Production Process of Hexagonal Bolts above M30 sizes

Hex head bolts above M30 are classified as large-specification fasteners. Due to their large diameter, their production process differs from that of small bolts, primarily utilizing hot forging (red forging) . The core process mainly consists of five key steps: material selection and cutting, hot heading of the head, thread processing, heat treatment, and surface treatment.  

1. Material Selection and Cutting

The performance of the bolt is primarily determined by the raw material. For large high-strength hexagonal bolts of M30 and above, the appropriate steel must be selected based on the required strength grade.

· Common Materials: For example, 35VB, 20MnTiB, 45# steel, or alloy steels like GH4169 (superalloy).
· Cutting: Round steel bars are precision-cut to the required dimensions, resulting in metal cylindrical billets. Some processes may also include chamfering after cutting.

2. Head Forming

This is the core step that distinguishes it from smaller bolts. Due to the large diameter of bolts above M30, cold heading machines are unsuitable, so the hot forging process is widely adopted.

· Heating: The billet is heated using medium frequency induction heating or in a furnace until it reaches a plastic state.
· Upsetting: The heated billet is placed into a die for upsetting. This is typically done in two or more stages, such as initial warm upsetting to roughly form the head, followed by secondary warm upsetting for complete forming. This hot forming process ensures the integrity of the metal flow lines in the head, significantly enhancing the bolt's mechanical properties.
· Trimming/Forming: The standard hexagonal head is formed by die extrusion or trimming.

3. Thread Processing

There are two main methods for forming threads on the bolt shank:

· Thread Rolling: This is the most common method for high-strength bolts. Before heat treatment or in a semi-finished state, a thread rolling machine is used to cold-form the threads by extrusion. This process induces work hardening, enhancing the thread's strength and surface finish.
· Thread Cutting: For certain special materials or extremely large bolts, thread cutting might be used, although material utilization is lower.

4. Heat Treatment

To achieve a strength grade of 10.9 or higher, heat treatment is the critical process determining the bolt's mechanical properties.

· Process Type: Typically involves quenching and tempering (quenching + high-temperature tempering).
· Quenching: The bolts are heated to the phase transformation temperature (e.g., 900°C – 920°C) and then rapidly cooled in oil or water to increase hardness. Some process innovations mention introducing water quenching.
· Tempering: Quenching must be followed by tempering (e.g., at 200°C – 300°C or higher) to eliminate brittleness and achieve a good balance of strength and toughness. The final microstructure obtained is typically tempered sorbite.

5. Surface Treatment

To prevent corrosion during service and meet specific torque coefficient requirements, surface treatment is necessary.

· Conventional Rust Prevention: Phosphating is a common process that helps meet torque coefficient requirements.
· High Corrosion Resistance Treatment: For applications demanding high corrosion resistance, Dacromet (zinc-chromium coating) is used. Advanced processes like "three-dip, three-bake" dip coating can meet the requirements of a 1000-hour neutral salt spray test.
· Galvanizing: Electro-galvanizing may also be used, involving steps such as degreasing, pickling, plating, and passivation.

Supplement: Quality Control

Throughout the production process, rigorous quality control measures are implemented, including:

· Hardness Sorting: Hardness testing of bolts after heat treatment.
· Non-Destructive Testing: Magnetic particle inspection is used to detect surface and near-surface defects like cracks.
· Torque Coefficient Testing: For steel structure connection pairs, the average torque coefficient must be ensured to fall within the range of 0.110 to 0.150.