Illustration: the marking on a screw and its test results provide different information.
Understanding bolts · Property classes
Bolt property classes: understanding 8.8, 10.9 and 12.9
The numbers on a bolt head indicate its property class. They tell you which mechanical properties the bolt is expected to have. Appropriate tests establish whether the delivered bolts actually meet those requirements. The test report must clearly identify the bolts examined.
Testawell offers Vickers, Rockwell and Brinell hardness testing, as well as core hardness, surface hardness and hardness profiles on metallographic sections. We are not currently accredited to DIN EN ISO/IEC 17025. If you need an accredited test report, this must be arranged with a laboratory accredited for the relevant tests before placing the order.
What do the two numbers in a property class mean?
For the steel bolts considered here, multiply the first number by 100. This gives the nominal tensile strength in MPa. For 8.8, the calculation is 8 × 100 = 800 MPa. Here, “nominal” means the value represented by the property class designation.
Divide the second number by ten to find the share of nominal tensile strength used as the nominal yield strength or proof stress. For 8.8, this is 80 %. For 10.9 and 12.9, it is 90 %.
Example for 8.8: the second 8 gives 8 ÷ 10 = 0.8. The nominal yield strength is therefore 800 MPa × 0.8 = 640 MPa. [1]
This calculation explains the nominal values in the designation. The applicable standard and component specification set the limits that a particular bolt must meet during testing. They also specify the required tests and test conditions.
Class
Nominal tensile strength
Share of tensile strength
Nominal yield strength or proof stress
8.8
800 MPa
0.8 = 80 %
640 MPa
10.9
1,000 MPa
0.9 = 90 %
900 MPa
12.9
1,200 MPa
0.9 = 90 %
1,080 MPa
Why does the marking alone not prove the properties?
The marking helps identify a bolt. If its origin is unknown, batches have been mixed or a complaint is being investigated, its actual properties need to be tested. We agree which tests are needed and clearly identify each sample and its results.
Depending on the component, tests may be needed for strength, elongation, hardness and other properties. The required tests also depend on whether the whole bolt can be tested and on the load its design is intended to carry. [2] Learn more on our page about screw testing to DIN EN ISO 898-1.
What hardness should an 8.8, 10.9 or 12.9 bolt have?
A property class does not give a hardness value in HV, HBW or HRC. A hardness test needs a specified method, measurement location and limit. Values measured on the surface cannot automatically be compared with values inside the bolt.
Before testing, we therefore agree the required hardness measurement and the limit set by the applicable specification. A conversion from hardness to tensile strength is useful only for certain materials and value ranges. It does not fully verify a property class.
If hardness values seem unusual, we also check where the measurements were taken, the condition of the surface and how the sample was prepared. If the cause lies near the surface, several measurements across a prepared section can help. Together, they form a hardness profile.
How do A2-70 and A4-80 differ from 8.8 and 10.9?
Stainless steel bolts use different markings. A2 and A4 identify steel grades; the following numbers, 70 or 80, give the property class. ISO 3506-1 applies to these bolts and screws. Similar tensile strength does not mean that they can replace quenched and tempered steel bolts in every application. [3]
Other properties and the intended use also matter when choosing a bolt. A hardness measurement shows neither its exact chemical composition nor its corrosion resistance. State in your order whether you need to identify the material or test its mechanical properties.
Does a higher property class mean a better joint?
Even with a higher property class, the bolted joint must be designed for its intended use. Geometry, the mating component, nut, coating and assembly all affect how the joint works. Testing an individual bolt therefore answers only part of these questions.
The numbers on the head alone do not tell you the tightening torque or the permitted load in service. If you need to establish whether the assembled joint meets its requirements, agree a separate test or calculation. Our fastener testing overview helps you choose the right approach.
How to request a strength assessment
State the expected or ordered property class and include a drawing and photographs of the marking. Dimensions, coating, batch identification and available delivery documents are also important. Tell us whether you need to investigate unusual hardness values or provide specific evidence required by your customer.
Testawell can directly test bolt and screw hardness. Other tests or specific inspection documents must be agreed separately. ISO 16228 describes different types of inspection documents for fasteners. A report on individual measurements does not automatically meet the requirements of every document type. [4]
Are 800, 1,000 and 1,200 MPa the required test limits for every bolt?
No. These are the nominal tensile strengths represented by 8.8, 10.9 and 12.9. The applicable standard and component specification set the limits and test conditions for your bolt.
Is a hardness test sufficient to confirm property class 12.9?
No. A hardness test shows whether the required hardness is met. The other specified tests are also needed to confirm the property class.
Is A4-80 equivalent to 8.8?
No. The markings describe different materials and property classes. Similar tensile strength does not mean that the bolts can replace one another in every application.
Can I read the tightening torque from the property class?
No. Additional information on the bolted joint, friction, geometry and assembly method is needed.
What information helps when requesting hardness testing of unidentified bolts?
Photographs of the marking, dimensions, coating, origin or batch, and the question you need answered. Measured values can be recorded without agreed requirements. To confirm a property class, however, the applicable requirements must be known and the required tests must have been carried out.
Have the properties of your bolts checked
Send photographs, a drawing, the property class and your test objective. We will agree the appropriate hardness test and required evidence with you.
