Complete Fastener Manufacturing Process: A Comprehensive Industrial Guide
2026-04-02
1. Fastener Core Definition and Classification
Fasteners are standardized mechanical components used to connect two or more parts together. Their primary feature is that they are removable and reusable, which distinguishes them from permanent connection methods such as welding or bonding. Often referred to as the “rice of industry,” fasteners play a vital role across all sectors of manufacturing.
1.1 Core Classification
Threaded Fasteners: The most widely used type, they are removable and provide strong clamping forces. This category includes:
Bolts: Paired with nuts, used for high-strength connections.
Screws: Inserted into threaded holes of the parts being connected, do not require nuts, ideal for precision assemblies.
Nuts: Paired with bolts, generate preload through the thread engagement.
Studs: Often used in situations requiring a fixed anchor with external threading on both ends.
Non-Threaded Fasteners: Primarily used in specific applications, some of which are permanent or semi-permanent connections. This category includes:
Rivets: Used for permanent connections by deforming the fastener itself, suitable for high-vibration environments like aerospace and steel structures.
Pins: Used for positioning or transferring lateral forces, such as cotter pins, dowel pins, and spring pins.
Clamps and Brackets: Used to secure pipes, cables, or other components.
Auxiliary Components: While they don’t provide direct fastening, they are critical to the reliability of the fastening system. These include:
Washers: Flat washers distribute pressure and protect surfaces; spring washers and lock washers prevent thread loosening.
Anti-loosening devices: such as threaded locking elements that prevent fasteners from loosening under vibration.
Fastener
1.2 Categorization by Standardization and Customization
Standard Fasteners: Manufactured according to national (GB), international (ISO), or industry standards (such as DIN, ANSI). These fasteners adhere to uniform specifications regarding size, performance, materials, and markings, allowing for full interchangeability. They are ideal for mass production and general applications.
Custom Fasteners: Custom-designed to meet the specific needs of certain products or operating conditions. There are no unified standards, and these fasteners are tailored in size, structure, and performance to fit unique requirements. Typically, non-standard fasteners are used in industries such as aerospace, medical devices, and other high-end customized applications, where complexity and low-volume production are common.
Non-Standard Fastenersstandard fastener
2.Comprehensive Fastener Manufacturing Process
The fastener manufacturing process is a closed-loop procedure from raw material to finished product. The process can be divided into seven major steps, with each step requiring strict control over the craftsmanship. Although the process may vary slightly depending on the type of fastener (standard/non-standard, ordinary/high-strength), the core logic remains the same. The entire process must ensure precision in size, mechanical performance, and surface quality.
2.1 Raw Material Preparation (Source Control)
The core of this phase is selecting suitable metal wire (coil stock/wire rod) and conducting an initial inspection to eliminate substandard materials, laying the foundation for subsequent processing.
Material Screening: Check the wire surface for defects such as cracks, corrosion, inclusions, and scale. Verify wire diameter and material composition to ensure compliance with production requirements (e.g., for high-strength bolts, focus on carbon and manganese content).
Material Storage: Store materials according to their type and specification. Ensure they are kept dry and ventilated to prevent mixing, moisture absorption, or corrosion. Regularly inspect the material’s condition.
2.2 Raw Material Pretreatment (Key Preprocessing)
The purchased coil stock cannot be directly used for processing. It needs to be converted into “precision wire” or “blanks” through pretreatment. The core goal is to remove surface defects, improve material ductility, and precisely control dimensions to fit subsequent forming processes.
Descaling: Remove surface oxidation (rust) through mechanical methods (e.g., bending or shot blasting, suitable for regular bolts) or chemical acid pickling (immersing in acid solution for high surface cleanliness; this method is often used for high-strength bolts via “mechanical descaling + acid pickling” composite process).
Spheroidizing Annealing: Mainly for high-strength bolts made from medium-carbon steel or alloy steel, this process transforms coarse pearlite into fine spheroidal pearlite, reducing hardness and improving plasticity. This reduces the risk of cold heading cracks. Regular low-carbon steel bolts typically don’t require this step. Process parameters: medium-carbon steel at 715-735°C, medium-carbon alloy steel at 740-770°C, with isothermal cooling to 500°C.
Cold Drawing (Wire Drawing): This process pulls the wire through a drawing die to achieve the desired diameter. It also strengthens the material by deformation, improving surface smoothness and preparing it for future processes such as spheroidizing annealing for medium-carbon steel or alloy steel.
Cleaning and Degreasing: Remove oil and grease from the wire surface. Ordinary bolts can undergo conventional cleaning, but high-strength bolts require chemical cleaning to prevent contaminants from affecting the subsequent cold heading and heat treatment quality.
2.3 Forming Process (Core Operation, Defines Product Structure)
By applying external force to the metal wire, it undergoes plastic deformation to form the head and shank of the fastener. Depending on the product specifications, material, and production scale, the main forming processes include cold heading (room temperature forming) and hot heading (high-temperature forming).
