Metallographic Cutting for accurate, low-deformation sectioning

Metallographic cutting or sectioning usually marks the first step of the metallographic sample preparation process. In most cases it is necessary because the part or solid body is too large for the following metallographic grinding and polishing steps carried out on a laboratory scale.

Depending on part geometries and material hardness as well as employed analytical techniques, different cutting techniques and consumables are used.

QATM is a leading manufacturer and supplier of high-quality consumables as well as a range of innovative metallographic cutting machines with a choice of manual or automatic axes. The QATM application experts combine decades of experience with thousands of processed materialographic samples and will be happy to assist with your application.

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QATM offers metallographic cut-off machines for any requirement

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Sampling for metallographic analysis

Typical methods to sample materials for microscopic analysis are:

Microtomy (common for polymers and very soft metals, can only be applied for thin sectioning)
Sawing using band saws (low-cost method with low surface qualities)
Wet Abrasive Cutting

Industry and academia commonly prefer wet abrasive cutting, a machine-based process, due to its very low impact on the material's structure, compared to other methods. In the field of metallographic sampling, this usually means the mechanical segmentation of components to be examined.

For this purpose, wheels of different thickness, structures and bonds are used on an appropriate cutter. Normally, they consist of a phenolic resin or rubber bond of a certain hardness and porosity in which aluminium oxide or silicon carbide grains of a certain grain size are mounted.

The following guidelines should be applied to the sectioning process in metallography:

Sectioning by wet abrasive cut-off machine with sufficient cooling and correct rotation speed of the disc for the application. Otherwise, the deformation in the surface is so high that it cannot be rectified without long grinding steps.
Optimum cooling is indispensable for evaluation. An excessively heated surface gives rise to misconceptions – artefacts simulate a false result.
A fitting corrosion protection agent must be added to the coolant, otherwise there is a risk of surface oxidation at the surface. This is also important to protect the interior of the machine.
The size and design of the cutter depend on the sample size – the main criterion is the so-called cutting capacity.

Metallographic cutting machines

In metallographic sample preparation, mostly abrasive cut-off machines are used for sectioning of workpieces. Optimum cooling is just as important for the sectioning result as precise movement. Common cooling media consist of a mixture of a boric and amino acid-free corrosion protection agent with water. An appropriate concentrate is diluted with water in a ratio of e. g. 1:30. The manufacturer's instructions should be carefully observed. Cleanliness (generally the top priority in metallographic sample preparation) must be ensured for the inside of the chamber as well as the draining system – this influences the precision and the service life of the machine.

The different metallographic sectioning techniques

_{Producten overzicht: Metallographic cutting machines}

Traverse Cut

Two versions are possible in this case:

The sample table with the workpiece previously fixed on is moved to the fixed wheel manually or automatically.
The wheel is moved accordingly.

Chop Cut

The workpiece is firmly fixed on the table. The wheel is then manually or automatically guided from top to bottom into the fixed workpiece.

Step Cut X

The workpiece is not sectioned in one step, but in layers (stepwise). This type of sectioning is particularly suitable for large, thick workpieces of solid material.

Step Cut Y

The workpiece is not sectioned in one step, but in layers (stepwise). This type of sectioning is particularly suitable for large, thick workpieces of solid material that must be clamped on edge (plates, angles, etc.).

Diagonal Cut

In case of a diagonal sectioning, the table moves backwards (X axis) while the wheel moves downwards (Y axis). This type of sectioning is particularly suitable for asymmetrical workpieces, or for compensating the wear of the wheel in order to obtain a constant cutting depth.

Zig-Zag Cut

The workpiece is not sectioned in one step, but in layers (stepwise). The zig-zag cut is made in the middle of the component.

Care Cut

During manual operation, the clamping table (X axis) moves back and forth until the cut is completed. It starts with the movement from front to back. This type of sectioning is particularly suitable for workpieces where a clean edge and material-friendly processing (temperature, deformation) is required due to the smallest possible contact surface.

