The specimen is examined in the as-polished condition for voids of different types, such as porosity from gas evolution or shrinkage cavities; cracks that may be present in either the weld metal or the heat affected zone; regions where the weld did not exist (lack of fusion or lack of penetration); and nonmetallic inclusions associated with the welding operation, chiefly slag-type in nature. Then the metallographer will etch the specimen to study both the macrostructure and the microstructure using an etchant appropriate for the alloy. In some cases, the weld metal is of sufficiently different composition that an etchant chosen to etch the base metal and heat-affected zone will not reveal the weld-metal structure, and vice versa. If the specimen has been polished, the macrostructural details are usually adequately revealed by the etchant used to reveal the microstructure. In some studies, the metallographer will macroetch the specimen after grinding and study the macrostructure. This specimen is not suitable for microstructural examination.
Most etchants used to reveal the structure of welds are standard general-purpose etchants. After examination with such an etch, it may prove to be valuable to use a color-etching technique, as these can be far more sensitive for revealing grain structure, segregation, residual strain and deformation. However, these etchants are not widely used. Their use does require a very well-prepared specimen for good results. But this level of perfection is easily achieved with modern equipment and consumable products. Figures 1a and 1b show an example of the superiority of color etching over standard etchants in revealing the grain structure of a low-carbon steel weld.