High-Strength Bolt Materials & Selection Guide

In the fastener industry, high-strength bolts—with excellent tensile strength, yield strength, and toughness—are widely used in wind power, bridges, nuclear power, heavy machinery, automotive manufacturing, and other critical sectors. Compared to ordinary bolts, high-strength bolts must withstand greater loads and more complex conditions, making material selection extremely stringent. Material composition and properties directly determine load capacity, fatigue resistance, and corrosion resistance.

I. Common Materials and Their Characteristics

Material selection must balance strength, toughness, processability, and suitability. Alloy structural steels are most widely used, supplemented by stainless steels and high-temperature alloys for special conditions.

(A) Alloy Structural Steels – Mainstream Materials

40Cr: Most widely used. Medium carbon alloy structural steel (0.37%-0.44% C) with chromium for hardenability and strength. After heat treatment: tensile strength ≥800MPa, yield strength ≥785MPa. Good toughness and processability at moderate cost. Suitable for medium loads, normal temperatures: machinery, building steel structures. Advantages: cost-effective, easy to process. Disadvantages: poor corrosion resistance, not for wet or corrosive environments.

35CrMoA: Medium carbon low alloy (0.32%-0.40% C) with chromium and molybdenum for improved strength, toughness, hardenability, and heat resistance. After quenching and tempering: tensile strength ≥980MPa, yield strength ≥835MPa. Excellent fatigue resistance and impact toughness. Suitable for alternating loads, moderate temperatures: wind turbine towers, heavy machinery drive shafts. Better corrosion and heat resistance than 40Cr, but higher cost.

42CrMoA: High-end alloy structural steel for high-strength bolts (0.38%-0.45% C). Higher chromium and molybdenum than 35CrMoA. After heat treatment: tensile strength ≥1080MPa, yield strength ≥930MPa. Very high strength, toughness, and hardenability. Suitable for high loads, complex conditions: nuclear power equipment, aerospace, large bridges. Excellent all-around properties but difficult to process and higher cost.

20MnTiB: Low carbon alloy structural steel (0.17%-0.24% C) with manganese, titanium, boron. Good cold heading properties and hardenability. After heat treatment: tensile strength ≥800MPa. Suitable for mass-produced small high-strength bolts: automotive engines, chassis. Cost-effective for high-volume production.

(B) Specialty Materials for Special Conditions

Stainless Steel – for corrosive conditions

• 304: Good corrosion resistance and toughness. Suitable for wet, mildly corrosive environments: outdoor construction, appliances.

• 316: Molybdenum addition provides superior corrosion and chemical resistance. Suitable for marine, chemical, strongly corrosive environments.

Advantages: excellent corrosion resistance. Disadvantages: slightly lower strength than alloy structural steels, higher cost.

High-Temperature Alloys – for high-temperature conditions

• Inconel, Hastelloy series. Excellent high-temperature resistance and oxidation resistance. Maintain high strength and toughness at elevated temperatures. Suitable for nuclear power, aerospace, high-temperature equipment: boilers, turbines. Superior performance but very difficult to process and extremely high cost—only for high-end specialty applications.

II. Key Selection Criteria

Consider four factors for accurate material selection:

1. Load requirements: Match strength grade to tensile and shear loads.

   • Low to medium loads: 40Cr, 20MnTiB

   • High loads: 35CrMoA, 42CrMoA

   • Extreme loads: High-temperature alloys

2. Service environment:

   • Normal temperature, dry: Alloy structural steels (cost-effective)

   • Wet, corrosive: 304 or 316 stainless steel

   • High temperature, high pressure: High-temperature alloys

   • Consider specific media (acids, alkalis, salt spray)

3. Processability:

   • Mass-produced small bolts: 20MnTiB, 40Cr (easy to process)

   • Large, high-precision bolts: 35CrMoA, 42CrMoA (require precise heat treatment)

4. Cost control: Prioritize cost-effective materials while meeting performance requirements.

   • Ordinary machinery: 40Cr is sufficient (42CrMoA is overkill)

   • Non-aggressive environments: 304 stainless steel is sufficient (316 unnecessary)

III. Quality Control & Industry Considerations

Material selection: Source qualified raw materials from manufacturers. Require material certificates and chemical composition reports. Perform ultrasonic testing and metallographic analysis to detect internal porosity, inclusions, or cracks.

Processing: Optimize processes based on material characteristics. Heat treatment is critical—different materials require different quenching and tempering temperatures. For example:

• 40Cr quenching temperature: 850-880°C

• 42CrMoA quenching temperature: 820-850°C

Precisely control temperature, holding time, and cooling rate to achieve required strength and toughness while avoiding quench cracks or uneven hardness.

Avoid blind selection:

• Do not use ordinary alloy structural steel bolts in strongly corrosive environments (will rust and fracture).

• Do not use stainless steel bolts in ultra-high load applications (insufficient strength).

• Follow industry standards (e.g., GB/T 3098) for material selection.

Material selection for high-strength bolts is a systematic process balancing performance, conditions, and cost. Alloy structural steels are the mainstream choice, with specialty materials for special conditions. Mastering material properties and selection criteria, combined with strict quality control, enables production of high-strength bolts that meet requirements and provide reliable safety for critical equipment.