Cold heading process
2.3.1 Cold Heading (Mainstream Process for Standard and Mass Production of Fasteners)
In this process, pressure is applied to the blank between molds, causing plastic deformation and redistributing the material volume. This forms the fastener’s head and shaft. It is suitable for fasteners in the M3-M30 range, offering a material utilization rate of over 99% and high production efficiency.
Core Process (Automatic Cold Heading Machine): Automatic wire feeding → Length cutting → Multi-stage punching (pre-forming, final forming, hole punching) → Shaping → Finished product. Fully automated, capable of producing 300-600 pieces per minute.
Key Parameters: Heading ratio (length of the heading section / diameter) determines the number of heading stages—h/d ≤ 2.5 (single stage), 2.5 < h/d ≤ 4.5 (two stages), 4.5 < h/d ≤ 6.5 (three stages).
Application: Standard fasteners like bolts, screws, and nuts for mass production, offering low cost and high precision.
Hot Forging Process
2.3.2 Hot Forging (Adapted for Large-Scale, High-Strength Fasteners)
Hot forging involves heating wire material to temperatures above its recrystallization point (typically between 800-1200°C). This process reduces material hardness and enhances plasticity, enabling shaping through forging. It is used primarily for large-diameter and high-strength fasteners, such as M30 and above, that are difficult to form with cold heading, including wind power bolts, railway fasteners, and nuclear power studs.
Core Process:
Heating: Induction heating (medium-frequency) ensures uniform temperature and fast heating speed.
Forging: Shaping is done by a hot forging press, where the forging force is higher than that in cold heading.
Cooling: Cooling occurs naturally or with controlled temperature to avoid cracking due to rapid cooling.
Rough Blank Finishing: Post-forging, rough edges and material excess are removed.
Key Considerations:
Control heating temperature and time carefully to avoid overheating (which leads to grain coarsening and performance degradation) or overburning (which causes material waste).
The cooling process must be slow, with cooling rates maintained between 50-100°C per hour, to prevent thermal stress and cracking.
Applicable Scenarios:
Large-diameter, high-strength non-standard fasteners for advanced equipment, heavy machinery, and high-end industrial applications.
Specific use in wind power bolts, nuclear power studs, large construction anchor bolts, and other demanding applications in heavy-duty industries.
2.3.3 Costom Fastener Forming
For structurally complex or low-volume non-standard parts, a combination of cold heading + turning/milling or CNC machining is employed. This process is flexible and supports the production of fasteners with irregular heads or multi-step shafts.
2.4 Thread Processing (The Core to Achieving Fastening Function)
Thread processing is the critical step in creating the fastening function for bolts, screws, and nuts. It involves either external threading (for bolts and screws) or internal threading (for nuts). The mainstream approach uses thread rolling, replacing traditional cutting methods, which enhances thread strength and production efficiency.
2.4.1 External Threading (For Bolts and Screws)
Thread Cutting: Involves using two flat molds with thread profiles that reciprocate, pressing metal from the shaft to form the thread. Suitable for mass production of standard threads with accuracy up to 6g level, cost-effective and high-efficiency.
Thread Rolling: Utilizes two or three threaded rollers to rotate in the same direction, pressing the shaft to form the thread. This process offers higher thread precision, stability, and is more suited for high-precision and long-thread products like aerospace fasteners or lead screws. Equipment includes CNC triple-roll thread rolling machines (for high precision, large diameter parts), fully automatic CNC thread rolling machines, and hydraulic triple-roll thread rolling machines (for heavy-duty, large-size parts).
Processing Timing: For standard bolts, thread rolling before heat treatment is common (since the material is softer, and the tool life is longer). For high-strength bolts (such as wind turbine fasteners), thread rolling after heat treatment is preferred (for better thread form retention and improved fatigue resistance).
2.4.2 Internal Threading (For Nuts)
Tapping: This process uses a tapping machine to rotate a tap to form internal threads in a nut. It is suitable for small to medium batch production of various specifications, with both manual tapping (for non-standard, low-volume) and machine tapping (for mass production and standard parts).
Thread Rolling: A mainstream process for mass-producing nuts, where a thread roller presses the nut’s inner hole to form the internal threads. This method has no cutting, is more efficient, and produces smooth, high-quality threads with better material utilization.
2.4.3 Thread Precision Control
Thread precision directly impacts the fastening effect. Common precision grades include 6H (internal threads) and 6g (external threads), while high-end applications (such as aerospace) require 4H or 4g precision. Throughout the process, thread diameter, pitch, and profile angle must be tightly controlled to avoid issues like thread stripping or misalignment.
2.5 Heat Treatment (Endowing the Product with Ultimate Mechanical Properties)
Heat treatment involves heating and controlled cooling processes to adjust the internal structure of the metal, balancing the hardness, toughness, and strength of the fasteners. It is the key differentiating process between regular and high-strength bolts, directly determining the product’s service life and load-bearing capacity. The heat treatment process differs significantly for fasteners of varying strength grades.
