Comprehensive Guide on Bolt and Nut Grades

Selecting the correct grade of bolts and nuts is crucial for the success and safety of any mechanical or structural project. Bolt grades indicate their material properties, strength, and resistance to environmental factors like corrosion. In this blog, we’ll explore different bolt grades, their applications, and how to choose the right ones for your needs.

What are Grades of Bolts and Nuts?

Bolt and nut grades reflect their tensile strength, hardness, and material composition. These grades help engineers and builders choose the right fasteners for applications based on load-bearing capacity, corrosion resistance, and environmental conditions.

Grades are often standardized by organizations such as ASTM (American Society for Testing and Materials), SAE (Society of Automotive Engineers), and ISO (International Organization for Standardization).

 Common Grades of Bolts and Nuts

Carbon Steel Grades

• Grade 2 (Low Carbon Steel):
  • Tensile Strength: ~74,000 psi.
  • Applications: Used for non-critical applications like furniture, shelving, and low-stress environments.
  • Marking: No head markings.
  • Characteristics: Inexpensive, commonly available, but lower strength.
• Grade 5 (Medium Carbon Steel, Quenched and Tempered):
  • Tensile Strength: ~120,000 psi.
  • Applications: Common in automotive parts, machinery, and engine components.
  • Marking: 3 radial lines on the head.
  • Characteristics: Moderate strength, used in applications that require durability.
• Grade 8 (Medium Carbon Alloy Steel, Quenched and Tempered):
  • Tensile Strength: ~150,000 psi.
  • Applications: Heavy machinery, motors, and industrial applications where high strength is required.
  • Marking: 6 radial lines on the head.
  • Characteristics: High strength, good for load-bearing structures under high stress.

Stainless Steel Grades

• Grade 304 (18-8 Stainless Steel):
  • Tensile Strength: ~70,000 psi.
  • Applications: Used in construction, food processing equipment, and general corrosion-resistant environments.
  • Marking: Usually marked as “304” or with a manufacturer’s code.
  • Characteristics: General corrosion resistance but not ideal for marine environments.
• Grade 316 (Marine Grade Stainless Steel):
  • Tensile Strength: ~80,000 psi.
  • Applications: Marine applications, chemical processing, and environments with high chloride exposure.
  • Marking: Often marked as “316”.
  • Characteristics: Superior corrosion resistance, especially against chloride and saline environments.

Alloy Steel Grades

• ASTM A193 Grade B7 (Chromium-Molybdenum Steel):
  • Tensile Strength: ~125,000 psi.
  • Applications: High-temperature and high-pressure applications are commonly used in the petrochemical and oil industries.
  • Marking: Stamped with “B7.”
  • Characteristics: High heat and corrosion resistance, excellent for flanged joints, valve bodies, and other pressure systems.
• ASTM A320 Grade L7:
  • Tensile Strength: ~105,000 psi.
  • Applications: Suitable for low-temperature environments, used in cryogenic systems and oil platforms.
  • Marking: Stamped with “L7.”
  • Characteristics: Ideal for low-temperature toughness and strength.

Specialty Grades

Special alloys like Inconel, Monel, Titanium, and Hastelloy bolts are used for highly specific and demanding applications. These materials offer exceptional resistance to extreme temperatures, chemicals, and corrosive environments.

• Inconel Bolts (Grade 600/625):
  • Applications: Aerospace, chemical processing, and marine industries.
  • Characteristics: High resistance to oxidation and corrosion at extreme temperatures.
• Titanium Bolts (Grade 2/Grade 5):
  • Applications: Used in aerospace, medical, and sports industries.
  • Characteristics: High strength-to-weight ratio and excellent corrosion resistance.

Factors to Consider When Choosing Bolt Grades

• Strength Requirements: Ensure that the chosen bolt grade can handle the expected load or stress. For instance, use Grade 8 or ASTM A193 B7 for high-strength needs.
• Corrosion Resistance: In corrosive environments like marine or chemical industries, opt for stainless steel Grade 316 or specialty materials like Inconel or Monel.
• Temperature Tolerance: For high or low-temperature applications, alloy steel grades like A193 B7 for high heat and A320 L7 for low temperatures are ideal.
• Cost Efficiency: While specialty grades like Inconel or Titanium offer superior performance, they come at a higher price. For general applications, carbon steel Grade 5 or stainless steel 304 may be more cost-effective.

How to Identify Bolt Grades

• Grade 2: No head markings, often found in household and non-critical uses.
• Grade 5: Three radial lines on the head.
• Grade 8: Six radial lines on the head.
• Stainless Steel 304/316: Markings include “304” or “316” along with manufacturer codes.
• Alloy Grades: Typically marked with grade-specific designations like “B7” or “L7.”

These markings are typically found on the bolt head and help ensure you’re using the correct grade for the job.

Applications of Different Bolt and Nut Grades

• Construction: Carbon steel grades (Grade 2, 5, 8) for general construction and infrastructure.
• Automotive: Medium strength bolts like Grade 5 are commonly used for engine parts, while Grade 8 is used for suspension and load-bearing components.
• Marine & Chemical Processing: Stainless steel Grade 316 or Inconel bolts provide superior corrosion resistance.
• High-Temperature Environments: ASTM A193 B7 or Inconel 625 bolts are ideal for high heat or pressure applications.

Conclusion

Choosing the right grade of bolts and nuts is crucial for ensuring the safety, strength, and longevity of your projects. Whether you need standard carbon steel bolts or specialty alloy bolts for demanding conditions, understanding the grades will help you make an informed choice.