Chemical, microstructural and thermal analyses

Analysis of the chemical composition

The microstructural and technological characteristics of cast irons and in general, of all metal alloys, depend primarily on their chemical composition. Even small variations in the content of elements such as Carbon, Silicon and Magnesium can completely upset the microstructure and performance of a cast iron. For these reasons, control over the composition is essential and is done using different methods, also depending on the chemical elements to be identified and quantified. The accurate determination of the presence and quantity of the various elements is done spectroscopically using quantometers; the determination of elements such as carbon and sulphur is done by oxidising the sample and detecting the oxides obtained using infra-red cells.
An undoubtedly thorough, but also more time-consuming, procedure is the “wet method”. This type of analysis consists of dissolving the sample in acid and analysing the concentration of elements being investigated in the solution obtained using classic and instrument analysis techniques: spectrophotometry, gravimetry or titration; atomic absorption; optical emission spectroscopy; mass spectrometry.

Zanardi Foundries Product Testing

Microstructural investigations

The microstructure of a cast iron (type and shape of graphite, presence or otherwise of cementite, amount of ferrite, pearlite and ausferrite, but also the possible presence of inclusions and defects) is essential in determining its operating behaviour. Precisely for this reason, it is necessary to systematically analyse the microstructural characteristics of cast irons using various metallographic procedures and instrumentation. Cast iron, after polishing and eventual chemical treatment, is observed under an optical microscope, highlighting and identifying the various microstructural constituents. Using image analysis systems interfaced with the metallographic microscope, it is possible to quantify the percentages of the constituents and the morphological parameters of graphite (nodularity, equivalent diameter, etc.). More specific and “local” information on microstructure and composition can be obtained by scanning electron microscopy (SEM), combined with an EMP microprobe.

Microscopic analysis of cast iron in Zanardi Foundries
Metallography – Macroscopic techniques
Metallography – Macroscopic techniques
Electron microscope

Thermal analyses

The microstructure of a cast iron is progressively developed during solidification and subsequent cooling. Moreover, all phase transformations are associated with the development or absorption of heat, which occurs in addition to heat exchange phenomena induced by the mould and external environment. A cooling curve thus describes the temperature trend from the moment the cast iron starts to solidify until the moment it reaches ambient temperature. Cooling curves can therefore be used to identify the critical transformation temperatures, monitor the genesis of the various microstructural constituents and consequently understand whether the qualitative level of the cast iron being produced complies with the design requirements.

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