Guide to Selecting Fastener Materials: Material Determines Performance, Heat Treatment Determines Strength, and Surface Treatment Determines Service Life!

A single sentence that captures the essence of the fastener industry:

Choose the wrong material, and even the strongest fastener will break;

Choose the wrong heat treatment, and even the highest-rated fastener is merely a false claim;

Choose the wrong surface treatment, and even the best screw will rust and become unusable.

I. Core Comparison of the Industry’s Four Major Materials

1. Carbon Steel

Advantages: Lowest cost, widest range of strengths, highest production volume, most stable supply

Disadvantages: Naturally prone to rust; poor corrosion resistance

Main Applications: Construction, automotive, machinery, home appliances, general industry

2. Stainless Steel

Advantages: Naturally rust-resistant, no electroplating required, hygienic and aesthetically pleasing, exceptionally long service life

Disadvantages: High cost, moderate maximum strength, prone to seizing and jamming

Primary Applications: Food, medical, chemical, outdoor, and marine equipment

3. Alloy Steel

Advantages: Ultra-high strength, fatigue resistance, impact resistance, high-temperature resistance

Disadvantages: Requires heat treatment, poor rust resistance, high processing costs

Primary Applications: Wind power, bridges, mining, heavy-duty trucks, construction machinery, high-voltage equipment

4. Titanium Alloys

Advantages: Ultra-lightweight, ultra-strong, corrosion-resistant, non-magnetic, and highly biocompatible

Disadvantages: Expensive, extremely difficult to machine

Primary Applications: Aerospace, defense, medical, racing, and high-end new energy lightweight applications

When selecting materials for fasteners, the most expensive option is never the best choice; instead, four core criteria are considered: operating environment, load requirements, service life requirements, and cost budget.

II. Carbon Steel Fasteners

Carbon steel is by far the dominant material in the fastener industry. It accounts for approximately 70% of global industrial fasteners and is the most widely used and versatile base material in industrial manufacturing and infrastructure projects.

Advantages

• The lowest overall cost among the four major materials, offering the best value for money
• Excellent ductility, easy to cold-forge, and low production difficulty
• Covers the full range of strength levels, suitable for applications ranging from general consumer use to medium- and high-strength industrial scenarios
• Mature global supply chain, ample inventory, and stable lead times

Disadvantages

Inherently poor corrosion resistance; susceptible to water, moisture, and salt spray. When used without protection, it rusts very easily and must be treated with a surface rust-preventive coating.

Three Core Heat Treatment Processes for Carbon Steel

1. Quenching and Tempering (Q&T)

The core process for all Grade 8.8 high-strength carbon steel bolts.

Function: Balances tensile strength and toughness, enhances fatigue resistance, and eliminates the risk of fracture.

2. Carburizing

Specifically used for self-tapping screws and drill-point screws

Effect: High surface hardness and high core toughness; the surface layer can penetrate steel plates, while the interior is resistant to brittle fracture.

3. Spheroidizing Annealing

An essential pretreatment process prior to cold heading production

Function: Softens the steel, reduces hardness, prevents cracking during forming, and ensures production yield.

Carbon steel has no natural rust-proofing ability; its service life depends entirely on surface treatments:

Electrogalvanizing (blue-white zinc, colored zinc, black zinc), hot-dip galvanizing, blackening, phosphating, Dacromet, Geomet zinc-aluminum coating, mechanical zinc plating, and Teflon coating.

III. Stainless Steel Fasteners

Stainless steel does not require electroplating for rust protection and is suitable for various humid, corrosive, and sanitary applications.

• Naturally resistant to oxidation, acids, alkalis, and salt spray corrosion
• Smooth, attractive appearance; non-toxic and hygienic, suitable for food and medical applications
• Designed for long-term use in outdoor environments, with a service life far exceeding that of carbon steel

Disadvantages

• Raw material costs are significantly higher than those of carbon steel and alloy steel
• Conventional heat treatment of austenitic stainless steel cannot increase strength
• Prone to thread seizing and cold welding lock-up, resulting in a high rate of assembly errors

Over 90% of stainless steel products in the fastener industry are still primarily made of 304 (A2) and 316 (A4) austenitic stainless steel; 410 stainless steel is used only for products requiring special hardness, such as self-tapping and self-drilling screws, and does not represent the characteristics of mainstream stainless steel grades.

Key Points on Stainless Steel Strength

The strength of 304 and 316 austenitic stainless steels cannot be enhanced through heat treatment, but their mechanical strength can be improved through cold working (work hardening). High-strength stainless steel fasteners on the market, such as A2-70 and A4-80, achieve their upgraded grades through work hardening processes.

