What is Metallography – Simply Explained
micrograph creation | microstructure analysis | hardness testing
Metallography is the scientific examination of metals to assess the quality of the structure. These examinations are now an integral part of quality assurance in the production and processing of metals. The quantitative and qualitative microstructure analysis requires a great deal of experience and is usually carried out by trained metallographers. The details of metallography are explained in a simple manner below.
Tutorial Part 1
Tutorial Part 2
Microstructure Analysis | Metallography Preparation
To produce a material of Structural examination This must be done according to the requirements of metallography Only after flawless preparation can a look inside be made using a microscope. This requires separating, grinding and polishing the sample.
For the microstructure analysis the sample must according to the rules of metallography This preparation is a “science” in itself and requires experience and a careful selection of the material used. consumables. To learn the basics of metallographic preparation, it is recommended to take a practical seminar to book.
Incorrect preparation can distort the properties of the preparation, for example through surface hardening or burning of the material. Example: If the steel is ground incorrectly, it turns dark blue due to the heat and burns. Metallography – Preparation is understood as the gentle preparation of the sample: separating, embedding, grinding and polishing, etching and the subsequent quantitative and qualitative microscopic structural analysis or structural examination. The special thing about metallography preparation machines and the creation of micrographs is that this preparation is carried out without heat input (= troughing with extremely large amounts of cooling water) and with low contact pressure during grinding and polishing (sharp abrasives, water cooling, lubricants, diamond suspensions required). In addition, the metallographic sample preparation is often the basis for further investigations. For example, the Hardness test according to Vickers a metallographic sample preparation up to the "mirroring" surface (requirement for the exact optical measurement of the Vickers indentation). In addition, the degree of purity of metals / materials is determined (proportion of foreign particles and contamination). In addition to the hardness test, Weld seam inspection often also be determined: penetration, weld structure, lack of fusion, weld penetration, crack formation, microstructure and A-dimension. In addition, weld seams are measured using Tensile test With a Tensile testing machine checked.
Even beginners in metallography are familiar with non-metallic crystals: e.g. table salt, rock crystal, precious stones (including diamonds). This crystalline form is also present in metals. In order to examine metals, mechanical processing with special metallography preparation machine necessary because it is usually not of interest what the surface of the material is like, but rather the internal structure must be assessed.
Short description: For metallography analysis, the workpiece must be prepared
- Cutting: Using a metallographic wet cutting grinding machine, the workpiece is cut up and possibly sectioned into even smaller pieces using an extremely large amount of cooling water and special cutting discs.
Remember: The more perfect the cut is (smooth, unburned surface), the less effort is required for subsequent grinding and polishing. Anyone who thinks they can work efficiently and roughly with a normal saw or milling machine will be "punished" because the time required for planing and grinding is increased many times over.
- Embed: This can be done in two different ways:
Cold embedding: The sectioned test piece is placed in a plastic mold and the cold investment compound is poured over it. This consists of a two-component plastic (powder and hardener) and hardens after about 10 minutes. As an alternative to a two-component plastic, liquid plastics that harden under blue light can also be used.
Warm embedding: The sample is placed on the flask of a hot mounting press and lowered into the cylinder. Now thermo-elastic plastic granulate is filled in. After closing the cylinder, the hot investment material is heated under high pressure to up to 200. The liquefied Plastic encloses the test piece seamlessly and is then solidified by cooling (water circulation cooling around the cylinder).
- Ribbons: Despite the fact that a test piece has been sectioned very gently and with a good separation surface, it must first be sectioned on a special surface grinder or (more complex) grinding machine + polishing machine (sandpaper 180 or 320 grit or diamond grinding wheel). This surface is then sanded in several steps with increasingly fine sandpaper - often up to 1200 or even 2000 grit.
