What is the difference between micro and macro examination in metallurgy field?

Answer 1

Macro Examination

Macroetching is the procedure in which a specimen is etched and evaluated macro structurally at low magnifications. It is a frequently used technique for evaluating steel products such as billets, bars, blooms, and forgings. There are several procedures for rating a steel specimen by a graded series of photographs showing the incidence of certain conditions and is applicable to carbon and low alloy steels. A number of different etching reagents may be used depending upon the type of examination to be made. Steels react differently to etching reagents because of variations in chemical composition, method of manufacturing, heat treatment and many other variables.

Macro-Examinations are also performed on a polished and etched cross-section of a welded material. During the examination, a number of features can be determined including weld run sequence, important for weld procedure qualifications tests. As well as this, any defects on the sample will be assessed for compliance with relevant specifications. Slag, porosity, lack of weld penetration, lack of sidewall fusion and poor weld profile are among the features observed in such examinations. It is normal to look for such defects either by standard visual examination or at magnifications of up to 50X. It is also routine to photograph the section to provide a permanent record. This is known as a photomacrograph.

Micro Examination

This is performed on samples either cut to size or mounted in a resin mold. The samples are polished to a fine finish, normally one micron diamond paste, and usually etched in an appropriate chemical solution prior to examination on a metallurgical microscope. Micro-examination is performed for a number of purposes, the most obvious of which is to assess the structure of the material. It is also common to examine for metallurgical anomalies such as third phase precipitates, excessive grain growth, etc. Many routine tests such as phase counting or grain size determinations are performed in conjunction with micro-examinations