Bolts: The Rice of Industry

Bolts, often referred to as the "rice of industry," are indispensable in daily life and industrial production. Their applications span a wide range of fields, including electronics, machinery, digital products, electrical equipment, electromechanical devices, as well as in ships, vehicles, hydraulic engineering, and even chemical experiments. As long as industrial activity exists on Earth, bolts will remain critical. This article provides a concise introduction to the production and processing techniques of bolts, offering useful insights.

1. Common Materials for Bolt Processing

The materials used for bolts depend on their strength grade. The three primary materials available in the market are carbon steel, stainless steel, and copper.

Carbon Steel

Low Carbon Steel:

Contains ≤0.25% carbon, commonly known as A3 steel domestically, and as 1008, 1015, 1018, or 1022 abroad. This material is primarily used for grade 4.8 bolts, grade 4 nuts, and small screws with no specific hardness requirements (e.g., self-tapping screws).

Medium Carbon Steel:

Contains 0.25%-0.45% carbon. In China, it is known as 35# or 45# steel, while abroad, it is referred to as 1035, CH38F, 1039, or 40ACR. This material is used for grade 8 nuts, grade 8.8 bolts, and grade 8.8 hex socket bolts.

High Carbon Steel:

Contains >0.45% carbon, though it is rarely used in the market.

Alloy Steel:

Adds alloying elements to standard carbon steel to enhance specific properties, such as plasticity, strength, hardness, and corrosion resistance. Common examples include 35CrMo, 40CrMo, SCM435, and 10B38.

Stainless Steel

Categorized by performance levels:

Austenitic (e.g., 18% Cr, 8% Ni): Excellent heat and corrosion resistance, good weldability.

Martensitic (e.g., 13% Cr): High strength, wear resistance, but weaker corrosion resistance.

Ferritic (e.g., 18% Cr): Better corrosion resistance than martensitic steel and good ductility.

Imported stainless steel materials, such as SUS302, SUS304, and SUS316, dominate the high-end market.

Copper

Commonly used grades include H62, H65, and H68 brass. Zinc-copper alloys are also frequently used for standard parts.

2. Spheroidizing (Softening) Annealing

Purpose:

For countersunk screws and hex socket bolts produced using cold forging, the original steel structure significantly affects formability.

Requirements:

Localized plastic deformation during cold forging can reach 60%-80%, necessitating steel with excellent plasticity. Fine spheroidized pearlite is preferable for enhancing plastic deformation, as coarse lamellar pearlite hinders it.

Application:

Medium-carbon steel and medium-carbon alloy steel used for high-strength fasteners undergo spheroidizing annealing before forging to achieve uniform, fine pearlite structures, optimizing production needs.

Process Details:

For medium-carbon steel, the annealing temperature is carefully controlled near the critical point. Excessive heating may lead to grain boundary carbide precipitation, causing cold forging cracks.

Medium-carbon alloy steel undergoes isothermal spheroidizing annealing, with gradual furnace cooling from ~700°C to ~500°C to refine the steel structure and minimize cracking risks.

3. Thread Processing

Cold Processing:

Bolt threads are typically formed through cold processing, where thread blanks are shaped by rolling or pressing between dies, ensuring continuous plastic flow, enhanced strength, high precision, and uniform quality.

Thread Blank Diameter:

Determined by thread precision and whether the material has coatings.

Rolling Process:

Thread formation involves compressing a cylindrical blank between dies with matching thread pitch and shape. As the blank rotates, the die transfers its pattern, forming the thread.

Efficiency Factors:

Excessive rolling rotations reduce efficiency and may cause thread defects like separation or misalignment.

Insufficient rotations can lead to uneven thread diameter or increased pressure on the die, shortening its lifespan.

Common Defects:

Surface cracks or scratches.

Misaligned threads.

Out-of-round threads.

Effective quality control during production can minimize these issues.

By understanding these materials and processing techniques, manufacturers can produce high-quality bolts that meet industrial demands efficiently and reliably.