What are the differences between cold heading and hot forging of bolts?

In the field of industrial fasteners, bolts, as the key connectors of the "mechanical skeleton", their manufacturing process directly determines the strength, precision and service life of the product. Cold heading and hot forging, as the current mainstream processes, are reshaping the detailed standards of modern industry with completely different production methods. This article will deeply analyze the essential differences between the two technologies and explore their industrial application logic.

1. Comparison of process principles: Technical paths determined by temperature

1.1 Cold Heading Process

Temperature conditions: Room temperature environment (usually 15-30℃)

Core process:

① Wire cutting → ② Multi-station die extrusion forming → ③ Thread rolling

Energy transfer: The metal undergoes plastic flow through an instantaneous pressure of up to 2,000 tons

Microscopic changes: The grains refine along the direction of force application, forming dense fibrous streamlines

1.2 Hot Forging

Temperature condition: Heat above the recrystallization temperature (about 1100-1250℃ for carbon steel)

Core process:

① Billet heating → ② Forging hammer impact forming → ③ Heat treatment and quenching

Energy transfer: Thermal energy softens the material, and the impact force is approximately 60% lower than that of cold heading

Microscopic changes: Reorganization of austenite structure, eliminating the original casting defects

2. Performance Difference Map: The product genes shaped by the process

In comparison, cold-forged bolts and hot-forged bolts have high mechanical strength, surface hardness (HRC35-45), good core toughness (HRC22-32), dimensional accuracy ±0.02mm (up to 6g grade) ±0.1mm (usually 4g grade), fatigue life of 2 million cycles (R=-1), 5 million cycles (R=-1), residual stress, and compressive stress on the surface layer as a whole Uniform stress distribution, surface quality Ra0.8-1.6μm (no machining required) Ra3.2-6.3μm (turning required)

3. Cost-benefit Analysis: The Economics Hidden Behind Temperature

3.1 Direct Cost Comparison

Cold heading: Mold cost accounts for 40% (the lifespan of precision molds is approximately 500,000 pieces)

Hot forging: Energy cost accounts for 55% (800-1200 kilowatt-hours of electricity consumed per ton of bolts)

3.2 Hidden Cost Considerations

The material utilization rate of cold heading process can reach 95%, while that of hot forging is only about 80%

The subsequent processing time of hot forgings is approximately three times that of cold heading parts

3.3 Batch Economic Critical Point

When the order volume is less than 100,000 pieces, the comprehensive cost of cold heading is 15-20% lower

When the order volume exceeds 500,000 pieces, the cost advantage of hot forging gradually emerges

4. Typical application scenarios: Process selection that matches the requirements

4.1 Leading Fields of cold heading

Automobile engine connecting rod bolts (requiring high-precision fit)

Micro fasteners for electronic products (threads below M1.6)

Aerospace standard parts (AMS/EN standard certification)

4.2 Irreplaceable scenarios of hot forging

Wind power tower flange connection bolts (M64 and above super-large specifications)

Special bolts for oil drilling and production equipment (subject to hydrogen sulfide corrosion)

Nuclear power main pipeline fastening system (requiring irradiation stability)

5. Technological development trends: Intelligence and composite processes

Innovation in cold heading technology：

Multi-station linkage machine tool (Efficiency increased by 300% at 8 stations)

Nano-coated mold (extended lifespan to 2 million times)

Evolution of Hot Forging process：

Electromagnetic induction precise temperature control (±5℃ temperature difference control)

Isothermal forging technology (Breakthrough in Mass Production of titanium alloy bolts)

The rise of hybrid processes:

Warm forging technology (balancing cost and performance in the 300-800℃ range)

Laser-assisted forming (Local heating reduces deformation resistance)