- Polishing: This final step is carried out using a polishing cloth. A suspension of carrier fluid/lubricant and synthetically produced diamonds is sprayed onto the polishing cloth as an abrasive. The diamond particles, which vary in size depending on the polishing level (6 / 3 / 1 / 0,3 µm diameter), find a hold in the mesh of the fabric of the polishing cloths of varying “coarseness”.
- Etching: Depending on the base material, the structure is etched with a variety of acids. This causes carbon and other particles to be dissolved from the mirror-like surface after polishing. The dissolution of these particles makes the grain boundaries and inclusions visible in the first place.
- Microscopy: Under the light microscope at 50 to 1.000 times magnification (typically and usually sufficient: 500 times magnification), the microstructure formation, the metallic components, their bonding to other components, the crystal formation (e.g. in cast iron: spheroidal graphite / lamellar graphite), grain sizes, grain boundaries and possible heat influences (weld seam testing) are analyzed.
METACUT 302
- Hand lever – cross cut: clamp and start
- cutting discs Ø 250 + 300 mm
- cuts round Ø 90 + 115 mm
- square cuts 50 x 165 or 195 mm
- best cutting machine (!)
- very robust + proven 1000 times
- protective cover aluminum floor assembly
ECOPRESS mounting presses
- ECOPRESS 52 – one press cylinder, electronic control: 1 program
- ECOPRESS 102 – one press cylinder, 25 programs SIEMENS control
- ECOPRESS 202 – two press cylinders, 25 programs SIEMENS control
FORCIPOL grinding/polishing machine
FORCIPOL 102 a grinding wheel
FORCIPOL 202 two grinding wheels
- variable speed
- without head manual preparation
- with head semi-automatic preparation
- grinding wheels Ø 200 / 250 / 300
- easy operation with Quickstart
- affordable premium quality
Metal Microscope NAZAR
- ATTENTION MetLab-C 3240
optics 50x 100x 200x 500x - NAZAR MetLab-C 3245
Optics 50x 100x 200x 500x 1000x - LED lighting
- solid precision mechanics
- 10 year manufacturer guarantee
cutting process with wet cutting machines
In order to be able to examine the material properties inside the workpiece, the test piece must first be sectioned (small piece) / separated / divided. During this separation process, it must be ensured that the separation process does not cause heat to influence the interface and thus cause a change to the material / structure / microstructure. It is therefore imperative that the separation takes place without heat being generated. To ensure this, special separation machines / wet cutting machines are used. The machines specially designed for metallography have a circulating cooling system in which the interface is supplied with a large amount of cooling water during the separation process. In addition, depending on the hardness of the material, special corundum or diamond cutting wheels (for extremely hard material) are used.
Note: For extremely hard materials, cutting discs are used in which the sharp grain (corundum) is bound in a particularly soft material (synthetic resin). The particularly soft material is removed very quickly (abrasive cutting) and sharp grains are released. For soft materials, hard synthetic resins are used to reduce wear (reduce costs).
Although the cut surface already appears to have an almost mirror-like, very smooth surface after this separation cut, further metallographic preparation steps are required. Depending on the requirements of the examination, small segments (the size of sugar cubes) are cut out of large workpieces. The purpose of this segmentation is that these small material sections (segments of a crankshaft, small pins, screws, nuts, etc.) must be embedded so that they are easier to handle during subsequent grinding and polishing (manual preparation) or can be fixed in the holder of a sample mover.
Hot Embedding in Metallography
Small workpieces or segments of large workpieces are usually embedded. In Europe, embeddings with a diameter of 30 / 40 / 50 mm are common. The purpose of this embedding is to make the embedded samples easier to handle. Because the sample shape is uniform after embedding, they can be clamped into standardized holders and automatically ground and polished on grinding and polishing machines. But embedding is also useful for hand preparation in order to be able to grip small samples better and achieve better grinding and polishing results. In addition, injuries caused by abrasives are reduced.
