Complete Manufacturing Process of Long Bolts Over One Meter

1. Raw Material Selection: Laying the Foundation

The performance of large bolts fundamentally depends on raw materials. Since they must withstand immense tensile and shear forces, often in harsh working conditions, material selection must strictly adhere to industry standards to eliminate any defects. Currently, long bolts over one meter primarily use high-strength alloy structural steels such as 40Cr, 35CrMoA, and 42CrMoA. These steels possess excellent tensile strength, toughness, and wear resistance, meeting the load-bearing demands of heavy equipment after subsequent processing.

The screening process involves multiple inspections. First, the chemical composition is analyzed to ensure elements like carbon, chromium, and molybdenum meet standards, preventing performance issues caused by deviations. Second, ultrasonic flaw detection identifies internal defects like pores, inclusions, or cracks, which severely reduce crack resistance and can cause fractures during processing or use. Finally, visual inspections remove materials with surface scratches, rust, or unevenness. Additionally, raw materials undergo pretreatment through annealing to eliminate internal stress and improve the microstructure, laying a solid foundation for subsequent processing.

2. Forging: Shaping the Bolt and Ensuring Density

Long bolts cannot be formed using ordinary cold heading processes; instead, hot forging is required. This involves heating the steel to soften it and then using forging pressure to shape the preliminary bolt form, directly determining the quality of the blank and the density of its internal structure.

The core hot forging process is as follows: First, qualified steel is cut into billets of corresponding lengths, calculated precisely based on finished dimensions and machining allowances. Next, the billets are placed in a heating furnace at temperatures between 1100-1250℃ to ensure complete softening without overheating or burning. Uniform heating and heat preservation are maintained to prevent stress from excessive temperature differences. Once heated, the billets are quickly transferred to a forging press. Using precisely designed molds for the head shape and shank dimensions, controlled forging force and speed ensure full deformation, resulting in a full head, straight shank, and dense internal structure free from looseness or delamination.

After forging, the bolt blanks undergo slow cooling in a pit or insulated box to prevent quenching cracks from rapid cooling and to relieve internal stresses generated during forging. Once cooled, the blanks are preliminarily ground to remove oxide scale, flash, and burrs, creating a standardized shape ready for further processing.

3. Heat Treatment: Enhancing Performance and Load Capacity

Heat treatment is the core stage in manufacturing large bolts. By precisely controlling heating, holding, and cooling, the microstructure is altered to significantly improve key properties like tensile strength, hardness, and toughness, ensuring the bolts can withstand long-term loads from heavy equipment.

The process mainly includes quenching and tempering, with some high-end bolts undergoing additional conditioning. During quenching, blanks are heated to 850-900℃ and held until fully austenitized, then rapidly cooled in media like oil or water to transform austenite into martensite, boosting hardness and strength. Strict control over temperature, holding time, and cooling rate is crucial to avoid cracks or uneven hardness—a major technical challenge where parameter failure can lead to entire batch rejections.

Following quenching, immediate tempering is required. The bolts are reheated to 200-300℃, held for a period, and slowly cooled to eliminate internal stress from quenching, reduce brittleness, and enhance toughness. This ensures the bolts possess both high strength and good ductility, preventing brittle fracture during use. Post-tempering, tensile strength typically reaches Grade 8.8 or higher.

4. Precision Machining: Ensuring Accuracy and Compatibility

Large bolts demand extremely high precision, particularly in shank diameter, length, and thread accuracy, as these directly affect mating precision with nuts and connection stability. Therefore, precision machining requires strict control to ensure every dimension meets standards.

The process begins with turning the bolt shank on CNC lathes to precisely control diameter and length, keeping dimensional errors within ±0.02mm. Thread processing for long bolts typically uses thread rolling rather than cutting. Rolling produces smoother, stronger threads with higher efficiency. Controlled rolling pressure and speed ensure regular tooth profiles and uniform pitch without stripping or broken teeth. For ultra-high-precision bolts, thread grinding is added to further enhance accuracy.

After machining, surface treatments like hot-dip galvanizing, Dacromet, or phosphating are applied to improve rust and corrosion resistance, extending service life—especially critical for bolts used in outdoor, humid, or corrosive environments. A final inspection of appearance and dimensions ensures no surface defects and compliance with specifications.

5. Final Inspection: Rigorous Quality Control

Since long bolts are used in critical scenarios where quality directly impacts equipment safety, final inspections must strictly follow industry standards to comprehensively identify defects and prevent non-conforming products from entering the market.

Final inspections include visual, dimensional, mechanical property, and non-destructive testing (NDT). Visual checks use the naked eye or magnifiers to find surface cracks, scratches, rust, or thread defects. Dimensional checks use calipers, micrometers, and projectors to measure length, diameter, and thread accuracy against design requirements. Mechanical tests, including tensile, impact, and hardness tests, verify tensile strength, yield strength, hardness, and toughness. NDT methods like ultrasonic and magnetic particle testing detect hidden internal and surface cracks that could compromise safety.

Only after these five stages can a long bolt over one meter officially leave the factory. From raw material selection to final inspection, every step requires strict control of process parameters and standardized operations; any oversight can lead to product rejection. With the rapid development of industries like wind power, nuclear energy, and bridge construction, the demand for large bolts is growing, imposing higher requirements on manufacturing processes. In the future, fastener enterprises will continue to leverage technological innovation to further improve the precision and performance of large bolts, supporting the high-quality development of the heavy equipment industry.