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Metallography deals with the relationship between phase diagrams, structural constitution and the properties of metals and alloys. The task of metallography is the qualitative and quantitative determination of the structure of metallic materials. Metallography is used in the area of material development and for the monitoring of ongoing production. Together with scanning electron microscopy it belongs to the most successful examination processes for the detection of processing flaws and damage causes of metallic materials.

Performing metallographic testing
The preparatory steps required for the metallographic assessment of the material conditions include specific sample taking, grinding and polishing and, if applicable, partial etching of the samples.

Sample taking
Already sample taking must be performed properly to prevent heating and deformation which may result in structural changes. Wet abrasive cutting is a suitable sample taking method. A motor-powered cutoff wheel, consisting of a cutting means with certain grain size and binder cuts the material through abrasive cutting using a coolant.

Embedding
To facilitate handling and to maintain the marginal sharpness the samples are embedded in plastic after cleaning them in the ultrasonic bath. A distinction is made between cold and warm embedding.
Different types of cold-curing embedding plastics are used with cold-embedding.
With hot-embedding thermoplastics or thermosets are partially enriched with fillers, e.g. glass fibres, and processed under pressure and heat in embedding presses.

Grinding and polishing
The samples are ground by hand or using automatic preparations with abrasive paper or other suitable removal methods. Heating of the sample is to be avoided also during grinding.
Polishing is done with polishing disks covered with suitable polishing cloths. The polishing agents, preferable diamond and aluminium oxide, are applied to the polishing cloths.

Light-microscopy evaluation
After polishing the generated polished metal section can be assessed with light-microscopy.
The determination and analysis of non-metallic contaminations of a material, such as oxide, silicate, sulfide, etc. is made on the non-etched polished face.

Etching is generally required to develop the microstructure. Etching agents are prepared for the different materials whereby a distinction is made between etching of the grain boundary and the grain surface.

The structure is evaluated with a reflected light microscope. Apart from bright field viewing the contrast methods dark field, polarisation contrast and differential interference contrast are available.

Material flaws such as pores, bubbles, segregation, rolling and forging flaws, microcracks, etc. and deviations from the preferred structure condition, e.g. surface decarburization, coarse grain structure, surface oxidation, grain boundary carbide deposits, etc. are identified.

Microhardness test
Apart from the common hardness testing methods (Brinell, Vickers, Rockwell) microhardness testing according to Vickers is mainly used in metallography. The hardness measurement of thin layers or individual structure sections is possible by applying small test loads. The hardness depths generated through surface hardening are measured on the microsection and represented in hardness progression curves.

Portable component metallography
If components in operation, which would sustain unreasonable damages by cutting out a sample, are to be subjected to metallographic assessment the portable component metallography via structure impression technology is applied.
The tests are carried out on site. The section is generated in the lab through grinding, polishing and etching. This is followed by making an impression of the structure using the replica technique and analysed under the light-optical microscope.
The light-microscopy analysis can then provide information on time-fracture damages, microcracks or structural changes.

Damage analysis
With damage analyses a statement is made on the type of breakage. Furthermore, the cause of the breakage is ascertained. The condition of the material and the stress status are important here.
The microscopic damage patterns must be captured for a useful damage analysis.
This is followed by a microfractographic analysis on the scanning electron microscope, however, which is usually complemented by metallographic analysis.

Material analysis / material identification
INCOS GmbH uses spectroscopy devices of Spectro Co. and X-ray fluorescence devices by Niton Co. for the material analysis or material identification (PMI). The tests primarily conducted on site are performed with a Spectrotest or a Niton XLt 898.


With spectroscopy the qualitative and quantitative chemical composition is determined using a spectral analysis or an excited specimen. Excitation can take place either through spark or ark discharge. This moves electrons to an electron orbit further away from the nucleus from which they fall back to their original orbit after a very brief residence time. Light is thereby emitted which is characteristic for the respective atom and electron orbit. The chemical composition can be determined through splitting and analysis of this light.


The device listed above is used to analyse alloys on Fe-base and Ni-base. Using special adapters this is not only possible on plane surfaces but even on fillet welds. To prepare the sample the surface must be free from interfering layers (paint, scales, rust, etc.).


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