Metallographic, thermal and elemental characterization of metals

The properties of metal alloys are determined by transformation through thermomechanical treatments. Controlling the effects of these treatments and managing their transformation processes requires knowledge of microstructures and how they evolve.

Two types of data provide access to this information. They are measured using two categories of methods:

• analysis of physicochemical transformation points: thermal, dilatometric, and magnetic analyses, as well as quantitative metallographic methods; these are described in this article;

• Analysis of microstructures using optical or electron microscopy and related techniques [M 91] .

Quantitative methods can be used to determine chemical components, but also, to a large extent, the metallographic structures present and their proportions. The most widely used laboratory equipment for metal analysis is undoubtedly the spark spectrometer (optical emission spectrometry, OES). Measuring the residual austenite content after quenching steels has become possible thanks to the X-ray diffraction measurement technique.

The analysis of transformation points makes it possible to construct phase diagrams or identify the phase transformations of an alloy whose diagram is known. DSC (differential scanning calorimetry) and dilatometry are the leading techniques in this field. Current equipment can often reach temperatures of around 1,500°C, which, in the case of DSC, allows the solidus and liquidus to be identified for steels and many other metals.

Metallographic, thermal, and elemental characterizations of metals lead to the optimization of their manufacturing processes, but also, and above all, to the optimization of their performance and, ultimately, to the maximization of the product, from both a technical-economic and environmental standpoint.