116 Fastening solutions EJOT MAXXtip® Direct fastening in ultra-high strength materials By Prof. Dr. Ralph Hellmig, Director Technology Management Fasteners, EJOT SE & Co. KG Direct fastening, where a screw forms its own female thread, is an economical and process-reliable alternative especially for highly stressed joints. The range of applications is limited by the maximum strength of the screw tip, which must not deform plastically during thread forming. Images: © EJOT SE & Co. KG In order to increase the hardness of a selftapping tip made from low-alloy carbon steels commonly used as screw material, different hardening mechanisms are combined. On the one hand, the maximum hardness is influenced by the carbon content in the material, which should be as high as possible. Such a condition can be set in the thread flanks by so-called carburization, during which the carbon diffuses into the surface while being heat treated. On the other hand, the material in the screw tip can be induction-hardened. If subsequent tempering can be avoided, especially in case of galvanic processes, the tip hardness achieved has a beneficial effect on the fastening process. However, subsequent tempering can only be omitted if the structure in the head and bearing area of the screw is completely insensitive to hydrogen embrittlement. The screw tip is only stressed for a short time during fastening so that a delayed hydrogen brittle fracture is excluded, provided that the tip is not or only slightly mechanically stressed in further use. There is actually a high-strength microstructure that is very insensitive to the influence of hydrogen. This so-called austempered structure can be obtained by first exposing the screw to a carbon-enriched atmosphere during heat treatment and then cooling it within a salt bath in such a way that a bainitic microstructure is formed across the crosssection, with the parameters being selected in such a way that the strength of the screw in the head and bearing area is around 1000 MPa. It could be proven that such a microstructure still shows ductile fracture behaviour after hydrogen-loaded galvanic processes (e.g. zinc coating) under destructive slow mechanical testing, whereas a screw after conventional 10.9 heat treatment shows hydrogen embrittlement under the same conditions. Therefore, a screw that is austempered in the first step and inductionhardened in the second step can fulfil exactly the requirements mentioned in the Potential application areas of the MAXXtip® screws in the automotive sector
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