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How Are the Strength Grades of Torx Screws Classified?

How Are the Strength Grades of Torx Screws Classified?

From the perspective of international standards, Torx screws made of carbon steel or alloy steel are usually classified by strength grades in accordance with the ISO 898-1 standard; if they are made of stainless steel, they comply with the ISO 3506-1 standard. These standards provide unified specifications for the production and quality control of Torx screws worldwide, ensuring consistency and interchangeability of products across different regions and manufacturers.
Specifically, common strength grades for carbon steel and alloy steel Torx screws include Grade 4.8, Grade 8.8, Grade 10.9, and Grade 12.9. Taking Grade 4.8 as an example: the number "4" before the decimal point indicates that the nominal tensile strength of the screw material reaches 400 MPa, while the number "8" after the decimal point means the yield ratio of the bolt material is 0.8. From this, the nominal yield strength can be calculated as 400 × 0.8 = 320 MPa. As the grade number increases, the tensile strength and yield strength of the screw increase accordingly. For Grade 10.9 screws, the nominal tensile strength reaches 1000 MPa, the yield ratio is 0.9, and the nominal yield strength is 900 MPa. Such screws are usually made of medium-carbon alloy steel and undergo heat treatment to enhance their mechanical properties. They are suitable for applications requiring high fastening strength, such as the connection of key components inside automobile engines.
For stainless steel Torx screws, common grades include A2-70 and A4-80. Here, the letter "A" denotes austenitic stainless steel, and the meaning of the subsequent number combination differs from that of carbon steel screws. In "A2-70", "A2" represents a specific type of austenitic stainless steel (e.g., 304 stainless steel), and "70" indicates that the minimum tensile strength of the screw is 700 MPa. Grade A4-80 screws are typically made of 316 stainless steel, with a minimum tensile strength of 800 MPa. Compared with Grade A2-70, they have better corrosion resistance and are often used in environments requiring high corrosion protection, such as chemical equipment and marine engineering.
The strength grade of a Torx screw is reflected not only in its mechanical property indicators but also closely linked to the production material and processing technology. Screws of different strength grades vary in the composition of the steel used and the heat treatment method. Low-strength Grade 4.8 screws are generally made of low-carbon steel and do not require complex heat treatment processes. In contrast, high-strength Grade 12.9 screws are made of high-carbon alloy steel and undergo heat treatment processes such as quenching and tempering to refine grain structure and improve hardness and strength.
In practical applications, the strength grade of a Torx screw is usually marked on the screw head. The heads of carbon steel and alloy steel screws may have indentations, numbers, or other marks, where the number of indentations or the digit corresponds to the respective strength grade. Stainless steel screws, on the other hand, are marked with a combination of letters and numbers, allowing users to quickly identify the grade.
The classification of Torx screw strength grades is based on scientific standards and practical requirements. Every link—from material selection and processing technology to the determination of mechanical property indicators—undergoes rigorous design and verification. Only by fully understanding the differences between these strength grades can users correctly select suitable Torx screws for different engineering scenarios, ensuring the safety and stability of equipment and structures.

Supplementary Notes on Key Terms

  1. ISO 898-1 / ISO 3506-1:
    • ISO 898-1 is the international standard for "Mechanical properties of fasteners made of carbon steel and alloy steel" (focusing on non-stainless steel fasteners), specifying test methods and performance requirements for strength grades.
    • ISO 3506-1 applies to "Mechanical properties of fasteners made of corrosion-resistant stainless steels", adapting to the unique mechanical and corrosion-resistant characteristics of stainless steel materials.
  2. Yield ratio: Translated as "yield ratio", referring to the ratio of a material’s yield strength to its tensile strength (yield strength / tensile strength), a key indicator reflecting the material’s ductility and safety margin in load-bearing.
  3. Quenching and tempering: A two-step heat treatment process ("quenching + tempering")—quenching increases material hardness, while tempering reduces brittleness, balancing strength and toughness, which is critical for manufacturing high-strength screws (e.g., Grade 12.9).
  4. Austenitic stainless steel: A type of stainless steel with an austenitic crystal structure (e.g., 304, 316), featuring good corrosion resistance and ductility, hence its wide use in fasteners for harsh environments.

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