Metric Fastener Markings: What Do They Mean?

Most people would be amazed by the amount of information that can fit on the head of a bolt. As you may already know, information about the diameter, length, and thread pitch of a fastener can often be found on its head. However, there’s a lot of other information that manufacturers put on the head of a bolt that you may not know about. That information can make a big difference in the type of bolt you choose for your application. Here, we’ll look at some of the markings, and what they mean in terms of real-world applications

Property Class Marking
You may have noticed some numbers on the head of your bolt. This is typically represented as a number, followed by a decimal point, and then another number. These digits can tell you a world of information about your fastener.

Metric Hex Bolt 8.8Tensile Strength
The number that appears before the decimal point indicates the approximate tensile strength of the bolt. This is the minimum load at which the bolt can be expected to fail. To ascertain the load of your bolt, multiply this number by 100. That will give you the approximate tensile strength in Megapascals (MPa). For example, a fastener with a 10 before the decimal point has a tensile strength of 1,000 MPa.

Proof Load

Proof load is defined as the maximum tensile force that can be applied to a bolt that will not result in plastic deformation. In other words, the material must remain in its elastic region when loaded up to its proof load. Proof load is typically between 85-95% of the yield strength.

Tensile strength tells you when a bolt is likely to fail, but there’s a lot of room between whole and broken. Bolts can flex and stretch quite a bit before they fail. Within limits, a bolt will return to its original shape after deforming. The range that a bolt can endure before permanent deformation occurs is the proof load. While that number isn’t usually stamped on the bolt, you can figure it out using the yield load.

Yield Load

Yield strength can be defined as the tensile force that will produce a specified amount of permanent deformation (most commonly 0.2%) within a specific fastener.

The number following the decimal point tells you the yield point of the bolt. When a bolt reaches the yield point, permanent deformation occurs. The bolt is not broken, but it is damaged to the point where it will not return to its original shape when the load is removed. Multiplying this number by 10 gives you the percentage of the tensile strength at which the bolt will reach yield load. On a bolt with 10.9 stamped on the top, the yield load is 90% of the 1,000 MPa tensile strength. If a load of approximately 900 MPa were applied to this bolt, the bolt would likely suffer permanent deformation.

Bolt Materials
These same numbers can also give you an indication of the type of steel used to make the bolt. For bolts with an 8.8 or lower marking, the metal is usually a medium carbon steel. Bolts marked higher than this are usually made from harder alloy steels. Many people assume that a higher class bolt is better for every application, however this isn’t always the case.

Strength vs. Ductility
Strength is very important in a bolt, but it’s not everything. As strength increases, so does brittleness. This makes higher class bolts less desirable for applications where a lot of flexing or movement is expected. A bolt with higher ductility can flex and distort to a greater degree before reaching the yield load or breaking altogether.

Manufacturer Markings
The head of the bolt should also have a marking indicting which company manufactured the bolt. This is the company that has tested the bolt and can verify that the other markings accurately reflect the strength of the bolt. For example, a bolt stamped FNL 8.8 is a bolt made by Fastenal with a tensile strength of 830 MPa and a yield load of 660 MPa.

Still not sure which bolt or screw is best for your job? Contact Mr. Metric or call us at 866-501-9504 and one of our trained technicians will help you find the right metric fasteners for your job

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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