Rotation Cut

This process is most often used on precision cutters. It can also be used for large workpieces with corresponding rotary sample holders. The sample is turned clockwise or anti-clockwise towards the cut-off wheel. Quarter or half rotations are also possible. For a round workpiece with a diameter of 50 mm, only a cutting distance of 25 mm is required.

The area between sectioned workpiece and the wheel should be as small as possible.

Large contact zone

Small contact zone

How to select a suitable cutting wheel

As a rule of thumb, the following principles help to choose the right material:

For hard materials, a soft bond is recommended
For medium-hard materials, a medium-hard bond is best suited
For soft materials, a hard bond should be used

Most manufacturers specify either a manufacturing or best-before date. This must be observed since the binders of the bond are hygroscopic, i. e. they absorb water. Water in the bond can lead to embrittlement of the material after a few months or years. In this condition, the wheel is still usable, but with significantly higher wear and risk of breakage.

For very ductile materials, such as titanium or plastics, wheels with silicon carbide as abrasive grain (also resin-bonded) are generally used.

Diamond is used for sectioning very hard materials (ceramics, glass fiber reinforced plastics, minerals, glass, etc.). Here, two main bond-types exist: metal-bonded where the diamonds are usually mounted in a bronze bond, or those with a synthetic resin bond. The latter are particularly suitable for very brittle materials.

Cut-off wheels, in comparison to saw-blades, generate smoother and less deformed surfaces and breakouts on their entry and exit locations. This strongly reduces the thickness of the deformation layer, which has to be removed through metallographic grinding.

Consumables for materialographic cutting

Cutting wheel materials used in metallography

Wear (i. e. abrasion resistance), service life and cutting performance are important quality factors. Abrasive agents (aluminium oxide, silicon carbide, diamond, cubic boron nitride (CBN), etc.) and bonding (metal, synthetic resin or rubber) are characteristics composition. The following tendencies can be taken into account for choosing the appropriate material for a specific application:

Aluminium oxide (Al₂O₃) with synthetic resin bond for all steels. Depending on hardness, optimized binders are used.
Silicon carbide (SiC) for soft and medium-hard non-ferrous metals as well as hard non-metallic materials (glass, stone).
Diamond (C) for hard ceramic materials, ceramic composites and geological materials (rocks).
Boron nitride (CBN – Cubic Boron Nitride) for hard and tough Co- and Ni-based alloys, hard metals, hard composite materials, and hard to very hard carbon steels.

Grain size of the abrasives

The grain size of the abrasive in the cutting wheel is an aspect that must not be neglected in metallographic sample preparation. It should be between 45 and 180 µm and will result in a surface that allows further metallographic preparation without removal-oriented planar grinding, if all parameters are set correctly. A very fine grain size is recommended when using thin wheels. A fine grain size is also important if burr-free sectioning is required. The degree of hardness of the wheel itself does not impact the hardness of the abrasive grains, but rather the hardness of the bond, i. e. about the resistance of the individual abrasive grain to breaking out of the bond material.

A softer bond should be used for large-area contact points. This also ensures that worn abrasive grains are broken out. This type is also well suited for heat-sensitive materials.
In the case of smaller contact areas, a harder and more stable wheel material is suitable. Those are also used for sectioning tubes and profiles. The advantage here is that it has less contact with the workpiece since the wall thickness is usually small.

Typical errors in metallographic cutting

Correctly selected tools and settings will produce edge shapes like this:

In case of solid material, rounded edges indicate the desired result.

Sharp-edged corners are the sign of a correctly selected wheel for solid or profiled material as well as tubes of medium wall thickness.

Concave corners are the expected result when sectioning thin tubes and cross-sections.

Incorrectly selected tools or settings form the following edge shapes:

If the edges are sharp, an excessively hard wheel material was used. The tapering effect can cause breakage and burn marks.

Bevelled edges indicate incorrect or insufficient cooling. This can cause thewheel to lateral run-out and shorten its service life.

Glazed corners are formed if the abrasive grit is not broken out and the wheel may get clogged. In addition to an incorrectly selected material, excessively slow feed rate or insufficient rotational speed may also be the cause.

QATM Producten & Contact

QATM offers a wide range of innovative machines for metallographic cutting, from manual machines 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!

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