Fastener Testing

Schlötter, together with iChemAnalytics, has developed a test rig that reduces testing time for hydrogen embrittlement, with the ability to test up to 20 items simultaneously – in just 180 seconds.

The market for fastening parts is increasingly demanding high strength materials and special attention must be paid to possible hydrogen embrittlement when electroplating them – because when you increase the material strength, the risk of hydrogen induced brittle fracture also increases. 

For this reason, Schlötter, together with iChemAnalytics, has developed a new practical test rig that enables versatile C-body testing, compliant with the latest standards, in a fraction of the time otherwise required. The WSRME TWIN precision measuring device uses C-ring samples to detect changes in the material condition that can occur, for example, due to liquid metal-induced, hardness, tempering or hydrogen embrittlement. Comparable tests used to take several days, however, thanks to the special design and precise testing concept, it now only takes two to three minutes. This enables process manufacturers and contract coaters to independently integrate standard compliant work, as well as genuine process monitoring, and quality assurance, into their own production without having to rely on external testing service providers. 

The new test method from Schlötter is based on a mechanical load test where force displacement curves are recorded. The individual measuring cells are provided with specifically parameterisable measuring programs that can be called up in parallel. Thanks to the high-resolution force sensors, and very precise measurement curves, high-quality characteristic values can be reproductively generated and used as a basis to further develop in-house processes and products. 

The WSRME TWIN precision measuring device contains two test field openings, as well as several electric drives with adjustable feed to precisely implement the desired load levels of the test procedures. Depending on the variant, up to 10 measuring cells can be equipped with a maximum of 20 C-rings, which can be measured in parallel and independently of each other.

Depending on the requirements, test specimens individually tailored to components or processes can also be manufactured and used. Various load and positioning options are simultaneously available to the operator – from force or distance controlled stepwise loading to defined holding times with subsequent lifting at a constant stretch rate. Schlötter explains that this saves a lot of time that would otherwise be needed for testing in a row. In the course of this, certain characteristic values are automatically determined and monitored, which result from the freely selectable measuring methods. At the same time, high-resolution force sensors and precise displacement measurement ensure optimum data quality.

Thanks to the integrated evaluation algorithms, which are controlled by a powerful microprocessor, the measuring device can measure or calculate a multitude of material-specific characteristic values, such as the transition from elastic and plastic material behaviour, the hydrogen induced material deconsolidation, as well as a material specific deformation index for the process. The measurement and characteristic values generated in the process are automatically linked and stored with the other metadata, such as pre-treatments and material conditions. “A high statistical significance is achieved by parallel testing on several measuring stations at the same time. This allows valuable conclusions to be drawn about how the component quality or the composition of the starting steel can be improved for machining alone,” explains Thomas Haberfellner, head of the chemistry and development of new markets business unit at Dr Ing. Max Schlötter GmbH & Co KG.

All measured values are stored in a database and can be imported into programs such as Microsoft Excel. These results can help customer specific products be developed or the existing portfolio optimised, such as additions to coating baths (e.g for electroplating) or new ‘bath additives’ (e.g pickling inhibitors). Schlötter is already successfully using the test rig for this purpose in its own research and development. The results that are quickly accessible with the new measuring technology has enabled Schlötter to develop the in-house SLOTOCLEAN BEF 1790 pickling degreaser, which has a new generation of inhibitors. These not only maximise the inhibition value, but also minimise the process related tendency to hydrogen embrittlement – while more than tripling the service life of the pickle compared to the first generation systems. This makes an active contribution to conserving resources and reduces the CO₂ footprint in contract electroplating. 

“With the material properties of high strength steels increasingly being optimised to become lighter and more resistant. Cathodic corrosion protection applied by electroplating is often used to give these steels the best protection against corrosion. A pickling step must be carried out during the pre-treatment of the component to achieve the best possible adhesion of the coating,” explains Dr Michael Zollinger, managing director of Dr Ing. Max Schlötter GmbH & Co KG. 

During the pickling process oxides and scales are removed, resulting in a metallically flawless surface for the subsequent coating process. However, if the acid used in pickling attacks the base material, hydrogen is also produced in atomic form. Physical processes can cause parts of this hydrogen to recombine to form non-diffusible molecular hydrogen on the surface or diffuse into the material. The storage of atomic hydrogen weakens the metal lattice. Depending on the alloy, the component can eventually become brittle and, in the worst case, this stressed state can lead to what is known as brittle fracture. 

“The pre-treatment and subsequent coating of higher strength steel components, with a tensile strength greater than 800 MPa, therefore repeatedly poses the question to both specialist companies and users of how high the maximum hydrogen concentration in the component may be for material damage to be avoided,” mentions Michael. “To date, despite numerous scientific papers, no user-friendly in-process testing methods have become established for the user in practice; any such methods are often time consuming and complicated.” 

He concludes: “The WSRME TWIN precision measuring device not only makes in-process testing for hydrogen embrittlement easily accessible to every manufacturer and contract coater. The versatile parameter recording, and automated evaluation, also help to reduce process costs in the long-term through targeted material selection, the optimisation of heat treatment parameters and the use of chemicals in-line with requirements.”