Metallography. New laboratory launched

Secrets of inner metal structure revealed

Our Metallography Laboratory, already set in operation as part of Rawlplug’s Research and Development Department, features comprehensive equipment and instrumentation allowing them to prepare samples (cutting, mounting, grinding, and polishing) and test them (macro and microstructure analysis, hardness testing) with the highest level of accuracy. It is noteworthy that adequate preparation of metallographic specimens is of paramount importance to reliable results. 

Equipment enabling preparation of metallographic specimens

• Automatic feed cutting machine

Cutting is the first stage in the preparation of metallographic specimens. It is crucial for further processing of samples, while having an effect on the examination itself. When not done properly, it makes the preparation process significantly longer. It can also cause formation of artefacts, i.e. induce changes to the specimen structure (consequently, the test results obtained later will not reflect the actual structure or properties of the material tested). 

With manual feed cutting machines in use, any change to the pressure applied by the cutting disc on the specimen causes faults in the cross-section, making it necessary to extend the grinding and polishing process. This problem is no longer the case in automatic feed cutting machines. A professional metallographic cutting machine, such as the one we have installed at our laboratory, must also feature a system for in-cutting sample cooling, making it possible to avoid the formation of a thermally transformed layer. This enables the actual microstructure of the metal the test piece contains to be identified without modifying it during the preparation process. Interestingly, the quality of cutting with highly specialised equipment is comparable to the outcomes of time-consuming grinding with abrasive paper 200–300 in grit size.

• Press for hot in-resin mounting of specimens

After cutting, specimens are taken to a hot mounting press which allows setting them in resin discs with a diameter of 40 mm. Thus prepared, the specimens are standardised in terms of dimensions, making them perfect for mounting in the press holder as well as for high-precision grinding and polishing.

• Grinding and polishing machine with sample holder and individual clamping mechanism

Polishing is usually the longest and most demanding stage in the process of metallographic specimen preparation. It frequently follows grinding with 200–300 grit size discs. An important factor affecting the quality of this process is making sure that the sample is pressed evenly against the polishing disc with uniformly distributed force, since only this guarantees that the material’s inner grain structure has been cut exactly across. Otherwise, the information on the grain size and composition (and consequently the material properties) will be distorted. Such an outcome is very difficult to achieve with manual polishing, while it is much easier when using a polishing machine with a sample holder and automatic clamping mechanism. 

The duration of the polishing process and the number of polishing stages is determined by the choice of tests to be performed on a given specimen. For macro-hardness testing (e.g. Rockwell C scale hardness – the HRC method), grinding alone is sufficient. Samples for micro-hardness testing (e.g. by the Vickers method) should be polished using a 3 µm diamond slurry. The final polishing step in the case of samples intended for microstructure identification and observation should involve using a suspension of oxides with grain 0.25 µm in diameter (such a surface is as smooth as a perfect mirror). 

Testing equipment

• Micro- and macro-hardness testing machine

Micro- and macro-hardness tests are typically conducted using two separate devices. Micro-hardness testers feature high-precision optical systems and a motorised table, but they only allow small loads to be applied on the indenter. Macro-hardness testers often have no optics and no moving table at all, but they make it possible to apply considerable loads on the indenter. 

The hardness testing machine on stock at our laboratory, on the other hand, combines all the aforementioned features. It is capable of generating a very wide range of forces (from 50 g to 250 kg), enabling hardness testing by a variety of methods: Vickers (even from HV 0.05), Rockwell (HRC – 150 kGf), Knoop, or Brinell’s. Among the wide range of characteristics, it can be used to test case hardness and depth of carburising, e.g. in the R-LX concrete screws or the R-HLX induction-hardened screws, but also in the R-PTX wood screws or the R-FSM screws for plasterboard fixing to steel profiles. 

Micro- and macro-hardness tests help us determine a number of relevant characteristics, including the mechanical properties of the final product, as well as the capacity of screws to tap the substrate in which they are to be set. 

• Digital microscope with up to x1,000 of magnifying power

It allows us to examine everything that cannot be examined with the hardness testing machine, and the microstructure of steel (and its type) in the first place. Its automatic grain count function enables very rapid and accurate determination of the share of individual phases in the material tested as well as estimation of the carbon content in steel. Making it possible to run observations and measurements at very high magnification, the device shows all pores and reveals all defects. It also allows us to check whether a given test piece has been adequately heat treated (read more about the new microscope in the dedicated article).