Micro Analysis / Examination

Micro analysis (microstructural examination / metallography) is a destructive testing method based on revealing the internal structure of a metallic material by examining its polished and suitably etched surface under an optical microscope, generally at magnifications of 50–1000X. The physical basis of the method is that different phases, grain boundaries and discontinuities reflect light at different angles or react at different rates to the chemical etchant. During etching, grain boundaries and second-phase regions are attacked selectively; this difference in topography and colour makes the grain structure, phase distribution, precipitates and micro discontinuities visible under the microscope. The manufacturing, heat treatment and welding history of the material, which cannot be read from its external appearance, thus becomes directly observable.

The reliability of the examination depends largely on the quality of specimen preparation. A small section cut from the region to be examined is generally mounted in cold (acrylic) or hot (bakelite) resin to preserve edge retention and ease of handling. The specimen surface is then flattened with progressively finer abrasive papers (for example from P240 to P1200/P2500) and brought to a mirror finish with diamond suspensions or alumina/colloidal silica. The aim at these stages is to obtain a surface that represents the true microstructure without leaving scratches or a deformation layer; faulty preparation can create structures that do not actually exist (artefacts) or conceal those that do.

The polished surface is etched with a reagent suited to the type of material. The most common example is nital (nitric acid + alcohol), used for carbon and low-alloy steels; different reagents are preferred for stainless steels, cast iron, copper and aluminium alloys. The examination is carried out with a metallurgical (reflected-light) microscope, which allows the general structure of the specimen to be scanned at low magnification and the region of interest to be examined in detail at high magnification. The illumination technique (bright field, dark field, polarised light, differential interference contrast) can be varied to accentuate phase distinctions. Results are documented with a digital camera and image analysis software; where measurements are to be made, the microscope is calibrated with a certified stage micrometer.

The information obtained by micro analysis answers different engineering questions depending on the application. Grain size measurement (by the comparison or intercept method) is used to predict the strength and toughness behaviour of the material. Phase proportions such as ferrite–pearlite, martensite, bainite and austenite show whether the heat treatment has been applied correctly; carbide precipitation at grain boundaries or sensitisation indicates a corrosion risk. The type and density of non-metallic inclusions (sulphides, oxides) in steel, the form of graphite in cast iron (spheroidal, lamellar), and porosity and segregation in castings can all be evaluated. On weld seams, micro analysis verifies the suitability of the welding procedure by revealing the change in structure between the weld metal, the heat-affected zone (HAZ) and the parent material, together with grain coarsening and micro cracks.

The findings are evaluated qualitatively (identification of the structure) or quantitatively (grain size number, phase percentage, inclusion rating) according to the standard applied, and are generally reported together with representative micrographs. The acceptance criteria are set by the customer specification, the product standard or the welding procedure standard (WPS/PQR), and the result is interpreted as conforming/non-conforming against these criteria. The value of the method lies in revealing, on an evidence-based footing, the metallurgical history of a material or joint that cannot be seen at macro level. TÜV AUSTRIA SILA KALİTE provides this service as a laboratory service accredited within the scope of TS EN ISO/IEC 17025, in accordance with internationally recognised specimen preparation and evaluation standards.