Full Production of Meter-Long Bolts

[Abstract]:Meter-scale extra-long bolts are widely used in wind power, bridges and heavy machinery, whose production overcomes challenges in long blank forming, precision control and residual stress relief.

Full Production of Meter-Long Bolts

Meter-class extra-long bolts serve as core structural fasteners for wind turbine towers, bridge reinforcement and heavy machinery assembly. Distinct from standard short bolts, they withstand extreme load and persistent vibration with stringent requirements on dimensional precision and mechanical performance. Their manufacture follows a systematic workflow: raw material preparation → forming → precision machining → thermal stabilization → comprehensive inspection.

I. Raw Material Preprocessing

High-strength alloy steels including 35CrMo and 42CrMo are prioritized with tensile strength above 800MPa to satisfy heavy-duty service. Three pre-treatment procedures are mandatory:

Low-temperature annealing: Steel bars are heated at roughly 740℃ for over 15 hours to reduce hardness and eliminate rolling residual stress, then air-cooled to ambient temperature against machining cracking.
Shot blasting: High-speed pellets remove scale and contaminants while optimizing surface finish for subsequent processing and lubrication compatibility.
Precision straightening: Five-axis straighteners calibrate bent bars within linearity tolerance ≤0.5mm/m.

II. Core Forming

Two mainstream processes address dimensional deformation of lengthy blanks based on final configurations:

Hot heading for headed bolts (wind tower hex bolts): Local bar end is induction-heated near 2100℃ within 30s, then hot-forged above 60-ton press into integrated heads to avoid welding weakness. Precise heating duration prevents over-oxidation or insufficient softening.
Cold drawing plus recrystallization annealing for full-thread studs: Cold drawing modifies bar diameter and improves compactness via plastic deformation; subsequent annealing relieves forming stress and refines grain structure. The chip-free process produces studs up to 3.7m long while cutting raw material consumption by 15%–20%.

III. Thread Machining

Thread rolling replaces conventional cutting for long-size thread fabrication. Tungsten carbide rolling dies extrude continuous metal grain flow to form complete thread profile, boosting thread strength by 20%–30% with tight pitch tolerance ±0.02mm. Custom dies accommodate varied thread specifications, and dedicated guiding fixtures guarantee uniform machining along full bolt length for high throughput.

IV. Thermal Enhancement

Vacuum or controlled-atmosphere furnaces prevent uneven heating-induced distortion during quenching & tempering:

Quench: Heat bolts to 850–890℃ with 1–2h holding before quenching oil cooling to form martensite for upgraded hardness and strength.
High-temperature tempering: Soak at 550–600℃ for 2–3h and cool gradually to generate tough tempered sorbite balancing strength and ductility. Hydraulic straightening corrects post-heat-treatment deformation.

Select premium bolts adopt strain hardening plus recrystallization annealing instead of conventional quenched tempering to meet ASTM A193 B7 specifications with lower energy consumption.

V. Surface Finishing & Final Inspection

Surface Treatment

Marine/humid service: Hot-dip galvanizing or DACROMET coating for over 1000-hour salt spray resistance.
High-temperature operating surroundings: Silver plating combining corrosion resistance and thermal conductivity.
General industrial conditions: Black oxide finish for improved wear and rust protection.

Full-Scope Quality Testing

Dimensional check: CMM for dimension parameters, laser detector for straightness.
Mechanical test: Tensile & impact sampling to verify rated strength (≥1040MPa for Class 10.9 bolts).
Internal flaw detection: Ultrasonic scanning for inclusions and cracks.
Torque verification: Assembly compatibility and fastening reliability confirmation.

Products pass all indexes before delivery.

Conclusion

Extra-long meter-size bolts integrate material science, precision machining and thermal treatment technologies. Strict process control across all production phases secures dependable connection for large-scale engineering and heavy equipment. Driven by new-energy and wind-power expansion, relevant manufacturing evolves toward higher strength, precision and eco-efficiency via chip-free forming and vacuum heat treatment.