2.5.1 Core Heat Treatment Processes
Quenching and Tempering (Austenitizing + High-Temperature Tempering): A crucial process for high-strength bolts (≥8.8 grade). The fastener is heated to 800-900°C (medium carbon steel 820-860°C, alloy steel 840-880°C) and then rapidly cooled (quenching, oil quenching to reduce cracking). This increases hardness (HRC 38-45). Next, it is reheated to 500-650°C for tempering, maintaining the temperature for a specific period depending on the product size. This process alleviates internal stresses and improves toughness, ultimately forming tempered sorbite structure with a balance of hardness and toughness.
Carburizing Treatment: This process is used for low-carbon steel fasteners such as self-tapping screws and self-drilling screws. Carbon atoms are introduced into the surface of the steel to achieve a “hard surface, tough core.” The surface hardness can reach HRC 58-62, enhancing wear resistance and fatigue strength, making it suitable for dynamic load components (e.g., gear bolts, engine connecting rod bolts).
Dehydrogenation Annealing: A necessary process for high-strength bolts (hardness ≥ HRC 38) after surface treatments. The fasteners are annealed at around 200°C for 2-4 hours and then slowly cooled to room temperature. This process removes hydrogen atoms absorbed during pickling and electroplating, preventing hydrogen embrittlement and delayed fracture. It is a crucial step for automotive safety parts and wind power bolts production.
Standard Bolt Heat Treatment: For low-carbon steel standard bolts (grade 4.8 and below), a simple annealing treatment is used to relieve processing stress without the need for quenching and tempering. This reduces production costs. The annealing temperature is 600-650°C, with a holding time of 1-2 hours, followed by natural cooling.
2.5.2 Key Considerations
Decarburization Control: It is essential to strictly control the furnace atmosphere using protective gases (nitrogen, hydrogen) during heating to prevent the loss of carbon from the surface of the fasteners. This ensures that the hardness and strength are maintained, and decarburization layer thickness should be controlled to within 0.01mm.
Deformation Control: Long bolts (length > diameter × 10) must be straightened after heat treatment. For wind power bolts, the linearity error must be ≤0.0025 × L + 0.05 (where L is the nominal length). After straightening, the bolts need to be re-annealed to relieve residual stresses.
Temperature Control: Heating and cooling rates must be precisely controlled to avoid overheating or overcooling, which can lead to cracks or uneven microstructures. Temperature control precision should be within ±5°C to maintain product quality and consistency.
2.6 Surface Treatment (Corrosion Resistance, Aesthetics, and Performance Enhancement)
The primary objective of surface treatment is to prevent corrosion, enhance surface hardness and wear resistance, and improve the appearance of fasteners. Depending on the application environment (corrosive, high-temperature, low-temperature), the appropriate treatment process should be selected. Different processes vary significantly in terms of corrosion resistance and cost, and the selection should be based on the specific application.
Galvanization
The most commonly used and cost-effective surface treatment, galvanization is available in two forms:
Hot-Dip Galvanization: Offers superior corrosion resistance, with a zinc coating thickness of 80-120μm. This process is ideal for outdoor and humid environments, such as construction and photovoltaic fasteners. The coating is highly resistant to corrosion, making it a popular choice for applications exposed to the elements.
Electroplating (Electrogalvanization): Provides a smooth, aesthetic surface with a thinner zinc layer (10-20μm). It is suitable for indoor or lightly corrosive environments, such as electronic and home appliance fasteners. Following electroplating, a passivation treatment (colored or black passivation) is applied to further enhance corrosion resistance, with salt spray testing reaching 24-48 hours.
Nickel and Chrome Plating
Both nickel and chrome plating provide excellent corrosion resistance and high surface smoothness, but with distinct advantages:
Nickel Plating: Offers better corrosion resistance than zinc and has a coating thickness of 5-10μm. Nickel plating is non-magnetic, making it suitable for precision instruments and aerospace applications where magnetism must be avoided.
Chrome Plating: Provides high hardness and excellent wear resistance with a coating thickness of 3-5μm. It is commonly used in high-end machinery and automotive parts due to its ability to withstand abrasion and harsh conditions.
Dacromet Coating
Dacromet is a unique, non-hydrogen embrittling treatment that provides superior corrosion resistance, especially under high-temperature conditions (up to 200°C) and salt spray exposure (up to 72 hours). The coating thickness is 8-12μm. Dacromet is ideal for high-strength, high-corrosion-resistant applications such as automotive chassis and marine equipment. Its main drawback is higher cost, and the finish is silver-gray.
Phosphating
Phosphating forms a thin, protective phosphate film (2-5μm thick) on the surface, which improves adhesion for subsequent coatings, lubrication, and enhances paint adhesion. It is commonly used for fasteners that will be painted (e.g., automotive body fasteners). After phosphating, the parts must be dried to prevent rusting.
Black Oxide Treatment
A cost-effective and straightforward process, black oxide provides a black-colored finish that offers moderate corrosion resistance. Salt spray testing for black oxide treatment typically reaches only 8-12 hours. It is most suitable for indoor, dry environments, such as internal machinery fasteners or non-load-bearing components.
2.7 Inspection and Packaging (Final Line of Defense for Finished Products)
Throughout the production process, fasteners undergo various inspections to ensure that only qualified products are shipped. Packaging must ensure that the fasteners are protected from damage and corrosion during transportation and storage.
