Metric Property Classes vs. Imperial Grades

A fastener’s strength is determined by two primary factors: 1) the material it is made from, especially the percentage of hardening carbon in the steel, and 2) the process used to create it – usually heat treatments such as tempering, galvanizing, annealing, and quenching. Treating steel fasteners to harden them necessarily reduces their ductility. Bolts, nuts, and screws can be made to fulfill tasks that require specific strengths and ductility, and identifying these properties aids contractors and manufacturers in selectig the right fastener for the job.

Metric Property Classes vs. Imperial GradesGrades and Property Classes

Metric fasteners adhere to strength ratings, or Property Classes, mandated by the International Standards Organization (ISO). Imperial, or SAE fasteners’ ratings, or Grades, are assigned by the American Society for Testing Materials (ASTM). Grades and classes are denoted by markings on the heads of externally threaded hardware and the sides or faces of internally threaded parts:

  • Metric Bolts
    Metric bolts and large screws will feature two numbers separated by a dot, such as “4.8” The first number indicates the bolt’s tensile strength in hundreds of mega Pascals, approximately 400 MPa in this case. The second number provides the bolt’s yield strength. In this case, “8” tells users the bolt’s yield strength is 80 percent of its tensile rating, or approximately 320 MPa (.80 x 400).
  • Metric Nuts
    Strength indicators on metric nuts may be a single number on the face or side, corresponding to its tensile strength. Or it may be denoted by a “clock face” pattern. In this case, a dot indicates 12 o’clock, and a hash mark represents the hour hand. A “time” of 8 o’clock means the nut is Class 8. The nut used should always be equal to or higher than the bolt’s tensile strength.
  • Imperial Bolts
    Imperial bolts and large screws made from low-carbon steel are not marked. Those made of medium carbon and alloy steel have their grades indicated by hash marks. Common strengths include Grade 5, with three hashes on the face, and Grade 8, with six hashes. ASTM also assigns minimum yield strengths for each grade, depending on the fastener’s length.
  • Imperial Nuts
    Imperial nuts will have their grade stamped as a numeral on the face or the side.

Correlation

Note that grade and class numbers are not equal. That is, a Class 8.8 metric bolt is not as strong as a Grade 8 imperial bolt. However, the relative strengths of metric and imperial bolts can be compared. For example, ASTM mandates Grade 8 fasteners have a minimum tensile strength of 150 ksi (thousands of pounds per square inch). This converts to 1,034 MPa – roughly the strength of a Class 10.9 metric bolt. Using the same conversion, we can determine these rough equivalents:

  • Class 4.6 or 4.8 = Grade 2. Both are made of low or medium carbon steel.
  • Class 8.8 = Grade 5. Both are made from medium quenched and tempered carbon steel.
  • Class 10.9 = Grade 8. Both constructed from quenched and tempered alloy steel.
  • Class 12.9 = Exceeds grade 8. Constructed from quenched and tempered alloy steel.

Conclusion

As you might expect, the stronger the fastener, the higher the cost. In any application, safety should be the primary consideration. If there is any chance a class 8.8 bolt would fail and potentially cause injury or significant property damage, prudent manufacturers would upgrade to class 10.9 hardware. On the other hand, if class 8.8 bolts will do the job, there may be no reason to pay more for stronger fasteners, especially when products require several fasteners and/or are mass-produced, such as in the automobile industry.

Have a question about which metric fastener to use for a particular application? Mr. Metric can help! Contact us online or call our customer service experts at 1-408-286-8816 for guidance on our vast selection of screws, bolts, nuts, and the tools to install them for maximum results.

About the author

Mr. Metric is the spokesperson for www.mrmetric.com, an online store for metric fasteners. Get $7.50 flat shipping with no minimum orders, plus advice and best practices for using metric fasteners in any application.

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