Fasteners, such as bolts, nuts and studs play a ubiquitously significant role in daily life. They are employed in the construction of complex machinery and equipment, as well as vehicles and buildings. Given that mechanical stress is concentrated at these connection points, their integrity is of paramount importance and must adhere to stringent safety requirements.
For fastener assemblies that are not amenable to tensile testing, hardness testing is conducted to verify the heat treatment process and ensure that the hardness does not exceed the maximum permissible hardness for the grade. Hardness testing can be accomplished on the appropriate surface (head, shank, or end) and/or across the cross-section through the threaded region.
Hardness testing can be carried out using the Vickers, Brinell, or Rockwell hardness scales in accordance with BS EN ISO/ASTM standards. The Vickers hardness test involves pressing a right-angle pyramid-shaped diamond indenter with a square base into the prepared surface of the test specimen, and subsequently measuring the diagonal length of the indentation left on the surface after the removal of the test force.
In accordance with BS EN ISO 898-1:2013, the following hardness tests are typically performed on fasteners:
Surface and core Vickers hardness
The hardness test should be executed on the flat surface of the head, shank, or end of the fastener after the removal of any coatings/finishes and the implementation of appropriate surface treatment. For core hardness, the cross-section at the end of the thread (1d) should be appropriately prepared, and the hardness test conducted within the half-radius region. The minimum test load for the Vickers hardness test should be 10 kilograms (98 N) for determining the surface and core hardness.
Carburizing and Decarburizing
During the heat treatment process, a thin hard carbon layer or a thin soft layer may form in the threaded area, potentially reducing strength. Carburization and decarburization evaluations can be conducted on metallographic sections of the threaded area. This entails mounting the longitudinal section in an appropriate resin to safeguard the edges, followed by a series of grinding and polishing stages, and then conducting a hardness test.
Carburization testing is undertaken to confirm that the fastener conforms to the requisite hardness specifications and does not exceed the maximum hardness. The test is performed on the threaded area around the pitch line at a distance of approximately 0.12 mm from the surface. Highly carburized areas can lead to surface embrittlement, thereby diminishing fatigue resistance.
Decarburization during the heat treatment process can result in carbon loss, giving rise to a microstructure with lower hardness compared to the harder microstructure of martensite. This operation is performed to determine whether the quenched and tempered fastener surface has decarburized and to ascertain the depth of the decarburized zone. The hardness method only detects partial decarburization and is applicable to fasteners with a pitch of ≥ 1.25 mm and a performance grade ranging from 8.8 to 12.9. A 300g (low force Vickers) hardness test load should be utilized to determine carburization and decarburization.
Tempering Inspection
A tempering inspection is conducted to ensure that the minimum tempering temperature is achieved during the heat treatment process. This test involves tempering the fastener and holding it at a temperature 10°C below the minimum tempering temperature for 30 minutes. Vickers hardness tests are performed to create three indentations at the same location on the same fastener. The average reduction in Vickers hardness (if any) should be less than 20 Vickers units.
All high-strength fasteners produced by Wanbo Fasteners undergo hardness testing to guarantee that the quality meets the standard requirements.
Post time: Dec-04-2024