2.7.1 Inspection Process (Full Project Testing)
Dimensional Inspection: Inspect head dimensions, shaft diameter, length, thread precision, and pitch. Tools include calipers, micrometers, thread ring gauges, and plug gauges. For large-scale production, optical sorting machines can be used for high-speed inspection.
Mechanical Property Testing: Measure tensile strength, yield strength, hardness, toughness, and fatigue strength. Testing equipment includes tensile testing machines and hardness testers (Rockwell, Brinell). High-strength bolts must undergo 100% hardness testing, with tensile and yield strength tested on samples.
Surface Quality Inspection: Check for defects such as cracks, burrs, corrosion, or coating defects. Visual inspection is combined with magnification (10x magnifier), while non-destructive methods like magnetic particle inspection or penetrant testing are used for critical products.
Corrosion Resistance Testing: Salt spray testing is used to evaluate the corrosion resistance of coatings like galvanizing and Dacromet. The testing time can range from 24 hours to 72 hours depending on the application.
2.7.2 Packaging Process
Classification Packaging: Fasteners are packaged by specification, material, and model. Common packaging methods include plastic bags, paper boxes, and cartons, while bulk packaging may use woven bags for large-volume fasteners.
Protection Measures: Desiccants are placed inside packages to prevent moisture absorption. The surface is wrapped in protective film to avoid damage to coatings and corrosion.
Clear Labeling: Packaging includes product name, specification, material, strength grade, production date, and manufacturer information for easy identification and traceability.
3. In-Depth Explanation of Core Fastener Processing Techniques (Industrial Level)
This section focuses on the key processing techniques used in fastener manufacturing, breaking down the principles, key operational points, parameter controls, common issues, and solutions. The details also highlight differences between standard/non-standard, and ordinary/high-strength fasteners, making this section suitable for practical training and technical guidance.
3.1 Cold Heading Process (Key Focus)
3.1.1 Process Principle
At room temperature, a cold heading machine uses a punch and die to apply high pressure to the pre-treated wire, causing plastic deformation. The metal flows through the die cavity, forming the head and shaft of the fastener. The metal flow is continuous and uncut, resulting in a final product with superior strength compared to cut parts.
3.1.2 Key Operational Points
Die Selection: Choose appropriate dies (punches and cavities) based on product specifications and material. The dies must be made of high-strength alloy steel and surface-treated with nitriding to enhance wear resistance and lifespan.
Lubrication Control: Special cold heading lubricants are applied during the cold heading process to reduce friction between the die and the blank. This helps prevent die jamming and blank cracking while improving surface finish. The lubrication amount should be controlled; excessive lubricant may cause oil residue, affecting the process.
Feeding Precision: The feeding speed and length of the automatic wire feeder must be accurately controlled, with cutting length deviation ≤ ±0.02 mm, ensuring uniform blank size to avoid dimensional deviations during forming.
Multi-Station Control: For multi-station cold heading machines, it is crucial that the punching pressure and speed are coordinated across stations. Pre-forming and final forming pressure should gradually increase to avoid excessive force at once, which could lead to blank cracking.
3.1.3 Frequently Asked questions And Solutions
Problem 1: Head Cracking
Cause: Insufficient material plasticity, incomplete spheroidizing annealing, excessive forming pressure, or small die radius.
Solution: Optimize spheroidizing annealing process, adjust forming pressure, increase die radius, and switch to materials with better plasticity.
Problem 2: Dimensional Deviation
Cause: Inaccurate feeding length, die wear, or uneven forming pressure.
Solution: Calibrate the feeding mechanism, replace worn dies, and adjust the forming pressure at each station.
Problem 3: Excessive Burrs on the Surface
Cause: Die wear, insufficient lubrication, or defects on the blank’s surface.
Solution: Replace worn-out dies, increase lubricant application, and improve raw material pre-treatment.
3.2 Hot Forging Process (Key Focus)
3.2.1 Process Principle
In hot forging, the wire material is heated to its austenitizing temperature (800-1200°C, 800-900°C for low carbon steel, 900-1200°C for alloy steels), which reduces the material hardness and increases its plasticity (elongation ≥30%). Pressure (ranging from 100-800 MPa) is then applied through the use of a hot forging machine’s die and punch to cause plastic deformation, forming large, complex-shaped fastener blanks. This process is particularly suited for products that are difficult to form through cold forging.
3.2.2 Key Operational Points
Heating Control: Use medium-frequency induction heating to achieve uniform temperature (temperature difference ≤±10°C). The heating time is adjusted according to the wire diameter (larger diameters require longer heating times; for example, a wire with a diameter of 50mm requires about 5-8 minutes). Avoid overheating (temperature exceeding 1250°C) or overburning (temperature exceeding 1300°C), as these can degrade material properties.
Cooling Control: After forming, the fastener blanks must be slowly cooled (either naturally or with controlled cooling). The cooling rate should be kept at 50-100°C per hour to prevent thermal stress and cracking due to rapid cooling. High-strength hot-forged parts should be annealed promptly after cooling to eliminate internal stresses.
