neutralized with an alkaline bath to remove residual acid.

3. Heat Treatment: Spheroidize Annealing (SA)

To improve the steel’s formability and ductility, which is essential for cold forming, the raw material undergoes a softening heat treatment known as Spheroidize Annealing (SA).

• Purpose: Annealing converts the lamellar (layered) pearlite structure of the steel into globular (spheroidal) cementite particles within a ferrite matrix. This spheroidized microstructure significantly lowers the material’s deformation resistance, making it much softer and more malleable, which is critical for complex cold heading operations.
• Process: The steel is heated to a temperature just below the lower critical temperature for a prolonged period, or subjected to slow cooling. This lengthy process is a major factor in preparation cost and time.

4. Wire Drawing (Cold Reduction)

The final pre-forming step is wire drawing, a cold working process that achieves the precise diameter, dimensional tolerance, and surface finish required by the fastener-forming machines (headers).

• Mechanism: The cleaned and annealed wire rod is pointed at one end and then pulled, not pushed, through a series of successively smaller, hard dies (typically tungsten carbide). As the diameter decreases, the length increases, and the cross-sectional area is reduced (draft).
• Benefits:
  • Dimensional Accuracy: Achieves tight diameter tolerances and improves roundness.
  • Surface Finish: Imparts a smoother surface required for the cold heading dies.
  • Work Hardening: The cold drawing slightly increases the material’s strength and yield properties, which can be balanced with the preceding annealing process.

5. Lubrication and Coating

Before the material enters the cold heading machines, a specialized coating is applied to the drawn wire to act as a lubricant carrier.

• Purpose: The coating reduces friction between the steel wire and the dies, preventing scoring or scratching of the wire’s surface and protecting the costly tooling from excessive wear during high-pressure forming.
• Common Coatings:
  • Phosphate Coating: The material is coated with manganese, zinc, or iron phosphate, which chemically reacts with the steel surface.
  • Soaping/Liming: A final coat of lubricant, such as metallic soap or lime, is applied over the phosphate. This combination provides a barrier that adheres well to the wire and facilitates smooth drawing and heading.

Summary of Preparation Steps

The prepared steel wire—now with a precise diameter, low internal stress, and a protective lubricant coating—is ready to be coiled and fed into the high-speed cold heading machines for the next stage of fastener production.

1. Selection & Testing (Chemical composition, mechanical properties)
2. Descaling & Cleaning (Removal of mill scale via chemical pickling or mechanical processes)
3. Spheroidize Annealing (Heat treatment to maximize ductility and formability)
4. Wire Drawing (Cold reduction to achieve final, precise diameter and surface finish)
5. Lubrication & Coating (Application of phosphate and soap to minimize friction during forming)