Causes of Seizing in Stainless Steel + Solutions

Core Causes of Seizing

Austenitic stainless steel has high ductility. The friction generated during thread tightening produces high temperatures, leading to cold welding of the metal. This causes the threads to stick together and seize, making disassembly impossible.

Practical Solutions

• Apply a stainless steel-specific anti-seize compound or lubricant before assembly
• Reduce the tightening speed to avoid heat generation from high-speed friction
• Select precision-machined threads that have undergone surface polishing and passivation treatment
• Control assembly torque to prevent excessive force during tightening

Stainless Steel Surface Treatments

Stainless steel does not require galvanization for rust prevention. Mainstream processes include: acid pickling, passivation, electrolytic polishing, mechanical polishing, mirror polishing, and sandblasting

IV. Alloy Steel Fasteners

Ultra-high-strength screws used in wind power, bridges, heavy-duty trucks, and high-voltage equipment all use alloy steel as their core base material.

By adding rare metals such as chromium, molybdenum, nickel, and vanadium, alloy steel overcomes the shortcomings of carbon steel in terms of strength, toughness, and fatigue resistance, making it the core material for high-end, heavy-duty applications.

Common Alloy Steel Grades

SCM435 (equivalent to 35CrMo), 35CrMo, 42CrMo, 4140, 4340

Advantages

Through proper chemical composition design and precision heat treatment, alloy steel can more easily achieve ultra-high strength, high toughness, and excellent fatigue and high-temperature resistance, far exceeding the performance limits of conventional carbon steel. It is suitable for extreme conditions involving heavy loads, vibrations, and high pressure.

Disadvantages

• Extremely dependent on heat treatment processes, resulting in high technical barriers and production costs
• Lacks inherent rust resistance and must be paired with specialized anti-corrosion treatments

Mainstream Heat Treatment for Alloy Steel

Almost exclusively uses quenching and tempering (quenching + high-temperature tempering)

High-end products may also incorporate: induction hardening, nitriding, carburizing, and carbonitriding

Capable of consistently producing ultra-high-strength fasteners of Grade 10.9, Grade 12.9, and above

Alloy Steel Surface Treatment & Avoiding Hydrogen Embrittlement Pitfalls

Core Risk: Hydrogen Embrittlement Fracture

For high-strength carbon steel and alloy steel fasteners of Grade 10.9 and above, if hydrogen removal and dehydrogenation treatments are inadequate during standard electrogalvanizing processes, hydrogen embrittlement risks may arise, leading to delayed fractures during use—a major safety hazard in the engineering, automotive, and wind power industries.

Currently, in high-end sectors such as automotive, wind power, railways, and bridges, traditional electrogalvanizing has been fully replaced by Dacromet and Geomet zinc-aluminum coatings. This approach eliminates the risk of hydrogen embrittlement at its source while extending corrosion resistance.

Mainstream Surface Treatment Processes

Dacromet, Geomet zinc-aluminum coatings, phosphating, blackening, and high-end hydrogen-free galvanizing (dual protection against corrosion and hydrogen embrittlement)

V. Titanium Alloy Fasteners

Titanium alloys represent the pinnacle of lightweight and corrosion-resistant materials in the fastener industry, primarily used in high-end precision applications and extreme operating conditions.

Representative grades: TA2, TC4 (Ti-6Al-4V)

Advantages

• Density of approximately 4.5 g/cm³, which is only about 57% of that of steel (approximately 7.85 g/cm³), resulting in extreme lightweight design
• Extremely high specific strength, comparable to high-strength alloy steel, while significantly reducing weight
• Exceptional corrosion resistance in the vast majority of industrial environments (corrosion occurs only in special environments such as strong acids and hydrofluoric acid)
• Non-magnetic, heat-resistant, and highly biocompatible, making them suitable for medical and aerospace applications

Only Drawback

Expensive raw materials, difficult machining, long production cycles, and extremely high overall costs

Heat Treatment of Titanium Alloys

Unlike the quenching and tempering process used for steel, the mainstream approach involves solution treatment followed by aging treatment to optimize material stability and mechanical properties

High-End Surface Treatment for Titanium Alloys

Anodizing (customizable colored finishes), sandblasting, passivation, PVD coating, and DLC wear-resistant coating

VI. Key Data: Salt Spray Life of Surface Treatments

The corrosion resistance of different surface treatments varies significantly. The following are reference data from neutral salt spray tests (subject to coating thickness and formulation; provided for industry selection purposes only):