Mold Protection: The hot forging dies need to withstand high temperatures (temperature tolerance ≥1200°C) and should be made from high-temperature alloy steel (e.g., H13). A high-temperature lubricant should be applied to the die surface to prevent the die from sticking to the high-temperature blanks, thereby extending the die’s lifespan. Typically, the die can process between 10,000 and 20,000 parts before requiring replacement.
3.2.3 Frequently Asked Questions & Solutions
Issue 1: Cracking of Blanks
Cause: Overheating or underheating, excessively fast cooling, or die wear.
Solution: Adjust the heating temperature and time (keep within the austenitizing temperature range), optimize the cooling process (using controlled cooling), and replace worn dies.
Issue 2: Severe Surface Oxidation
Cause: Excessive exposure to air during heating or prolonged heating times.
Solution: Use a protective atmosphere for heating (such as nitrogen gas), shorten the heating time (keep within a reasonable range), and immediately apply forging after heating to minimize oxidation.
Issue 3: Large Dimensional Deviations
Cause: Uneven heating, insufficient pressing force, or inaccurate die positioning.
Solution: Optimize the heating process (adjust induction coil positioning), increase the pressing force, and calibrate the die positioning (adjust die gaps) to improve accuracy.
3.3 Thread Processing Technology (Key Focus)
Thread processing is the critical step in ensuring the fastener’s fastening function. This section covers various methods for creating threads, including thread rolling (for external threads), thread cutting (for internal threads), and tapping (for nuts), highlighting their advantages and common issues.
3.3.1 Thread Rolling Process (External Threads)
Thread rolling uses rotating rollers to apply pressure on the blank, causing the metal to plastically flow and form external threads. This process is a chipless method, and the resulting thread strength is 20%-30% higher than cutting processes. It is suitable for large-scale, high-precision threading.
Key Parameters:
Roller Speed: Adjust based on wire diameter; excessive speed may lead to thread stripping.
Pressing Pressure: Should be moderate. Excessive pressure may deform threads, while insufficient pressure results in incomplete threads.
Solution: Adjust roller speed and pressing pressure, and use high-precision rollers.
Problem 2: Thread Stripping
Solution: Ensure the wire diameter is within specification and adjust roller speed.
Problem 3: Pitch Deviation
Solution: Check and adjust the pressing pressure, and ensure uniform material flow during the process.
3.3.2 Thread Cutting Process (External Threads)
Thread cutting uses two opposite plates with thread profiles to move reciprocally, applying pressure to form external threads. This method is high-efficiency and cost-effective, ideal for producing standard bolts and screws. The precision of this process is slightly lower than thread rolling.
Key Parameters:
Plate Stroke: Adjust based on the thread length.
Cutting Speed: Avoid excessive speed to prevent burrs on the threads.
Plate Gap: Should be moderate. Too large a gap leads to loose threads, while too small a gap can deform the threads.
Common Problems:
Problem 1: Burrs on Threads
Solution: Adjust plate gap and speed, and grind the plate thread profiles.
Problem 2: Irregular Thread Profiles
Solution: Fine-tune plate gap and improve raw material surface preparation.
3.3.3 Tapping Process (Internal Threads for Nuts)
Tapping involves rotating a tap within the nut’s internal hole to create the threads. There are two types of tapping: manual tapping for small batches and non-standard nuts, and machine tapping for mass production and standard nuts.
Key Points:
Tap Selection: Choose the appropriate tap based on thread specifications and material (e.g., high-speed steel taps for alloy steel, and carbide taps for stainless steel).
Tapping Speed: Avoid excessive speeds, which can cause tap breakage or thread stripping.
Lubrication: Use tapping oil during the process to reduce friction and heat.
Common Problems:
Problem 1: Thread Stripping
Solution: Ensure correct tap selection and adjust tapping speed.
Problem 2: Tap Breakage
Solution: Improve lubrication, select proper taps, and adjust tapping speed.
Problem 3: Internal Thread Quality Issues
Solution: Check the internal hole dimensions, improve raw material treatment, and ensure accurate tap handling.
3.4 Heat Treatment Process (Key Focus)
3.4.1 Tempering Process (Core for High-Strength Bolts)
Process Parameters:
Quenching Temperature: 820-860°C for medium carbon steel, 840-880°C for alloy steel.
Quenching Medium: Oil quenching, with cooling rates of 50-100°C/s.
Tempering Temperature: 500-650°C.
Tempering Time: Adjusted based on the product size. For example, a bolt with a diameter of 10mm typically requires 1-2 hours of tempering, while a 50mm diameter bolt requires 3-4 hours.
Critical Controls:
The quenching temperature must be uniform to avoid local overheating.
The tempering temperature must be precisely controlled; excessive temperature leads to a decrease in hardness (below HRC38), while insufficient temperature results in poor toughness (impact toughness < 10 J/cm²).
Cooling speed needs to be controlled to avoid cracks, and cooling should be done at a rate of 50-100°C per hour after quenching.
Common Issues:
Hardness Issues: Hardness may fail to meet standards.
Lack of Toughness: Insufficient toughness can cause brittle failure.
