Cold Heading Screw Manufacturing Guide

Introduction to Cold Heading Process

Cold heading, also known as cold forming, is a high-efficiency manufacturing process used to produce screws and other fasteners at room temperature. This method involves deforming metal wire through a series of dies to form the desired shape, offering advantages such as material savings, enhanced strength due to work hardening, and precise tolerances. Compliant with standards like GB/T 3098.1 for fastener mechanical properties, cold heading is ideal for mass production in automotive, construction, and electronics industries.

The process begins with raw wire rods and progresses through annealing, surface preparation, drawing, forming, threading, heat treatment, and finishing. Each step is critical to ensure the final product meets specifications for tensile strength, hardness, and corrosion resistance. Proper execution minimizes defects like cracking or dimensional inaccuracies, promoting reliability in applications where failure is unacceptable.

Guidance: Select materials based on end-use requirements, such as low-carbon steel for ductility or alloy steels for high strength. Regular quality checks, including hardness testing per GB/T 230.1, are essential throughout production.

Annealing

Annealing softens the wire rod by heating it to a specific temperature, holding it, and cooling slowly to adjust the crystalline structure, reduce hardness, and improve machinability at room temperature. This step is vital for materials like 1018, 1022, 10B21, 1039, and CH38F steels.

Procedure: Load up to 7 coils (about 1.2 tons each) into the furnace, seal tightly. Heat gradually over 3-4 hours to 680-715°C for 1018/1022 or 740-760°C for others, hold for 4-7.5 hours, then cool slowly over 3-4 hours to below 550°C, followed by furnace cooling to room temperature.

• Quality Control: Post-annealing hardness should be HV120-170 for low-carbon steels and HV120-180 for medium-carbon. Surfaces must be free of oxide films or decarburization.
• Guidance: Monitor temperature uniformity to prevent uneven softening, which could lead to forming defects. Adhere to GB/T 699 for premium carbon structural steel standards.

This process enhances plasticity, reducing the risk of cracking during subsequent cold deformation steps.

Pickling

Pickling removes oxide films from the wire surface and forms a phosphate coating to minimize tool wear during drawing and forming. This chemical treatment is crucial for surface quality and lubrication.

Procedure: Immerse in 20-25% hydrochloric acid tanks for several minutes to remove oxides, rinse with water, treat with oxalic acid for metal activation, apply phosphate solution to form Zn2Fe(PO4)2·4H2O film, rinse again, and apply lubricant like sodium stearate for enhanced lubricity.

• Guidance: Control immersion times to avoid over-etching, which can weaken the wire. Ensure environmental compliance by treating wastewater per industry regulations.
• Benefits: The phosphate layer reduces friction, extending die life and improving surface finish.

Proper pickling ensures uniform coating, vital for consistent performance in high-volume production.

Wire Drawing

Wire drawing reduces the coil diameter to the required size through cold pulling, often in rough and fine stages for certain products. This step prepares the wire for forming by achieving precise dimensions.

Procedure: After pickling, draw the coil through dies to the target diameter. For large screws, nuts, or rods, use dedicated drawing machines.

• Guidance: Maintain reduction ratios of 10-15% per pass to avoid excessive work hardening or cracks. Lubrication is key to prevent surface defects.
• Standards: Align with GB/T 6478 for cold heading steel, ensuring elongation and tensile properties meet forming needs.

Effective drawing enhances material strength via deformation while preserving ductility for cold heading.

Forming

Forming shapes the wire into semi-finished screws via cold or hot forging, achieving desired geometry and dimensions. This core step uses multi-station machines for efficiency.

For hex bolts (four or three-die processes): Cut blank, initial upset, secondary forming, hex trimming. For screws: Cut, preliminary head forming, final shaping. Hot forming heats to 7-15 seconds for larger sizes, followed by shank reduction.

Nut forming: Cut, initial shaping in multiple punches, final piercing.

• Guidance: Use dies with Ra ≤0.2 μm surface roughness for precision. Install knockout devices to prevent jams. Control flip angles for complex shapes.
• Standards: Per ISO 898, ensure fiber flow continuity for strength.

This process maximizes material utilization, producing parts with superior mechanical properties compared to machined alternatives.

Threading

Threading forms threads on semi-finished parts via rolling or tapping, creating the functional screw profile. This enhances strength through plastic deformation.

Procedure: Roll threads using fixed and moving plates for bolts; tap for nuts; roll for rods. Optimize revolutions to avoid defects like cracks or out-of-roundness.

• Guidance: Adjust blank diameter for accuracy, considering plating effects. Inspect for surface cracks per GB/T 3098.1.
• Common Defects: Cracks, random threads, non-roundness—control via process parameters.

Thread rolling preserves grain structure, improving fatigue resistance in load-bearing applications.

Heat Treatment

Heat treatment optimizes mechanical properties through quenching and tempering, tailored to material and purpose: high-temperature temper for quenched steels (500-650°C), medium for springs (420-520°C), low for carburized (150-250°C).

Procedure for structural steels: Normalize, quench at 850°C, temper at 400-500°C or 200°C for high strength. For springs: Oil quench at 830-870°C, temper at 420-520°C. For carburized: Carburize, quench, low-temper.

• Guidance: Use continuous furnaces with atmosphere control to prevent decarburization per GB/T 3098.1. Monitor for uniform hardness and avoid cracking.
• Defects: Insufficient hardness, unevenness, deformation, cracking—mitigate via precise controls.

Advanced equipment ensures consistent quality, crucial for high-strength fasteners.

Surface Treatment

Surface treatments provide corrosion resistance and aesthetics: electroplating (zinc, nickel, etc.), hot-dip galvanizing, mechanical plating.

Quality Control: Appearance free of defects; thickness 4-12 μm for electroplating, 43-54 μm for hot-dip; uniform distribution; hydrogen embrittlement mitigation via baking at 176-190°C for 3-24 hours; adhesion testing.

• Guidance: Bake high-hardness parts promptly to avoid embrittlement. Comply with ISO 4042 for electroplated coatings.

These treatments extend service life in corrosive environments.

High-Strength Bolt Specifics

High-strength bolts follow: Hot-rolled wire – drawing – spheroidizing annealing – mechanical descaling – pickling – drawing – cold forging – threading – heat treatment – inspection.

Material Design: C 0.25-0.55%, Mn 0.45-0.80%, Si ≤0.30%, P/S ≤0.030/0.035%, B ≤0.005% per GB/T 6478 and JIS G3507.

Spheroidizing: Heat to Ac1 +20-30°C, cool to ~700°C isothermally, then to 500°C. For 35/45 steels: 715-735°C; SCM435: 740-770°C, isothermal 680-700°C.

Descaling: Mechanical (bending/spray) + pickling for >8.8 grade.

Drawing: 10-15% reduction per pass to minimize hardening.

Forming: Multi-station, precision dies.

Threading: Rolling for strength.

Heat Treatment: Automated lines for quality.

• Guidance: Fine-tune for grades 8.8/9.8 to balance strength and ductility per ISO 898-1.

Manufacturing Diagrams and Visuals

Visual aids illustrate key forming stages:

These stages, often visualized in animations, demonstrate progressive deformation. Videos show real-time forming and threading, highlighting precision and speed.