Structural Analyses Automated Grinding and Polishing of Metal Samples

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In metallography, metal samples are meticulously prepared, and their microstructure is examined during structural analysis. A grinding and polishing machine can also automate the sample preparation.

In the steel mill, a temperature sensor is defective, causing the oven temperature to be too low and the heat treatment of the material to be incorrect. As a result, the metal's structure is flawed, despite the raw material being in good condition. The issue goes unnoticed, the components pass the visual inspection, and are installed in vehicle suspension systems. Months later, cracks appear: the consequences are tens of thousands of recalls, a damaged reputation for the manufacturer, and massive costs. To avoid such a scenario, all process steps in metal production are monitored, as every processing stage has the potential to cause errors.

The supplier must provide proof of quality assurance, for example, as part of audits. Quality testing of metals not only includes tensile and tearing tests; metallography (microstructure analysis) is used to verify that the crystalline basic structure of the material meets the requirements after different manufacturing steps or whether a faulty heat treatment has caused undesirable changes.

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Technically used metals are alloys, mixtures with varying proportions of different elements. These are crucial for quality. Metallography examines whether the metal has solidified in the correct structure or specific arrangement, ensuring that iron has become qualified steel and successful hardening can be verified. For this purpose, the microstructure of the metal is analyzed under a microscope, including the type and size of grains, their number and boundaries, and the structure types. The analysis provides insights into manufacturing processes and material properties and detects defects such as cracks, segregations, shrinkage cavities, or inclusions.

The Sample Preparation

For metallography, metal samples must be prepared. In the first step, the metal sample is divided using a special cut-off grinding machine. To prevent spark formation and the extreme heat released from affecting the material properties, special cutting discs and a cooling system are used to keep the surface cool.

Afterward, the sample, roughly the size of a sugar cube or with an edge length of about ten millimeters, is embedded in plastic for fixation, ground, polished, and then treated with acids, meaning etched. The acid removes particles such as carbon, making the grain boundaries of the metal visible under the light microscope.

The manual preparation of samples entails potential errors: the human hand is not a stand, and for grinding, the operator must hold the sample in their hand, which is not an exact position. Each employee works differently, applies more or less pressure, and doses the suspensions needed for grinding differently.

If the sample is ground at an angle, the grind is not flat but convex, this affects the evaluation. The metallographer works with a magnification of 500 to 1000 times. The depth of field is correspondingly tiny and less than a micron. If the sample is convex because of unsteady hand movement during preparation, the necessary image sharpness is not achieved. A sample prepared in this way leads to incorrect results in the examination. The metallography is then not reliable.

Machine Sample Preparation

The grinding and polishing of metal samples can be automated with the Velox 102 and 102-JR grinding and polishing systems from Schütz+Licht Prüftechnik GmbH. It takes over all process steps from planar to fine grinding, polishing and fine polishing, cleaning, and drying.

The samples embedded in plastic are placed in the sample holder of the machine, which, depending on its configuration, holds four or six samples. This allows the processing to begin. The clamped samples go through various grinding steps, from 180 to 320 to 800 grit. The necessary discs are changed automatically, and the cleaning of the samples is also part of the process: the particles from the coarse grinding must not contaminate the sample, so the samples are cleaned with spray water and immersed.

A sample surface that is so smooth it reflects is not sufficient—therefore, the next step involves polishing, for which differently woven cloths are used. Depending on the type of metal, up to six polishing agents are used in the process. The next processing step before microscopy, etching, is then carried out manually again.

The Advantages

Foundries and steel mills must prepare and examine numerous metal samples for quality assurance. If approximately 30 to 40 grinding samples are produced daily, machine preparation saves significant time and costs: preparing samples individually takes about 15 minutes per sample. While the machine prepares multiple samples simultaneously, the employee can already begin microscopy; their working time is no longer tied up with simple tasks.

This allows the machine to partially compensate for the shortage of skilled workers. It can operate during an unmanned night shift, and in the morning, the desired number of samples is ready for use. Preparation can thus be automated, and sample preparation standardized, which halves the costs of quality assurance. With daily utilization, the investment in a grinding and polishing station pays off within two years at the latest.

Additionally, the reproducibility of the grinding quality no longer requires a highly skilled operator—the machine reliably produces the same sample quality. As a result, the outcomes are no longer dependent on the individual but are defined by the input parameters—everyone has the same conditions, ensuring identical surface quality. The machine also contributes to sustainability by optimizing the use of consumables and grinding materials and minimizing water consumption. Only as many resources as necessary are used, thereby reducing pollutants to a minimum.

Quality assurance of metal samples is a central component of industrial manufacturing, as any defect in the microstructure or material properties can lead to significant follow-up costs. The combination of precise sample preparation, microscopic analysis, and machine assistance ensures reproducible results, reduces sources of error, and helps use resources more efficiently. In times of labor shortages and increasing demands for sustainability, the automation of metallography offers an economically and qualitatively viable solution.

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