In modern industry, screws are often called the "cells of industry." From aircraft and high-speed trains to smartphones and home appliances, nearly every product relies on these small yet essential fasteners.
While screws may seem simple, high-strength industrial screws that meet ISO or ASTM standards require precise manufacturing processes such as cold heading, thread rolling, heat treatment, and surface finishing. This article provides a professional overview of how modern screws are manufactured, from material selection to final testing.
1. Raw Material Selection & Pre-treatment
High-quality screws originate from premium metal raw materials. The mechanical performance, tensile strength, and fatigue life of a screw depend heavily on the purity of its base material and its initial processing.
1.1 Scientific Selection of Wire Rods
The source of screw production consists of coiled metal wire rods, commonly referred to in the industry as "wire rods" or "coils." Depending on the final application scenario and strength grade of the screw (such as ISO Grade 8.8, 10.9, or 12.9), factories select distinct steel compositions. Common materials include:
Low-Carbon Steel: Commonly used for manufacturing ordinary, low-strength screws.
Medium-Carbon Steel: Frequently utilized for manufacturing Grade 8.8 high-strength fasteners.
Alloy Steel: Featuring exceptional hardenability, this is the premier choice for Grade 10.9 and 12.9 ultra-high-strength screws.
Stainless Steel: Focused on providing excellent corrosion resistance.
1.2 Pickling and Rust Removal Process
During the hot-rolling process at the steel mill, a dense layer of iron oxide scale inevitably forms on the surface of the wire rods. If this oxide scale directly enters subsequent processing, it will severely wear down the dies and cause surface cracking on the screws. Therefore, the wire rods must undergo chemical rust removal in a pickling tank (typically utilizing diluted sulfuric acid or hydrochloric acid). Once pickling is complete, water rinsing, phosphating treatment (forming a phosphate film on the metal surface to minimize friction during subsequent drawing), and saponification lubrication are required.
1.3 Precision Wire Drawing and Annealing
The pre-treated wire rods are forcefully drawn through the carbide dies of a wire drawing machine to reduce them to the exact wire diameter required for screw production (with tolerances typically controlled within ±0.02MM). For medium-carbon and alloy steels, the internal crystal lattice undergoes strain hardening after drawing. To restore ductility for subsequent forming processes, the wire coils usually enter a bell-type annealing furnace for prolonged spherical annealing under a protective atmosphere.
2. Core Forming Processes of Fasteners
Once the wire material is ready, it officially enters the structural processing stage of the screw. Modern screw manufacturing relies primarily on cold forming technology, the core purpose of which is to allow the metal to flow rather than cutting the metal fibers, thereby maximizing its mechanical strength.
2.1 Cold Heading: Shaping the Screw Head and Shank
Cold heading is a process executed at room temperature where massive axial pressure is applied to cut wire blanks using high-speed cold heading machines and precision dies, forcing the metal to undergo plastic deformation within the die cavities.
One-Blow / Two-Blow Process: Standard hex screws or cross-recess (Phillips) screws are generally completed on multi-station cold heading machines. The first blow utilizes a punch to flatten the head, upsetting it into a preliminary shape; the second blow uses a forming punch to compress it into the final hex or round head while simultaneously punching out internal drive structures like cross recesses or Torx configurations.
Multi-Station Cold Heading: For complex, custom-shaped screws or automotive-grade high-strength fasteners, a 3-station, 4-station, or even 5-station cold heading machine is used. Through progressive extrusion across multiple stages, it ensures that the metal fiber flow lines follow the continuous distribution of the screw's outer shape, which significantly enhances the screw's resistance to fatigue cracking.
2.2 Thread Rolling: Generating Precision Threads
The threads of a screw are typically not "cut" out with cutting tools; rather, they are "rolled" or "formed" through plastic deformation. The cold-headed blank with a smooth shank is fed into a thread rolling machine.
Thread Rolling Plates Process: The blank passes between two thread rolling plates engraved with specific thread profiles. One plate remains fixed while the other performs a reciprocating linear motion. Relying on immense mechanical thrust, the surface layer of the smooth shank is forcefully extruded into the grooves of the thread plates, forming the threads.
Thread Rolling Dies Process: For large-diameter, long-shank screws or high-precision studs, two cylindrical thread rolling dies rotating in the same direction are used to compress the blank circumferentially. This extrusion process subjects the metal grains at the thread surface layer to compressive stress, thereby substantially reinforcing the shear strength and surface hardness of the threads.
3. Core Technical Metrics in Screw Forming Processes
To clearly contrast cut threads against cold-formed (rolled) threads, as well as the functions of different manufacturing stages, the following table organizes the core forming processes and quality control factors in modern screw manufacturing:
Processing Stage / Process Type
Processing Principle
Metal Fiber Flow Line State
Surface Quality & Mechanical Strength Performance
Typical Applications & Limitations
Cold Heading
Axial pressure is applied to the wire blank at room temperature via high-pressure punches and dies, causing strain hardening.
