Metallographic Grinding Mechanical Sample Preparation

Metallographic grinding is one of the process steps in mechanical sample preparation for subsequent microscopic examination. The principle is based on the fact that abrasive particles in successively finer grain sizes are used to remove material from the surface until the desired result is obtained. These particles are present in bonded form during grinding and in unbonded form during polishing.

The quality to be achieved depends on the application purpose. It is not always necessary for the true structure to be available in the highest degree of perfection – often a satisfactory result is sufficient to enable subsequent testing (e. g. a hardness test).

In any case, the metallographic preparation must be carried out systematically and reproducibly. This is the only way to achieve optimum results at the lowest possible cost.

In metallography, a distinction is made between macro and micro grinding.

Product Overview: Metallographic Grinding

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	Macro-section	Micro-section
Fault size	>100 µm	<100 µm
Surface	Non-processed or ground	Fine-ground and polished, etched if necessary
Microscope	Stereoscopic microscope	Light microscope incident light, bright field
Examples	Cracks, shrink holes, heat-affected zone	Non-metallic impurities, layers, transformation micro-structure, grain boundaries
Hardness measurement	Macro-, small-force hardness	Small-force, micro hardness

A macro-section is understood to be the preparation of a sample for observation and evaluation, either without any microscopic aid or under a stereoscopic microscope with a magnification range between 6x and 60x, e. g. for welded and soldered joints.

A micro-section is understood to be the preparation of a sample for observation and evaluation under a light microscope with Köhler's illumination in the magnification range from 50:1 to 1000:1, i. e. for classical structural analysis.

This distinction is useful because a finely ground surface is usually sufficient for evaluation of a macro-section. For a micro-section, a deformation-free and polished surface is essential.

Metallographic grinding is divided into three separate operations:

Planar - grit P60-P120
Fine - grit P320-800
Ultra-fine - grit P1000-P4000

In the metallographic preparation process it must be ensured that the grit levels are selected such that a good grinding result is achieved. The next step (grit) should rectify or remove the roughness of the previous step in a reasonable time.

Metallographic planar grinding

The aim of metallographic planar grinding is to remove roughness from the metallographic cutting process and to achieve flatness of the ground surfaces.

The abrasive itself depends on the material to be ground. For soft materials, this is essentially silicon carbide (SiC) as paper, foil, disc or stone. For harder materials ( 300 HV, e. g. hardened steel or ceramics), diamond discs are however used. These are usually fixed on a magnetic carrier disc. One advantage is that the achieved flatness is maintained from the beginning to the final polishing. Furthermore, several steps are not required. For larger samples and sample quantities, an aluminium oxide-based (Al₂O₃) stone can also be used. It must be noted that a special stone grinder is required.

Metallographic fine and ultrafine grinding

The surfaces produced with finer-grained silicon carbide paper already have only small residual deformations that can be removed directly by polishing.

Alternatively, so-called fine grinding discs can be used for metallographic sample preparation. These work by adding diamond suspension and lubricant. This is especially helpful for thin coatings, nitrided layers and decarburized edges to ensure an optimal evaluation after polishing.

With special diamond grinding discs, it is now also possible to grind down to an area that was previously reserved for metallographic polishing. Discs with down to 3 µm diamond particles are available. With suitable materials, this is a possibility that should be considered from an ecological and economic point of view.

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QATM offers a wide range of innovative machines for metallographic grinding, from manual grinders to fully automated systems. Accompanying QATM consumables are thoroughly tested and selected for perfect interaction with our machines. Contact us for a consultation, quote or to talk to one of our dedicated application specialists!

Metallographic Polishing - Product Overview