Vickers hardness indentation: an illustrative image of a measurement at a single location.
Surface region · Heat treatment · Hardness profile
Testing screws for decarburization and carburization
Even when the core hardness of a screw is within the expected range, hardness near the surface may differ. If decarburization or carburization is suspected, measurement locations must therefore be chosen carefully. Testawell measures core and surface hardness, as well as hardness profiles on ground and polished cut surfaces, known as metallographic sections.
Testawell performs hardness testing. We are not currently accredited to DIN EN ISO/IEC 17025. If you need an accredited test report, you must arrange the testing with a laboratory accredited for those tests.
What changes near the surface of a screw?
Decarburization reduces the carbon content near the surface of the steel; carburization increases it. This can happen during heat treatment. Whether such a change is acceptable depends on the screw type and its requirements. In case-hardened screws, the surface layer is deliberately hardened. This must be assessed differently from unintended carburization. Research on altered surface regions in screws.
A difference in hardness can indicate changes, but it does not measure carbon content. To find the cause, the manufacturing process, coating and, where needed, the microstructure must also be considered. Microstructure means the internal structure of the material. A hardness comparison is therefore only one part of a more comprehensive investigation of decarburization.
Which measurements can examine the surface region?
We perform Vickers testing, including HV 0.3 and HV 10, and measure hardness profiles on prepared sections. We choose the test force and measurement locations to suit the agreed objective. If values were measured with different test forces or at different locations, we need to check whether a comparison is valid. The Vickers method is described in DIN EN ISO 6507-1.
Investigation
Information it provides
Core hardness
Hardness at a specified location inside the screw
Near-surface and surface hardness
Hardness of the steel near the surface, beneath the coating on coated screws
Hardness profile on a metallographic section
Differences in hardness between documented measurement locations
Additional microstructure examination
Further information from the internal structure of the material; this examination must be agreed separately
Why is careful sample preparation important?
Preparing the section must preserve the shape of the edge being examined. The location of the cut, mounting the sample in a supporting material and polishing all matter. If preparation rounds off the edge, the region that matters may no longer be suitable for assessment.
Dr. Mike Keeble describes this difficulty in his technical article Metallographic Preparation of Fasteners: mounting protects the edges. However, gaps between the mounting material and the screw can interfere with preparation and later assessment. His article appeared in the American Fastener Journal in 2021. For test planning, this means that measurement locations and section quality should be documented together. Keeble: preparation of fasteners.
Which standards apply to the assessment?
If the screw falls within the scope of DIN EN ISO 898-1, it is assessed using the agreed edition of that standard. DIN EN ISO 3887:2023-12 describes how to determine the depth of decarburization in steel in general. The product requirements still determine which results are acceptable for a particular screw.
Before testing, we use the applicable requirements to define the permitted deviations, measurement locations and test conditions. For customer-specific parts, we also need the drawing or test specification as well as the standard designation.
What does research on actual screw specimens show?
Hideharu Shimizu, Kunio Funami and Gendai Etoh investigated carburized and decarburized surface regions in screw specimens in 1989. In the published summary, or abstract, they report a beneficial effect of decarburization on delayed fracture in their experiments — fracture that occurs after some time. They found no effect on fatigue fracture caused by repeated loading. Their findings therefore do not support a general statement that decarburization always reduces fatigue strength. Original paper and DOI.
A hardness profile shows how hardness is distributed within the screw examined. Whether the screw is suitable for its intended use and meets the product standard needs a separate assessment. A research finding does not replace that assessment of the actual screw.
How to prepare for testing the surface region
Tell us why the investigation is needed and label suspect parts and unaffected comparison parts separately. The following information is helpful:
Screw type, material, property class and any available drawing.
Heat treatment and coating details, where known.
Which delivery batch or manufacturing condition each sample represents.
Previous hardness results, including method, test force and measurement location.
Requested investigation: individual values, surface-to-core comparison or hardness profile.
Applicable test specification and permission for the necessary cutting, grinding and polishing of samples.
Standards and technical sources
DIN EN ISO 898-1:2013-05 – Product requirements for steel bolts and screws with specified property classes.
DIN EN ISO 3887:2023-12 – General standard for determining the depth of decarburization in steel.
Is a core hardness measurement sufficient to rule out surface decarburization?
No. A measurement inside the screw does not show enough about what happens near the surface. Suitable additional measurement locations and, where needed, further investigations must be agreed.
Is every harder surface region a defect?
No. A hard surface layer is intentional in certain types of screws. Whether it is acceptable depends on the material, manufacturing process and product requirements.
Does hardness testing measure carbon content?
No. It measures hardness at the test location. A hardness difference alone is not chemical evidence of carburization or decarburization.
Can a screw still be used after testing on a metallographic section?
The screw is cut or ground to prepare the section. Plan for samples that will no longer be used as functional screws afterwards.
Can Testawell measure hardness profiles on metallographic sections?
Yes. Testawell measures hardness profiles as well as core and surface hardness. We agree the test force, measurement locations and questions the report should answer before testing.
Compare the surface and core condition of your screws
Describe what you have noticed and provide the material, screw type and available requirements. We will agree the hardness measurements and samples needed with you.