Surface Decarburization: Carbon loss at the surface may occur during heat treatment.
Solutions: Adjust quenching and tempering temperatures and times; optimize the furnace atmosphere (use a protective atmosphere such as nitrogen) to prevent decarburization. Control the decarburized layer thickness to no more than 0.01mm.
3.4.2 Hydrogen Annealing Process
Process Parameters:
Heating Temperature: 180-220°C.
Soaking Time: 2-4 hours.
Cooling: Slow cooling to room temperature (cooling rate ≤ 50°C/hour).
Key Points:
The soaking time must be sufficient to ensure complete hydrogen release from the material (hydrogen content ≤ 2 ppm).
The cooling rate must be slow to avoid the reintroduction of internal stresses.
Common Issues:
Hydrogen Embrittlement: This can occur if hydrogen is not fully removed.
Solutions: Extend the soaking time (add 1-2 hours) and optimize the annealing process. Improve hydrogen control during surface treatment (e.g., ensuring that parts are cleaned and dried immediately after acid pickling).
Zinc Bath Temperature: 440-460°C. (Too high results in a thick zinc layer, too low leads to a porous or non-dense zinc layer).
Immersion Time: Adjusted according to product size. For example, bolts with a 10mm diameter typically require 1-2 minutes of immersion in the zinc bath.
Passivation Solution Concentration: 5%-10%. (Ensures the passivation film is uniform and dense).
Common Issues:
Zinc Layer Peeling Off: The zinc layer may detach due to poor adhesion.
Surface Blisters: The surface may develop blisters, affecting the finish.
Optimize the pickling process to thoroughly remove oxide scales and oils from the surface.
Adjust zinc bath temperature and immersion time for better zinc layer adherence.
Strengthen passivation treatment by extending passivation time to 3-5 minutes for a more uniform and protective layer.
3.5.2 Dacromet Coating Process
Process Flow:
Degreasing → Rust Removal → Dacromet Coating → Drying (120-150°C for 10-15 minutes) → Sintering (300-320°C for 15-20 minutes).
Key Parameters:
Coating Thickness: 8-12μm (ensure uniformity).
Sintering Temperature and Time: 300-320°C for 15-20 minutes (ensures tight bonding between the coating and the base material).
Common Issues:
Uneven Coating: The coating may be applied inconsistently.
Coating Peeling Off: Poor adhesion can lead to peeling.
Failure to Pass Salt Spray Test: Insufficient corrosion resistance.
Solutions:
Adjust coating thickness by ensuring uniform application of the Dacromet solution.
Optimize the sintering process by controlling both temperature and time for a consistent, durable bond.
Ensure degreasing and rust removal are thorough (10-15 minutes for degreasing, 5-10 minutes for rust removal).
4. Fastener Material Selection (Industrial-Level Guide)
Materials play a crucial role in determining the performance of fasteners. The right material must be selected based on application scenarios such as load, environmental factors, temperature, strength requirements, and cost constraints. This section focuses on the material differences between standard and high-strength fasteners, as well as the latest high-performance materials.
4.1.1 Carbon Steel (Mainstream Material for Ordinary Fasteners)
Carbon steel is cost-effective, easy to process, and offers good plasticity. It is suitable for fasteners of grade 4.8 and below but is not recommended for high-strength applications or corrosive environments.
Low Carbon Steel (Carbon content: 0.08%-0.25%): Examples: 1005, 1010, 1018, Q195, Q235 Suitable for common screws, bolts, and nuts. It has low strength but high plasticity and is easy to cold forge.
Medium Carbon Steel (Carbon content: 0.25%-0.60%): Examples: 1045, 45#, SWRCH35K Suitable for grade 6.8 bolts and screws. It offers medium strength and can be heat-treated for enhanced properties.
High Carbon Steel (Carbon content: 0.60%-1.40%): Example: 1095 Suitable for high-strength, high-hardness fasteners such as spring washers and self-tapping screws. However, its plasticity is poor, requiring optimized processing to avoid cracking.
4.1.2 Alloy Steel (Mainstream Material for High-Strength Fasteners)
Alloy steels contain alloying elements such as chromium, manganese, molybdenum, and vanadium to improve strength, hardness, toughness, and wear resistance. These materials are ideal for grade 8.8 and above high-strength fasteners, suitable for heavy-duty and vibration-prone environments.
Chromium-Molybdenum Steel (35CrMo, 42CrMo): Suitable for grade 10.9 and 12.9 high-strength bolts. These materials offer high strength, toughness, and high-temperature resistance, making them ideal for applications in automotive engines, wind power, and nuclear power.
Chromium-Vanadium Steel (40CrV): Suitable for high-strength, tough fasteners such as aerospace and construction machinery bolts. They have excellent fatigue resistance.
Manganese Steel (16Mn, 20MnTiB): Suitable for grade 8.8 and 10.9 bolts. This material is less expensive than chromium-molybdenum steel and offers medium strength, commonly used in automotive chassis and construction.