The metal flow lines conform to the screw's outer profile, remaining completely intact without breaking.
The surface structure is dense, exhibiting highly exceptional anti-fatigue and impact resistance properties.
Broadly applicable to the vast majority of standardized, mass-produced industrial fasteners.
Thread Rolling
Relies on the relative motion of carbide rolling dies or thread plates to forcefully extrude the smooth shank without chip generation.
The metal fibers are continuous at the thread crests and roots, and the grains are highly compressed.
Surface hardness is significantly elevated, surface roughness is minimized, and fatigue strength is high.
Ideal for highly efficient processing of standard external threads; unsuitable for ultra-hard or extremely brittle materials.
Thread Cutting
Utilizes cutting tools or taps to mechanically peel the metal matrix, forcing cut grooves into the component.
The metal flow lines are completely cut through at the threads, resulting in multiple fractured cross-sections.
Stress concentration easily occurs at the thread roots, micro-tool marks are often present, and overall strength is relatively lower.
Mostly used for low-volume custom shapes, ultra-large-diameter fasteners, or internal thread processing.
4. Heat Treatment & Surface Protection
After forming, the screws possess the correct geometric shapes, but their hardness and toughness frequently fall short of the demands of high-load industrial environments. Consequently, heat treatment and surface corrosion protection are the pivotal steps that endow a screw with its functional durability.
4.1 Quenching & Tempering: Unleashing Metal Potential
For fasteners of Grade 8.8 and above, a quenching and tempering heat treatment is mandatory (consisting of hardening followed by high-temperature tempering).
Continuous Mesh Belt Furnace Quenching: Screws are uniformly conveyed into a mesh belt heat treatment furnace and heated to between 850℃ and 880℃ under an endothermic protective atmosphere (which prevents surface decarburization or oxidation). This converts the internal structure of the steel into austenite. Subsequently, the screws rapidly drop into quenching oil or water to cool, transforming into a highly hard but extremely brittle martensite structure.
Tempering to Relieve Internal Stress: Quenched screws must immediately enter a tempering furnace to be heated to between 500℃ and 650℃. This stage is designed to eliminate the massive internal stresses induced by quenching, transforming the martensite into a uniform tempered sorbite structure, which enables the screw to achieve excellent impact toughness while maintaining its high hardness.
Exposed carbon steel screws rust very easily under atmospheric exposure. To enhance their corrosion resistance, factories apply different surface treatment processes based on the end-user's environmental requirements:
Black Oxide: Forms a thin layer of iron oxide film on the surface. While its anti-corrosion capability is relatively weak, it is cost-effective and does not alter the dimensional tolerances of the screw, making it common for internal machinery components.
Zinc Plating: Electro-deposits a layer of zinc onto the screw surface, followed by a passivation treatment. This is the most prevalent anti-corrosion method, yielding a bright appearance and exceptional cost-effectiveness.
Hot-Dip Galvanizing: Screws are immersed in molten zinc at approximately 450℃, forming a thick zinc-iron alloy layer. It provides an exceptionally long corrosion-free lifespan and is widely utilized in outdoor electrical towers and steel building structures.
Dacromet Coating: A modern anti-corrosion coating offering superior salt spray resistance (often exceeding 500 to 1,000 hours). Because the process avoids acid pickling, it completely eliminates the risk of "hydrogen embrittlement," a phenomenon that can cause catastrophic, sudden fractures in high-strength screws.
5. Quality Control & Final Testing
In industrial applications, the failure of even a single screw can trigger catastrophic systemic accidents. Therefore, modern fastener manufacturing features an extremely rigorous quality control network.
On the production line, optical sorting machines equipped with high-resolution cameras and laser sensors conduct 100% full-count inspections of screw attributes like total length, head diameter, recess depth, and thread major diameter while moving at high speeds. Go Gauges and No-Go Gauges are strictly employed to cross-check the mating precision of the threads, ensuring that every screw leaving the factory can smoothly thread into standard tapped holes.
5.2 Destructive and Non-Destructive Mechanical Testing
Samples are drawn from every production batch and sent to laboratories to verify extreme mechanical performance metrics:
Tensile Testing: The screw is secured in a universal material testing machine and stretched until it fractures, measuring its ultimate tensile load capacity (for example, a Grade 10.9 screw dictates that its tensile strength must reach 1000 MPa).
Hardness Testing: Measures the hardness gradient from the screw's core to its surface to verify that the heat treatment is entirely uniform.
Metallographic Microscopic Analysis: The screw is sectioned, polished, and etched to observe its fiber flow lines and grain size under a microscope, while strictly checking for surface decarburization layers or microscopic cracks.
The birth of a tiny screw represents the synthesis of material science, mechanics, precision die manufacturing, and surface chemistry. From a coil of ordinary steel wire rod to ahigh-strength fastener arriving at the assembly line, every punching station, every micro-adjustment of heat treatment temperatures, and every quality checkpoint is executed under precise scientific and industrial specifications.
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