Steel fasteners are essential components across various industries, from automotive and construction to aerospace. The performance and durability of these fasteners depend significantly on their material properties, which are often specified by their grades. In this blog, we’ll delve into the differences between steel fastener grades 4.6, 8.8, 10.9, and 12.9, exploring their tensile strength, yield strength, hardness, and suitable applications.

Steel Fastener Grades Explained

Steel fasteners come in different grades, each characterized by its tensile strength, yield strength, and hardness. These properties determine how well the fastener performs under stress and environmental conditions. Let’s break down the specifications and applications of each grade:

Grade 4.6

Tensile Strength: 400 MPa
Yield Strength: 240 MPa
Vickers Hardness (HV): 110-220
Rockwell Hardness (HRC): 10-22

Applications:
Grade 4.6 fasteners are commonly used in light-duty applications. Their relatively lower tensile and yield strengths make them suitable for tasks where high strength is not critical. You’ll often find these fasteners in furniture assembly, general construction, and other low-stress environments.

Key Points:

  • Tensile Strength: 400 MPa indicates the maximum stress the material can withstand while being pulled.
  • Yield Strength: 240 MPa denotes the point at which the material begins to deform plastically.
  • Hardness: With a Rockwell hardness ranging from 10 to 22 HRC, these fasteners are relatively softer, making them easier to machine but less suitable for high-stress applications.

Grade 8.8

Tensile Strength: 800 MPa
Yield Strength: 640 MPa
Vickers Hardness (HV): 250-320
Rockwell Hardness (HRC): 22-34

Applications:
Grade 8.8 fasteners are made from medium carbon steel and are quenched and tempered to achieve higher strength. These fasteners are widely used in the automotive industry, machinery, and structural applications where higher tensile strength is required.

Key Points:

  • Tensile Strength: 800 MPa provides a higher level of strength compared to Grade 4.6, making it suitable for more demanding applications.
  • Yield Strength: 640 MPa indicates a significant increase in strength, making these fasteners more resistant to deformation.
  • Hardness: With a Rockwell hardness of 22 to 34 HRC, Grade 8.8 fasteners are harder and more durable than Grade 4.6.

Grade 10.9

Tensile Strength: 1,000 MPa
Yield Strength: 900 MPa
Vickers Hardness (HV): 320-380
Rockwell Hardness (HRC): 34-39

Applications:
Grade 10.9 fasteners are designed for heavy-duty applications where higher strength and durability are crucial. They are commonly used in heavy machinery, automotive components, and high-stress structural applications.

Key Points:

  • Tensile Strength: 1,000 MPa signifies even higher strength, making these fasteners suitable for extreme conditions.
  • Yield Strength: 900 MPa provides excellent resistance to deformation, which is essential for high-stress environments.
  • Hardness: The Rockwell hardness of 34 to 39 HRC ensures that these fasteners can withstand significant stress without deforming.

Grade 12.9

Tensile Strength: 1,200 MPa
Yield Strength: 1,080 MPa
Vickers Hardness (HV): 380-450
Rockwell Hardness (HRC): 39-45

Applications:
Grade 12.9 fasteners represent the highest strength grade in common use. They are used in high-performance automotive and aerospace applications, as well as in critical structural applications requiring maximum strength and durability.

Key Points:

  • Tensile Strength: 1,200 MPa is the highest tensile strength among the grades, ensuring superior performance in the most demanding environments.
  • Yield Strength: 1,080 MPa reflects the extreme resistance to deformation, making these fasteners ideal for high-stress applications.
  • Hardness: With a Rockwell hardness of 39 to 45 HRC, Grade 12.9 fasteners are extremely hard and durable, designed for the toughest conditions.

Grade Tensile Strength (MPa) Yield Strength (MPa) Vickers Hardness (HV) Rockwell Hardness (HRC) Applications
4.6 400 240 110-220 10-22 Light-duty applications, such as furniture assembly, general construction, and low-stress environments.
8.8 800 640 250-320 22-34 Automotive industry, machinery, structural applications, and other high-stress environments.
10.9 1,000 900 320-380 34-39 Heavy machinery, automotive applications, high-stress structural applications.
12.9 1,200 1,080 380-450 39-45 High-performance automotive and aerospace applications, critical structural applications, high-stress environments requiring maximum strength.

Here's a table summarizing the chemical compositions of the raw materials for steel fastener grades 4.6, 8.8, 10.9, and 12.9:

Grade Carbon (C) Manganese (Mn) Phosphorus (P) Sulfur (S) Chromium (Cr) Nickel (Ni) Molybdenum (Mo) Vanadium (V)
4.6 0.05% - 0.25% 0.3% - 1.0% ≤ 0.05% ≤ 0.06% < 0.5% < 0.5% < 0.3% -
8.8 0.15% - 0.35% 0.6% - 1.65% ≤ 0.05% ≤ 0.06% < 0.5% < 0.5% < 0.3% -
10.9 0.20% - 0.55% 0.3% - 1.7% ≤ 0.025% ≤ 0.035% 0.5% - 1.0% 0.3% - 0.7% 0.1% - 0.3% Up to 0.1%
12.9 0.20% - 0.50% 0.4% - 1.7% ≤ 0.025% ≤ 0.035% 0.8% - 1.2% 0.3% - 1.0% 0.15% - 0.3% 0.1% - 0.2%

Notes:

  • Carbon (C): Influences the hardness and tensile strength. Higher carbon content increases strength.
  • Manganese (Mn): Enhances hardness and strength. It also improves steel’s ability to be hardened.
  • Phosphorus (P): Excess phosphorus can make steel brittle; thus, it is kept low.
  • Sulfur (S): Low sulfur content is important to avoid brittleness and maintain ductility.
  • Chromium (Cr): Improves hardness, tensile strength, and corrosion resistance.
  • Nickel (Ni): Increases toughness, strength, and corrosion resistance.
  • Molybdenum (Mo): Enhances strength at high temperatures and wear resistance.
  • Vanadium (V): Improves strength, toughness, and overall performance.

Conclusion

Selecting the right steel fastener grade is crucial for ensuring the strength and durability of your projects. Understanding the differences between grades 4.6, 8.8, 10.9, and 12.9 allows you to make informed decisions based on the specific demands of your application. For light-duty tasks, Grade 4.6 may be sufficient, while for high-stress environments, Grades 10.9 and 12.9 offer superior strength and hardness. By considering the tensile strength, yield strength, and hardness values, you can ensure that your fasteners will perform reliably and effectively in their intended applications.