Macro Analysis / Examination

Macro analysis (macroscopic examination) is a destructive testing method in which the sectioned surface of a metallic specimen is examined with the unaided eye or with low-magnification optical aids (magnifier, stereo microscope) up to a maximum of 50X, in order to evaluate the general internal structure of the material and of the weld. The basic physical principle of the method is that the different metallurgical regions (parent metal, heat-affected zone, weld metal) and the discontinuities react differently to a suitable etching process. Because the etchant selectively attacks grain boundaries, segregation lines, cast structure and weld pass boundaries, these regions reflect light differently and create a contrast on the section that is distinguishable by eye. Direct and holistic information about the macro-scale integrity of the component is thus obtained without going down to the microscopic level.

The examination begins with destructively removing the section to be examined from the component. On welded joints the specimen is generally cut perpendicular to the weld axis; this transverse section makes all layers of the weld visible in a single plane. Cooled cutting is preferred so that the heat generated during cutting does not alter the microstructure. The surface is then taken through coarse and fine grinding to obtain a flat surface free of scratch marks; for macro examination a suitable surface preparation is usually sufficient without going as far as polishing. The prepared surface is brought into contact with an etching reagent suited to the type of material (for example dilute nitric acid or hot hydrochloric acid solutions for steels), by immersion or swabbing, and after rinsing and drying it is ready for evaluation.

In terms of equipment the method relies less on complex instruments than on correct specimen preparation and experienced interpretation. The essential equipment consists of a precision cutting machine, a grinding/polishing bench, suitable chemical reagents, a fume hood, a stereo microscope or low-magnification magnifier, and a calibrated measuring scale together with a photographic recording arrangement. The limitation of magnification to 50X is the fundamental criterion that separates the method from microstructural (microscopic) examination: macro analysis evaluates the overall geometry of the weld and macro discontinuities, not individual grains or phase distinctions. The etching time, temperature and choice of reagent are adjusted to the material; insufficient etching prevents contrast from developing, while excessive etching darkens the surface and causes detail to be lost.

On welded joints, macro analysis is one of the most direct methods of verifying weld quality. On the transverse section the root penetration, the depth of fusion, the sequence and arrangement of passes, the transition between the weld and the parent metal and the width of the heat-affected zone (HAZ) can be seen clearly. On the same section, volumetric and planar discontinuities such as porosity, slag inclusions, incomplete penetration, lack of fusion, undercut, cracks and overlap can be measured. Within weld procedure qualification (WPS/PQR), geometric dimensions such as the a-dimension (effective throat) and the leg length are also verified on this section. On cast and forged parts the method is used to evaluate segregation, porosity, shrinkage cavities, flow lines (forging fibre orientation) and the general homogeneity of the structure.

In the evaluation stage the observed discontinuities are compared with the acceptance criteria of the relevant product or welding standard and a conforming/non-conforming decision is made for the specimen; weld quality levels may, for instance, be interpreted with reference to standards such as ISO 5817. The report contains the specimen identification, the location and orientation of the section, the material, the etchant and magnification used, the macrograph, the measured dimensions, the discontinuities detected and the result according to the applied acceptance criterion. TÜV AUSTRIA SILA KALİTE performs macro analysis as a testing laboratory accredited to TS EN ISO/IEC 17025, carrying out every step from specimen preparation to reporting in accordance with the methods and acceptance criteria defined in the standards.