Bolt manufacturing process

Bolts are categorized into ordinary carbon steel bolts, high-strength bolts (grades 8.8, 10.9, and 12.9), and stainless steel bolts. While the basic manufacturing processes are largely similar, high-strength bolts require an additional heat treatment step, whereas stainless steel bolts undergo pickling and passivation. 

I. Incoming Raw Material Inspection

Raw materials: Wire rod (coiled steel); material grades include Q235, 45#, 40Cr, 35CrMo, and 304/316 stainless steel.

Visual inspection: Check for cracks, rust, or surface delamination.

Physicochemical testing: Material spectral analysis, tensile strength, and hardness.

Dimensional verification: Ensure wire diameter meets bolt blank specifications.

II. Wire Drawing (Cold Drawing)

The initial wire rod diameter is oversized and must be drawn down to the standard blank diameter.

Pickling: Hydrochloric acid immersion to remove surface scale and rust.

Neutralization, water rinsing, and phosphating: Formation of a lubricating phosphate film on the surface.

Saponification: Application of lubricating powder to reduce wear on cold-heading dies.

Cold drawing: Multiple drawing passes to reduce wire diameter to the target processing size.

Coiling: Wire is wound into coils, ready for cold heading.

III. Cold Heading (Core Process: Forming bolt head and shank blank in one operation)

Uses a multi-station cold-heading machine; forming occurs via extrusion at room temperature (no heating).

Feeding and cutting: Wire stock is cut to the required total bolt length.

First punch (pre-heading): Initial extrusion to form the head shape.

Second punch (finish heading): Forms the hex head, round head, or pan head; simultaneously reduces shank diameter and forms the chamfer.

Ejection: Produces a smooth-shank bolt blank (unthreaded).

Defect control: Checks for head cracking, off-center heads, inconsistent shank lengths, and burrs.

IV. Thread Rolling (Thread Processing)

Distinguished by thread rolling (for high-precision, high-strength bolts) vs. flat-die thread rolling (for standard bolts).

Blank loading, positioning, and clamping.

Two thread-rolling dies rotate against each other to extrude standard external threads onto the shank.

Thread gauge inspection (Go/No-Go): Ensures thread fit accuracy.

Deburring: Removal of burrs from thread edges.

V. Heat Treatment (Required only for high-strength bolts, Grade 4.8 and above; optional for standard Grade 4.8 bolts)

Purpose: To enhance strength, hardness, and toughness, achieving... 8.8/10.9/12.9 Strength Grades

1. Quenching

Heated in a mesh-belt furnace to 800–900°C, followed by rapid oil cooling to increase hardness.

2. Tempering

Reheated and held at medium temperature to relieve internal quenching stresses and prevent brittle fracture.

3. Cooling & Discharge

Post-heat treatment inspection of hardness and microstructure; non-conforming parts are reworked.

VI. Surface Treatment (Corrosion/rust protection; process selected based on requirements)

1. Electro-galvanizing (Standard bolts)

Degreasing → Pickling → Activation → Zinc plating → Passivation (Blue-white, iridescent, black) → Sealing

Advantages: Low cost; Disadvantages: Susceptible to hydrogen embrittlement at high temperatures; electro-galvanizing is prohibited for high-strength 10.9/12.9 bolts.

2. Hot-dip Galvanizing (Power industry & heavy-duty outdoor bolts)

Pickling & fluxing → High-temperature zinc dipping; thick coating, excellent corrosion resistance; threads require post-process cleaning.

3. Dacromet / Geomet (For high-strength bolts)

No hydrogen embrittlement; superior rust resistance compared to galvanizing; salt spray resistant; mainstream choice for new energy and automotive high-strength components.

4. Blackening / Bluing (Low-cost indoor bolts)

Alkaline oxidation; short-term rust protection only; no coating layer.

5. Stainless Steel Specific: Pickling & Passivation

Removes processing scale/oxides; enhances stainless steel's rust resistance.

VII. Full Inspection & Sorting

Visual inspection (100%): Cracks, dents, burrs, coating peeling.

Dimensional inspection: Total length, hex width across flats, thickness, thread outer diameter.

Mechanical spot checks: Tensile strength, yield strength, hardness, torque testing.

Optical sorting machine: Automatic sorting to remove defective parts.

VIII. Hydrogen Relief Baking (Mandatory for galvanized high-strength bolts)

Baked at 200–230°C for 2–4 hours within 24 hours of plating to eliminate hydrogen embrittlement and prevent stress-induced fracture.

IX. Packaging & Warehousing

Packaged by specification and grade; labeled (specification, strength, material, batch number). Moisture-proof packaging; warehousing and shipment.