Metallography is the science which research into the metal materials’ structure in connection with their chemical – physical and also mechanical properties.
It develops into two phases: the preparation of the metallographic specimen for the view under the microscope and observation the study of the specimen’s structure.
PREPARATION OF THE METALLOGRAPHIC SPECIMEN
To obtain a metal surface suitable for the view under a metallographic microscope, a sequence of operations are necessary:
- Metallographic cut
- Metallographic polishing
- Use of the acid to highlight the specimen’s structure
These are the 5 operations which make the specimen’s surface extremely polished and shining.
Let’s speak about every single step.
The metallographic cut is one of the most important and delicate phases of the specimen’s preparation. The cut must be precise and the obtained surface must not be overheated, otherwise the specimen’s structure will be irreparably changed and establishing its real characteristics will be impossible.
Of course the use of a good metallographic cutting machine like those supplied by companies who deal with metallographic equipments and consumables (for instance Winter, Struers, ATM, Skyward, Presi, Metkon, LamPlan, Buehler, Leco, Remet, Cloren Technology and so on) is an essential requirement.
The metallographic cutting wheel must be installed inside the cutting machine. The cutting wheel consists in different kind of abrasives (AI203, aluminium oxide or corundum, silicon carbide or SIC, cubic boron nitride or CBN, diamond), it depends on the kind of metal to be sectioned. Sizes of the metallographic cutting wheels are standardized, therefore they’re compatible with all cutting machines.
Another important aspect of the surface, to be obtained, is the level of roughness. Lower is the roughness value (Ra), sleeker is the surface and reaching the final result of the preparation of the metallographic specimen is easier.
For this reason, the choice of the right cutting wheel is a decisive aspect .
It’s the practice to protect the borders of the metallographic specimen (most of all if it has suffered a treatment of superficial hardening). It consists in the drowning of the specimen into a mixture of fluid resin.
Once the resin hardens, we obtain a metallographic specimen which is more compliant and protective than the metal to be worked.
This operation can be carried-out through the use of a metallographic mounting press (in case of immediate incorporation) or through the use of silicone moulds (in case of cold working).
As for the metallographic cutting, also for the incorporation the use of several kinds of resins is necessary (phenolic, epoxy, glass-fiber, acrylic). Obtaining the optimum adhesion between the resin and the specimen’s borders is important to protect its surface.
Smoothing or pre-smoothing
The surface of the metallographic specimen , deformed, must be polished using several times the abrasive. Only some slightest alterations will remain on it . Then everything will be completely leveled during the following operation of polishing.
During the smoothing the use of a metallographic grinding machine is necessary. The operator uses silicone-carbide paper abrasive wheels (SIC) mixed granulometries (P80, P120, P240, etc.). The dimensions of wheels change depending on the quantity of metallographic specimen to be smooth at the same time. Anyway sizes are standard to allow their use on metallographic equipment of several brands and models (Struers, ATM, Skyward, Presi, LamPlan, Metkon, Leco, Buehler, Remet, Clore Technology and so on).
As already told, this phase is the sequel of the previous operation. The metallographic equipment to be used is the same used to smooth the surface (a grinding machine). The difference compared to the previous phase are the polishing cloths used. They are available in several and different materials and kinds (silk, polyester, disposable fabric, waved, staple, etc.). The kind of cloth must be chosen depending on the kind of metal to be polished, on the abrasive’s granulometry and of course on the surface’s finishing to be obtained.
The abrasive’s granulometries used during this phase are decidedly lower (¼ - 45 micron) if compared to those which can be found on the SIC abrasive wheels.
A reflecting surface is obtained after several polishing operations using cloths with a more and more thin abrasive, properly lubricated with metallographic fluids.
After the last operation (the use of the acid), the specimen will be fit for the view under a metallographic microscope.
Use of the acid to highlight the specimen’s structure
Even if well smooth and polished, the view of the specimen’s structure under the microscope is often difficult.
The process with the acid is therefore necessary to point out the structure’s architecture. The dipping of the specimen into the acid (of different compositions depending on the material’s nature: Nitral, Picral, Keller, Cogne, and so on) for a few seconds is enough.
At the end of the process, after the rinsing and the drying, the specimen is finally ready to be observed under the microscope.
OBSERVATION AND STUDY OF THE METALLOGRAPHIC SPECIMEN
Thanks to the study of the metal’s structure we can understand its reactions during and after a possible working (mechanical, heating treatment, chemical treatment). The main features to be studied are: the dimensions of the crystalline grains, the inclusions, the structural orientation, the precipitates, the phases.
Once defined the structure of the studied specimen we have the possibility to change some of its features to improve its physical, chemical or mechanical performances (addition or change of the chemical element’s percentage, treatment of surface hardening , etc.).
The specimen’s observation is also necessary to detect possible fractures, to identify the causes of a mechanical detail’s breaking or its wear and tear. It’s also important because it makes possible the failure analysis (the study of the deterioration which is the cause of a material’s breaking).