4.1.3 Stainless Steel (Best for Corrosion-Resistant Applications)
Stainless steel offers excellent corrosion resistance and rust protection, making it ideal for humid, corrosive, and high-temperature environments. It is more expensive than carbon steel and alloy steel, and it is classified into austenitic, martensitic, and ferritic types.
Austenitic Stainless Steel (304, 316, 316L): Non-magnetic with excellent corrosion resistance. 316L provides superior corrosion resistance compared to 304, making it suitable for highly corrosive environments such as food, medical, marine, and electronics applications.
Martensitic Stainless Steel (410, 420): Magnetic, and can be heat-treated for enhanced strength. Suitable for high-strength, corrosion-resistant fasteners such as mechanical and automotive components.
Ferritic Stainless Steel (430): Magnetic with moderate corrosion resistance. It is less expensive than 304 stainless steel and is ideal for light corrosion environments such as interior decorations and ordinary machinery.
Non-ferrous metals offer advantages like lightweight, excellent corrosion resistance, and good conductivity. They are often used in specialized applications such as aerospace, electronics, and renewable energy.
Copper and Copper Alloys (H62, H65): Excellent electrical and thermal conductivity. Suitable for electronic and electrical fasteners such as screws and rivets, with good corrosion resistance.
Aluminum Alloys (6061, 6063): Lightweight with moderate strength. Commonly used in aerospace, electric vehicles, and electronic devices to reduce product weight.
Titanium Alloys (TC4, TA2): High strength, lightweight, corrosion-resistant, and heat-resistant. Ideal for high-end applications such as aerospace, nuclear power, and medical devices. However, the cost is extremely high.
These alloys are designed for extreme environments, such as high temperature, high pressure, strong corrosion, and radiation. They are commonly used in aerospace, nuclear power, and chemical industries. Examples include Inconel alloys, Hastelloy alloys, and Waspaloy alloys. These materials are difficult to process and have very high costs.
4.2 Material Selection Principles (Industrial-Level Practical Guidance)
4.3 Material Acceptance Standards (Source Quality Control)
Material Composition Testing: Using a spectrometer, analyze elements like carbon, manganese, chromium, and molybdenum to ensure compliance with material standards (e.g., GB/T 699, GB/T 3077).
Mechanical Property Testing: Test raw materials for tensile strength, yield strength, and plasticity to ensure they are compatible with subsequent processing and final product performance requirements.
Surface Quality Testing: Inspect raw materials for defects such as cracks, rust, inclusions, and oxide scales. Materials that do not meet standards must not be used in production.
Equipment is crucial to ensuring processing efficiency and product precision. The right equipment must be selected based on production scale (mass production or small batch), product type (standard or custom, regular or high-strength), and process requirements. This section provides insights into equipment configurations across the entire processing workflow, covering both manufacturing and inspection equipment. Additionally, it includes key selection points and maintenance guidelines.
5.1 Raw Material Pre-Treatment Equipment
Wire Drawing Machine: Used for cold drawing of wire rods to the required diameter. There are standard wire drawing machines and CNC wire drawing machines. The latter offers higher precision and efficiency, making it suitable for high-precision fastener production.
Cold heading machines are used for batch production of standard fasteners. There are single-station and multi-station cold heading machines (2-6 stations). Multi-station machines enable integrated operations like “feeding – cutting – pre-heading – final heading – punching,” offering high automation and efficiency.
Core Parameters: Number of stations, production speed, heading force, feeding precision.
Selection Tips: Choose machines with high automation, precise temperature control, and easy mold changes for batch production. For high-strength fasteners, select CNC cold heading machines with high heading force and precision.
5.2.2 Non-Standard Forming Equipment
CNC Lathes/Milling Machines: Suitable for turning and milling non-standard fasteners, allowing for complex shapes like irregular heads or multi-step shafts. They offer high precision and flexibility, making them ideal for small batch custom production.
CNC Machining Centers: Used for non-standard fasteners with complex structures and high precision requirements. These centers can perform multiple operations in one go, reducing manual intervention and improving both precision and efficiency.
5.3 Thread Processing Equipment
Thread Rolling Machines: Used for external thread rolling. There are two-axis thread rolling machines, three-axis thread rolling machines, and CNC thread rolling machines. CNC machines offer higher precision and automation, making them suitable for large-scale, high-precision thread production.
Core Parameters: Rolling wheel speed, thread precision, production speed.
Selection Tips: For large diameters and high precision, CNC three-die thread rolling machines are preferred.
Thread Chasing Machines: Used for external thread chasing. There are standard thread chasing machines and fully automated thread chasing machines, the latter being suitable for high-efficiency, low-cost mass production.
Selection Tips: Choose fully automated machines for high-volume production to improve efficiency.
Tapping Machines: Used for internal thread processing. There are manual tapping machines, semi-automatic tapping machines, and fully automatic tapping machines, with the latter being ideal for mass production of nuts.
Core Parameters: Tapping speed, thread precision, compatibility with specifications.
Hot-Dip Galvanizing Production Line: Available in semi-automatic and fully automatic versions. Fully automatic lines are more suitable for batch production and include processes like pickling, fluxing, hot-dip galvanizing, passivation, and drying.
Core Parameters: Zinc bath temperature, immersion time, production efficiency.