Basically, a distinction is made between cold embedding and hot embedding. In cold embedding, the samples are cast in two-component plastics. A liquid hardener is stirred into a powder and poured over the sample, which is in a plastic embedding mold. After the epoxy resin reacts with the hardener, this initially honey-thick embedding mass hardens within about 10 - 15 minutes. The fully embedded sample is then pressed out of the embedding mold.
Hot embedding is carried out without an embedding mold. The sample is placed directly on the piston of an embedding press. The piston/cylinder has a diameter of 30/40/50 mm. The piston is then lowered approximately 50 mm down into the cylinder. A thermoplastic granulate/powder that liquefies when heated is poured into the cylinder. An additional intermediate piston may be used to embed a second sample at the same time. Once the second chamber has been filled, the cylinder is closed, the thermoplastic is heated and melted. Together with a high pressure, a perfectly embedded sample is created that is free of air pockets and encloses the sample without any gaps.
The embedding time is similar for both methods. However, with cold embedding, several samples can be embedded at the same time by mixing a larger amount of investment material. On the other hand, the embedding material for hot embedding is significantly cheaper and the embedding quality (especially for further processing) is of higher quality. In addition, the end faces of the hot-embedded sample are perfectly parallel to one another (important for microscopy and Vickers hardness testing of hardening depth testing).
A disadvantage of cold embedding is that it can sometimes produce a significant odor. The resulting emissions are often so intense or even dangerous to health that an exhaust fume hood is necessary.
Metallographic grinding of samples
For grinding and polishing, a grinding machine with extremely quiet operation and a water inlet and outlet is required (e.g. METKON Type FORCIPOL 102). Note: The higher the quality of the previous cut (wet cutting machine), the less effort is required for subsequent grinding and polishing. Traditionally, SiC sandpaper (silicon carbide sandpaper) in several, increasingly finer grain sizes is used for planing and grinding. Sandpapers in the following grain sizes are common:
- Sanding for planing: Grit 180, 320
- Grinding for fine grinding: grain 600, 800, less often 1.000, 1.200 or even 2.000
The most common type of sandpaper for grinding and polishing is still sandpaper that is placed loosely on a grinding wheel and secured with a clamping ring. In addition to the clamping, there is additional adhesion through a film of water (adhesion) under the sandpaper. Alternatively, sandpaper with a self-adhesive back can be used. These are usually glued to a sheet metal carrier disc (self-adhesive back of the sandpaper), which in turn is placed on the grinding wheel with a glued-on magnetic film. The magnetic force between the grinding wheel and the sheet metal carrier plate ensures that the sandpaper is held in place sufficiently.
The system has been changing for a few years now: a permanently "sticky" adhesive contact disc made of a special, soft plastic is glued to the grinding disc. The sandpaper coated with a plastic back "sticks" very securely to this adhesive contact disc and no longer needs to be clamped. If the adhesive effect of the adhesive contact disc wears off, it is washed off using pure water and permanently regains this adhesive property. Depending on the mechanical wear, this disc must be replaced - but this is no longer referred to as a consumable.
If the materials have a minimum hardness (>30 HRC), so-called diamond grinding wheels can be used today. These consist of synthetic diamond grains embedded in plastic. As the plastic wears down, new, sharp diamond particles are constantly released - the diamond grinding wheel remains sharp until it is completely used up. The service life of these grinding wheels is significantly longer than with sandpaper (approx. 300 - 1000 sandpapers per wheel and more) and also reduce the effort of constantly changing the sandpaper due to wear (after about 30 seconds, sandpaper is almost blunt - depending on the grain). The use of diamond grinding wheels is particularly useful when planning when extremely large volumes have to be removed, which means that new sandpaper would have to be applied up to 10 times. Diamond grinding wheels with diamonds bonded in nickel can be used for hardnesses of >55 HRC. The rigid bond with fixed diamond grain results in an extremely high removal rate.