Selection Tips: Fully automatic lines are more efficient and ensure consistency in coating quality.
Electroplating Equipment: Used for high-precision, aesthetic fasteners. Offers high surface smoothness.
Core Parameters: Current, voltage, plating time, coating thickness.
Selection Tips: For high-quality finishes, choose equipment that ensures precise control over electroplating parameters.
Dacromet Coating Equipment: Used for fasteners requiring high corrosion resistance. The process includes degreasing, rust removal, coating, drying, and sintering.
Core Parameters: Coating thickness, sintering temperature, production efficiency.
Selection Tips: Opt for fully automated Dacromet coating lines for consistency and high throughput.
Cleaning and Drying Equipment: Used for cleaning and drying before and after surface treatment to ensure cleanliness and prevent contamination that could affect coating adhesion.
Core Parameters: Cleaning time, drying temperature.
Selection Tips: Ensure equipment is compatible with the type of coating and surface treatment.
5.6 Inspection Equipment (Core of Quality Control)
Dimensional Inspection Equipment: Includes calipers, micrometers, thread rings, and plug gauges for manual inspection. Automated options like optical inspection machines are used for batch measurements to check for dimensional deviations and surface defects.
Tensile Testing Machines: Used for measuring tensile strength and yield strength.
Hardness Testers (Rockwell, Brinell, Vickers): Used for measuring surface hardness and core hardness.
Impact Testing Machines: Measure toughness by simulating impact forces.
Fatigue Testing Machines: Test fatigue strength by simulating cyclic loading.
Selection Tips: Ensure machines are calibrated regularly for accuracy and that they can handle the specific materials used in fasteners.
Surface Quality Testing Equipment:
Magnetic Particle Testers and Penetrant Testing Machines: Used for detecting surface cracks, particularly for critical products.
Microscopes: Used for inspecting surface burrs, coating thickness, etc.
Selection Tips: Opt for non-destructive testing equipment for high-precision or critical applications.
Corrosion Resistance Testing Equipment:
Salt Spray Test Chambers: Used for testing the corrosion resistance of coatings.
Core Parameters: Salt concentration, test duration, temperature.
Selection Tips: Choose equipment with consistent environmental control to ensure reliable corrosion testing.
Material Composition Testing Equipment:
Spectrometers: Used for detecting material composition and ensuring material quality.
Selection Tips: Opt for advanced spectrometers that can quickly analyze the key elements in fastener materials.
5.7 Auxiliary Equipment and Molds
Auxiliary Equipment:
Automatic Feeders and Dischargers: Improve automation by reducing manual handling.
Mold Grinding Machines: Used for maintaining mold precision and extending mold life.
Automatic Counting and Packaging Machines: For efficient mass packaging and reduced human error.
Storage Bins and Bulk Feeders: Provide automated supply of raw materials to production lines.
Molds:
Cold Heading Dies, Thread Rolling Dies, Thread Chasing Dies, Tapping Tools: Made from high-strength alloy steel with nitriding treatment to improve wear resistance.
Selection Tips: Choose molds with high precision and durability for mass production. Custom molds may be needed for non-standard fasteners.
5.8 Equipment Selection Points and Maintenance Guidelines
5.8.1 Selection Points
Match Production Scale:
Mass Production (Standard Fasteners) → Fully automated, high-capacity equipment.
Small Batch/Custom Production (Non-Standard Fasteners) → Flexible, high-precision equipment (e.g., CNC machining centers).
Match Product Specifications:
Small Sizes → High-precision, high-speed equipment.
Large Sizes → High heading force, stable equipment.
Focus on Precision and Stability: Prioritize equipment with precise temperature control, high positioning accuracy, and low failure rates, especially for high-strength, high-precision fastener production.
Balance Automation and Intelligence: Choose equipment with
5.8.2 Equipment Maintenance Guidelines
Daily Maintenance:
Perform daily checks on the equipment’s operational status and lubrication system.
Clean the equipment surfaces to remove debris and oil residues.
Regularly replace worn-out components such as molds and thread rollers to ensure optimal performance.
Routine Maintenance:
Conduct a comprehensive weekly inspection, including lubrication of key parts.
Calibrate equipment precision monthly, especially for measuring instruments and heat treatment systems.
Perform annual overhauls to replace any aging or damaged components, ensuring long-term, stable operation.
Mold Maintenance:
Regularly grind molds to remove surface wear and burrs, ensuring precision and proper function.
Store molds with protective coatings, such as rust inhibitors, and keep them covered to prevent dust and damage.
Safety Maintenance:
Periodically check safety devices, including protective shields and emergency stop buttons, to ensure safe operation.
Ensure that all operators undergo professional training to handle equipment properly and respond to emergency situations effectively.
Author: Fiona Huang
Fiona Huang is a Sales Engineer with 16+ years of experience in the mechanical and precision manufacturing industry. She specializes in CNC machining components, custom fasteners, and engineering-driven manufacturing solutions. With strong expertise in international trade and technical project support, she focuses on transforming complex engineering requirements into high-precision, production-ready solutions for global clients.