Depending on the material, this preparation can require different steps and consumables: Soft steel, copper, aluminum, etc. "always" require the use of sandpaper (with the exception of SiC grinding pads), as these soft materials immediately clog the pores of a diamond grinding wheel. A completely new development (2017) is the use of SiC grinding wheels with SiC particles (silicon carbide) embedded in plastic. These use the principle of diamond grinding wheels (previous paragraph) and allow for permanent use without changing. If the soft metal soils this wheel ("smearing removal of the soft metal"), it can be freed of these contaminants using a standard nail brush.
Manual preparation is still widely used. The (large or embedded) sample is pressed onto the rotating grinding wheel by hand until the desired removal has been achieved (planning). Further processing steps then take place in several grinding and polishing stages using changing, increasingly fine sandpaper/grinding pad or by polishing.
At the beginning we said: “This preparation is a science in itself and requires great experience and careful selection of the consumables used.” At this point we would like to draw your attention to our seminars:
Metallographic polishing
The finest removal of materials is achieved by polishing the surfaces prepared by grinding. Cloths of different densities are used for polishing. Diamond abrasives or polishing agents with synthetic diamonds of different sizes (depending on requirements, grain sizes 6 µm | 3 µm | 1 µm | 0,3 µm) and lubricant are applied to these polishing cloths. Combinations of diamond abrasives and lubricants (so-called suspensions) are often used. It is extremely important when polishing that the samples are cleaned very carefully under water after each polishing stage in order to avoid introducing coarser abrasive particles into the next polishing stage.
The finer the polishing level, the smaller the diamond grains and denser the polishing cloths used. What is new (in 2017) is that diamond grinding wheels (see previous paragraph) can replace polishing cloths up to grain sizes of 6 µm and 3 µm, making polishing even more efficient.
A higher density of the polishing cloths (finer mesh) prevents the smaller diamond grains from sinking completely into the fabric: only the protruding part of the diamond produces the removal power. The aim is a finely polished, scratch-free surface. This is required, among other things, in order to be able to remove the grain boundaries or to make inclusions visible. A similarly fine polishing step is also required for the Vickers hardness test.
Etching of metals for preparation for microscopy & hardness testing
Acid etching is required to make the metal structures of the microstructure visible. Without acid etching, the metallographically prepared surface appears as a mirror under a microscope. This is only desirable if this preparation is used for hardness testing (Figure 1: reflective surface with minor grinding errors and a Vickers pyramid impression).
Only when the sample is etched do the grains, crystalline forms, martensite formation, pearlite formation and the spaces between the grains (grain boundaries) become visible. Depending on the type of metal, different acids / concentrations or dilutions of the acids are used.
Some examples of common etchings:
eagle etching – for welds, coarse grain, “all” steels, low + high alloyed, stainless, cast iron, nickel alloys:
200cm² hydrochloric acid HCl
100cm³ water H²O
60g iron(III) chloride FeCl3 * 6 H2O
12g copper ammonium chloride (NH4)2[CuCl4] * 2 H²O
Etching time: A few minutes – observe the surface
Aim of sample preparation: Microscopy | Hardness testing | Macro-microscopy
Microscopic examinations begin at low magnification. Higher magnifications are used as required. For a structural analysis, magnifications of 50x, 100x, 200x, 400x, 500x are common and sufficient. In rare cases, a magnification of 1000x is required. At 1000x magnification, one is at the limit of the resolution of light microscopes (Wikipedia: Abbe – resolving power)
Especially for the Hardness Testing According to Vickers, finely ground and polished metal surfaces are extremely important, since the evaluation according to Vickers is carried out using a microscope. For example, with the CHD hardness curve, the last step is polishing with approx. 3µm diamond suspension.
A weld seam test requires etching the sample, followed by macroscopic evaluation and hardness testing at defined positions. Welding causes heat to be introduced (heat-affected zone HAZ). This can lead to an increase in hardness, which leads to premature cracking/damage to the connection.
